Marie-Laure Mugnier

Marie-Christine Rousset

Federico Ulliana

Ontology-Mediated Queries for NOSQL Databases (Extended Abstract)

Introduction

Ontology-based data access (OBDA) is a well-established paradigm for querying incomplete datasources while taking into account knowledge provided by a domain ontology [PLC + 08]. Today, the main applications of OBDA can be found in data integration as well as in querying the Semantic Web. So far, OBDA has been studied for relational structures and deployed on top of relational databases, and then transposed to RDF. The database queries used in OBDA are (unions of) conjunctive relational queries (or basic graph pattern queries), while the ontologies are specified in either a description logic (e.g., the lightweight DL-Lite [CGL + 07][KLT + 10], or the expressive Horn-SHIQ [EOS + 12]), or a rule-based fragment of first-order logic (e.g., Datalog± [START_REF] Calì | Towards more expressive ontology languages: The query answering problem[END_REF] and existential rules [START_REF] Baget | On Rules with Existential Variables: Walking the Decidability Line[END_REF]). Decidability and complexity of ontologymediated query answering within this framework have been extensively studied and many algorithms have been designed and implemented.

Whether this paradigm can be used in conjunction with other kinds of query languages is a still an open question. The naive way to deal with non relational datasources is to define mappings for translating them into relational structures, and then use the classic OBDA framework as it is. However, this approach would induce a significant performance degrade as it would add a step for converting the data using the mappings and, most importantly, it would make impossible to take advantage of the low level query optimizations provided by native systems. This can be particularly acute for NOSQL systems, like key-value (KV) stores, that have been specifically designed to scale when dealing with very large collections of data.

Our goal is to study ontology-based data access directly on top of NOSQL systems. The term NOSQL (NotOnly SQL) defines a broad collection of languages. KV stores are NOSQL systems adopting the data model of KV records (also called JSON records). These records are processed on distributed systems, but also increasingly exchanged on the Web thereby replacing semistructured XML data and many RDF formats (see JSON-LD [SLK + 04]). KV records are non-first normal forms where values are not only atomic (in contrast with relational databases) and nesting is possible [START_REF]Foundations of Databases: The Logical Level[END_REF].

This extended abstract summarizes our work on OBDA for KV stores [START_REF] Mugnier | Ontologymediated queries for nosql databases[END_REF]. We introduce NO-RL, a rule language to express lightweight ontologies on top of KV records, and provide first (un)decidability and data complexity results for ontologymediated query answering in our setting. The NO-RL rule language operates directly on KV records and can be compared to existing languages designed for reasoning on nested structures like F-Logic [START_REF] Kifer | Logical foundations of object-oriented and frame-based languages[END_REF][START_REF] Kifer | Rules and Ontologies in F-logic[END_REF], Elog [BFGK01,BCG + 03,GK04] and Active XML [START_REF] Abiteboul | Positive Active XML[END_REF]. However, up to our knowledge, none of this existing work follows an OBDA approach.

Data and Queries

Our first contribution is a formalization of the data model of KV stores (sets of KV records) and the associated basic queries. In this abstract, we will just illustrate these notions by an example. The KV record below contains ten different keys: most of them are associated with atomic values, except phone associated with a record, professor and course associated with sequences (of records and of sequences, respectively) and director associated with an unspecified value null .

{ department : "CS", professor : [ { name : "Alice", reachable : "yes", boss : "Charles" } { name : "Bob", phone : {office : "5-256"}, speciality :

"Logics"} ] course : [ ["C123", "Java"] , ["C310", "C++"] ] director : null }
KV stores feature a limited set of low-level highly parallelizable queries based on paths, and leave other functionalities (such as joins between records) for the application accessing the data. An example of query retrieving the names of professors is get(professor.name). This expression returns the values "Alice" and "Bob" once evaluated on the example record. Another feature supported by KV stores is the possibility to check structural properties of a record before selecting some content. Thus, we further look at queries with a check() construct like the expression check(director). get(professor.name). The get part of the query is evaluated at the only condition that the key director also exists in the record. The check() construct is particularly useful when dealing with incomplete information expressed by null values, like for the key director in the example.

The NO-RL Rule Language

To define rules on KV records, we consider atoms of the form κ(x), where κ is a (finite) sequence of keys, called a key-path, and x a variable. NO-RL rules can be of two kinds: κ (x) -→ κ(x) or κ (x) -→ ∃yκ(y), as illustrated below:

1. "every phone number is a contact (any value for the key phone is a value for the key contact) " phone(x) -→ contact(x) 2. "if there is a reachable field then there is a phone number" reachable(x) -→ ∃y. phone(y) 3. "if a professor has a boss, then this last one is a director of the department" professor.boss(x) -→ director(x) 4. "the department defines a teaching specialty for all of its professors" department(x) -→ professor.specialty(x)

We define three rule fragments: NO-RL(1), composed of rules that employ only key-paths of length one (like Rules 1 and 2 in the example), NO-RL(2) and NO-RL(3), which both extend NO-RL(1) by allowing respectively for rule bodies of arbitrary length (like Rule 3) and for rule heads of arbitrary length (like Rule 4).

The following record is obtained from the previous one by application of the four above rules until fixpoint (process called "saturation").

{ department : "CS", professor : [ {name : "Alice", reachable : "yes", boss : "Charles", phone : null, specialty : "CS" }, {name : "Bob", phone : {office : "5-256"}, contact : {office : "5-256"}, specialty : ["CS", "Logics"]} ] course : [["C123", "Java"], ["C310", "C++"]] director : "Charles" } Rule application inserts a path into the existing record, which requires to merge information (e.g., the application of Rule 4 inserts the path professor.specialty : "CS"). We show that the inference operator associated with rule application is confluent (up to a notion of record equality), hence the order in which rules are applied is irrelevant. Finally, a value is an answer to a query Q over a knowledge base (I, Σ), where I is a set of records and Σ a set of NO-RL rules, if it is an answer to Q on a record inferred from a record in I by the rules in Σ.

