Variation Of Sound Pressure Research Articles

Sound pressure distribution and power flow within the gerbil ear canal from 100Hzto80kHz

Sound pressure was mapped in the bony ear canal of gerbils during closed-field sound stimulation at frequencies from 0.1 to 80 kHz. A 1.27-mm-diam probe-tube microphone or a 0.17-mm-diam fiber-optic miniature microphone was positioned along approximately longitudinal trajectories within the 2.3-mm-diam ear canal. Substantial spatial variations in sound pressure, sharp minima in magnitude, and half-cycle phase changes occurred at frequencies >30 kHz. The sound frequencies of these transitions increased with decreasing distance from the tympanic membrane (TM). Sound pressure measured orthogonally across the surface of the TM showed only small variations at frequencies below 60 kHz. Hence, the ear canal sound field can be described fairly well as a one-dimensional standing wave pattern. Ear-canal power reflectance estimated from longitudinal spatial variations was roughly constant at 0.2-0.5 at frequencies between 30 and 45 kHz. In contrast, reflectance increased at higher frequencies to at least 0.8 above 60 kHz. Sound pressure was also mapped in a microphone-terminated uniform tube-an "artificial ear." Comparison with ear canal sound fields suggests that an artificial ear or "artificial cavity calibration" technique may underestimate the in situ sound pressure by 5-15 dB between 40 and 60 kHz.

Underwater sound pressure variation and bottlenose dolphin (Tursiops truncatus) hearing thresholds in a small pool

Studies of underwater hearing are often hampered by the behavior of sound waves in small experimental tanks. At lower frequencies, tank dimensions are often not sufficient for free field conditions, resulting in large spatial variations of sound pressure. These effects may be mitigated somewhat by increasing the frequency bandwidth of the sound stimulus, so effects of multipath interference average out over many frequencies. In this study, acoustic fields and bottlenose dolphin (Tursiops truncatus) hearing thresholds were compared for pure tone and frequency modulated signals. Experiments were conducted in a vinyl-walled, seawater-filled pool approximately 3.7 x 6 x 1.5 m. Acoustic signals were pure tone and linear and sinusoidal frequency modulated tones with bandwidths/modulation depths of 1%, 2%, 5%, 10%, and 20%. Thirteen center frequencies were tested between 1 and 100 kHz. Acoustic fields were measured (without the dolphin present) at three water depths over a 60 x 65 cm grid with a 5-cm spacing. Hearing thresholds were measured using a behavioral response paradigm and up/down staircase technique. The use of FM signals significantly improved the sound field without substantially affecting the measured hearing thresholds.

Measurement of sound power and absorption in reverberation chambers using energy density

Reverberation chamber measurements typically rely upon spatially averaged squared pressure for the calculation of sound absorption, sound power, and other acoustic values. While a reverberation chamber can provide an approximately diffuse sound field, variations in sound pressure consistently produce uncertainty in measurement results. This paper explores the benefits of using total energy density or squared particle velocity magnitude (kinetic energy density) instead of squared pressure (potential energy density) for sound absorption and sound power measurements. The approaches are based on methods outlined in current ISO standards. The standards require a sufficient number of source-receiver locations to obtain suitable measurement results. The total and kinetic energy densities exhibit greater spatial uniformity at most frequencies than potential energy density, thus requiring fewer source-receiver positions to produce effective results. Because the total energy density is typically the most uniform of the three quantities at low frequencies, its use could also impact the usable low-frequency ranges of reverberation chambers. In order to employ total and kinetic energy densities for sound absorption measurements, relevant energy-based impulse responses were developed as part of the work for the assessment of sound field decays.

Acoustic field measurements and bottlenose dolphin hearing thresholds using single-frequency and frequency-modulated tones

Studies of underwater hearing are often hampered by the behavior of sound waves in small experimental tanks. At lower frequencies, tank dimensions are often not sufficient for free-field conditions, resulting in large spatial variations of sound pressure. These effects may be mitigated somewhat by increasing the frequency bandwidth of the sound stimulus, so effects of multipath interference average out over many frequencies. In this study, acoustic fields and bottlenose dolphin (Tursiops truncatus) hearing thresholds were compared for pure-tone and frequency-modulated stimuli. Experiments were conducted in a vinyl-walled, seawater-filled pool approximately 4×5×1.5 m. Sound stimuli consisted of 500-ms tones at 13 carrier frequencies between 1 and 100 kHz. Frequency-modulated stimuli featured both linear and sinusoidal modulating waveforms with 5%, 10%, and 20% bandwidths. Acoustic fields were measured (without the dolphin present) at three depths over a 60×65-cm grid with a 5-cm spacing. Hearing thresholds were measured using a behavioral response paradigm and up/down staircase technique. Frequency-modulated stimuli with a 10% bandwidth resulted in significant improvements to the sound field without substantially affecting the dolphins hearing thresholds. [Work supported by ONR.]

Transverse variations of sound pressure in an ear canal occluded by a hearing aid

The sound field in a model ear canal has been investigated experimentally and theoretically. The canal was cylindrical, 7.5-mm diameter, and terminated with a Zwislocki coupler to represent absorption by the human middle ear. The outer end of the canal was driven by a test hearing aid, with the acoustic output from a receiver entering the canal through a 1-mm port. The hearing aid was provided with a 20-mm-long vent, either 1 or 2 mm in diameter. The sound field inside the canal was measured using a specially designed 0.2-mm-diam probe microphone [Daigle and Stinson, J. Acoust. Soc. Am. 116, 2618 (2004)]. In parallel, calculations of the interior sound field were performed using a boundary element technique. Both calculations and experiment reveal large transverse variations of sound pressure level, as much as 25 dB at 8 kHz, near the inner face of the hearing aid. Variations are greatest near the outlet port of the receiver and the vent hole. Deeper into the canal, the transverse variations are less significant and, at depths greater than 5 mm, only a longitudinal variation remains. [Work supported by Gennum Corp.]

Comparison of an analytic horn equation approach and a boundary element method for the calculation of sound fields in the human ear canal

The sound field inside a model human ear canal has been computed, to show both longitudinal variations along the canal length and transverse variations through cross-sectional slices. Two methods of computation were used. A modified horn equation approach parametrizes the sound field with a single coordinate, the position along a curved center axis-this approach can accommodate the curvature and varying cross-sectional area of the ear canal but cannot compute transverse variations of the sound field. A boundary element method (BEM) was also implemented to compute the full three-dimensional sound field. Over 2000 triangular mesh elements were used to represent the ear canal geometry. For a plane piston source at the entrance plane, the pressure along the curved center axis predicted by the two methods is in good agreement, for frequencies up to 15 kHz, for four different ear canals. The BEM approach, though, reveals spatial variations of sound pressure within each canal cross section. These variations are small below 4 kHz, but increase with frequency, reaching 1.5 dB at 8 kHz and 4.5 dB at 15 kHz. For source configurations that are more realistic than a simple piston, large transverse variations in sound pressure are anticipated in the vicinity of the source.

The spatial distribution of sound pressure within the human ear canal

The sound field inside a human ear canal has been computed using 2 approaches. A simple model, a modified Webster horn equation approach, can accommodate the curvature and varying cross section of the ear canal. Calculations using the horn equation demonstrate the formation of standing waves within the canal. The pressure may be interpreted as either the pressure along the center axis of the canal or the average pressure within a cross-sectional slab. To investigate possible spatial variation through a cross section, the sound field has also been computed using the boundary element method (BEM). Over 2000 triangular mesh elements, 1 mm or less in size, were used to represent the canal geometry. For a plane piston source at the canal entrance and both a rigid and a resistive impedance condition at the eardrum position, the computed sound pressures along the center axis of the ear canal are in good agreement with the horn equation calculations, up to 15 kHz. The BEM approach, though, reveals spatial variations of sound pressure through each canal cross section, increasingly significant as frequency increases. Further, for source configurations that are more realistic than a simple piston, large transverse variations in sound pressure are anticipated.

