摘要:

A method is proposed to calculate and measure impulse responses in an enclosure along closely spaced receiver arrays. Hence, instead of using a sparse distribution of receiver positions with single microphones, as is common practice now,arraysof microphones are applied to register the complex sound fields within enclosures. This way, there is a strong spatial correlation between adjacent responses, enabling one to analyze individual reflected wavefronts. It is shown that visualization of the recorded data in a two-dimensional domain, defined by detector position and travel time, gives a significantly improved insight in the structure of complex sound fields. This insight is further increased by applying the linear Radon transform (plane wave decomposition), yielding a representation of the data in the so-called ray parameter versus intercept time domain.

ISSN:0001-4966    

DOI:10.1121/1.420330

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Sound-pressure level (SPL) fluctuation caused by air conditioners is measured using pure tones as the sound source, in a gymnasium. When the air conditioner is turned on, the mean standard deviation of SPL changes about 0.9 dB at 2 kHz, 2.5 dB at 4 kHz, 3.3 dB at 8 kHz, and 3.8 dB at 16 kHz, while that in the case with air conditioning off is less than 0.6 dB at all frequencies. The cumulative density of mean-squared amplitude corresponds well with the gamma distribution. Based on this statistical analysis, the SPL change is thought to be related to the impulse response which is represented by the direct sound (regular) and changing delay time of reflection sound (irregular). The cumulative density of mean-squared amplitude for this model using pure tones also indicated a gamma distribution. It is found that the frequency characteristic and the cumulative density of the SPL change have the same tendencies as the method of modulation for the delay time of reflected sound.

ISSN:0001-4966    

DOI:10.1121/1.420331

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The speeds of sound in water measured by Del Grosso and Mader [J. Acoust. Soc. Am.52, 1442–1446 (1972)], Kroebel and Mahrt [Acustica35, 154–164 (1976)], and Fujii and Masui [J. Acoust. Soc. Am.93, 276–282 (1993)] were compared. A fairly good agreement was found. A new fifth-order polynomial describing the dependence of the speed of sound in water on temperature (ITS-90) within the limits 0–95 °C was proposed. The importance of the effect of impurities of water on the systematic errors of the measurement results was pointed out. The accuracy of the relative measurements of speed of sound in liquids was discussed in the context of the proper choice of a standard (“true”) value of the speed in water.

ISSN:0001-4966    

DOI:10.1121/1.420332

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The influence of the low-frequency roll-off and phase distortion of sound level meters (SLMs) on rocket noise metrics based on measured data is studied. Specifically, during the take-off of a space launch vehicle (SLV) called the Taurus, environmental sound pressure was measured with a carrier microphone system. This system has a virtually flat frequency response at low frequencies. In this study, the low-frequency response of SLMs was simulated using high pass filters. Two sets of filters with the same response magnitude were used: with and without phase distortion. It was shown that, for SLVs with small engines (e.g., the Delta, Scout, and Taurus), the low-frequency distortion resulting from measurements with (simulated) SLMs is apparent only in sound pressure power spectral densities (PSDs) of high resolution at frequencies below 10 Hz. This is where PSD levels are already more than 10 dB below the peak levels. There is no significant effect on the skewness of the time domain data nor on the time-energy averaged sound pressure level (SPL). It is shown that for SLVs with large engines the low-frequency response of the typical SLM results in a significant increase in the skewness of the data as peak (positive) shock pressures are increased. Comparisons of the effects of the two high pass filters indicate that the phase of the frequency response is a critical factor in the distortion of the data.

ISSN:0001-4966    

DOI:10.1121/1.420333

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Acoustic talker direction finders have potential applications to camera pointing for teleconferencing and to microphone array beam steering for audio communication and voice processing systems. This paper describes a laboratory setup and computer interface that was developed for testing talker direction finder algorithms in the Bell Labs Varechoic chamber, a room with computer-controlled absorbing panels. The procedure exploits the full capability of the facility by automatically stepping through a sequence of room panel configurations, outputting a digital speech signal, running the processor, and collecting the data. The advantage of this technique is that it allows for testing under a multitude of different acoustic conditions in the same physical location, thereby enabling a general characterization of the algorithm under evaluation. As an example of the technique, we have implemented the Fischell–Coker talker direction finder algorithm using real-time C-code running on an SGI workstation, which is the same machine that is used to orchestrate the automatic testing procedure.