Decidability and Complexity Results

Despite the apparent simplicity of the NO-RL language, answering check/get queries under NO-RL rules is generally undecidable (which we prove by reduction from the word problem in a semi-Thue system). Intuitively, this is due to the unrestricted interaction between NO-RL(2) and NO-RL(3) rules. In particular, decidability can be recovered by restricting the rule language to NO-RL(2) or NO-RL(3) only.

A soon as a single NO-RL(3) rule is involved, saturation may be infinite. If we consider NO-RL(2) rules, saturation is always finite since these rules do not increase the depth of records. However, with a single NO-RL(1) rule already, saturation can be exponential in the size of the input record. This observation advocates for query reformulation techniques in the spirit of the OBDA paradigm. We define a sound and complete reformulation technique (i.e., a value is an answer to Q over (I, Σ) iff if it is an answer to a Σ-reformulation of Q over I). Dually to saturation with NO-RL(2) rules, query reformulation with NO-RL(3) rules is always finite since these rules do not increase the depth of records. It follows that the data complexity of (the decision problem associated with) query answering with NO-RL(3) rules is the same as basic query answering on KV stores, i.e., it is in the low complexity class AC 0 . Query reformulation can be infinite with NO-RL(2) rules, however, given a record of depth d, only reformulations of depth less or equal to d are useful. There is an exponential number of useful reformulations, nevertheless each of them can be produced with a polynomial number of steps ((d + 1) × |Σ|), hence query answering over NO-RL(2) rules is in NP (for both data and combined complexities -the exact lower bounds being open).

Future work includes further clarifying query answering complexity, identifying decidable / tractable cases mixing NO-RL(2) and NO-RL(3) rules, as well as designing algorithms that would exploit the parallelization features of KV stores. Acknowledgments. This work has been partially supported by projects PAGODA (ANR-12-JS02-007-01) and Labex PERSYVAL-Lab (ANR-11-LABX-0025-01).


A Gourbil

P Duru

F Fichot

M Quintard

M Prat

EXPERIMENTAL STUDY OF BOILING IN POROUS MEDIA: EFFECT OF THE VOID FRACTION ON THE CRITICAL HEAT FLUX

Forced-convection boiling is experimentally studied in a two-dimensional heat-generating model porous medium. Visualizations of the two-phase flow and thermal measurements inside the test section are performed. Boiling curves show that the critical heat flux on a specific heating element decreases as the global thermal power released by the porous medium increases. Also, visualizations show that the void fraction in the test section increases with the global heating power. An image processing program was developed to measure the void fraction, in order to investigate its influence on the critical heat flux. Such a measurement is validated using a second test section where air is injected at a known flow rate inside the device, in isothermal conditions, and where flow regimes comparable to those observed in the boiling experiment are obtained.

NOMENCLATURE q CHF

Critical heat flux (W/m 2 ) T cylinder Wall temperature of the cylinder under study (°C) T inlet Liquid inlet temperature (°C) y Height in the test section (m) ROI Region of interest

INTRODUCTION

Reflooding during a reactor severe accident is an important topic in nuclear safety studies. Following a loss of coolant accident in a pressurized water reactor, the materials of the core can crumble and form a porous, hot debris bed that must be efficiently cooled. This is done by reflooding the debris bed with water. However, this operation can cause hazardous oxidization reactions between water steam and zircaloy contained in the fuel cladding [START_REF] Hering | Degraded core reflood: Present understanding and impact on LWRs[END_REF], and its success to cool the core by extracting the nuclear residual power is not guaranteed because dry-out phenomena can occur. Since the seventies, many studies have been made to investigate the coolability of a hot debris bed, [START_REF] Hering | Degraded core reflood: Present understanding and impact on LWRs[END_REF][START_REF] Lee | Flow visualization in heat generating porous media[END_REF][START_REF] Lipinski | Model for boiling and dryout in particle beds[END_REF][START_REF] Schulenberg | An improved model for two-phase flow through beds of coarse particles[END_REF][START_REF] Broughton | A Scenario of the Three Mile Island Unit 2 Accident[END_REF][START_REF] Tutu | Debris bed quenching under bottom flood conditions[END_REF][START_REF] Catton | Experimental study of debris-bed coolability under poolboiling conditions[END_REF][START_REF] Tung | A hydrodynamic model for two-phase flow through porous media[END_REF] to cite a few. They are reviewed and discussed by Fichot et al. [START_REF] Fichot | The impact of thermal nonequilibrium and large-scale 2D/3D effects on debris bed reflooding and coolability[END_REF]. Many of them tried to determine the dry-out conditions of a reflooded debris bed, and to define a dry-out critical heat flux. Indeed, because of the low thermal conductivity of the vapor, a dry zone in the debris bed may heat up to a point where particles can melt and form a mixture called corium. Thus, the dry-out heat flux is considered to be the highest thermal flux that water can extract from a debris bed [START_REF] Lipinski | Model for boiling and dryout in particle beds[END_REF][START_REF] Groeneveld | The 1995 look-up table for critical heat flux in tubes[END_REF].

