Microphone Locations Research Articles

Measurement of acoustic power output in the active control of sound

Active sound control can be used to obtain large global reductions in low-frequency harmonic enclosed sound fields. The secondary sources of sound in such applications have typically been adjusted to minimize practical approximations to the total acoustic potential energy in the enclosure. Minimizing the total acoustic power output of the primary and secondary sources has recently been theoretically shown to give very similar results. In this paper, some of the physical consequences of minimizing total power output are discussed, specifically the implications this has on the power output of the secondary sources. In the single channel case, it is found that the acoustic power output of the secondary source is exactly zero when adjusted to minimize the total power output of both sources. A method of adjusting the secondary source to achieve this objective is presented, in which the volume velocity of the secondary source is adjusted to be out of phase with that of the primary source, and the secondary source amplitude is gradually increased. The amplitude is adjusted using measurements of the acoustic power output of the secondary source only. This initially becomes negative as the amplitude is increased, indicating that the secondary is absorbing power, but then increases until it is exactly zero at the amplitude required to minimize the total power output. This method has been verified by experiments in an enclosure in which the acoustic power output of two loudspeakers acting as the primary and secondary sources were monitored, together with the sum of the squared pressures at 32 microphone locations in the enclosure.

INVESTIGATION OF THE AUDIBILITY OF LOCOMOTIVE HORNS

An investigation was conducted to determine the audibility of locomotive horns and, in particular, the effect of masking when other sources of sound may be present. The investigation was triggered by an accident in which two workers were struck by a moving train. The workers were engaged in routine maintenance work at the time. Tests were conducted with a stationary locomotive as well as one in motion (pass-by tests). The overall sound levels and the frequency spectra of the locomotive horn were measured. During the final pass-by test, a portable hand grinder was in operation. The combined sound level was measured, and the subjective response of persons assembled in the vicinity of the test site was sought regarding the audibility of the locomotive horn above the sound of the grinder. The sound level of the horn of a stationary locomotive, which was about 103 dBA at a distance of 50 m (164 ft) from the microphone location, attenuated at a rate of 7.25 dB per doubling of distance up to the final measuremen...

An energy-density-based control strategy for minimizing the sound field in enclosures

Squared pressure has generally been the quantity chosen to minimize when controlling the sound pressure level (SPL) in enclosures. However, difficulty in selecting the best possible microphone location makes this method unattractive for arbitrary enclosures. As an alternative control strategy, minimizing the total energy density in an enclosure has been investigated. This method is a logical alternative because the total energy in the field is composed of both pressure and velocity components, as opposed to simply a pressure component. Computer simulations and experimental measurements of reducing the SPL in a rigid‐walled duct have been conducted to compare the two strategies. The results of the experiments show that minimizing the total energy provides greater attenuation of the total potential energy with the added advantage of being less sensitive to microphone placement than when minimizing the squared pressure.

Near-field pressure, intensity, and wave-number distributions for active structural acoustic control of plate radiation: Theoretical analysis

This paper presents a theoretical analysis of near-field time-averaged intensity and pressure distributions of actively controlled plate-radiated sound. A harmonic point force was considered as a noise source, and a piezoelectric element bonded to the plate was applied as control actuator. A single microphone that measures sound pressures in the radiated acoustic far field serves as an error sensor. The optimal input voltage to the piezoelectric actuator is obtained by minimizing a quadratic cost function, defined as the mean square of the error sensor signal. The influence of the location of the error microphone on near-field pressure and normal intensity was studied. Fourier transforms in the wave-number domain are also used to study the mechanisms of control. This work provides a better understanding of the behavior inherent in controlling structurally radiated sound using piezoelectric actuators. In particular, an understanding of the near-field behavior under closed loop control is inherent in the design of sensor arrays located near or on the plate surface.

