Loudspeaker Positions Research Articles

Active control of double-glazed windowsPart I: Feedforward control

This paper describes an experimental investigation of an actively controlled double-glazed window. It is the first part of two companion papers, and it shows the results with a feedforward controller. In the second part about results obtained with adaptive feedback control will be reported. A laboratory version of the window was equipped with loudspeakers and microphones inside the cavity. Various positions of loudspeakers and microphones were tested with band-limited white noise excitation. Different combinations of distributed loudspeakers were realized driving them in parallel by single channels of the controller. Similarly the signals of microphones were summed to realize simple modal filters. Plane mode control as well as control of higher cavity modes were performed with a feedforward controller and the reference signal taken directly from the primary signal. Additionally tests with different traffic noise examples were performed showing the ability of the actively controlled window to enhance protection against traffic noise.

Audio analysis method of a car stereo

Presently, a car stereo system plays an important part in choosing a car, as people spend more time in their cars and pursue comfortable space, including audio. Thus, the sound quality analysis of a car becomes more conspicuous. We made an analysis of acoustic characteristics with 4-channel loudspeakers of three car types and carried out a subjective listening test with a test reference CD based on sound factors using a scale of seven. This paper proposes how the sound reproduction characteristics are changed according to position of loudspeakers, interior space of the cars, and the characteristics of sound reproduction equipment, in particular, a delay time and frequency response. Moreover, we examined the relation between measurement data and human listening sense.

AN EXPERIMENTAL INVESTIGATION OF ACOUSTIC PARAMETERS OF THE LABORATORY FOR MEASURING SOUND INSULATION OF BUILDING ELEMENTS/STATINIO ELEMENTŲ GARSO IZOLIACIJAI MATUOTI SPECIALIAI ĮRENGTOS LABORATORIJOS AKUSTINIŲ PARAMETRŲ TYRIMAS IR DERINIMAS

The report deals with a task of investigating and adjusting acoustic parameters of the reverberant chambers of recently built laboratory for measuring sound insulation properties of building elements (windows, doors). The reverberation time values in the source and receiving empty rooms were measured at 16 one-third-octave frequency bands. It was determined that they significantly exceed the recommended ones. After a number of measurements the reverberation time was adjusted to the allowable limits. An optimum combination of loudspeaker positions in the source room with respect to the best acoustic field diffusivity has been experimentally found. Repeatability values r were calculated from the measurements of sound reduction index of plastic window conducted under repeatability conditions. At the 16 one-third-octave frequency bands they show a good precision of the test equipment.

AN EXPERIMENTAL INVESTIGATION OF ACOUSTIC PARAMETERS OF THE LABORATORY FOR MEASURING SOUND INSULATION OF BUILDING ELEMENTS

Summary The report deals with a task of investigating and adjusting acoustic parameters of the reverberant chambers of recently built laboratory for measuring sound insulation properties of building elements (windows, doors). The reverberation time values in the source and receiving empty rooms were measured at 16 one-third-octave frequency bands. It was determined that they significantly exceed the recommended ones. After a number of measurements the reverberation time was adjusted to the allowable limits. An optimum combination of loudspeaker positions in the source room with respect to the best acoustic field diffusivity has been experimentally found. Repeatability values r were calculated from the measurements of sound reduction index of plastic window conducted under repeatability conditions. At the 16 one-third-octave frequency bands they show a good precision of the test equipment.

Experimental evaluation of cross-talk cancellation regarding loudspeakers' angle of listening

The reproduction of three-dimensional (3-D) sound (either from binaural recordings or virtual source synthesis) through loudspeakers, also known as transauralization, adds difficulties to the traditional method using headphones: the cross-talk effect between speakers and the listener's ears must be taken into account. The common way of removing this distortion consists of placing a filter bank before reproduction through the loudspeakers, but these crosstalk cancellers are notoriously nonrobust to slight movements of the head. Recent theoretical works studied the influence of loudspeaker positions relating to robustness of the system. This paper presents a series of experiments that validate the theory and add new ideas for the design of crosstalk cancellers.

Speaker placement for robust virtual audio displaysystem

Based on a robustness analysis of the multichannel crosstalk canceller, a physical insight into a 3D audio system is presented, and hence a novel method for placing multiple speakers is introduced to increase the robustness of a sound reproduction system. Furthermore, the optimum loudspeaker positions are derived and simulation results are presented.