At the French Institute for Radiation Protection and Nuclear Safety (IRSN), the ICARE/CATHARE code used to simulate reflooding is based on a thermal-hydraulic two-phase flow model in porous media [START_REF] Fichot | The impact of thermal nonequilibrium and large-scale 2D/3D effects on debris bed reflooding and coolability[END_REF][START_REF] Petit | Écoulement diphasique en milieu poreux : modèle à nonéquilibre local[END_REF][START_REF] Duval | A local thermal non-equilibrium model for two-phase flows with phase-change in porous media[END_REF][START_REF] Bachrata | Quench front progression in a superheated porous medium : experimental analysis and model development[END_REF]. To validate and improve this model, an experimental research program has been set up. Two test facilities have been made, called PRELUDE and PEARL. These experiments consist in reflooding with water a packing of steel beads heated by an induction coil [START_REF] Repetto | Experimental program on debris reflooding (PEARL) -Results on PRELUDE facility[END_REF]. They allow to test the model used, providing data such as steam production rate, pressure drop, temperature at different points in the test section. Yet, these are essentially macroscopic data, and local information about the flow at the pore scale is also needed to improve the model [START_REF] Sapin | Reflooding with internal boiling of a heating model porous medium with mm-scale pores[END_REF]. Thus, a collaboration has been undertaken between IRSN and the Fluid Mechanics Institute of Toulouse (IMFT) in order to characterize boiling phenomena at the pore scale. A nearly two-dimensional heat-generating model porous medium has been set up [START_REF] Sapin | Etude expérimentale de l'ébullition en masse dans un milieu poreux modèle[END_REF], which allows the monitoring of the thermal power released by the heating elements, the measurement of their temperature, and visualizations of the flow patterns. This experimental device is introduced in a first part of this paper and then, the results of typical boiling experiments are detailed. An important finding is that the critical heat flux on a given heating element decreases as the global thermal power released by the porous medium increases. Also, visualizations show that the void fraction around the heating element under study has a great influence on the critical heat flux. Finally, an image processing method aiming at estimating the void fraction in the test section is presented.

EXPERIMENTAL SETUP

The experimental setup is fully described in [START_REF] Sapin | Reflooding with internal boiling of a heating model porous medium with mm-scale pores[END_REF]. The test cell is composed of 392 cylinders positioned between two ceramic plates, one of which is transparent, allowing direct flow visualizations (see Fig. 1). The staggered arrangement of the 2 mm diameter cylinders is slightly randomly disturbed. The spacing between the plates is 3 mm. The cylinders are Pt-100 resistance temperature detectors (RTD) heated by Joule effect. They perform a dual function: they are used as both temperature probes and heating elements. It is the main original feature of this experimental device, which gives the possibility of heating the porous medium while controlling the thermal power released by the heating elements and measuring their temperature.

The heat flux applied to each cylinder is controlled by a microelectronic servo-control system. The test section is held vertically, the liquid is injected at the bottom at a controlled flow rate, pressure, and inlet temperature. The working fluid is the HFE-7000, which boils at 34°C at atmospheric pressure. It was chosen because of its low saturation temperature, which allows to study all the boiling regimes, in particular film boiling, while keeping the thermal constraints in the setup at a relatively low level. The size of the cylinders has been calculated so that the ratio of their diameter to the capillary length of the HFE-7000 is the same as the ratio of the steel beads diameter to the capillary length of water in the PRELUDE experiment: thus, bubble size distribution and confinement effects are comparable in both experiments.

EXPERIMENTAL RESULTS

In the present paper we only present some results obtained in steady-state boiling regimes. Results in transient, reflooding experiments can be found in Sapin et al. [START_REF] Sapin | Reflooding with internal boiling of a heating model porous medium with mm-scale pores[END_REF]. Forcedconvection boiling curves, later on referred as Nukiyama curves [START_REF] Nukiyama | The maximum and minimum values of the heat Q transmitted from metal to boiling water under atmospheric pressure[END_REF] have been established for a particular heating element, while all the others dissipated a fixed, controlled thermal power. Figure 2 displays the heating configuration of the model porous medium. Initially, the test section is saturated by the liquid, flowing upward at a flow rate fixed by a gear pump (the filtration velocity is 3.6 mm/s), while a constant thermal power is applied to the whole bundle of heating cylinders, except one. The cylinder under study is then progressively heated so that all boiling regimes are obtained at its surface, from nucleate to film boiling. A steady-state cylinder temperature is measured for the different heating powers. On Fig. 3, we compare two boiling curves obtained in two different configurations. In a first reference case, labeled "single probe" in Fig. 3, only the cylinder under study is heated. Then in another second case, labeled "probe in a bubble swarm", the Nukiyama curve for the cylinder under study is obtained while a fixed heat flux is applied to the surrounding cylinders.

Both curves display the typical shape of a Nukiyama curve: the evolution of the heat flux applied as a function of the cylinder wall temperature in forced-convection boiling in porous media is similar to the one obtained in pool boiling. However, we note that the critical heat flux q CHF at which the boiling crisis occurs is lower when the cylinder under study is within the vapor bubbles swarm generated by boiling at the surface of the others heating elements. Values of critical heat In addition to thermal measurements, high-speed camera visualizations show that the presence of bubbles in the vicinity of the heating cylinder under study favors the formation of a vapor film around it, thus triggering the transition to the film boiling regime at lower imposed heating flux, compared to the situation without bubble swarm. To discuss this point quantitatively, we are currently looking at determining by image processing the void fraction in the test section, as a function of the thermal power released by the heated cylinders. This is detailed in the next section.

