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[N466]
Concepts Behind Sound Quality: Some Basic Considerations
Jens Blauert
Institut für Kommunikationsakustik
Ruhr-Universität Bochum , D-44780 Bochum, Germany blauert@ika.rub.de
Ute Jekosch
Institut für Kommunikationsakustik
Ruhr-Universität Bochum , D-44780 Bochum, Germany
ABSTRACT
In this paper, basic concepts behind sound quality are reconsidered in a systemic way. This appears to be of some use as sound quality is a mental construct which is often insufficiently defined and, conse-quently, not understood properly by many. Ambiguous use of the term - even among experts - provides ample evidence of this fact. After a discussion of the assessment of the character of sounds, product-sound quality is considered, where it is often the mission of the product-sound attached to a product to relay confidence in the quality of the product itself. Thus, the actual issue here is the "sound of quality". The next step is a discussion of the auditory quality of speech. From there it is relatively easy to generalize to other sounds, such as the auditory quality of musical sounds. The issue, then, is the "quality of sound". Finally, different kinds of audio transmission and reproduction will be dealt with, where a ref-erence sound may or not be available, and where we should talk about "sound-transmission quality" rather than sound quality. Effort is put into an attempt to discuss all these different aspects of sound quality in a concerted way, such as to show that the general procedure in which sound quality is formed is relatively invariant across specific applications. Rather, it is predominantly the character of the references of the quality judgement that requires adaptation to the specific applications. It is hoped that such a unified view of the formation process of sound quality will support current attempts to esti-mate sound quality instrumentally.
INTRODUCTION
Around 1985, a discussion started in the noise-control community as to whether the A-weighted sound level is a sufficient measure to specify noises [1]. The reason for the discus-sion was the observation that different noise sounds, although having the same A-weighted level, may have quite different perceptual effects. Hence, it was concluded that further pa-rameters besides the A-level should be used to specify noises. Instead of a single measure, a set of adequate measures was called for. As a result, different parameter sets have been pro-posed to be used in noise metrics. These parameters, as a rule, are either physically or psycho-acoustically ones, i.e. either based on instrumental measurements of the physical sound signals or on psycho-acoustic measurements of the accompanying sound sensations. Exam-ples for acoustic (physical) measures are: differently-weighted sound-pressure levels (A, B, C, D, slow, fast, impulse, peak), power spectra, spectral centroids, amplitude statistics and enve-lope spectra. Well known examples for psycho-acoustic (auditory) measures are: loudness,
sharpness, roughness, fluctuation strength, and tonalness [2]. We shall show in the following that this approach cannot completely capture the problem of sound quality in its complexity, but is a necessary first step into the right direction.
THE CHARACTER OF SOUNDS
The application of a metric composed from acoustical and/or psycho-acoustic measures re-sults in the portrayal of a noise in terms of a set of parameter values. In the language of the 80th, this analytical aspect of a noise was called the "quality" of the noise concerned. In other words, the term "noise quality" was used for what is, at best, a parametric profile of a noise. In view of the following sections which consider the quality-forming process in more depths, we strongly advice to avoid wasting the term "quality" already here and in such a way. In-stead, the term "character" of the noise seems to be a more appropriate term in this context [3, 4, 5], with the "noise character" being specifiable by a profile of acoustic and/or psycho-acoustic parameter values. What holds for the character of a noise can in the same way be ap-plied to other sounds. Hence, we speak of the "sound character" when we mean the parametric representation of a sound.
The reason of why this issue has been discussed at this point is as follows: To assess the char-acter of a sound, a valid metric has first to be set up. To achieve this, an analytic process has to take place with the goal of identifying a suitable parameter set to specify a sound appropri-ately. In order to identify psycho-acoustic parameters systematically, special methods for multidimensional analysis can be employed [see, e.g., 6, 7]. Multi-dimensional analysis is ca-pable of providing parameter sets which reveal the orthogonal factor of perceptual spaces. Currently, it seems that there is some agreement among experts that the psycho-acoustic measures as mentioned in the introduction [2] are of relevance in this context. Yet, they are neither exhaustive nor orthogonal with respect to each other. Further research is thus neces-sary to identify the parameters which are suited best to represent the character of sounds. Once a metric has been defined, values have to be assigned to each parameter in the metric by way of measuring procedures. Measurement means assigning numbers to objects in a rela-tional way, - e.g., by comparison with a standardized quantity of the same dimension (a so-called unit). To this end, specific instruments and/or a panel of expert listeners are needed. As to the psycho-acoustic measures, in order to arrive at results which have inter-individual va-lidity ("objectivity"), it is mandatory that the listeners are trained to judge consistently and in kind of an "unbiased way" - as any "objective" judge should do. By the way, what holds for this kind of analytic auditory testing, can also be observed with tests in other sensory modali-ties, e.g., testing of fragrances, wine, food, etc. [8]. To determine the "sensory profile" of a product such that the results shows reasonable objectivity, trained and experienced panels of experts are, in fact, indispensable. As an alternative, some algorithms have been developed to substitute psycho-acoustic measurements by instrumental procedures that evaluate the wave-form of the sounds - e.g., algorithms to predict loudness, sharpness or roughness. Such algo-rithms may be useful for some applications, but must be used with care as they only provide estimates of auditory quantities, and their range of validity is limited.