Adaptive audio systems and sound reproduction systems

A sound reproduction system comprises a plurality of loudspeakers (S1, S2) spaced from a listener at a location (M1, M2), and a loudspeaker drive means (H) for driving the loudspeakers (S1, S2) in response to a plurality of channels of a sound recording (x) of the type being suitable for playing normally through a plurality of reference speakers that are optimally positioned at locations that are displaced from the actual positions of the loudspeakers (S1, S2). The loudspeaker drive includes a filter (H), having a filter characteristic selected by minimising the difference between a desired sound field that would be created by playing the unfiltered sound recording (x) through the reference speakers and sound field reproduced at the listener location (M1, M2) by playing the recording through the speakers (S1, S2). This results in creating a local sound field at the listener location (M1, M2) which is substantially equivalent to the local field that would result from playing the unfiltered sound recording (x) through the reference speakers.

Effect of loudspeaker position on the robustness of acoustic crosstalk cancellation

Acoustic crosstalk cancellation is a signal processing technique whereby two (or possibly more) loudspeakers are used to deliver desired signals exactly at the listener's ears. Such a system is useful for three-dimensional (3-D) audio applications, and removes the requirement for the listener to wear headphones. However, crosstalk cancelers are notoriously nonrobust to slight movements in head position. Recently, a system consisting of two closely spaced loudspeakers (referred to as the stereo-dipole) has been proposed to improve the robustness of the crosstalk canceler. In this letter we present a robustness analysis of the two-channel crosstalk canceler, and derive optimum loudspeaker positions.

Hands-free car kits—A performance discussion of ‘‘state-of-the-art’’ solutions

The increasing demand for hands-free solutions in vehicles partly results from safety reasons. In several countries, the use of handsets is already prohibited for drivers. From the technical point of view, not only the ‘‘hands-free problem’’—acoustical stability and echo—has to be solved. The environmental conditions, i.e., noise level and position of microphone and loudspeaker, are extremly critical. Technical implementations include various signal processing methods such as echo cancellers, noise reduction algorithms, and level switching devices. Typically they work in conjunction with codecs (GSM or others). Consequently, speech transmission quality is influenced in various ways. Comparative measurements carried out with eight different hands-free car kits are discussed in this presentation. The test setup guarantees reproducible conditions using the auditory part of a driving simulator, artificial head technology, and a codec simulator. The determination of important objective parameters was carried out with adapted test signals, while real speech samples were used for subjective evaluation. Measurement setup and relevant parameters concerning speech transmission quality are briefly discussed. A number of listening examples demonstrate differences between various implementations.

Sound localization after transection of the commissure of Probst in the albino rat.

1. The ability of rats to localize sounds in space was determined before and after cutting of the commissure of Probst. The commissure of Probst was transected at its midline decussation with a microknife inserted into the brain according to stereotaxic coordinates. Six animals were tested after extensive lesions that destroyed all of the commissure of Probst fibers. An additional animal was tested after a smaller lesion that destroyed most of the commissure of Probst but left some fibers intact. Three control animals were tested before and after surgical intervention that did not involve the commissure of Probst. 2. The animals were tested in a semicircular apparatus with loudspeakers located on the right or left of midline. They were trained to make a response toward the left or right in the direction of the active loudspeaker. Correct reponses were rewarded by delivery of a small quantity of water from spouts located at +30 and -30 degrees azimuth. Tests of sound localization were conducted with a single broadband noise burst, 45 ms in duration, presented at the beginning of each trial. The position of the active loudspeakers was varied from trial to trial and performance at different speaker angles was calculated to determine psychometric curves. Minimum audible angles were estimated by interpolation from a performance level of 75% correct. 3. After postoperative testing was completed, the effectiveness of the lesions was confirmed by cell counts to determine the extent of retrograde degeneration in the dorsal nucleus of the lateral lemniscus (DNLL). These data showed that most of the contralaterally projecting neurons in DNLL underwent retrograde degeneration and the number of neurons was reduced by 60-65%. Ninety to 95% of the contralaterally projecting neurons in the DNLL disappeared within 7 wk after transection of the commissure of Probst. 4. The condition of damaged commissural fibers was further confirmed by tract tracing methods. A unilateral Fluoro-Gold injection was made into the inferior colliculus and the auditory brain stem was examined for retrograde label. No labeled neurons were seen in the contralateral DNLL in cases with complete transection of the commissure of Probst. In addition, the distribution of Fluoro-Gold labeling in other brain stem auditory structures was similar to that seen in normal animals. This result confirmed that the transection of the commissure of Probst was successful and that projections to other auditory structures remained intact. 5. Transection of the commissure of Probst produced marked deficits in midline sound localization. Although sound localization was still possible, there was a degradation in the ability of rats to localize sounds in the horizontal plane. The mean minimum audible angle was elevated 22.1 degrees after destruction of the commissure of Probst compared with a shift of only 2.0 degrees in control animals.