IMAGE PROCESSING: TOWARDS AN ESTIMATION OF THE VOID FRACTION

Direct visualizations

Two kinds of direct visualizations by video image recording have been performed, in three different steady-state forced-convection boiling cases where one, two, or three consecutive horizontal lines of cylinders are heated, see Fig. 2. Each cylinder dissipates a 400 mW power, the fluid filtration velocity and pressure being held constant (at 3.6 mm/s and 1.23 bar respectively). In both cases, images were first processed by subtracting a background image obtained while a one-phase liquid flow was set in the test section. Consequently, the bubbles are displayed in the clearest grey levels on the obtained images, as shown on Fig. 5. First, image sequences were obtained by high-speed visualization with a Dimax PCO camera. The acquisition frequency was then 500 frames per second, with a 35.5 pixels per millimeter spatial resolution. An annular region of interest (ROI) is defined around a given cylinder, as shown on Fig 4 and the mean grey level (i.e. the average on all the pixels contained in the ROI) is computed for each image. This mean value is close to zero when no bubbles are present in the ROI and increases sharply when one or more bubbles are located in the ROI. A typical time-resolved signal is shown in Fig. 6: each spike corresponds to one or many bubbles passing through the ROI. The mean value of this signal, integrated over the observation period, is therefore an increasing function of the time-averaged void fraction inside the ROI. Such a void fraction indicator has been calculated around each of the 232 cylinders downstream the heated lines, for each boiling configuration studied, over a 2 seconds period. Averaging the indicator over a given line of cylinders allows one to plot this mean value as a function of the height in the test section, as well as a function of the number of heated lines, see Fig. 7. The results displayed in Fig. 7 clearly show that the largest the number of heated lines is, the larger the time-averaged void fraction is, as expected. More interestingly, they also suggest that the bubble size distributions reach a steady state beyond a certain height, as the indicator mean values seem to converge. We are currently looking to calibrate this indicator value against the void fraction.

Second, images were acquired at a 0.5 Hz frequency with a Sensicam camera. Whatever the boiling regime is, a bubble takes less than 1 second to travel through the whole test section, from bottom to top. Taking pictures every 2 seconds ensure that we obtain a statistically meaningful set of images, in order to perform ensemble averaging. Three sets of 1500 images were then processed, heating respectively one, two, and finally three lines of cylinders. For each set, a mean image was calculated, see Fig. 8. FIG. 6. Typical temporal evolution of the mean grey level in a Region Of Interest (10 pixels wide ring, displayed in green in Figure 4) located around a given cylinder.

On such an image, the mean grey level over horizontal stripes was calculated as a function of the height in the test section. The results, plotted on Fig 9 , show the same trend as the one presented in Fig 7. This shows that, as far as void fraction evolution with height is concerned, the pieces of information brought by time-resolved fast speed imaging and ensemble averaging over uncorrelated images are the same. In this last case, a calibration of the mean grey level as a function of the void fraction must also be done. First steps in that direction are presented below.

Void fraction measurement

To our knowledge, the actual state of the art concerning two-phase flow quantitative visualization in porous media is limited to finely dispersed bubbly flows, where bubble size is smaller than the throats of the pore space [START_REF] Jo | Bubble mechanisms and characteristics at pore scale in a packed-bed reactor[END_REF][START_REF] Jo | Effect of coalescence and breakup on bubble size distributions in a twodimensional packed bed[END_REF]. In our case, intense boiling regimes with greater bubble sizes are investigated. As already seen on Figure 5, bubbles shapes are often complex, with contours displaying convex and concave parts due to the bubbles confinement between the neighboring cylinders. Also, the contrast between the bubbles and the test section ceramic back-plate is quite low. Moreover, contours of bubbles as well as their interiors display light and dark areas, because of the many reflections due to the surrounding cylinders. For all these reasons, processing such images is particularly challenging.

The main steps of the image processing algorithm are listed below: -Subtraction of the background image -Application of a mask to hide the cylinders -Thresholding to binarize the image -Dilatation-erosion operations to smooth the bubbles edges -Labeling: allocation of a unique number for each identified bubble To perform these image processing steps, specific Matlab scripts have been written. The two key steps affecting the final results are the thresholding and dilatation-erosion operations. The binarization threshold value and then the size of the structuring element applied are operator dependent because there is no quantitative criterion to fix them. The dilatationerosion operation is essential to smooth the bubble edges after the thresholding. As shown in Fig. 10, it consists in scanning the bubble edges with a "structuring element", a circle in our case, which successively enlarge the contours (dilatation) and then shrink them (erosion). This can result, according to the circle radius, into a more or less pronounced smoothing of the edges, and even in a numerical coalescence of some neighboring bubbles. The threshold value and the structuring element size are estimated on a few images by trial and errors and visual inspection with a naked eye. Once the images have been processed as explained above, a projected bubble area is easily determined, allowing to calculate the ratio of the total surface of all the bubbles to the pore-space projected area. This surface fraction can be considered as a first indicator of the order of magnitude of the void fraction in the test section, as proposed by Jo and Revankar [START_REF] Jo | Effect of coalescence and breakup on bubble size distributions in a twodimensional packed bed[END_REF], see Table1. To estimate a real mean void fraction corresponding to the vapor volume fraction in the test section, bubbles volumes, and not only projected areas, have to be estimated. For this purpose, we plan to classify bubbles into three categories depending on their projected areas. A small bubble will be considered to be spherical and its volume will be estimated from the determination of an equivalent radius based on its projected area. A large bubble, extending over two or more pores, will be assumed to be in contact with the test section front and back plates. A major difficulty will lie in the determination of the volume of the intermediate size bubbles.

In order to calibrate the volume bubble estimation and to evaluate the uncertainty, a test section with the same geometry as the heating model has been 3D printed. It was designed to permit air to be injected downstream the section at a known flow rate, in isothermal configuration, while a HFE-7000 flow is imposed from bottom to top. Bubbling tests have been performed to check that two-phase flow patterns similar to those obtained with the boiling experiment are recovered. The translucent test section allows to use a backlight, offering a good contrast between the bubbles and the porous medium and thus easier image processing. Images from the isothermal experiment are currently being processed with the same algorithm that the one used for the boiling experiment. The idea is to recover the imposed air flow rate from image processing, in order to validate the parameters used in the algorithms to estimate the bubbles volumes (grey level threshold, structuring element size, and bubble classification in three main types according to their size). This methodology will also allow us to estimate rigorously the uncertainty on the volume void fraction measurement.