It is necessary to note at this point that auditory perception can vary with non-acoustic factors. In other words, what we hear does not only depend on the sound waves which impinge upon our ears. Moderating factors such as emotion, knowledge about the situation, and action may play a role. A complication here is that listeners tend to associate meanings with the sounds and to modify their responses accordingly. Trained subject have learned to discriminate these factors and to process them selectively. For example, when measuring auditory quantities which reflect the function of the sub-cortical auditory system, such as loudness and rough-ness, trained subjects can dismiss factors such as associated meanings and emotions. By the
way, in praxi, it is often useful in such cases not to disclose to the subjects what the actual sound sources are, thus assisting their selective processing of the different perceptual compo-nents of auditory events.
Besides the acoustic and psycho-acoustic parameters of sounds, further parameters can be processed analytically and, in many cases, selectively. The results can, again, be described by means of adequate profiles. Examples are semantic profiles (denoting the meanings transmit-ted by the sound) or psychological profiles (denoting the associatransmit-ted emotions). Yet, in con-trast to the "sound character", where specialized instruments and/or expert listeners are man-datory for the analytic profiling, there are further aspects of the perception and judgement as-pects of sounds which can only be captured by non-expert listeners, as the experts would be biased or too analytically minded to get the point. Non experts, as a rule, tend to more global rather then analytic judgements. In fact, these more global judgements are of paramount im-portance for the formation and measurement of that specific mental construct for which we would like to reserve the term "sound quality". We start the discussion about quality with a more complex shade of the meaning of this term, as given in the phrase "product-sound quality".
PRODUCT-SOUND QUALITY
It is widely accepted these days that the sound of an (industrial) product should fit the prod-uct. It is further hypothesized that the "goodness of fit" may determine the quality of the product sound. To discuss this matter in more detail, we start with a current definition:
Product-sound quality is a descriptor of the adequacy of the sound attached to a product. It results from judgements upon the totality of auditory characteristics of the said sound - the judgements being performed with reference to the set of those desired features of the product which are apparent to the users in their actual cognitive, ac-tional and emoac-tional situation [9].
Obviously, product-sound quality does not exist per se, but evolves in a multi-step process. The stages of the quality-forming process are as follows: To begin with, a product must emit acoustical waves. These can be measured instrumentally, rendering acoustical profiles of the waves. The sound waves, upon reaching the ears of listeners, usually give rise to auditory per-ception, resulting in auditory events. The character of the auditory events can be quantified via selective listening tests with experts, leading to profiles, such as psycho-acoustic, emo-tional or semantic ones. Building upon auditory events with a recognized character being available, the forming of quality proceeds as follows: As to the definition above, the subjects -usually the users of the product - compare the actual character of the perceptual representation of the product sound - the auditory event - with a reference that they have in their mind regarding the product. The reference consists of a set of desired features of the product, be it real or conceptual ones, not necessarily restricted to the auditory modality. We, thus, can again speak of a character here, namely, a "reference character" - following the same terminological concept as in "sound character". The comparison then continues by an assignment of the re-sulting quality rating, based on how close the sound character is to the reference character. In other words: the better the features of the auditory event "product sound" match the desired features which form the reference of the subject, the higher the quality of the product will be rated [10]. Both comparison and quality assignment are cognitive processes primarily, but may be moderated by the actual emotional and actional situation that the subjects are exposed to at that specific point in time [9, 10]. It goes without saying that here actual and/or potential users of the product under evaluation will render more valuable results than a panel of trained, analytically listening experts.