Natural algorithms for choosing source locations in active control systems

Natural algorithms have been considered as a method of finding optimal secondary loudspeaker positions in an active noise control system. Several genetic algorithms and simulated annealing algorithms were developed and tested for different size searching problems. The performance of each algorithm was investigated and compared with a simple random search. The performance was also tested by using the results obtained from exhaustive searching. It was found that the speed of convergence of the best genetic algorithm and that of the best simulated annealing algorithm were similar, with the latter generally being slightly faster on average. In the case of choosing eight loudspeaker positions from a possible 32 locations, for which there are more than[formula]possible combinations, the simulated annealing program was reliably able to find a set of positions which gave an overall reduction at the microphones which was within 0.5 dB of the best achievable, by searching only about 2000 loudspeaker combinations. The sets of secondary source positions which gave the greatest reduction, without excessive control effort, were then used in experiments in an enclosure, and the results showed good agreement with the theoretical predictions. These secondary source positions were also used in experiments in which the frequency of the primary source was changed slightly and good performance was again observed, indicating that the search procedure is reasonably robust.

Temporal constraints on apparent motion in auditory space.

The hypothesis that the extent of spatial separation between successive sound events directly affects the perception of time intervals between these events was tested using an apparent motion paradigm. Subjects listened to four-tone pitch patterns whose individual tones were sounded alternately at one of two loudspeaker positions, and they adjusted the alternation rate until they could no longer distinguish the four-tone ordering of the pattern. Four horizontal and two vertical loudspeaker separations were tested. Results indicate a direct relation between horizontal separation and the critical stimulus onset asynchrony (SOA) between successive tones within a pattern. At the critical SOA, subjects reported hearing not a four-tone pattern, but two pairs of two-note groups overlapping in time. The findings are discussed in the context of auditory spatial processing mechanisms and possible sensory-specific representational constraints.

Auditory Localization: Effects of Reflecting Surfaces

Based on anatomical and evolutionary conceptions of the human ear, an experiment was conducted in which forty-eight human subjects were asked to localize sounds (a human voice) emitted by one of twenty-seven stationary loudspeakers in an anechoic chamber. The position of the active loudspeaker varied with respect to azimuth, distance, and elevation in three steps each. The position of a single sound-reflecting surface (about 6 m2) was varied: on the floor, on the ceiling, to the left, and to the right. The accuracy of identifying the active loudspeaker for each position of the sound-reflecting surface was compared intraindividually with the absence of reflection. The results show an overall increase in correct localizations with a sound-reflecting surface on the floor. Especially the elevation of the sound source can be detected with greater precision. Additionally, the percentage of correct localizations decreased systematically with the presence of a sound-reflecting ceiling, while the presence of sound-reflecting walls did not systematically affect the localization performance. Judgments in the horizontal plane and those of distance were not systematically influenced by the presence of a sound-reflecting surface.

The variance of reverberation time measurements due to loudspeaker position variation

This paper describes an investigation of the effect of loudspeaker position variation on the precision of reverberation time measurements. It is shown that the residual loudspeaker spatial variance is similar in magnitude to the microphone spatial variance in the frequency range 100 to 315 Hz. In the frequency range 400 to 5000 Hz the residual loudspeaker spatial variance is only one-sixth of the microphone spatial variance. Because most measurement set-ups do not use more than six microphone positions, this means that they will not have to use more than one loudspeaker position above 315 Hz.

Sound reproduction in a car cabin using a digital filter network

Designs to reproduce stereo sound fields in a car cabin have many problems, such as the asymmetrical positions of loudspeakers (for the listener) and the complex characteristics of the sound transmission. Therefore, a technique is investigated in which the sound field in a car cabin is transformed using a digital filter network, keeping in mind the interaural sound pressure in both ears of the listener. The digital filter network can be realized from the transfer functions measured by a head and torso simulator in the car cabin and in the desired sound field. The purpose of using the digital filter network is to equalize the sound pressure in both ears of the listener (when the digital network is not used) to that obtained in the desired sound field. The realizability of the desired sound field in a car cabin was confirmed by computer simulation.

