Auralization of psychoacoustic phenomena

Citation for published version (APA): Houtsma, A. J. M. (1997). Auralization of psychoacoustic phenomena. In Proceedings of the International Symposium on Simulation, Visualization and Auralization for Acoustic Research and Education, ASVA-97, Tokyo, Japan, April 2-4 (pp. 97-102)

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97 International Symposium on Simulation, Visualization and Auralization for Acoustic Research and Education 2-4 April 1997 Tokyo, Japan

AURALIZATION OF PSYCHOACOUSTIC PHENOMENA Adrianus J.M. Houtsma

IPO: Center for User-System Interaction Research Eindhoven University of Technology P.O. Box 513, 5600 MB Eindhoven The Netherlands

ABSTRACT Psychoacoustics deals with descriptions of the relationship between acoustic stimuli and auditory sensations. The acoustic stimulus is strictly speaking, the pressure waveform at each ear drum. In most cases, however, it is assumed that it is equivalent to a voltage waveform at the terminals of a set of headphones, which are used for almost all psychoacoustic experiments. Only for studies on spatial perception and localization, free-field stimulation in an anechoic room is necessary, or, as has become possible in recent years, use can be made of simulated free-field situations through headphones. In some situations, however, one is forced to use loudspeaker sound reproduction under conditions where the use of headphones would be preferred, e.g. in classroom demonstrations of psychoacoustic effects. We will review some cases where the effect to be shown is artificially enhanced by the use of loudspeakers, some cases where the effect is mostly lost, and some cases where against all expectations things work out equally well as they would with headphones.

INTRODUCTION

Psychoacoustics deals with the transformation of acoustic stimuli to auditory sensations. The acoustic stimuli are typically specified as waveforms of sound pressure measured at the eardrum in the case of monaural stimuli, or both eardrums in the case of binaural stimuli. Eardrum pressure can only be controlled in a reasonable and repeatable manner by the use of calibrated headphones. This is the principal reason why headphones are used in almost all psychoacoustic studies. In everyday life, however, we freely move around in sound fields and listen to the world around us by continuously taking spatial samples with both ears. In doing this we make use of the absolute and differential frequency weightings by the outer parts of our ears, which change every time we move our head in the sound field. Such changes are bypassed when headphones are used. The main idea of auralization is the make the world sound as it would sound naturally, with the use of headphones. The way this is done in modern virtual reality systems is by presenting signals to the headphones that represent the sound pressure waveforms on either side of the head in the free field, and that are filtered by position-dependent head-related transfer functions.

http://www.zainea.com/aural.htm[19-8-2014 10:27:37] 97 In this paper on auralization of psychoacoustic effects we will interpret the term auralization in a broader sense, by considering what can happen if acoustic situations intended for headphones are represented through loudspeakers positioned in a listening room. Such a situation occurs very often when teaching psychoacoustics, because classroom demonstrations are much more conveniently done with a couple of loudspeakers than with a large number of headphone sets. We will review four kinds of cases. The first one is where psychoacoustic effects to be demonstrated are enhanced by the use of loudspeakers through purely physical artefacts. The second kind occurs when psychoacoustic effects are weakened or lost by the use of loudspeakers. A third class represents those cases where headphones should actually be used but, against all expectations, things turn out to work quite well with loudspeakers. The last case considered are effects that work only when presented through loudspeakers in an enclosed space, and that do not work when presentation is done through headphones or through speakers in an anechoic room.

CASE 1. USE OF LOUDSPEAKERS ENHANCES PSYCHOACOUSTIC EFFECTS

This case is about those instances where the psychoacoustic effect to be demonstrated or measured depends on aural integration of non-simultaneous sound events. If headphones are used, the sound events at the level of the eardrum are non-simultaneous, and observed temporal integration effects must reflect integration properties within the auditory system. If, however, loudspeakers are used in a space that has a reverberation time constant larger than the integration time constant of the auditory system, the acoustic integration effect will dominate and actually simulate the intended psychoacoustic effect. An unwary listener may easily be impressed by the salience of an effect, but is actually fooled.