Active Control of Radiated Sound from Ducts

The noise radiated by duct and pipe systems is modeled in the laboratory using a circular duct driven by a speaker at one end. Active control is achieved using a control speaker located adjacent to the open end of the duct. The objective of the investigation was to minimize the total sound power radiated by the duct and a single secondary source. However, the adaptive algorithm used by the controller for this investigation seeks to cancel the acoustic pressure only at a single location; that is, the location of the “error” microphone. Analytical studies predict that in order for the total sound power radiated by primary and secondary sources to be minimized, a single secondary source must radiate sound which is of approximately equal magnitude and opposite phase to the noise source and the error microphone must be placed somewhere in the plane of minimum pressure of an ideal acoustic dipole. These analytical results were verified in part, for the case of the secondary source facing the same direction as the primary source in the plane of the pipe outlet. Other cases were studied where the control speaker was located outside the plane of the duct outlet. The performance improved for these alternative orientations for a wide range of error microphone positions. Measured sound power with and without active control is shown for a range of frequencies and error microphone locations for three configurations of the duct and control source.

Individualized correction factors in the preselection of hearing aids.

This study investigated three issues involving corrections for individual ear acoustics in hearing aid prescriptions: (a) the extent to which inconsistencies in the sound-field reference position can affect comparative corrections for the real-ear unaided response (REUR); (b) the extent to which individual variability in the real-ear-to-coupler level difference (RECD) supports the use of individual measurements as opposed to an average-ear estimate; and (c) the adequacy of using KEMAR estimates of the effects of the location of the hearing aid microphone. In Experiment 1, KEMAR REURs using over-the-ear and under-the-ear reference positions were compared with KEMAR REURs using a center-of-head reference position. Maximum differences of 4-9 dB were found in the 1500- to 5000-Hz range, depending on test conditions. In Experiment 2, the ear canal response of an insert earphone was compared to the 2-cc coupler response of the same earphone to calculate the RECD. Individual RECDs for a population of hearing aid candidates were compared to the RECD for KEMAR. For 8 of the 15 subjects (9 of 18 ears), the RECD was more than 4 dB different from KEMAR at two or more third-octave frequencies between 500 and 4000 Hz. In Experiment 3, the effect of the location of the hearing aid microphone for in-the-ear (ITE) and in-the-canal (ITC) locations was compared with the over-the-ear (OTE) location for 18 ears and for KEMAR. The effects varied across individual ears, but all ears and KEMAR showed positive gain in the high frequencies for the ITE and ITC locations. The relevance of these results to hearing aid prescription practices is discussed.

Field testing of operable walls.

In addition to submitting laboratory test results prior to approving an operable wall assembly for a given project, it is common practice to require field testing of the noise reduction provided by operable partitions using ASTM E-336 to ensure that each partition is properly installed and adjusted. This paper discusses the various testing methods commonly used with particular attention to measurement problems typically encountered in the field. The dependence of NIC on the measurement microphone locations in the source and the receiving room is examined. The paper also proposes a simple normalizing procedure that should help reduce variations in the measured NIC values for different room arrangements and geometries.

An energy-based control strategy for minimizing the sound field in enclosures.

Squared pressure has generally been the quantity chosen to minimize when controlling the sound pressure level (SPL) in enclosures. However, difficulty in selecting the best possible microphone location makes this method unattractive for arbitrary enclosures. As an alternative control strategy, minimizing the total energy density in an enclosure has been investigated. This method is a logical alternative because the total energy in the field is composed of both pressure and velocity components, as opposed to simply a pressure component. Computer simulations and experimental measurements of reducing the SPL in a rigid-walled duct have been conducted to compare the two strategies. The results of the experiments show that minimizing the total energy provides greater attenuation of the total potential energy with the added advantage of being less sensitive to microphone placement than when minimizing the squared pressure.

Faith in natural systems

The faith in natural systems principle holds that if an engineering effort is being directed at duplicating or simulating a natural system, that effort is most efficiently directed at incorporating critical functions and structures of that system. The faith in natural systems principle would say that if a person would want to design a more effective hearing instrument, that effort is most efficiently directed at incorporating the critical functions of the hearing mechanism, and the enhancement of the anatomical structures of that natural system. Binaurality is supportive of the faith in natural systems principle. The choice of an in‐the‐ear microphone location, ear canal resonances, and compensation for the recruitment phenomenon are also supportive of the faith in natural systems principle. The use of real ear measurements utilizing a probe tube microphone will demonstrate support of these principles.