CONCLUSION

A nearly two-dimensional heat-generating model porous medium was used to conduct forced convection boiling experiments. The decrease of the critical heat flux around a given element due to the increase of the surrounding porous matrix heating has been experimentally demonstrated. The ongoing objective is to link this critical heat flux variation to the void fraction, using two-phase flow visualizations. So far, image processing is limited to the estimation of indicators linked to the local time-averaged void fraction, and to the calculation of the surface void fraction based upon the projection of the bubble areas into a vertical plane. Work is in progress to refine the analysis in order to estimate the volume void fraction. To this purpose, a calibration of the various parameters used in the image processing algorithm is being performed thanks to isothermal bubbling experiment visualizations for which the injected gas flow rate is known. When such a calibration will be performed, the influence of the void fraction on the critical heat flux will be quantified and investigated. These results will have to be taken into account in the reflooding numerical simulation tools used at the IRSN, where critical heat fluxes are so far estimated by using correlations established for convective boiling around one single heating tube [START_REF] Groeneveld | The 2006 CHF look-up table[END_REF][START_REF] Chikhi | Reflooding model for quasi-intact rod configuration: Quench front tracking and heat transfer closure laws[END_REF] and not in a bundle of heating tubes.

FIG. 1 .

 1 FIG. 1. Sketch of the test section. Each cylinder is a Pt100 resistance temperature detector.

FIG. 2 .

 2 FIG. 2. Heating configuration of the model porous medium.Nukiyama boiling curves: the cylinder under study is highlighted in red, the surrounding heating cylinders in green. Image processing: L1, L2 and L3 are the heated cylinders lines.

FIG. 5 .

 5 FIG.5. Close-up on a typical image obtained after background subtraction. A region of interest around a given cylinder is highlighted in green.

FIG. 10

 10 FIG. 10. a) Close-up on a binarized image just after thresholding; b) c) d) Same close-up after dilatation-erosion operation performed with different radii R of the circular structuring element.

TABLE 1 .

 1 Surface void fraction as a function of the total heating power.

		1 heated	2 heated	3 heated
		line	lines	lines
	Total heating power	5,6 W	11,6 W	17,2 W
	Surface vapor void	0,07	0,22	0,47
	fraction

ACKNOWLEDGMENTS

This work was supported in part by Electricité de France (EDF). Authors are also grateful to Ruddy Soeparno and Hervé Ayroles for their contribution to the experimental setup development.


Patrice Guyot
email: guyot.patrice@gmail.com

Julien Pinquier
email: julien.pinquier@irit.fr

BROWSING SOUNDSCAPES

Browsing soundscapes and sound databases generally relies on signal waveform representations, or on more or less informative textual metadata. The TM-chart representation is an efficient alternative designed to preview and compare soundscapes. However, its use is constrained and limited by the need for human annotation. In this paper, we describe a new approach to compute charts from sounds, that we call SamoCharts. SamoCharts are inspired by TMcharts, but can be computed without a human annotation. We present two methods for SamoChart computation. The first one is based on a segmentation of the signal from a set of predefined sound events. The second one is based on the confidence score of the detection algorithms. SamoCharts provide a compact and efficient representation of sounds and soundscapes, which can be used in different kinds of applications. We describe two application cases based on field recording corpora.

INTRODUCTION

Compact graphical representations of sounds facilitate their characterization. Indeed, images provide instantaneous visual feedback while listening sounds is constrained by their temporal dimension. As a trivial example, record covers allow the user to quickly identify an item in a collection. Such compact representation is an efficient means for sound identification, classification and selection.

In the case of online databases, the choice of a sound file in a corpus can be assimilated to the action of browsing. As proposed by Hjørland, "Browsing is a quick examination of the relevance of a number of objects which may or may not lead to a closer examination or acquisition/selection of (some of) these objects" [START_REF] Hjørland | The importance of theories of knowledge: Browsing as an example[END_REF].

Numerous websites propose free or charged sound file downloads. These files generally contain sound effects, isolated sound events, or field recordings. Applications are numerous, for instance for music, movie soundtracks, video games and software production. In the context of the CIESS project 1 , our work focuses on an urban sound database used for experimental psychology research.

Most of the times, on-line access to sound files and databases is based on tags and textual metadata. These metadata are generally composed of a few words description of the recording, to which may be added the name of its author, a picture of the waveform, and other technical properties. They inform about the sound sources, recording conditions or abstract concepts related to the sound contents (for example "Halloween").

Natural sonic environments, also called field recordings or soundscapes [START_REF] Schafer | The tuning of the world[END_REF], are typically composed of multiples sound sources. Such audio files are longer than isolated sound events, usually lasting more than one minute. Therefore, short textual descriptions are very hard to produce, which makes it difficult to browse and select sounds in a corpus.

The analysis and characterization of urban sound events has been reported in different studies. Notably, they can be merged in identified categories [START_REF] Guastavino | Categorization of environmental sounds[END_REF], which leads to a taxonomical categorization of environmental sounds (see [START_REF] Payne | Research into the practical and policy applications of soundscape concepts and techniques in urban areas[END_REF] for an exhaustive review). Outdoor recordings are often composed of the same kinds of sound sources, for instance birds, human voices, vehicles, footstep, alarm, etc. Therefore, the differences between two urban soundscapes (for example, a park and a street) mostly concern the time of presence and the intensity of such identified sources. As a consequence, browsing field recordings based on the known characteristics of a set of predetermined sound events can be an effective solution for their description.

Music is also made of repeated sound events. In instrumental music, these events can be the notes, chords, clusters, themes and melodies played by the performers. When electroacoustic effects or tape music parts come into play, they can be of a more abstract nature. In the case of musique concrete, the notion of "sound object" (which in practice is generally a real sound recording) has its full meaning and a central position in the music formalization itself [START_REF] Schaeffer | Traité des objets musicaux[END_REF]. As long as events are identified though, we can assume that the previous soundscape-oriented considerations hold for musical audio files as well.

The TM-chart [START_REF] Hiramatsu | The physcial expression of soundscape: An investigation by means of time component matrix chart[END_REF] is a tool recently developed to provide compact soundscape representations starting from a set of sound events. This representation constitutes a bridge between physical measures and categorization, including acoustic and semantic information. Nevertheless, the creation of a TM-chart relies on manual annotation, which is a tedious and time-consuming task. Hence, the use of TM-charts in the context of big data sets or for online browsing applications seems unthinkable.

