Diffuse Sound Field Research Articles

Measure of complexity of wave motions and crosscorrelation coefficient in reverberant fields.

In this paper, the complexity of wave motions has been formulated in order to more detailedly than previously consider the complexities of reverberant sound fields, and the difference of complexity between two separate points has been described in an attempt to find a measure of diffuseness in these fields. The reverberant fields are assumed to be sound fields where random plane waves propagate two- or three-dimensionally. Entropy H (=h2-h1)and energy-entropy product K are used to define the complexity of wave motions, where h1, h2 are entropies by sound pressures p1, p2, respectively. Then H is compared with the cross-correlation coefficient R calculated from same sound pressures. Furthermore, K is obtained by considering energy dissipation with propagation, and its properties are discussed as well as those of H. The results show that the entropy of sinusoidal wave is in a complementary relationship with the cross-correlation coefficient; H=1-R, and that it may be convenient to measure since the cross-correlation coefficient is inseparable into two entropies. Either the entropy or the energy-entropy product is also found to be required at audio frequencies as a measure of the complexity of wave motions (i.e. approximately K=A0H, A0=const. for this case). Because of the lack of a quantity giving degrees of directional distribution, our introduced measure is as yet insufficient to accurately measure the diffuseness of sound fields, but it may be fairly near to the corrected measure.

Determination of sound reduction indices in the presence of flanking transmission

Unlike the standard sound insulation test method, which requires diffuse sound fields in both source room and receiving room, the sound intensity method only requires the source room to be diffuse. Flanking may, in some cases, cause large errors in the sound reduction indices obtained by the standard method, even if the flanking power is measured separately and subtracted from the total power. The sound intensity method allows selective determination of sound reduction indices for test objects surrounded by flanking surfaces, provided the measurement surface in the receiving room is defined in such a way that it encloses nothing but the test object.

Recording control room design incorporating a reflection‐free zone and reflection phase grating acoustical diffusors

The acoustical diffusing properties of flat, mono, and bicylindrical surfaces, alternating absorptive and reflective panels, and a wide variety of reflection phase grating diffusors (RPG) will be discussed. Experimental TEF measurements including energy‐time, energy‐frequency, time‐energy‐frequency, and polar angle‐energy‐frequency curves plus theoretical Kirchhoff modeling calculations will be presented. A new control room design incorporating a reflection‐free zone (RFZ), in the front half of the room, and a diffuse zone, in the rear half of the room created with RPG diffusors, will be described and documented. The RFZ minimizes the speaker boundary interference over a wide volume surrounding the mix position. The RPGs provide a diffuse sound field which enhances the perception of spatial textures and helps maintain the stereo perspective across the entire width of the mixing console and in the rear of the room. The RFZ/RPG design minimizes frequency coloration, image shifting, and provides accurate stereo imaging.

Sound intensity in diffuse sound fields

The net sound intensity field in a room containing a random noise source is the vector sum of the direct field and the reverberant intensity field. If the room is diffuse, the net reverberant intensity will be zero and only the direct field will constitute a net intensity flow. The net sound intensity field in the vicinity of a source placed in a diffuse room will therefore be identical to that in a free field, decreasing by 6 dB per doubling of the radial distance from a simple source. In any other environment the reverberant field will also yield a flow and the resulting distortion of the free‐field radiation pattern will reflect a lack of directional diffusion. A simple descriptive model of the intensity field in various types of acoustic environment is presented.

Wave propagation in ground-structure systems with line contact

The response of a structure in contact with the surface of a viscoelastic half-space (VEHS) is considered. The system is modeled as a ground-structure system. Vibration and radiation of sound into the closed space of the structure, resulting from a harmonic line force applied on the surface of the VEHS, are investigated theoretically. Stress-displacement relationships of the contact area are formulated by using modified forms of Lamb's integral solutions for an elastic half-space as Green functions. A set of integral equations formulated by using this procedure is evaluated and numerically calculated. The structure is modeled as one of thin plates, and the response can be determined in the form of flexural and quasi-longitudinal waves. Also, acoustic coupling between the flexural modes of vibration and the air is presented as a spatially averaged sound pressure level based on the assumption of a perfectly diffuse sound field.

Effect of reverberation on the identification of stationary vowels

In a diffuse sound field, the amplitudes of the individual harmonics of a stationary vowel have an uncertainty with a standard deviation of 5.57 dB. It is shown that this phenomenon slightly reduces the identification score for a set of eight synthetic vowels derived from male utterances. In terms of variance in the one‐third octave band‐filter levels, the spectral differences introduced by reverberation are of about the same magnitude as the differences among the spectra of the same vowels pronounced by different male speakers.

Coherent active methods for applications in room acoustics.

An adjustment of reverberation time in rooms is often desired, even for low frequencies where passive absorbers fail. Among the active (electroacoustic) systems, incoherent ones permit lengthening of reverberation time only, whereas coherent active methods will allow sound absorption as well. A coherent-active wall lining consists of loudspeakers with microphones in front and adjustable control electronics. The microphones pick up the incident sound and drive the speakers in such a way that the reflection coefficient takes on prescribed values. An experimental device for the one-dimensional case allows reflection coefficients between almost zero and about 1.5 to be realized below 1000 Hz. The extension to three dimensions presents problems, especially by nearfield effects. Experiments with a 3 X 3 loudspeaker array and computer simulations proved that the amplitude reflection coefficient can be adjusted between 10% and 200% for sinusoidal waves at normal and oblique incidence. Future developments have to make the system work with broadband excitation and in more diffuse sound fields. It is also planned to combine the active reverberation control with active diffusion control.

Phase statistics and phase correlation of sinusoidal signals in reverberant rooms

Statistical properties of the phase in sinusoidal diffuse sound fields in reverberant rooms are considered. An exact form for the distribution density of the phase difference measured at two space points is obtained, and from second order phase statistics the spatial phase correlation function is derived. The theoretical results are examined experimentally.

Measuring sound attenuation capabilities of helmets using a KEMAR manikin

Sound attenuation measurements were made on three Army helmets and one headset. A Knowles Electronic Manikin for Acoustic Research (KEMAR) was instrumented to read the sound pressure level of one-third octave band filtered white noise. Sound pressure levels were measured using two different procedures. The first procedure employed a Zwislocki coupler microphone responding as a human eardrum and the second procedure used a probe tube located at the ear canal entrance of the manikin. All measurements were taken with the manikin in a diffuse sound field in a sound isolation room. Attenuation values were calculated by subtracting the sound pressure level measured with the helmet on the manikin's head from those sound pressure levels recorded without the helmet in place. These attenuation values compare favorably at most frequency bands with similar measurements made using human subjects in the same sound field wearing the same helmets. [Work supported by U.S. Army, Natick Laboratories, Contract No. DAAK 60-80-C-100.]

Evaluation of high-fidelity hearing aids.

An essential building block for any high-fidelity hearing aid is an amplifier-transducer-coupling combination that does not audibly degrade the sound, that is, provides high-fidelity sound reproduction as judged by someone with normal hearing. To demonstrate that such a combination is possible, two binaural pairs of hearing aids were assembled using available hearing aid transducers and electronic components, one pair of Over-The-Ear hearing aids with 8-kHz bandwidth and one pair of In-The-Ear hearing aids with 16-kHz bandwidth. Objective insertion-gain measurements on these aids, obtained with a KEMAR manikin in a diffuse sound field, revealed a frequency-response accuracy comparable to that available in expensive high-fidelity loudspeakers. Subjective fidelity ratings obtained from three groups of listeners judging prerecorded A-B-A comparisons (made from equalized eardrum-position microphones in a KEMAR manikin) produced a similar conclusion. We conclude that the important question for hearing aid research is no longer "What can a hearing aid be designed to do?" but "What should a hearing aid be designed to do for the hearing impaired?"

Comparison of stimuli used in sound field audiometric testing

The problems associated with performing pure-tone threshold measurements in reverberant or diffuse sound fields are illustrated with the use of three-dimensional representations of the sound field within a typical test booth. A microphone mounted on a motorized trolley is used to perform these measurements. A comprehensive comparison is then made of the efficiency with which FM tones, AM tones, damped wave trains, and narrow bands of noise provide a uniform sound field. The conclusion is reached that the bandwidth of the stimulus is the major factor determining the uniformity of the field. A decision about the most appropriate stimulus for sound field work must thus be based on factors other than field uniformity. When the constraints of obtaining suitable spectral distributions, and being able to relate thresholds obtained with complex stimuli to those obtained with pure tones are also considered, FM tones and suitably generated narrow bands of noise appear to be the most suitable stimuli. The selection of suitable parameters is discussed and an Appendix discusses the spectrum and bandwidth of FM signals with different modulation waveforms. The relative accuracy of testing in the direct and reverberant regions in a nonanechoic environment is also discussed.

Re-examination of the low-frequency (50-1000 Hz) normal threshold of hearing in free and diffuse sound fields.

In two experimental replications involving a total of 17 otologically normal subjects, hearing threshold levels were observed that were from 2.6 to 7.1 dB less sensitive in the 125-1000-Hz frequency range, than those specified in ISO R226-1961. The published data from 12 additional threshold experiments conducted since 1933 were reviewed. Variables that could affect the data, such as free versus diffuse sound fields, pure tones versus 1/3-octave noise band stimuli, psychophysical methods, pulsed versus continuous stimuli, binaural versus monaural listening, age, sex, race, and otological rejection criteria, were analyzed. For frequencies from 50-1000 Hz and ages up to 35, the only corrections found to be necessary were for sound field conditions and binaural versus monaural listening. A second-order polynomial regression line was fitted to the combined, corrected data. The resultant thresholds were 5-6 dB less sensitive than ISO R226 between 50 and 250 Hz, with the divergence dropping to +1.7 and -1.5 dB at 500 and 1000 Hz, respectively.

A reexamination of the low frequency (50–1000 Hz) normal threshold of hearing in both free and diffuse sound fields

A reexamination of the low frequency (50–1000 Hz) normal threshold of hearing in both free and diffuse sound fields

A summary of the sound pressure level transformations from free and diffuse sound fields to the human torso, head, and ear

The sound pressure level (Lp) measured at a particular location on the human head or torso differs from the level in the empty sound field because sound is scattered by the head and body. The difference between the level measured near the head or torso and the level measured in the empty field is defined here as the sound pressure level transformation. Previous measurements of these transformations are summarized and presented as a function of frequency, microphone location, type of incident sound field, and angle of incidence (in the horizontal plane). For the most part, these transformations are positive: the levels near the head or torso are greater than those in the empty sound field. Differences in the results of various studies are pointed out. Further studies are proposed to resolve the differences and provide additional information. Some simple guidelines for choosing a free or diffuse field transformation in a real environment are presented. The choice depends on source directivity, distance betw...

Walk away method versus reverberation time method in determining the room constant

The room constant is a quantity relating to the sound absorption in an enclosure. It is necessary to room acoustics engineering calculations based on the perfectly diffuse sound field theory, both in noise control work and sound system design. Normally, in the validation of the above-mentioned theory, in situ evaluations of the room constant are performed on the basis of reverberation time measurements in the relevant space. Such methods fail to give accurate results when uncertainties regarding the volumes and areas effectively involved in the reverberation process arise. In this case the walk away method is more advantageous in that it only needs steady state sound pressure level measurements at various distances from a small sound source whose directivity factor is known. In this paper both methods are discussed and experimental results relative to four different rooms are compared.

Acoustic field around a plate in diffuse sound field : Approach with geometrical acoustics

Acoustic field around a plate in diffuse sound field : Approach with geometrical acoustics

The measurement of hearing protector attenuation via ASA STD1‐1975 (ANSI S3.19‐1974)

E‐A‐R® Corporation has recently completed construction of a hearing protector attenuation measurement facility that complies with the requirements of the new real ear attenuation standard, ASA STD1‐1975 (ANSI S3.19‐1974). The test room is a 113‐m3 a double walled, mechanically isolated chamber. All significant acoustical, physical, and electrical requirements of ASA 1 will be discussed as they relate to the performance of the E‐A‐R facility. Topics will include sound field uniformity, diffusivity, and background noise level measurements. Experiments were also conducted to determine open ear hearing threshold levels of 12 subjects and compare them to ISO R226 (corrected for diffuse field listening). Agreement was good. Additional data was acquired for pulsed versus nonpulsed thresholds and “noisy” versus silent subject response switches. These results as well as those of some hearing protector attenuation data will be presented and finally comparisons will be made between attenuation data from at least two independent testing laboratories.

Acoustic field around a disc in the diffuse sound field

Acoustic field around a disc in the diffuse sound field

Acoustic field around a strip in the diffuse sound field

Acoustic field around a strip in the diffuse sound field

An experimental and computer study of the spatial distributions of p2 in a reverberant enclosure at low frequencies

In a diffuse sound field the spatial distributions of p2 are exponential. Because in an enclosure the diffusion of the sound field is not perfect, it is necessary for some applications, such as the sound power measurements, to know how much the statistical distributions deviate from exponential, particularly at frequencies lower than the critical frequency fc = 2000 (T/V)1/2. Therefore, both computer and experimental studies were conducted from below the first mode in an enclosure up to and above the critical frequency in 1-Hz frequency increments with p2 calculated and measured in 120 spatial positions. Both probability density functions and cumulative distribution functions as well as the deviation from the exponential distributions were obtained. Computer-generated second order statistics have been investigated to determine the effect of a finite number of points used to generate the distributions. Based on the frequency dependence of the deviation of the actual distributions from the exponential distributions, it can be concluded that the reverberation room is usable from a lower frequency than fc.