Ear Level Recordings of the Long-Term Average Spectrum of Speech*

The long-term average speech spectrum (LTASS) was measured at two different recording positions: 30 cm directly in front of the talker (reference position), and at the tragus of the talker's ear (ear-level position), for three groups of subjects: adult males, adult females, and children. Results indicated significant differences in the overall level and frequency spectra between the LTASS obtained at each microphone location. For all three groups of subjects the LTASS measured at the ear-level position consisted of more low frequency energy (i.e., below 1000 Hz) and less high frequency energy (i.e., above 2500 Hz) than did the LTASS measured at the reference microphone position. The findings suggest that the algorithms currently used to prescribe hearing aid gain may underestimate the sensation level of a hearing-impaired individual's own amplified speech productions at frequencies below 1000 Hz and overestimate the sensation level of a talker's own speech above 2500 Hz. The implications of these findings concerning selection of the electroacoustic characteristics of an amplification system for hearing-impaired individuals are discussed.

Current and future issues of active noise control.

Active control of sound fields is an emerging technology which has a great potential to become superior to many passive low frequency noise control methods. It is based on fundamental physical principles which include acoustical coupling of sources, destructive wave interference, and control of enclosure modes. The implementation of active noise control requires placing secondary sources and microphones into the controlled field. Most systems use adaptive filters to control the secondary sources to achieve maximum noise reduction. The engineering applications will require developing optimization techniques for secondary sources and controlling microphone locations. Also new types of robust actuators and controlling sensors will be needed. A brief summary of existing and future applications is provided.

The Effect of Reference Microphone Placement on Sound Pressure Levels at an Ear Level Hearing Aid Microphone

The present study was designed to investigate the effects of reference microphone location on probe tube microphone measures of hearing aid response. The reference microphone of a clinical probe tube microphone system was located either at a position on the cheek (position A) or in close proximity to the microphone of an ear level hearing aid (position B). With sound pressure level (SPL) held constant at the reference microphone, the SPL at the position of the hearing aid microphone was measured at 14 test frequencies using a 1/8-inch condenser microphone. Measures were obtained on ten male and ten female subjects. Results indicated large frequency-dependent deviations in SPL at the hearing aid microphone, compared to that measured at the reference microphone, when the reference microphone was at position A. In the 1200 to 2000 Hz range, the SPL at the hearing aid microphone was as much as 9.5 dB higher than at the reference microphone. There were no large frequency-dependent variations with the reference microphone in position B, but the SPL at the hearing aid microphone location was approximately 3 dB higher than at the reference microphone. Results suggest that estimates of hearing aid output can be affected markedly by the reference microphone location. Clinical implications of the impact of reference microphone location on probe microphone measures of hearing aid gain and saturation sound pressure levels are discussed.

On the estimation of the acoustic surface impedance of layered porous materials

The acoustic impedance of layered porous materials can be calculated using the Biot theory and a matrix formalism to characterize each layer. A comparison with experimental data will be made for layered systems of various types, including layered systems covered with a membrane. The experimental data have been obtained with a two-microphone technique, which calculates the acoustic impedance from pressure measurements at two different microphone locations above the sample.

Relative masking effectiveness of self-noise and ambient noise distributions in outdoor recreational settings

The audibility of man-made noise intrusions in park and wilderness environments is determined not only by masking of natural origin, but also by the self-noise of outdoor recreationists. The relative influences of self-noise and ambient noise on the audibility of aircraft noise, a matter of central concern for aircraft overflight management for purposes of Public Law 100-91, thus vary with the activity and location of outdoor recreationists. Comparisons of spectral distributions of acoustic energy from fixed and at-ear microphone locations document large differences between estimates of masking based on place-oriented rather than visitor-oriented acoustic measurements in outdoor recreational settings.

Teleconferencing—The acoustical environment and directional transducers

The performance of teleconferencing systems are largely a function of the acoustical environment, the type, and the location of loudspeakers and microphones. Acoustical treatment in most conference rooms is limited to carpeted floors and an acoustical ceiling. For the most part, present microphones are designed to work best under close talking conditions and have omnidirectional, or first-order unidirectional characteristics. Two small experimental conference rooms have been constructed in which the effects of room parameters such as reverberation time, noise level, and discrete echo control can be independently studied. Broadband (8-kHz) directional microphones and steerable directional arrays, mostly of second order, are being investigated for a variety of acoustical environmental conditions. The easily adjustable conference rooms allow us to investigate these microphone systems with respect to their directional properties, placement, and steering algorithms. Loudspeaker directional characteristics, and advantages of stereo transmission are also under investigation. The experimental conference rooms and new directional microphones will be described and a discussion of trade-offs between major parameters for speech reception and transmission will be given.