Besides sound visualization, automatic annotation of audio recordings recently made significant progress. The general public has recently witnessed the generalization of speech recognition system. Significant results and efficient tools have also been developed in the fields of Music Information Retrieval (MIR) and Acoustic Event Detection (AED) in environmental sounds [START_REF] Mesaros | Acoustic event detection in real life recordings[END_REF], which leads us to reckon with sustainable AED in the coming years.

In this paper, we propose a new paradigm for soundscape representation and browsing based on the automatic identification of predefined sounds events. We present a new approach to create compact representations of sounds and soundscapes that we call SamoCharts. Inspired by TM-Charts and recent AED techniques, these representations can be efficiently applied for browsing sound databases. In the next section we present a state of the art of online sound representations. The TM-chart tool is then described in Section 3, and Section 4 proposes a quick review of Audio Event Detection algorithms. Then we present in Section 5 the process of SamoCharts creation, and some applications with field recordings in Section 6.

SOUND REPRESENTATION

Temporal Representations

From the acoustic point of view, the simplest and predominant representation of a sound is the temporal waveform, which describes the evolution of sound energy over time. Another widely used tool in sound analysis and representation is the spectrogram, which shows more precisely the evolution of the amplitude of frequencies over time. However, spectrograms remain little used by the general public.

While music notation for instrumental music has focused on the traditional score representation, the contemporary and electro-acoustic music communities have introduced alternative symbolic representation tools for sounds such as the Acousmograph [START_REF] Geslin | Sound and musical representation: the acousmographe software[END_REF], and the use of multimodal information has allowed developing novel user interfaces [START_REF] Damm | Multimodal presentation and browsing of music[END_REF].

All these temporal representations are more or less informative depending on the evolution of the sound upon the considered duration. In particular, in the case of field recordings, they are often barely informative.

Browsing Sound Databases

On a majority of specialized websites, browsing sounds is based on textual metadata. For instance, freeSFX2 classifies the sounds by categories and subcategories, such as public places and town/city ambience. In a given subcategory, each sound is only described with a few words text. Therefore, listening is still required to select a particular recording.

Other websites, such as the Freesound project, 3 add a waveform display to the sound description. In the case of short sound events, this waveform can be very informative. On this website it is colored according to the spectral centroid of the sound, which adds some spectral information to the image. However, this mapping is not precisely described, and remains more aesthetic than useful.

The possibility of browsing sounds with audio thumbnailing has been discussed in [START_REF] Bartsch | Audio thumbnailing of popular music using chroma-based representations[END_REF]. In this study, the authors present a method for searching structural redundancy like the chorus in popular music. However, to our knowledge, this kind of representation has not been used in online systems so far.

More specific user needs have been recently observed through the DIADEMS project 4 in the context of audio archives indexing. Through the online platform Telemeta 5 , this project allows ethnomusicologists to visualize specific acoustic information besides waveform and recording metadata, such as audio descriptors and semantic labels. This information aims at supporting the exploration of a corpus as well as the analysis of the recording. This website illustrates how automatic annotation can help to index and organize audio files. Improving its visualization could help to assess the similarity of a set of songs, or to underline the structural form of the singing turns by displaying homogeneous segments.

Nevertheless, texts and waveforms remain the most used and widespread tools on websites. In the next sections, we present novel alternative tools, that have been specially designed for field recording representation.

TM-CHART

Overview

The Time-component Matrix Chart (abbreviated TM-chart) was introduced by Kozo Hiramatsu and al. [START_REF] Hiramatsu | The physcial expression of soundscape: An investigation by means of time component matrix chart[END_REF]. Based on a <Sound Source × Sound level> representation, this chart provides a simple visual illustration of a sonic environment recording, highlighting the temporal and energetic presence of sound sources. Starting from a predetermined set of sound events (e.g. vehicles, etc.), and after preliminary annotation of the recording, the TM-chart displays percentages of time of audibility and percentages of time of level ranges for the different sound sources. They constitute effective tools to compare sonic environment (for instance daytime versus nighttime recordings).

Method

Despite a growing bibliography [START_REF] Matsui | Timecomponent matrix chart as a tool for designing sonic environment having a diversity of sound sources[END_REF][START_REF] Bild | Habitats assen pilot: Testing methods for exploring the correlation between sound, morphology and behavior[END_REF], the processing steps involved in the creation of TM-charts as not been precisely explained. We describe in this part our understanding of these steps and our approach to create a TM-chart.

Estimation of the Predominant Sound

TM-charts rely on a preliminary manual annotation, which estimates the predominant sound source at each time. To perform this task, the signal can be divided in short segments, for example segments of one second. For each segment, the annotator indicates the predominant sound source. This indication is a judgment that relies on both the loudness and the number of occurrences of the sources. An example of annotation can be seen on Figure 1.

Afterwards, each segment label is associated to a category of sound event, which can be for instance one of car, voice, birds, or miscellaneous.

Computation of the Energy Level

An automatic process is applied to compute the energy of the signal and the mean energy of each segment (respectively in blue and red curves on Figure 1). We assume that the sound pressure level can be calculated from the recording conditions with a calibrated dB meter.

In this process, we can notice that the sound level of a segment is not exactly the sound level of its predominant source. Indeed the sound level of an excerpt depends upon the level of each sound sources, and not only the predominant one. However, we assume that these two measures are fairly correlated.

Creation of the TM-chart

We can now calculate the total duration in the recording (in terms of predominance) and the main sound levels for each category of sound. From this information, a TM-chart can be created.