One type of stimulus that may be used to illustrate this case is the so-called GoldPumphrey stimulus (Gold and Pumphrey, 1948). It consists of two sequential short tone bursts, both sinusoidal and with the same frequency, amplitude and starting phase, presented though headphones. If the silent gap between the bursts is an integral multiple of the sinusoidal period, a pitch is hard that matches that of the sinusoid. If, however, the silent gap is slightly more or less than this period, the pitch changes as a result of the discontinuity in phase. The pitch percept reflects a degree of aural integration over both tone bursts needed to achieve the spectral dependence on phase illustrated in Fig. 1. If presentation occurs through a loudspeaker, the required integration is achieved by the room reverberation, making the tone bursts (at least partially) simultaneous by the time they reach the ears. Another example is an effect where the pitch of the missing fundamental is perceived for a complex-tone comprising non-simultaneous successive harmonics (Hall and Peters, 1981). If one tests subjects' ability to recognize simple melodies or melodic intervals played with these stimuli through headphones, scores are generally very poor (Houtsma, 1983). If, however, loudspeaker presentation is used as one typically would do in a classroom, the effect suddenly becomes very convincing. Needless, of course, to say that such a demonstration has no value for showing the existence of aural temporal integration. http://www.zainea.com/aural.htm[19-8-2014 10:27:37] 97

CASE 2.

USE OF LOUDSPEAKERS WEAKENS PSYCHOACOUSTIC EFFECTS This case covers those instances where the psychoacoustic effect to be demonstrated is critically dependent on temporal details of the sound, which can be disturbed by a.n insufficiently controlled acoustic process. Examples are temporal (forward and backward) masking situations, binaural masking level difference (BMLD) conditions, and pitch effects that depend on the phase of tone partials such as tonal edge pitch (Kohlrausch and Houtsma, 1992). In forward masking of a short tone burst by broadband noise, for instance, the tone threshold depends critically on the length of the time gap between the offset of the noise masker and the onset of the tone target. The operating range of forward masking is for time gaps between 0 and 200 ms. Acoustic reverberation times of classrooms range between 400 and 1000 ms. Therefore, if loudspeakers are used to demonstrate temporal masking effects, the room reverberation will typically be so dominant that variations of the time gap are hardly noticeable by a change in the amount of masking. Thus the effect to be demonstrated is essentially destroyed.

BMLD demonstrations depend critically on the correct interaural phase relations of masker and target. Correct phase relation can be assured through the use of headphones, at least for frequencies below 1000 Hz. If one compares the detection threshold of a binaural homophasic 500-Hz tone masked by binaural in-phase noise with the threshold of a binaural antiphasic tone masked by inphase noise, one typically finds that the antiphasic threshold is between 10 and 20 dB lower. When loudspeakers are used to demonstrate this BMLD effect, however, results are very disappointing. This is because the phase shifts, caused by sound reflections from the walls of the listening room, basically randomize the interaural phase relations for target tone as well as masking noise, destroying most of the BMLD effect.

CASE 3. USE OF LOUDSPEAKERS WORKS, AGAINST EXPECTATION

This category of effects depends in principle very critically on time or intensity relations, both of which are poorly controlled if headphones are replaced by loudspeakers in a room with reflections. It sometimes happens, however, that poor control of a sound parameter in a listening space can be compensated. for by averaging observations of listeners located at different places within this space. It is also possible that an auditory effect is critically dependent on temporal features that are not affected by room acoustics, such as relative delays between onsets of primary tones. We will discuss one example from each of these cases. An example of the first case is a group measurement of the auditory threshold curve in a classroom. For pure tones, such a room will show a complex sound field with standing waves, nodes and antinodes at various places, dependent on the test tone's frequency. Hartmann (1993) observed in his tutorial on classroom use of auditory demonstrations that "a priori, the idea of measuring auditory threshold in a lecture hall or classroom situation seems only slightly less