Cross spectral measurement of head related speech transfer functions using speaker’s own voice

A cross spectrum method is applied to measure sound pressure variations around the head using the speaker’s own speech sounds. The variations are represented as transfer functions from the mouth reference point to a set of measuring points. Preliminary tests indicated that there are systematic frequency response variations depending on vowel colors. This vowel color dependency was not replicated in the classical measurement of speech radiation characteristics by J. L. Flanagan. However, taking into account the large (sometimes exceeding 20 dB) amount of variations, it is not likely to be negligible. A set of calibration and normalization procedures were introduced to reduce artifacts due to background noise, room acoustics, zeros in the speech spectra. A series of M-sequence based transfer function measurements were also conducted using a head and torso simulator to evaluate intrinsic errors in the cross spectral measurements. It was found that the standard errors in the cross spectral measurements using recorded speech sounds are around 1 dB. Based on these reference data and confidence interval calculations based on coherence, it is safe to conclude that the vowel color dependency is significantly modifying the transfer functions. [Work supported by JSPS.]

Experimental Study on Sound Pressure Generated by Wave-Submerged Structure Interaction

This study is intended to discuss the sound pressure generated by the wave-submerged structure interaction, which is considered to play an important role on fish gathering function of an artificial fish reef. Laboratory experiments were carried out to measure a sound pressure as well as the velocity and free surface variation. In the experiments, idealized simple structures were employed and flow visualization was also conducted. It is confirmed that the installation of the submerged structure generates the sound pressure on several frequency domains. Moreover, it is revealed that the variation of sound pressure depends much on vortex formation and its behavior.

Human Eeg Responses to Classical Music and Simulated White Noise: Effects of a Musical Loudness Component on Consciousness

The main purpose of the present study was to investigate the psychophysiological effects of music on human EEG. For this purpose, a sound modulator was developed which simulates the sound-pressure variations of a given piece of music by white noise (sim-music). Using this apparatus, the author tested the psychophysiological effects of music on human EEG. The electroencephalograms (EEG), electrocardiograms (ECG), and electrooculograms (EOG) of eight normal volunteers were recorded for a total of 21 min., 5 sec. per session for each subject under three sound conditions: silence for 5 min., two types of music (music) or two types of simulated noise (sim-music) for 11 min., 5 sec., followed by silence for another 5 min. Each subject was exposed to a total of 10 music and 10 sim-music conditions. At the low consciousness level (drowsiness, Stage S1), higher delta component power densities were observed with sim-music than with music. Thus, even in the same Stage S1, entire physiological consciousness levels may be higher when listening to music than to sim-music. While listening to music, many subjects reported that they felt pleasantly relaxed or comfortable. However, with the sim-music, they reported feeling unpleasantly weary and sleepy. It seems that the mental set toward two sound conditions differed greatly for many subjects. In Stage S1, the differences in EEG slow components showed that the differences in consciousness had a physiological aspect and indicated differences in mental set toward both sound conditions and mental activity during the listening conditions.

Spatial distribution of sound pressure and energy flow in the ear canals of cats

Spatial pressure distributions have been measured in the ear canals of ten cats and analyzed to obtain the energy reflection properties of the middle ear over a 10- to 25-kHz range of frequencies. Considerable intersubject variability is observed, much of which can be correlated with the condition of the tympanic membrane. For ears judged to be in good condition, reflection coefficients typically take values of about 0.2 between 15 and 25 kHz, indicating good matching of the dynamical properties of the auditory system to the ear canal sound field. At lower frequencies, the reflection coefficients tend to be somewhat higher and at higher frequencies the reflection coefficients increase quite rapidly with frequency. For ears judged to be in poorer condition, energy reflection coefficients of 0.5 and 0.9 were determined for the 15- to 25-kHz range. The variations of sound pressure along the canal (about 10 dB, even in well-coupled systems) confirm that single point pressure measurements may be inappropriate for defining the acoustical input at higher frequencies and new measures for specifying the input should be investigated. The net flow of acoustic energy into the auditory system, the sound power, is one possibility. Some initial measurements of sound power, obtained from analysis of the spatial pressure distributions, are presented.

Integrated-optical acoustical sensors

We have investigated the feasibility of a novel integrated-optical (IO) microphone. Sound-pressure variations acting on a small membrane directly modulate the phase of a guided wave in a (planar) optical waveguide by the IO nanomechanical effect. The phase modulation is transformed into an intensity modulation in an IO difference (or polarimetric) interferometer.