Figure 2 shows a TM-chart based on the example from Figure 1. It represents, for each category of sound, the percentage of time and energy in the soundscape. The abscissa axis shows the percentage of predominance for each source in the recording. For one source, the ordinate axis shows the duration of its different sound levels. For example, the car-horn is audibly dominant for over 5 % of time. Over this duration, the sound level of this event exceeds 60 dB for over 80 % of time.

Interpretation of the TM-chart

Charts like the one on Figure 2 permit quick interpretations of the nature of the sound events that compose a soundscape. We could infer for instance that the soundscape has been recorded close to a little traffic road, with distant conversations (low energy levels). From such interpretation, one can clearly distinguish and compare sonic environments recorded in different places [START_REF] Hiramatsu | The physcial expression of soundscape: An investigation by means of time component matrix chart[END_REF].

The main issue in the TM-chart approach is the need for manual annotation, a time-consuming operation which cannot be applied to big data sets. Therefore, the use of TM-charts seems currently restricted to specific scientific research on soundscapes. In the next sections we will show how recent researches and works on sound analysis can be leveraged to overcome this drawback.

AUDIO EVENT DETECTION

Various methods have been proposed for the Audio Event Detection (AED) from continuous audio sequences recorded in real life. These methods can be divided in two categories.

The first category of methods aims at detecting a large set of possible sound events in various contexts. For instance, the detection of 61 types of sound, such as bus door, footsteps or applause, has been reported in [START_REF] Mesaros | Acoustic event detection in real life recordings[END_REF]. In this work the author modeled each sound class by a Hidden Markov Model (HMM) with 3 states, and Mel-Frequency Cepstral Coefficients (MFCC) features. Evaluation campaigns, such as CLEAR [START_REF] Stiefelhagen | The CLEAR 2006 evaluation[END_REF] or AASP [START_REF] Giannoulis | Detection and classification of acoustic scenes and events[END_REF], propose the evaluation of various detection methods on a large set of audio recordings from real life.

The second category of methods aims at detecting fewer specific types of sound events. This approach privileges accuracy over the number of sounds that can be detected. It generally relies on a specific modeling of the "target sounds" to detect, based on acoustic observations. For example, some studies propose to detect gunshots [START_REF] Clavel | Events detection for an audio-based surveillance system[END_REF] or water sounds [START_REF] Guyot | Water sound recognition based on physical models[END_REF], or the presence of speech [START_REF] Pinquier | Robust speech/music classification in audio documents[END_REF].

These different methods output a segmentation of the signal informed by predetermined sound events. They can also provide further information that may be useful for the representation, particularly in the cases where they are not reliable enough. Indeed, the detection algorithms are generally based on a confidence score, that allows to tune the decisions. For instance, Hidden Markov Model, Gaussian mixture models (GMM) or Support Vector Machine (SVM), all rely on confidence or "likelihood" values. Since temporal confidence values can be computed by each method of detection, it is possible to output at each time the probability that a given sound event is present in the audio signal.

Based on these observations, we propose a new tool for soundscape visualization, the SamoChart, which can rely either on automatic sound event segmentation, or on confidence scores by sound events.

SAMOCHART

The SamoChart provides a visualization sound recordings close to that of a TM-chart. At the difference of a TMchart, it can be computed automatically from a segmentation or from temporal confidence values.

In comparison with TM-charts, the use of the automatic method overcomes a costly human annotation and avoids subjective decision-making.

Samochart based on Event Segmentation

Audio Event Segmentation

SamoCharts can be created from Audio Event Detection annotations. This automatic annotation is an independent process that can be performed following different approaches, as mentioned in Section 4. We will suppose in the next part that an automatic annotation has been computed from a set of potential sound events ("targets"). For each target sound event, this annotation provides time markers related to the presence or absence of this sound in the overall recording. In addition to the initial set of target sounds, we add a sound unknown that corresponds to the segments that have not been labeled by the algorithms.

Energy Computation

As in the TM-chart creation process, we compute the energy of the signal. However, if the recording conditions of the audio signal are unknown, we cannot retrieve the sound pressure level. In this case, we use the RMS energy of each segment, following the equation:

RM S(w) = 20 × log 10 N i=0 w 2 (i) (1)
where w is an audio segment of N samples, and w(i) the value of the i th sample.

SamoChart Creation

From the information of duration and energy, we are able to create a SamoChart. Figure 3 shows an example of a SamoChart based on event segmentation considering two possible sound events. Unlike TM-charts, we can notice from this method that the total percentage of sound sources can be higher than 100% if the sources overlap.

Samochart based on Confidence Values

Most Audio Event Detection algorithms actually provide more information than the output segmentation. In the following approach, we propose to compute SamoCharts from the confidence scores of these algorithms. We use for each target sound the temporal confidence values outputted by the method, which can be considered as probabilities of presence (between 0 and 1). The curve on Figure 4 shows the evolution of the confidence for the presence of a given sound event during the analyzed recording. We use a threshold on this curve, to decide if the sound event is considered detected or not. This threshold is fixed depending on the detection method and on the target sound. To obtain different confidence measures, we divide the upper threshold portion in different parts. With this approach, we infer the probability of presence for each sound event according to a confidence score. Figure 5 shows the SamoChart associated to a unique sound event. In this new chart, the sound level is replaced by the confidence score.

Implementation

We made a JavaScript implementation to create and display SamoCharts, which performs a fast and "on the fly" computation of the SamoChart. The code is downloadable from the SAMoVA web site 6 . It uses an object-oriented paradigm to facilitate future development.

In order to facilitate browsing applications, we also chose to modify the size of the chart according to the duration of the corresponding sound excerpt. We use the equation 2 to calculate the height h of the Samochart from a duration d in seconds.

h =        1 if d < 1 2 if 1 ≤ d < 10 2 × log 10 (d) if d ≥ 10 (2)
We also implemented a magnifying glass function that provides a global view on the corpus with the possibility of zooming in into a set of SamoCharts. Furthermore, the user can hear each audio file by clinking on the plotted charts.