Length error analysis for impedance tube measurements

Length error analysis of the transfer function method and improved standing wave ratio (SWR) method for impedance tube measurements is presented in this letter. The error of measuring the complex reflection coefficients R is caused by errors in measuring the positions of the two microphones (or the two pressure points for the improved SWR method). Results agree with Abom and Boden’s [J. Acoust. Soc. Am. 83, 2429–2438 (1988)] conclusions, i.e., errors will be large if s, the separation distance of the two microphones, is about one-half wavelength and will be small if s is about one-quarter wavelength. Results also show that in order to reduce the amplitude error of R, it is better to locate one microphone near a minimum pressure point and the other near the adjacent maximum pressure point. To reduce the phase error of R for the transfer function method, preference of the two microphone locations depends on the combination of length errors Δl1 and Δl2. Phase error reduction for the improved SWR method requires an accurate measurement of the minimum pressure point.

Prediction of helicopter rotor discrete frequency noise for three scale models

A new computer program that uses Farassat's most advanced subsonic time domain formulation has been written to predict helicopter rotor discrete frequency noise. A brief description of the program (WOPWOP), is followed by a comparison of predicted and experimentally measured acoustic pressure and spectra for a V* scale UH-1 model rotor blade and a V? scale OLS (AH-1G) model rotor blade. The rotorcraft flight simulation computer program C81 was used to predict the spanwise loading on the rotor for aerodynamic input into the acoustic prediction. Comparisons are made for different flight conditions and microphone locations with good results. In general, the acoustic pressure is underpredicted. The acoustic predictions for a tapered rotor blade and predictions for microphones well below the tip path plane show less underprediction. Finally, in-plane motion of the rotor blade is shown to significantly affect the peak-to-peak amplitude of the acoustic pressure for high advancing tip Mach numbers.

Active control of combustion instability

The principle of ‘antisound’ is used to construct a method for the suppression of combustion instabilities. This active instability control (AIC) method uses external acoustic excitation by a loudspeaker to suppress the oscillations of a flame. The excitation signal is provided by a microphone located upstream of the flame. This signal is filtered, processed, amplified, and sent to the loudspeaker. The AIC method is validated on a laboratory combustor. It allows the suppression of all unstable modes of the burner for any operating ratio. The influence of the microphone and loudspeaker locations on the performance of the AIC system is described. For a given configuration, domains of stability, i.e., domains where the AIC system parameters provide suppression of the oscillation, are investigated. Measurements of the electric input of the loudspeaker show that the energy consumption of the AIC system is almost negligible and suggest that this method could be used for industrial combustor stabilization. Finally, a simple model describing the effects of the AIC system is developed and its results compared to the experiment.

Acoustic Grain Moisture Meter

ABSTRACT Acontinuous flow grain moisture meter prototype was developed based on the sound pressure level produced by a stream of grain falling onto a grain filled funnel. The design parameters considered were funnel shape, housing surfaces, microphone location, and input grain flow rate. Measurements were made for the effect of wheat temperature, % cheat in wheat, and variety on the Leq-vs MC relationship.

Comparisons between A-weighted sound-pressure levels in the field and those measured on people or manikins

The A-weighted sound-pressure levels for 75 different industrial noise spectra were determined. Previously published sound-pressure level transformations by Kuhn and Guernsey [J. Acoust. Soc. Am. 73, 95–105 (1983)] from the field, either progressive or diffuse, to the body or head surface were applied to these noise spectra and the resulting A-weighted sound-pressure levels were calculated. For the most part, a microphone at the torso or head measures a larger A-weighted sound-pressure level than a microphone in the unobstructed sound field. The range of the resulting measurement error, due to the presence of the body or the head, is approximately − 1 dB(A) to + 5 dB(A). The magnitude of the error is sensitive to the type of sound field, the subject's orientation relative to the angle of incidence of the sound, the noise spectrum, and the microphone location. Other factors, for example, the microphone size and directivity, the distance between the microphone and the body, and the absorption of the sound by different types of clothing will also affect the measurement errors. Although these factors are implicit in some of the reported transformations, their individual, explicit effect on the measurement errors remains to be determined.