http://www.zainea.com/aural.htm[19-8-2014 10:27:37] 97 preposterous than putting a man on the moon with a large slingshot" . Nevertheless his attempt to do an absolute threshold measurement with a group of 100 students resulted in an average threshold curve that was remarkably close to those measured under controlled condition in a laboratory. The results are shown in Fig. 2. The cause of this unexpected success is of a .statistical nature. The presence of nodes and antinodes would make measurements on any individual highly questionable. In a group, however, listeners who sit at nodes and at antinodes will compensate each other, so that the average group results resemble the result of an individual measurement under controlled conditions.

An example of the second case is auditory streaming. This happens when a stimulus consists of a rapid sequence of short non-overlapping tone bursts of low and high frequencies. The auditory system parses such a stream into two separate perceptual streams, corresponding to the high and the low frequencies, provided that the ranges of these high and low frequencies is not larger than a critical band. If one plays such a sequence in a room with reflections, the non-overlapping tones at the sound source will temporally overlap by the time they reach the ear, due to acoustic reverberation: The streaming effect, however, appears to be very robust against this kind of acoustic disturbance. It seems that, among the temporal features of the tone bursts, the time relations between tone onsets are the most important ones. These are not affected by the acoustic properties of a listening room.

CASE 4. EFFECTS THAT WORK ONLY WITH LOUDSPEAKERS IN AN ENCLOSED SPACE Franssen (1960) reported a rather remarkable stereo effect established with two loudspeakers positioned in a listening room with normal reflections. A sinusoidal tone was abruptly turned on in the left loudspeaker and began to fade out immediately and linearly over a period of 20 to 40 ms, while an identical tone was faded in through the right loudspeaker. Such a stimulus is schematically illustrated in Fig. 3. Subjects reported to hear the entire tone as coming from the left loudspeaker, and were not even aware that the right loudspeaker had ever sounded. Franssen reported that an attempt to reproduce this effect with the use of headphones failed. This effect, which nowadays is know as the "Franssen effect" , is explained as arising from a conflict between two potential localization clues. >>>>>>>>> Franssen Effect (see AUDIO FILES for information on the *.wav files) (The effect can only be heard if the sounds are presented to two loudspeakers. The loudspeakers should be placed at about +45 degrees (right) and -45 degrees (left), where 0 degrees is straight ahead. The listener should be about 3 feet from the loudspeakers. The exact configuration is probably not important, as long as the loudspeakers are not too close together and the listener either too close or too far away). The Franssen Effect (see references below) is a strong auditory illusion demonstrating the power of the first arriving information in establishing the location of a sound source. The general stimulus configuration for the Franssen Effect is shown in the Figure. A sound is turned on abruptly at one loudspeaker and is then turned off slowly (with a 100-ms linear offset ramp). As this sound is going off, the sound is turned on at the other loudspeaker with the same envelope (with a 100-ms linear onset ramp). In this demonstration, this tone is left on for 5 seconds at this loudspeaker.

In the Noise Demonstration a broadband noise is used as the carrier sound for the temporal conditions shown in the Figure and explained above.

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In this case, you should hear a sudden noise appear at one loudspeaker and then "jump" to the other loudspeaker, where the source of the sound remains for the rest of the five seconds. This is what the temporal diagram in the Figure shows.