We would like to mention at this point that our concept of quality formation is certainly sim-plified, since, among other things, we assume a fully transversal process. In fact, the process may well include some recursive elements at all its stages. This, however, is not relevant for our discussion here. In any case, it becomes obvious at this point of discussion that designers of product sounds need clear conceptions of the character of the references that the product users refer to when rating quality - e.g. in the form of profiles of the desired features of the products in their specific application context. Accordingly, research into the character of such references is a paramount concern in the context of product-sound design. The following issue is widely accepted here: Among other things, a good product sound should certainly signal that the product itself is of good quality [1, 9, 10, 11, 12]. In other words, the product sound is used as a carrier of specific information to the user of the product, it stands for the quality of the product. Hence, when it comes to an evaluation of the quality of the product sound, the function of the sounds as signs is the crucial issue [13, 14]. Particularly, a product sound is then rated of high quality when it is conceived as the "sound of quality".
SOUND QUALITY
What holds for product sounds, certainly also holds for speech, namely, that speech is a car-rier of information. It is beyond doubt that a product sound can provide clues to the users re-garding the operational state of the product. For example, the sound of a car engine carries information about whether it is idling or accelerating. Yet, when judging upon the quality of speech, it is normally not the quality of the speech-generating organism which is at stake - be it a human or artificial speech synthesizer -, but the quality of the speech sounds themselves. The following definition takes regard of this scenario (liberally translated from German):
Speech Quality is the result of an assessment of the adequacy of a speech sample - con-sidering all of its recognized and nameable features and feature values - namely, as to which amount this speech sample complies with a reference arising from aspects such as individual expectations and/or social demands and/or pragmatic necessaries - con-sidering all recognized and nameable features and feature values of the reference [15].
This definition has noticeably been refined as compared to the earlier one which we have used above for product-sound quality. To begin with, quality is now defined as the result of an as-sessment. This makes the definition more precise, as it is explicitly said that the process in which quality evolves is assessment, i.e. relational assignment of numbers to objects. Quality is thus quantified. Actually, we might as well use the term "measurement" here. Along the same lines of thinking the parameters and parameter values under consideration are required to be recognized and nameable. This is again a useful constraint, especially in the engineering context, as it actually liberates the quality-forming process from anonymity. Thus, according to this definition, only what is explicitly recognized and nameable is considered further. In other words, the character of the speech sounds and the character of references - each speci-fied by their respective profiles - prompts the process which finally leads to quantispeci-fied quality [15, 16].
There is, however, an important distinction between the earlier and this more sophisticated definition. The distinction lies in the character of the references. In the first definition, it is the desired features of the product that are referenced against. Here, however, the references are defined in a far broader and more abstract, e.g., functional way. In particular, they do no longer directly refer to the sound source. Now besides individual taste, topics like tradition, economic and sociologic context, among many others, may come into play [17]. The result of the assessment, then, is no longer as to which amount the sound indicates a high quality of the
product, but rather, to which amount the sound itself is suitable to match given expectations, demands, and necessaries. The information which the speech sound transmits, the meanings that are associated to it, certainly may play a role in this process, but are not necessarily of primary concern. The main object of quality is now the sound itself, that is the sound as it sounds - in other words, the "auditory event".
In the course of the discussion it has, hopefully, already become apparent to the reader that the cited definition for speech quality can easily be generalized to sound quality at large. In con-trast to product-sounds, where the sound of quality is the dominant issue, here it is finally the quality of the sound itself, namely, "sound quality" in its specific sense. The quality of musi-cal sound is a good example for the application of the definition above. In passing by, it also reminds us to mention that aesthetic features are of high relevance for many judgements on sound quality.
SOUND-TRANSMISSION QUALITY
The definition of the reference in the above definition of speech quality and, hence, sound quality, has been left fairly open. In fact, it may suggest that the features to match only exist as abstract entities - such as, e.g., concepts and recollections in the listeners’ minds [14]. However, it is not excluded that the reference could as well be formed by actually existing sound samples - such as being physically present or available from recordings. The quality can then be measured by means of direct auditory comparisons of the sounds to be assessed in terms of quality with respective reference sounds. The term "sound-transmission quality" is appropriate for this quality aspect (sometime the term "sound- reproduction quality" is also used in this context, with the same or a slightly modified sense). There are different criteria which may govern the quality-assessment process in these cases, more important ones are "authenticity", "plausibility" and "enhancement", as discussed in the following. Please note again, that orthogonality of these factors is obviously not given, such requiring further re-search to explore the semantic space that they belong to.