APPLICATIONS 6.1 Comparison of soundscapes (CIESS project)

Through the CIESS project, we have recorded several urban soundscapes at different places and times. The sound events of these recordings are globally the same, for instance vehicle and footstep. However, their numbers of occurrences are very different according to the time and place of recording. As an application case, we computed several representations of two soundscapes. Figure 6 shows the colored waveforms of these extracts as they could have been displayed on the Freesound website. As we can see, these waveforms do not show great differences between the two recordings. We used AED algorithms to detect motor vehicle, footstep and car-horn sounds on these two example recordings [START_REF] Guyot | Soundscape visualization: a new approach based on automatic annotation and samocharts[END_REF]. Then, we computed SamoCharts based on the confidence score of these algorithms (see Figure 7).

The SamoCharts of Figure 7 are obviously different. They provide a semantic interpretation of the soundscapes, which reveals important dissimilarities. For instance, the vehicles are much more present in the first recording than in the second one. Indeed, the first recording was recorded on an important street, while the second one was recorded on a pedestrian street.

Browsing a corpus from the UrbanSound project

If the differences between two soundscapes can easily be seen by comparing two charts, the main interest of the SamoChart is their computation on bigger sound databases.

UrbanSound dataset7 has been created specifically for soundscapes research. It provides a corpus of sounds that are labeled with the start and end times of sound events of ten classes: air conditioner, car horn, children playing, dog bark, drilling, enginge idling, gun shot, jackhammer, siren and street music. The SamoCharts created from these annotations allow to figure out the sources of each file, as well as their duration and their sound level. They give an overview of this corpus. Figure 8 shows the SamoCharts of nine files which all contain the source car horn. The duration of these files range form 0.75 to 144 seconds.

SoundMaps

Other applications can be found from the iconic chart of a soundscape. Soundmaps, for example, are digital geographical maps that put emphasis on the soundscape of every specific location. Various projects of sound maps have been proposed in the last decade (see [START_REF] Waldock | Soundmapping. critiques and reflections on this new publicly engaging medium[END_REF] for a review). Their goals are various, from giving people a new way to look at the world around, to preserving the soundscape of specific places. However, as in general with sound databases, the way sounds are displayed on the map is usually not informative. The use of SamoCharts on soundmaps can facilitate browsing and make the map more instructive.

Music Representations

If the process we described to make charts from sounds was originally set up to display soundscapes, it could certainly be extended to other contexts. Indeed, Samocharts give an instantaneous feedback on the material that compose the sonic environment. Handled with the appropriate sound categories, they could provide a new approach to overview and analyze a set of musical pieces composed with the same material.

For example, Samocharts could be used on a set of concrete music pieces. The charts could reveal the global utilization of defined categories of sounds (such as bell or birds songs). In the context of instrumental music analysis, they could reflect the utilization of the different families of instrument (e.g. brass, etc.), representing the duration and musical nuances. Applied on a set of musical pieces or extracts, they could emphasize orchestration characteristics.

Figure 9 shows an analysis of the first melody (Theme A) of Ravel's Boléro, which is repeated nine times with different orchestrations. The SamoCharts on the figure display orchestration differences, as well as the rising of a crescendo. The main chart (Theme A-whole) shows how each family of instrument is used during the whole extract.

CONCLUSION AND FUTURE WORKS

In this paper, we presented a new approach to create charts for sound visualization. This representation, that we name SamoChart, is based on the TM-chart representation. Unlike TM-charts, the computation of SamoCharts does not rely on human annotation. SamoCharts can be created from Audio Event Detection algorithms and computed on big sound databases. A first kind of SamoChart simply uses the automatic segmentation of the signal from a set of predefined sound sources. To prevent eventual inaccuracies in the segmentation, we proposed a second approach based on the confidence scores of the previous methods. We tested the SamoCharts with two different sound databases. In comparison with other representations, Samo-Charts provide great facility of browsing. On the one hand, they constitute a precise comparison tool for soundscapes. On the other hand, they allow to figure out what kinds of soundscapes compose a corpus. We also assume that the wide availability of SamoCharts would make them even more efficient for accustomed users. In this regard, we could define a fixed set of color which would correspond to each target sound.

The concepts behind TM-charts and Samocharts can finally be generalized to other kind of sonic environments, for example with music analysis and browsing.

Figure 1 .

 1 Figure 1. Preliminary annotation of a sound recording for the creation for the creation of a TM-chart.

Figure 2 .

 2 Figure 2. Example of a TM-chart.

Figure 3 .

 3 Figure 3. SamoChart based on event segmentation.

Figure 4 .

 4 Figure 4. Confidence measures for a sound event.

Figure 5 .

 5 Figure 5. SamoChart based on confidence value.

Figure 6 .

 6 Figure 6. Colored waveforms of two soundscapes.

Figure 7 .

 7 Figure 7. SamoCharts of the two recordings of Figure 6, based on confidence values.

Figure 8 .

 8 Figure 8. Browsing recordings of the UrbanSound corpus.

Figure 9 .

 9 Figure 9. Analysis of the first melody of Ravel's Bolérol (repetitions number 1, 4 and 9, and global analysis). The horizontal axis corresponds to the percentage of time where a family of instrument is present. This percentage is divided by the number of instruments: the total reaches 100% only if all instruments play all the time. The vertical axis displays the percentage of time an instrument is played in the different nuances.

http://www.irit.fr/recherches/SAMOVA/ pageciess.html

http://www.freesfx.co.uk/

https://www.freesound.org/

http://www.irit.fr/recherches/SAMOVA/DIADEMS/

http://telemeta.org/

http://www.irit.fr/recherches/SAMOVA/ pageciess.html

https://serv.cusp.nyu.edu/projects/ urbansounddataset/

Acknowledgments

This work is supported by a grant from Agence Nationale de la Recherche with reference ANR-12-CORP-0013, within the CIESS project.