In the Tone Demonstration, a 1000-Hz tone is used as the carrier. In this case, almost all listeners report that the sound is always located at one loudspeaker. And this is the loudspeaker to which the brief tone was presented (the same loudspeaker from which you heard the short noise burst in the Noise demonstration). That is, you hear the full five seconds of sound coming from the location of the loudspeaker that only presented the sound for 100 ms. Or put another way, you hear the tone coming from a loudspeaker that is no longer presenting any sound. However, the location that you perceived as the sound's source is the loudspeaker that presented the sound first, and thus its location seems to dominate your perception of the sound's location. In public demonstrations the tone is often left on for many seconds while the person presenting the demonstration removes the wires from the loudspeaker that everyone is pointing to as the source of the sound. Even with no wires going to the loudspeaker (or in some cases, even with the loudspeaker removed from the room), the audience still reports that the source of the sound is at the location of the (missing) loudspeaker. The acoustics of the room in which the demonstration is being played will affect the strength of the illusion. For instance, it does not work in an anechoic room (see Hartmann and Rakerd in the references below).

Suggested References: Blauert, J., Spatial Hearing. Cambridge, MA: MIT Press, 1983. Hartmann, W. M., & Rakerd, B. (1989). Localization of sound in rooms IV: The Franssen effect. Journal of the Acoustical Society of America, 86(4), 1366-1373. Yost, William A. and Guzman, Sandra J. Sound Source Processing: Is There an Echo in Here?, Current Directions in Psychological Sciences, invited paper-under review. <<<<<<<<<<

One type of clue is the interaural intensity and phase difference which allows the binaural hearing system to determine the location of a sound source. These clues are reliable in an anechoic space and also in headphone listening (if we ignore the problem of externalization for the moment), but are very unreliable for pure tones in enclosed spaces where reflections occur. The second kind of clue is given by the law of the first arrival, where the apparent location of the source is determined by the direction from which the first sound (onset) arrives. For the case of the Franssen effect, the abrupt onset of the tone in the left loudspeaker creates a broadband transient that clearly is heard as coming from the left speaker. During the steady-state portion of the stimulus there is a conflict between a strong onset clue pointing to the left, and a weak and unreliable steady-state clue pointing to the right. Hartmann (1985) explored a "plausibility" hypothesis which in essence says that the more reliable clue dominates in a conflict situation. From this hypothesis he predicted Franssen's observation that the effect failed when headphones are used, since steady-state clues of interaural differences are actually quite reliable for pure tones played through headphones. He also predicted that the effect would fail for a pure-tone sound played through speakers in an anechoic chamber, and also for broadband noise stimuli in just about any acoustic space, even a reverberation room. In a subsequent experimental study (Hartmann, 1989) he showed that the effect is indeed greatly attenuated under these conditions. CONCLUSION In psychoacoustic experimentation, the proper method of presenting acoustic stimuli to the ear(s) is

http://www.zainea.com/aural.htm[19-8-2014 10:27:37] 97 seldom a problem because it can usually be derived from the empirical situation that is being studied. If one wants to demonstrate certain effects, however, as often happens in classroom situations, one must always be very careful when using loudspeakers. One might loose the effect altogether, which gives listeners little confidence in the subject matter. One might artificially enhance the effect, giving the listener a false impression of salience. Sometimes everything seems to work fine, as a result of one disturbing effect offsetting another. Proper evaluation of the acoustic situation beforehand is always a must, as it can prevent unpleasant surprises.

REFERENCES N.V. Franssen, Some considerations on the mechanism of directional hearing (Ph.D. Thesis, Delft University of Technology, 1960) T. Gold and R.J. Pumphrey, Proc. R. Soc. Lond. B 135, 462 (1948) J.W. Hall III and R.W. Peters, J. Acoust. Soc. Am. 69, 509 (1981) W.M. Hartmann, J. Acoust. Soc. Am. 78, 524 (1985) W.M. Hartmann, J. Acoust. Soc. Am. 86, 1366 (1989) W.M. Hartmann, J. Acoust. Soc. Am. 93, 1 (1993) A.J.M. Houtsma, Mus. Perception 1, 296 (1984) A. Kohlrausch and A.J.M. Holitsma, Phil. Trans. R. Soc. Lond. B 336, 375 (1992) Fig. 1. Waveform and power spectrum of a typical Gold-Pumphrey stimulus with gaps that have slightly negative and positive deviations from two complete cycle.

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