The Listener and the Acoustic
R. A. Rasch and R. Plomp
If a sound source and a listener are situated in an
open field without any soundreflecting surfaces in the neighborhood, the
emitted sound will reach the cars of the
listener only via the straight
line that-connects source and listener. The sound image that the listener
receives will roughly correspond to the sound emitted by the source.
However, that is not the usual situation in listening to
music. Producing musical Sounds and listening to them is almost always done in
rooms or halls-technically
speaking, in enclosed spaces. These enclosed spaces
have bounding surfaces (walls,
floor, ceiling) that reflect the incident sound. Because
of these reflections the emitted Sound dots not only reach the cars of the
listener via the straight line from source to
listener, but also via numerous
other paths. The sound that reaches the listener without any reflection is
called the direct sound; the sound
that arrives after one or more reflections is called
the indirect round or reverberation (sec Fig. 1). The presence
of an indirect sound field has a profound influence on
the sound image that the
Whereas the physical aspects of sound in an enclosed space have been studied for almost a century (see Beranek, 1954; Kuttruff, 1973; Meyer, 1970), the subjective effects cannot claim a long history of research. Research in subjective room acoustics begins after World War II, and its results up to now are mainly tentative. This chapter gives a summary of empirical evidence from the experimental literature on subjective musical room acoustics, which centers in the United States, Great Britain, Germany (West and East), and Japan (Rasch 1977).
We will first brifly examine the physical aspects of indirect sound. Fig. 1 shows several paths by which the sound of a source can reach a listener. Since all sound paths work equally well in both directions, source and listener can always be interchanged. The differences between a situation with and one without indirect sound may be summarized in three points:
1. The indirect sound adds sound energy at the position of the listener, resulting in a higher intensity than there would be without indirect sound. The gain can be substantial and depends, of course, on the sound absorption (and reflection) of the boundaries. It can be up to 10 or 15 dB.
2. The indirect sound arrives later than the direct sound because its path is always longer. If the velocity of sound is approximately 340 m/sec, it can be stated that every additional meter in a sound path causes a delay of 3.4 msec. Roughly, the time delays of indirect sound can he up to 100 msec per reflection. If the indirect sound includes some strong single late reflections with delays of more than 50 msec, these arc called echoes.
Usually, it is possible to distinguish some discretely traceable reflections from the walls and ceiling that arrive first after the direct sound and a mass of diffuse later reflections coming from all directions.
The corresponding subjective effects may be described as follows:
1. The increase in sound intensity is perceived as an increase in loudness.
2. The later arrival of indirect sound has the effect that the source seems to sound a little longer than it really does. The direct sound is followed by a "cloud" of indirect sound. This gives continuity to a stream of notes that may have small discontinuities, such as staccato -notes: But the indirect sound may also coincide with or even mask the direct sound of the succeeding notes, which may confuse the sound image to a lesser or greater extent. The temporal aspects of indirect sound correspond to the subjective attribute definition, the ability to distinguish and to recognize sounds.
3. The incidence of sound from all directions results in an impression of spaciousness. This is usually considered a positive quality, although it seems necessary that the position of the sound source should be recognizable in the sound field.
These three effects of indirect sound, both physical and subjective, can be quantified in scales, as will be shown later in this chapter. One of the aims of subjective musical acoustics is to relate subjective to objective scales. The objective and subjective effects are both based on one physical phenomenon (the indirect sound), and, therefore, the scales are not independent. Very often the values on different scales can be predicted from each other. Some scales have a positive extreme corresponding to a condition with a lot of indirect sound; some other scales have a positive extreme corresponding to a condition with no indirect sound.
The subjective effects of a room on the perceived sound can be separated in theoretical description. However, in practical situations and even under laboratory conditions, they can never be separated because they are all dependent on one physical source, the indirect sound. For this reason, and some other ones, the methodology of subjective acoustics research is a rather complicated affair. There are some methodological problems that are specific to subjective room acoustics. They give good insight into what kinds of research and results may be expected in this field. We will deal with three such problems.
First, there is the problem of subjective response. The most direct way to measure this response is to ask the listener to report verbally. his or her subjective impression of the acoustics of a room or hall. This method was used by Beranek (1962), who based his work on interviews with musicians (mainly conductors and soloists) and music critics. Hawkes and Douglas (1971) and Wilkens (1977) made extensive use of semantic differentials. They opened the way for statistical analyses, including correlation anti factor analyses. The subjective factors found can be, more or less successfully, related to measured physical factors (Yoshida, 1965). An objection to this method is that one cannot be sure how the subjects interpret the verbal scales, especially with terms that are not applied to acoustical aspects in normal use. Better in this respect are nonverbal multidimentional scaling techniques, in which the similarity or dissimilarity between various acoustical conditions has be to compared (Yamaguchi, 1972; Edwards, 1974), or only a preference has to be reported (Schroeder et al.,- 1974). These methods lead to unlabeled factors that can be filled in by comparing them with physical factors. Some researchers, such as Reichardt and co-workers in Dresden, made use of methods borrowed from psychophysics, such as detectability thresholds and difference limens. These methods permit only indirect conclusions concerning subjective aspects, but they arc more reliable and more reproducible, both intra- and inter-individually, than subjective methods, such as interviews and semantic differentials.
A second problem lies in the method of presentation of the acoustical situation for subjective evaluation. In a number of studies audiences at live performances have been interviewed. An advantage of this method is its directness. However, it is difficult to compare performances in different halls or different performances in the same hall. In order to cope with these difficulties, researchers have performed experiments with synthetic sound fields, with the aid of which the acoustics of a hall are simulated in the laboratory (see also Wilkens & Kotterba, 1978).
These synthetic sound fields are constructed with loudspeakers in an anechoic chamber. The positions of the loudspeakers determine the angles of incident sound, thus simulating direct sound and indirect sound coming from several directions. All loudspeakers, except for the one for direct sound, are connected to time-delay and attenuation circuits. The music reproduced in such a synthetic sound field must have been recorded without reverberation. A typical setup for a synthetic sound field is illustrated in Fig. 2. It contains the following elements:
1. A loudspeaker in front of the subject, simulating the direct sound. Sometimes two loudspeakers at short distances are used.
2. Two loudspeakers, obliquely placed to the right and to the left. They simulate the first reflections from the walls. These reflection arrive 10 to 50 msec after the direct sound.
3. A loudspeaker mounted above the subject, simulating a ceiling reflection.
4. Several loudspeakers placed on all sides, simulating the later diffuse reverberation (see Fig. 2).
The time and intensity patterns of a sound field can be represented by a echogram or rcflectogram. This is a diagram indicating the time delay and intensity of the various components of a sound field, determined relative to the direct sound. The reflectogram of a room or hall can be measured, for instance, by recording the acoustic response to an impulse (Fig. 3); the reflectogram of a synthetic sound field can be derived from its construction, as a matter of fact (Fig. 4). The reflection patterns used in synthetic sound fields arc inspired by possible live reflectograms. A recent developement in synthetic sound fields is to use headphones and to add the acoustic response of a hall to "dead" recorded music with the help of filtering techniques (Schrocder, 1975, 1979).
A third problem lies in the choice of subjects. Who gives the best judgment? Average concert goers arc often not aware of the acoustical properties of the hall in which they listen to musical performances. Acousticians and sound technicians do have such an awareness, but they may be less sensitive to the relevant musical criteria. Musicians have their own place in this respect, both literally and metaphorically. As a rule, the investigator tries to find subjects belonging to several of these categories mentioned, such as. music critics...acousticians and technicians with a musical background or interest, composers, conductors, and so on.
III. LEVEL EFFECTS OF INDIRECT SOUND: LOUDNESS
The increase in intensity caused by indirect sound is traditionally expressed in sound pressure levels (dB). However, our hearing system is not very sensitive to absolute levels unless these are outside the range of normal listening conditions. Much more important is the ratio between the intensities of the direct and the indirect sound fields. For this ratio we coin the term indirect l direct ratio (abbreviated as i/a~ given by the formula
where R is the indirect/direct ratio (in dB), Li the intensity of indirect sound (in dl3), and Ld the intensity of direct sound (in dB).
Wilkens's (1977) first subjective factor is clearly a level factor. It is characterized by the variables "large," "sounding," "loud," "brilliant," "strong," and "penetrating." Typical sound pressure levels of classical music performed in concert halls are within the range of 60 to 90 dB (Winekel 1962).
IV. TEMPORAL EFFECTS OF INDIRECT SOUND: DEFINITION
The subjective aspects of the temporal effects of indirect sound will be called definition. It is a negative scale in the sense that good definition implies little or no indirect sound. Other terms found in the literature are "clarity" (Beranek, 1962, pp. 36-40) and "clearness" (Mafune & Yoshida, 1968). The German term is Durchsichtigkeit (Reichardt, Abdel Alim, & Schmidt, 1975; Reichardt & Lehmann, 1976). Definition may be described subjectively as that which enables the listener to distinguish temporal details in the musical sound, and, as such, it is a 'necessary condition for listening to music. Beranek (1962) distinguishes between horizontal definition (holding for successive sounds) and vertical definition (holding for simultaneous sounds). The same distinction is made by Reichardt (1975) when he refers to temporal and register definition. The physical counterpart of vertical or register definition is not very clear, however. Hawkes and Douglas (1971) do not make this distinction. Their definition factor correlates with the variables "good definition," "clear," and "brilliant." Wilkens's (1977) second factor is a definition factor. Variables strongly loading this factor are "clear," "concentrated," and "definite." It is remarkeable that some evaluative variables like "pleasant," "liked," and "nice" had their highest loadings on this factor.
Mafune and Yoshida (1968) found a high correlation between definition and the intelligibility score for speech. There was also a high correlation with a subjective measure for listening comfort. Yamaguchi (1972) found two subjective definition factors, both correlated with intelligibility: speech definition (a d/i ratio taking the indirect sound within 50 msec after the direct sound as direct sound) and sound pressure level. The physical factors. supporting a good musical definition are evidently closely related to the factors ensuring a good understanding of speech.
Several physical scales can be given that quantify the temporal effects of indirect sound. It must be mentioned, however, that the relationship between the physical anti subjective aspects have not yet been worked out in detail for all scales. The classical scale is the reverberation time T, the time required for the sound intensity to decay by 60 dB after abruptly stopping the sound source. This can be estimated from the physical characteristics of a room or hall by the following formula:
T=V / 6áS (2)
where T is the reverberation time in sec, V, the volume in m3, S the surface of bounding areas in m2, and á the mean absorption coefficient of the boundaries (fraction of sound energy not reflected).
Optimal reverberation times have been reported in the literature. Kuhl (1954) mentions: 1.5 sec for classical and contemporary music and 2.1 sec for' romantic music. Beranek's figures (1962, pp. 425-431) differ only slightly: 1.5 sec for baroque music and Italian opera, 1.7 sec for classical music and Wagnerian opera, and 2.1 sec for romantic music. Optimal reverberation times for music are higher than for speech. A second physical measure of definition is the modulation transfer function on MTF (Houtgast & Steenckcn, 1973; Steenekën & Houtgast, 1980). If the intensity of a sound source is modulated, the modulation depth at a distance decreases to a greater or lesser extent because of the indirect sound. With a lot of indirect sound the valleys in the temporal envelope will be filled, the more so for higher modulation frequencies.
The degree of modulation retained at the listener's position may be used as a measure of the influence of •the indirect sound. The modulation transfer. depends on the modulation frequency so that the actual measure is a curve, the modulation transfer function. For speech the relevant modulation frequency range is from 0.4 to 20 Hz. By weighting the modulation frequencies, the information in the curve can be condensed to a single measure. Up to now, the modulation transfer approach has only been applied to speech communication problems, but application to musical acoustics, both objective and subjective, seems worthwhile. Macfadyen's (1970) confusion index, the minimum perceivable modulation depth expressed in dB of an amplitude-modulated white noise (modulation frequency 10 Hz), is related to the modulation-transfer approach. Macfadyen used his measure to assess the subjective definition of different seating positions in a hall under various acoustical conditions.
A third physical measure for definition is an adaptation of the i / d ratio, called clarity by Reichardt et al. (1975). It is actually a d / i ratio, in which the direct sound has been expended to include in addition the indirect sound coming within 80 msec after the direct sound:
C = L'd - L'i (3)
where C is the clarity (physical definition) (in dB), L'd is the intensity of direct sound plus indirect sound within 80 msec in (dB), L'i is the intensity of indirect sound, arriving more than 80 msec after the direct sound (in dB). With this measure a good prediction of subjective definition is possible. Definition seems to be optimal when there is no indirect sound. This is not realistic in practical situations because another indispensable, positive aspect, spaciousness, depends on indirect sound. Reichardt et al. (1975) state that clarity should be at least 1.6 dB.
V. SPATIAL EFFECTS OF INDIRECT SOUND: SPACIOUSNESS
The subjective aspeçts of the spatial effects of indirect sound are indicated here by the term spaciousness. In the literature no prevailing term has come up yet. One finds terms such as "liveness" (Maxfield & Albersheim, 1947; Beranek, 1962), "richness" (Mafune & Yoshida, 1968; Yoshida, 1965), "ambience" (Lochner & DeVilliers Keet, 1960), "fullness of tone" (Beranek, 1962), "spatial responsiveness" (Marshall, 1967), "spatial impression" (Baryon, 1971), "resonance" (Hawkes & Douglas, 1971), and "reverberance" (Hawkes & Douglas, 1971). In the German literature the list of terms is restricted to Raumeindruck, Rüumlichkeit and Halligkeit (room impression, spaciousness, and reverberance, respectively, in papers by Reichardt and co-workers; and Kuhl 1977, 1978). The German authors treat Raumeindruck as a generic term, with Rüumlichkeit and Halligkeit as special aspects.
The subjective aspects of spaciousness have been described by Maxficld and Albersheim (1947) as follows: (1) a change in the general tone quality, stated by musicians to be improved "resonance" or "roundness;" (2) the blending of the sound from the various instruments of an orchestra into a single coordinated sound; (3) the sense of acoustic perspective; and (4) the realization on the part of the listener of the approximate size of the auditorium., Beranek (1962, pp. 22-24) mentions the following aspects of "liveness" or a "live room": more uniform loudness, enhancement of bass and treble, fullness of tone, range of crescendo, sound diffusion, intimacy and texture. In this list level and temporal effects are also included. Reichardt et al. (1974), Kuhl (1977), and Reichardt and Lehmann (1978a) give lists that do not differ essentially from the items mentioned. Hawkes and Douglas (l971) describe a factor resonance / reverberance, characterized by the variables "resonant," "reverberant," "responsive," and "large dynamic range." Wilkens' (1977) third factor may be regarded as spaciousness factor, with the corresponding variables "weak," "round," "blunt," "dark,'' and "not-treble."
Maxfield and Albersheim (1947) first connected "liveness" with the i /d ratio. This approach has in particular been elaborated by Reichardt and co-workers at the Technological University of Dresden. They used the dli ratio, referred to as Hallabstand (Schmidt & Lehmann, 1974). Their first publication (Rcichardt & Schmidt, 1966) describes a spaciousness scale with 15 subjectively just-distinguishable points. The relation between the i / d ratio and subjective spaciousness is represented in Fig. 5. This scale was based on measurements with synthetic sound fields consisting of direct sound and diffuse reverberation with T = 2 sec.
In later research it became evident that not only the amount of indirect sound but also its temporal spread affects spaciousness-Sound fields with equal i/d ratios but different reverberation times may have slightly different spaciousnesses. Actually, the early discrete reflections that come before the diffuse reverberation act subjectively as direct sound, not as indirect sound. Also, the angle of incidence influences the subjective spaciousness. Indirect sound that comes from the frontal direction strengthens the direct sound subjectively.
- Reichardt et al. (1974, 1978) and Reichardt and Lehmann (1978a,b) summarize the spaciousness effects of the various indirect components of a sound field as follows: 1. Indirect sound arriving within 25 msec after the direct sound counts as direct sound.
2. Sound arriving between 25 to 80 msec after the direct sound must be divided into two components: (2a) the sound arriving with an angle up to 40° relative to the direct sound must be counted as direct sound, and (2b) the sound arriving from side and rear directions must be counted as indirect sound.