Performance of a large new anechoic chamber at the Canadian Bureau of Radiation and Medical Devices

The large new anechoic chamber is to be used for health effects research and standards development for noise, and for other acoustics research not feasible elsewhere in Canada. The chamber’s free-field performance was quantified using measured deviations from the 1/R law for the variation of sound pressure with distance, R, from a point source in a free field. In measurements at 46 locations throughout the chamber, deviations were typically less than 1 dB for frequencies ranging from the design cutoff of 50 Hz to 20 kHz. The largest deviation was 1.9 dB. Impulse response analysis was also used to examine sound reflections. Background noise was found to be less than the design criteria used, which ranged from 22 dB at 50 Hz to 4 dB at and above 1600 Hz. Comparisons will be made to the few other chambers in the world which are of a similar size.

Perception of the relative distances of nearby sound sources

The pressure of a sound varies systematically with a listener's distance from a sound source, providing a useful cue for perceiving changes in the distance between a listener and a sound-producing object. The pressure-discrimination hypothesis predicts that thresholds for discriminating changes in distance are constrained by the underlying ability to discriminate the resulting changes in sound pressure--specifically, that the smallest discriminable change in distance should be about 5% of the reference distance. Previous studies reported thresholds of about 5% for reference distances greater than a few meters but surprisingly worse thresholds for closer reference distances. In the present study, thresholds at two close distances, 1 and 2 m, were within the 5% range predicted from the pressure-discrimination hypothesis. Moreover, thresholds were significantly worse in a control condition in which the pressure cue was removed. Results of previous studies were adjusted to take into account the possibility of conservative response tendencies by the subjects. These adjusted findings agree well with the results of the present study and the pressure-discrimination hypothesis. It is concluded that variations in sound pressure are very useful for perceiving changes in listener-source distances, even at close distances.

Frequency response of headphones measured in free field and diffuse field by loudness comparison

Coupler measurement of headphone frequency response may be incorrect at high and low frequencies [T. Hirahara and K. Ueda, J. Acoust. Soc. Am. Suppl. 1 87, S142 (1990)]. Since sound pressure near the eardrum is difficult to measure physically, a psychophysical method was employed to assess real ear frequency response of headphones. Loudness comparisons were performed by four subjects, under two experimental conditions: free field (anechoic room) and diffuse field (reverberation room). These conditions were run to check the influence of field characteristics on the reliability of the measurements. Each subject adjusted the headphone level of critical band noise bursts until they were equally loud as those from a reference loudspeaker (7 dB SPL). Measurement scatter was smaller in the diffuse field than in the free field. One hypothesis to explain this result is that head movement causes less change in level in a diffuse field than in a free field. Support for this comes from measurement of the SPL distribution in the two rooms, which revealed that the sound-pressure variation was smaller in the diffuse field than in the free field. To examine the reliability of loudness judgments at high frequencies, another method—hearing thresholds by Békésy tracking—was employed. Each subject's threshold was measured with both loudspeakers and headphones. After compensation was made for loudspeaker transfer functions, headphone frequency response was deduced from the results. This method led to high-frequency responses similar to those from loudness comparison. Below 6 kHz, however, there were only slight differences, because equating loudness for loudspeaker sound and for headphone sound does not strictly correspond to equal sound pressure at the eardrum.

A brief tutorial on the fast field program (FFP) as applied to sound propagation in the air

The Fast Field Program (FFP), a computational technique originally developed for predicting acoustic wave propagation in the sea, has proved useful for calculating sound propagation in the air above the ground. The procedure involves the Hankel transformation of the Helmholtz equation in circular cylindrical co-ordinates, and the integration of the resulting ordinary differential equation by analogy with electrical transmission lines. Variation of the sound speed in the vertical co-ordinate is represented by horizontal stratification of the air into discrete layers, each with a different sound speed homogeneous and isotropic within the layer. The heights of source and detection points can be arbitrarily assigned, as can the ground impedance. It is assumed that there is no azimuthal variation of sound pressure and that there is no range dependence of ground impedance or atmospheric parameters. The computation yields the sound pressure, at a given detector height, for a continuous range of source to detector radii. This tutorial paper gives a detailed account of the theory of the FFP, describes the current implementation in terms of discrete variables, and references the original sources of the technique.