As to "authenticity", this is an aspect often encountered with in communication technology, where the task is to transmit sound signals across space and time. Transmission, frequently, is considered of highest possible quality when the output signal of a transmission system does not show any deviation from the input signal (straight-line transmission, absence of distor-tions). In the context of sound-transmission quality, this requirement is usually formulated with regard to the perceived sounds, the auditory events: Highest transmission quality, then, is postulated when listeners at the playback end of the transmission system hear no difference as to what they would hear at the recording end at the recording location and time [18].
Authenticity, however, is not always the preferred goal of sound transmission and/or repro-duction. If we think of hearing aids or public-address systems as examples, it becomes clear that we expect from such systems that they enhance the sound which they have picked up, for example to make it more intelligible. Also, in the sound-recording industry, authenticity, as a rule, is not the goal, but rather a maximum artistic effect is aimed at. Room-acoustics en-hancement systems (e.g., for reverberation assistance and/or provision of lateral reflections) can also be seen from this viewpoint. The transmission systems, in these cases, are expected to provide "enhancement" (augmentation) to the input sounds. Accordingly, the quality refer-ence is no longer straight-line transmission, but has to be defined specifically for each par-ticular application.
An issue worthwhile to touch upon at this point is the so called "acoustics" of spaces for mu-sical performances as, e.g., concert halls. In fact, what is denoted by good or bad acoustics is the transmission quality of a hall for acoustical signals, rated upon the basis of the musical
sounds that the concert goers perceive. This example shows how complicated the issue of sound-transmission quality can become. The transmission quality that a concert hall provides depends, among other things, on the shape of the hall, the type of music, the position of the musicians and the listeners, the amount and distribution of sound absorbing material - just to mention a few. Concert hall acoustics has been a research topic for a long time, and we can learn a lot about sound-transmission quality in three-dimensional spaces from the relevant lit-erature as well as from the experience of musicians and acoustical consultants. In any case, concert halls are built to "enhance" the sound of the orchestra as well as that of the soloists. Another important class of transmission systems are those which aim at inducing certain per-ceptual and/or behavioral effects in the listeners. Examples are auditory displays as are used to improve the situational awareness of, e.g., operators of machinery or pilots of aircraft. Also, the auditory representation in virtual-reality generators (e.g., in flight simulators) can be regarded in this context. Further, one could certainly argue that modern cinema with surround should also be considered here. In this class of transmission systems, the reproduced sounds can well deviate from the originally recorded sounds as long as they are taken by the listeners as being perceptually "plausible". As a measure of "plausibility" the amount of perceptual in-volvement (the sense of "presence") has been proposed - -among other perceptually, behav-iorally and psychologically based measures. Again we see that for such systems the references for the assessment of sound-transmission quality can be quite sophisticated.
THE CHARACTER OF REFERENCES
In all three definitions in the context of sound quality which we have introduced above, namely, product-sound quality, sound quality and sound-transmission quality, there is obvi-ously something in common: The quality formation always consists in a mental process of comparison and distance rating. The objects to be compared are the sounds on the one hand and the references on the other one. Both sound and references have characters specific to them. The respective characters can be quantitatively specified by profiles of values of, e.g., acoustic, psycho-acoustic, psychological and/or semantic parameters. As the references play such a paramount role in the quality-formation process, but are not readily accessible for physical or psycho-acoustic measurement, they obviously pose a problem - especially to en-gineers engaged in quality assessment. In fact, there is a large agreement among experts in the field of sound quality [19] that research into the formation, specification and classification of references should be assigned high priority.
A rough classification of references might start from the amount of abstraction given in the reference characters. This makes sense as the higher the abstraction level is, the more the cortex is involved in the quality-formation process. For further exploration, let us assume for a while that the level of abstraction can be defined in such a way, that a one-dimensional scale of abstraction level can be set up. Following this line of thinking, the low end of abstraction is certainly given when the reference is a real sound to be matched, e.g., in sound transmission where straight-line transmission is the goal. In fact, there is not abstraction at all. Real-sound references are, however, also used in other cases, such as target sounds in product-sound de-sign. One could certainly argue about whether the level of abstraction is different when the match is aimed at in the physical or perceptual domain, but this could lead to an episto-mological dispute beyond the scope of this paper.
The high end of abstraction can be assumed when not the sound itself is the reference, but the meaning that is stands for, i.e. when the function of the sound as a sign is the dominant issue. Meaning are concepts and, as such, objects of abstraction. Semiotics teaches that the assign-ment of meaning to a sound can be of symbolic, iconic or indexial nature [13, 14, 17]. The highest abstraction in this context is given at the symbolic level, as here the assignment of
meaning to the sound is arbitrary and, consequently, has to be learned like the elements of a language. At the indexial level, the sound has a natural relationship to an associated event in other modalities, e.g., the sound of a breaking glass falling on the floor. The breaking glass is a concept, to be sure, unless we actually see it, but a concept with a low level of abstraction. Icons provide a level of abstraction in between those of symbols and indexes.
As to emotions associated to sounds, the amount of abstraction in the character of the refer-ence is given by whether real emotions are expected to be induced by the sound (no abstrac-tion) or the imagination of emotions (specific emotions, lower abstraction, or concepts of emotions, higher abstraction). Aesthetic and ethic features may be considered in this category - with a relatively high level of abstraction. Sociological features, just as an example, may be more emotional or more cognitive, and thus, more ore less abstract.
Although we hope that the level of abstraction may turn out to be a useful aspect to categorize the character of references for quality-judgment purposes, is not possible for us at this state of exploration to go into more detail. One obvious reason is that we are not aware of experi-mental which could be used to confirm or reject our reasoning. Nevertheless, it certainly be-comes clear here that characters of references can cover a range from very simple to very complex, often combining attributes of physical nature and from sensory perception with cognitive, emotional and actional ones. Whenever the quality formation is performed by a human being, not only perception in different modalities is included, such as the function of the auditory tactile and visual systems, but also the central nervous system - and this is a bio-logical computer with massive parallel computation and a huge memory. This fact has to be kept in mind when attempting to understand and model the formation of sound quality.
DISCUSSION AND CONCLUSION
The sections above have rendered some insights into the processes of quality formation re-garding sounds. Be it product-sound quality, sound quality or sound-transmission quality, there is an inherent similarity in the processes as follows: Each quality formation starts with a sound sample to which, in the further course of the process, a quality rating is to be assigned. To this end, the sound sample has first to be explored and evaluated. The result of the evalua-tion is parameter profiles specifying the sound sample, such as an acoustic profile, a psycho-acoustic (auditory) profile, an emotional profile and/or a semantic profile. We call that aspect of the sound sample which is determined by these profiles, the character of the sound. The character of the sound has then to be compared to a reference which is composed from fea-tures taken as important with regard to quality. The specification of the reference can again be performed by profiles. Assuming that both the character of the sound sample under considera-tion and the character of an appropriate reference are at hand, we think of the final step for the quantitative assignment of quality as follows: The sound character is compared with the refer-ence character in that a measure of similarity (or, inversely, distance) is applied. The result of the distance measurement can then be converted into a quality rating.
The actual definition of the distance measure (or similarity measure) is a major issue in this context. Mathematically, it would be easiest if both the parameter set which constitutes the character of the sound sample and that one which constitutes the character of the reference, were orthogonal and sufficiently explanatory (e.g., explain more than 85% of the respective variances). This has unfortunately not yet been achieved in most cases reported in the litera-ture and, such, appropriate distance measurement still turn out to be an issue of concern. Nevertheless, the basic steps of the quality-forming procedure seems to be quite common to all sound-quality assessment tasks. What actually differentiates the individual tasks is the character of their reference. The reference is specific for each individual application context.
As we have tried to explore, one aspect of the reference characters is that the features estab-lishing it show different amounts of abstraction. The level of abstraction is important when attempting to model the quality-formation process - leading to consequences as follows: When constructing models of quality formation or algorithms for instrumental quality estima-tion, mimicking the acoustical and elementary psycho-acoustics processes will often not suf-fice. As soon as abstract quality features come into play, the model has to incorporate explicit knowledge at some point. Further, the architecture of the modeling process may have to be constructed in such a way that it can handle complex recognition procedures. In many cases this will lead to architectures that are not only signal-driven (bottom-up) but include hy-pothesis driven (top-down) components. Speech technology provides good examples for such architectures, as modern speech recognizers face similar tasks of similar complexity. Actually, it would not be too far from the mark if we considered instrumental quality-estimation algorithm as "quality recognizers".
REFERENCES TO THE LITERATURE
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