3. Sound arriving later than 80 msec after the direct sound must be counted as indirect sound.
With these rules a corrected i / d ratio can be constructed to predict spaciousness:
iR = L''i - L''d (4)
where R equals spaciousness, L''d the intensity of direct sound, plus indirect sound, within 25 msec from all directions and within 80 msec from front directions (in dB), and L''i the intensity of all other components of indirect sound (in dB).
Reichard and Lehmann (1978b) found the correlation between their i/d ratio and the subjectively judged spaciousness as determined in two concert halls to be 0.64 and 0.65, respectively. Ild ratios in these concert halls differed with seat position but were mostly within the range of 2 to 4 dB (Reichardt & Sarkov, 1972).
It is well known that traditional rectangular concert halls-like the Boston Symphony Hall, the Grosser Musikvereinsaal in Vienna, and the Concertgebouw in Amsterdam-have excellent acoustics, very often better than modern halls, which are wide and low. Marshall (1967) related this observation to the relatively strong early reflections. In high, rectangular halls these reflections come from the side walls; in wide, low halls, from the ceiling. Since our ears are located in the horizontal plane, lateral reflections introduce interaural differences that are important in contributing to the perception of spaciousness. For this reason, reflection panels hanging from the ceiling may affect speech intelligibility and musical definition positively, but their spaciousness effects are doubtful.
Barron (1971) conducted detailed research concerning early lateral reflections. He used synthetic sound fields consisting of direct sound and a single side reflection at 40° of varying intensity and delay time. His results may be summarized as follows:
1. Reflections within 10 msec after the direct sound are too early; they result in a subjective sideward shift of the sound source.
2. Relatively strong reflections later than 50 msec after the direct sound disturb the sound image. They arc perceived as echoes, distinct from the direct and early indirect sound.
3. Reflections of 20 to 25 dB weaker than the direct sound are below threshold. 4. In between the aforementioned effects there is a region of spatial impression. Most important for this are the reflections arriving between 40 and 100 msec after the direct sound. Reflections between 10 and 40 msec can give rise to a distortion of the timbre as an effect of the addition of the direct sound with its delayed repetition (see also Lochner & DeVilliers Keet, 1960 and Bilsen, 1968). Barron's results are illustrated in Fig. 6. The importance of lateral reflections was confirmed by Kuhl's (1978) experiments. .
VI. THE COMPROMISE BETWEEN DEFINITION AND SPACIOUSNESS
The right amount of definition and spaciousness are decisive for good subjective room acoustics. However, definition is negatively correlated with indirect sound while spaciousness is positively correlated. So, it will not come as a surprise that definition and spaciousness have a high negative intercorrelation. This means that in practice a compromise between requirements for definition and spaciousness is always necessary.
However, comparing the formulas for clarity (objective definition) and objective spaciousness, one will notice that one component of the sound field affects both definition and spaciousness positively. It is the sound coming from the sides and from the rear later than 40 msec and earlier than 80 msec after the direct sound. Coming curlier than 80 ìsec, it functions as direct sound with respect to definition. Coming from non front directions, it functions as indirect sound with respect to spaciousness. It may be concluded that these reflections are of great importance for good acoustics of a room or hall.
Subjective musical room acoustics is a relatively new field of scientific enquiry. It does not possess time-honored concepts, methods, or basic results. It is not yet a standard component of handbooks, textbooks, university curricula, or scientific institutions. Its nature is to a large extent interdisciplinary. Methods and concepts have been derived from physical acoustics as well as from psychology and from musicology. However, the rapid growth of the literature concerning its problem areas during the last decade shows that it will soon become an undispensable part of both room acoustics and psychophysics.
Out of the recent literature two important subjective features of room acoustics have emerged: definition and spaciousness. A number of definitions and measurement procedures for both the subjective attributes and its objective counterparts have been proposed, which include a wide range of ways of thinking. Much research will still be needed before the interrelation between the two basic concepts as well as their connection with physical-acoustical properties are fully understood. However, there are several methods available that promise good progress, such as simulation techniques for room acoustics, the indirect/direct ratio, and the modulation-transfer function. These methods have a mainly physical background. They should be applied along with methods such as factor analysis and multidimensional scaling, which have a predominantly psychological origin. The recognition of the hybrid nature of subjective musical room acoustics is essential for the solving of its questions and problems.
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