Middle ear dynamic characteristics of patients obtained by a newly developed measuring apparatus

A tympanometer is well known to be a valuable apparatus for the diagnosis of secretory otitis media (SOM). However, it is insufficient for the diagnosis of ossicular chain disorders. Therefore, an attempt was made to develop a new measuring apparatus, which had a function of sweeping both frequency and pressure and was measurable the absolute sound pressure variations at the external auditory canal, owing to the carefully designed probe. In the previous papers, the middle ear dynamical characteristics of normal subjects were measured by this apparatus, and the measurement results were compared with the theoretical analysis. In this paper, the middle ear dynamical characteristics of patients were measured, and the effects of various kinds of middle ear diseases upon its dynamical characteristics were examined. The measurement results of the patients with ossicular chain separation, ossicular chain fixation, SOM and tympanic membrane perforation had their own distinctive patterns, and were also clearly different from those of normal subjects. Therefore, this apparatus seems to have a high degree of the clinical applicability to the diagnosis of these diseases.

The spatial distribution of sound pressure within scaled replicas of the human ear canal

A theoretical model for calculating the variation of sound pressure within the ear canal is presented. The theory is an extension of the horn equation approach, and accounts for the variation of cross-sectional area and curvature of the ear canal along its length. Absorption of acoustic energy at the eardrum is included empirically through an effective eardrum impedance that acts at a single location in the canal. For comparison, measurements of the distribution of sound pressure have been made in two replica ear canals. Both replicas have geometries that duplicate, as nearly as possible, that of a real human ear canal, except that they have been scaled up in size to increase the precision of measurements. One of the replicas explicitly contains a load impedance to provide acoustical absorption at a single eardrum position. Agreement between theory and experiment was good. It is clear that at higher frequencies (above about 6 kHz in human ear canals), this theoretical approach is preferable to the more usual "uniform cylinder" approximation for the ear canal. At higher frequencies, there is no unique eardrum pressure; rather, very large variations of sound pressure are found over the tympanic membrane surface.

Specification of the acoustical input to the ear at high frequencies.

The sound fields that arise in the auditory canals of cats have been examined both experimentally and theoretically. Of particular interest was the spatial variation of sound pressure near the eardrum, where reference probes are typically located. Using a computer controlled data acquisition system, sound pressure was measured between 100 Hz and 33 kHz for constant driver input at 14 different locations in the ear canal of a cat, and the standing wave patterns formed. The shape of the patterns could be predicted quite well above 12 kHz using a theory that requires specification of only the geometry of the ear canal. This theory, an extension of the one-dimensional horn equation, applies to three-dimensional, rigid-walled tubes that have both variable cross section and curvature along their lengths. Large variations of sound pressure along the ear canal and over the surface of the eardrum are found above about 10 kHz. As a consequence it is not possible to define the acoustical input to the ear from sound pressure level measured at any single location. Even in comparative experiments, in which only the constancy of the acoustical input is important, any uncertainty in reference probe location would lead to an uncertainty in sound pressure level when different sets of measurements are compared. This error, calculated for various probe locations and frequencies, is especially large when the probe is near a minimum of the sound field. Spatial variations in pressure can also introduce anomalous features into the measured frequency response of other auditory quantities when eardrum sound pressure is used as a reference. This is illustrated with measurements of the round window cochlear microphonic.

Male Loud Calls in Macaca silenus and Presbytis johnii

Analysis of the male loud calls of free-ranging lion-tailed macaques (Macaca silenus) and Nilgiri langurs (Presbytis johnii), two sympatric primates of the South Indian rain forests, revealed remarkable structural similarities: In both species the calls are uttered in long phrases with regular time structure, the units consisting of a tonal exhalation phase with high sound pressure and a more noisy inhalation phase with low sound pressure, but significant sound pressure variations. In both phases the energy is distributed over discrete frequency bands and concentrates on low-frequency ranges. These characteristics are consistent with essential bioacoustic requirements for long-distance propagation of vocal signals in rain forest habitat. Such type of loud call, being typical for langurs and mangabeys, is the first to be found in the Macaca genus. The two species, which belong to quite different genera, resemble each other not only in the vocal patterns of their loud calls, they also display striking similarities in habitus, exclusively arboreal dwelling habits, and in their social organization.