Sound transmission loss of glass and windows in laboratories with different room design

Systematic research has been carried out in two Belgian laboratories on the sound transmission loss (STL) of glass panels and windows to investigate the influence of room design. The samples used were three glass panels with the same dimensions, but different thicknesses. Research has also been done on glass panels of the same thickness inserted into three different frames of PVC, aluminium and wood, which are normally used in practice. The different glass panels and windows were fixed in a niche between the transmitting and receiving room with the same edge dimensions. Systematic STL measurements have been done with the conventional two-room method in both laboratories and the sound intensity method has been used at the AHCL. The windows are of the type which can be opened and the airtightness, to get an optimum STL, has been controlled and improved if necessary. The design of the niche opening in the two laboratories is different and this results in some systematic discrepancies in the STL. Also the influence of loudspeaker and microphone positions has been investigated.

Loudness: Independent of the direction of the incident sound

Up to now it has been accepted that loudness is completely determined by the sound levels at the eardrum. However, recent investigations on spatial hearing indicate that the outer‐ear transfer functions, depending on the direction of the incident sound, have no effect on loudness and timbre. Addressing this discussion, a loudness comparison experiment was carried out, using two loudspeakers in an anechoic room. Situation 1: loudspeaker A and B are placed close together in front of the subject. Situation 2: loudspeaker A is placed in front of the subject, loudspeaker B beside the subject, at the same distance as A. In both situations the loudness of loudspeaker B was determined due to the reference loudspeaker A. The matching sound levels in the ear canals were measured using probe microphones. Furthermore, a corresponding loudness comparison test was carried out, using headphones and reproducing the same levels as in the experiment above, which means lateralization instead of localization. The result is: loudness is independent of the position of loudspeaker A and B. It is determined by the sound level of the loudspeakers, but not by the sound levels at the ear drums. The result supports the hypothesis that localization cues cannot be loudness cues. This is predicted by the so‐called “association model” in which an adaptive filter with a transfer function inverse to the outer ear transfer function is postulated.

The psychophysical basis of monaural localization.

Listeners were required to locate, monaurally, noise bursts emanating from the horizontal plane ipsilateral to the functioning ear. Loudspeakers were positioned from 0 through 180 degrees azimuth, separated by 15 degrees. Stimulus bandwidth was 1.0 kHz, and centered at 4.0-14.0 kHz in steps of 0.5 kHz. The location judgments were governed by the frequency composition of the stimuli, not by their place of origin. With a miniature microphone positioned at the entrance of the external ear canal, the relative amplification provided by the pinna was obtained for the stimuli employed in the localization tests. For each differently centered noise burst, that loudspeaker position re other positions which was associated with the greatest amplification of the stimulus was the one most likely to have been chosen as the source of that stimulus during the localization tests.

Behavioral evidence for central coding of azimuth as a function of stimulus frequency

In a previous report to the Society [Musicant and Butler, J. Acoust. Soc. Am. Suppl. 1 72, S93 (1982)] we presented data on the orderly progression of perceived azimuth of narrow bands of noise as a function of stimulus frequency. The patterns that resulted have been called Spatial Referent Maps (SRMs). A recent experiment suggests that these SRMs must involve a central mechanism for coding location in the horizontal plane. In this experiment, stimuli consisted of 1‐kHz‐wide noise bands with center frequency (CF) ranging from 4–14 kHz. There were 13 loudspeakers placed 15° apart at locations from 360° to 180° azimuth. Stimuli were presented from only that loudspeaker located at 270°. Subjects, listening monaurally, responded with the loudspeaker position from which they perceived the sound as originating. SRMs were then constructed. Subjects next performed the same task, but now listened monaurally with the pinna cavity of the open ear filled. The external meatus was open. Azimuthal judgements of stimulus location were collected and compared to the data collected earlier. Correlations between the means of the azimuthal judgements at each frequency for the two conditions ranged from 0.81–0.97 for seven subjects. The high degree of correlation suggests that, at least for narrow bands of noise, a central mechanism must exist that codes azimuthal sound source location as a function of stimulus frequency.

Monaural localization following exposure to different segments of acoustic space.

Four groups of eight monaural listeners received practice on locating sounds coming from different segments of the horizontal plane prior to a test in which all sounds originated within the same region. An additional eight monaural listeners were given the final localization test without the pretest practice. Knowledge of results was withheld. The main finding was that positive transfer of training was not equally apparent for all groups. That group for which the pretest and test involved the same ear and the same azimuthal positions of loudspeakers performed best. Practice in locating rearwardly positioned sounds did not benefit the localization of frontally positioned sounds even when the same ear was functioning in both situations. Experience in locating sounds from all segments of the horizontal plane appears to be required in order to build up an adequate internal representation of the acoustic surrounds.