ISSN:0001-4966    

DOI:10.1121/1.420334

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Audible electronic circuit noise generated within a hearing aid is distracting to a listener in quiet situations and, if the noise level is high enough, may cause listener irritation and rejection of the hearing aid. Thus for hearing aid specification and fitting purposes, it is useful to know the acoustic levels at which this internal noise may become audible and also at which it may become objectionable. For hearing aid amplifier circuit specification and design purposes, it is useful to know the same levels in electrical terms. This paper reports on a study that used an amplifier with no acoustic input and a hearing aid receiver output to simulate internally generated hearing aid circuit noise. Results are reported for testing eight subjects with high-frequency hearing loss for the perceived acoustic and electrical levels at which internal circuit noise became both audible and objectionable.

ISSN:0001-4966    

DOI:10.1121/1.420335

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

A simple method for reciprocity calibration of accelerometers and velocity sensors is presented. The useful frequency range is from about 1 Hz to a kilohertz or more and the method is capable of subpercent accuracy. Calibrations with excitation levels from 1 to300 mm/s2(0.1 mg to 0.03 g) were produced in prototype fixtures. The device to be calibrated and two geophones are used in the calibration fixture. Since the geophones are both reciprocal, a direct test of reciprocity can be done prior to the calibration to evaluate the apparatus. In contrast to standard techniques, the method requires only one relatively inexpensive mechanical setup.

ISSN:0001-4966    

DOI:10.1121/1.420336

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

A method of space–time array processing is introduced that is based on the model-based approach. The signal and measurement systems are placed into state-space form, thereby allowing the unknown parameters of the model, such as signal bearings, to be estimated by an extended Kalman filter. A major advantage of the model-based approach is that there is no inherent limitation to the degree of sophistication of the models used, and therefore it can deal with other than plane-wave models, such as cylindrically or spherically spreading propagation models, as well as more sophisticated representations such as the normal mode and the parabolic equation propagation models. Since the processor treats the parameters of interest as unknown parameters to be estimated, there is no explicit beamformer structure, and therefore no accuracy limitations such as fixed beam bin sizes and predetermined number of preformed beams. After a theoretical exposition of the underlying theory, the performance of the processor is evaluated with synthesized data sets. The results indicate that the method is a highly effective approach that is capable of significantly outperforming conventional array processors.

ISSN:0001-4966    

DOI:10.1121/1.420337

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The amplitude of the psychoacoustic distortion product2 f1−f2elicited by primariesf1andf2depends on the method of measurement. The cancellation-tone procedure gives consistently higher estimates of the distortion product than nonsimultaneous procedures. It was suggested that the difference could be attributed to suppression effects by the lower primaryf1[Smoorenburg, J. Acoust. Soc. Am.52, 615–632 (1972)]. Simulations carried out with a computational model of the auditory periphery including a single, but distributed, compressive nonlinearity confirm the hypothesis; the amount of suppression produced at2 f1−f2byf1equals the difference between simultaneous and nonsimultaneous methods of measuring the amplitude of the distortion product. This is in agreement with psychoacoustical data [Shannon and Houtgast, J. Acoust. Soc. Am.68, 825–829 (1980)]. The model also reveals the importance of suppression effects byf1to bring the phase of the traveling wave associated with the cancellation tone2 f1−f2in exact opposition with that of the distortion product2 f1−f2generated by the primaries. There is also evidence of suppression effects by the higher primaryf2whenL2≫L1.In the model, cancellation occurs over a substantial portion of the basilar membrane near the characteristic place for2 f1−f2.

ISSN:0001-4966    

DOI:10.1121/1.420338

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Detailed analysis of spontaneous otoacoustic emissions (SOAEs) in human subjects revealed that all stable SOAEs sufficiently above the noise floor to permit appropriate analysis have sidebands at multiples of approximately 1 Hz. This is consistent with the hypothesis that SOAEs are modulated by heartbeat. Simultaneous measurement of the rate of blood flow to the thumb and the separation of the spectral sidebands demonstrated that they covary (r=0.982,p<5×10−10). An adaptive least-squares fit (LSF) paradigm was developed to facilitate the measurement of the instantaneous frequency and amplitude of the signals. A combination of traditional spectral analyses and new LSF analyses showed that the sideband generation stems from frequency modulation of the emissions. If there is any amplitude modulation correlated with the blood flow, it is below the noise floor of the analysis. The frequency of the emission was at a minimum when the blood flow was maximal. Examination of alternative mechanisms using computer simulations suggests that these changes stem from changes of 10–100 ppm in the mass of the basilar membrane.

ISSN:0001-4966    

DOI:10.1121/1.420339

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP