Sound Pressure Measurements Research Articles

Sound Pressure Measurements under Airflow

Sound Pressure Measurements under Airflow

The Effect of Probe Tube Reference Placement on Sound Pressure Level Variability

This study examined the effect of external microphone reference placement on peak sound pressure level (pSPL) and measurement variability. Nine normal subjects were seated in a double-walled sound suite, 1 m and 0 degrees azimuth from a wall-mounted speaker. Digitized Gaussian noise was presented at 80 dB pSPL with a 500 msec duration and was measured through a probe tube microphone assembly. Replicated measurements were made at five locations external to the pinna. They were: anterior-superior and posterior-superior positions simulating hearing aid microphone placement (locations 1 and 2) and 2, 4, and 6 cm lateral to the lateral edge of the pinna (locations 3, 4, and 5). Means, standard deviations, and ranges were compared, and statistical analyses were performed. The highest pSPL values were recorded lateral to the pinna, and the lowest values were obtained at the simulated hearing aid positions. ANOVAs indicated a main effect for pSPL, and post hoc testing demonstrated a significant difference between the posterior-superior and 2 cm lateral to the pinna positions. Variability was largest at the posterior-superior and 6 cm positions, and lowest 2 cm from the pinna. From this study, we concluded that pSPL and variability are both important criteria for selecting an optimal reference microphone site and both can affect the accuracy of ear canal measurements. A reference site 2 cm from the pinna eliminates attenuation of the signal and is the least variable site.

Residual pressure intensity index and performance evaluation of instruments to measure sound intensity

Residual pressure intensity index measurements and performance evaluation of sound intensity probes and analyzers are discussed. The residual pressure intensity index was determined separately for the analyzers using identical electrical input signals, and for probe and analyzer combinations using identical sound-pressure inputs. Both FFT and digital frequency analyzers were examined in the investigations. The results show that the residual pressure intensity index depends on the input signal levels, and any phase compensation scheme should be considered in relation to these measurements. Performance of the probes was examined using a gating technique in a large enclosed space. The measured sound intensity levels using the sound intensity probe were compared with the levels derived from sound-pressure measurements from a single standard microphone. The results are in agreement with the predicted performance. Further discussions in the paper include the directivity of the probes and phase compensation schemes.

Optical measurement of sound pressure

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The application of the sound-intensity technique to defect detection in rolling-element bearings

The two-microphone sound-intensity technique has been used for the detection of defects in radially loaded ball bearings. The difference in the sound-intensity levels measured for bearings with no defect and for those with intentionally introduced defects of different sizes in their elements under various operating conditions of loads and speeds is demonstrated. A change in the intensity frequency spectrum because of the defects is observed. The results show that the detectability of an outer-race defect is much better than that of an inner-race or ball defect. It is difficult to detect defects at lower speeds. Sound-pressure measurements were also performed for comparison, and it is shown that the detectability of defects by sound-intensity measurements is better than that by sound-pressure measurements.

Low‐frequency sound intensity measurement and free‐field sound‐pressure propagation

Classical acoustical equations relate the sound power of a source to sound‐pressure level at a given distance from that source. Sound intensity of a source can be measured, sound power determined by integration over the radiation area, and the sound‐pressure levels at a distance from the source can be calculated. An assumption is made when the sound power to sound‐pressure calculation is made, that at low frequencies (20–100 Hz) the waves have propagated and the resultant sound‐pressure level is from the source. This work tests the reliability of that assumption. A speaker was suspended inside three different size boxes, and powered by a noise generator bandpassed at discrete frequencies. Sound intensity measurements were recorded near field at the same discrete frequency. Free‐field sound pressure measurements were then recorded at distances from the boxes. This paper reports the results of the measurements.

Laser diode interferometer for vibration and sound pressure measurements

A laser-diode interferometer for vibration measurement was constructed using two acoustooptical tunable filters (AOTFS) with slightly different frequencies of the diffracted light. A sound pressure measurement of a condenser microphone using a combination of a polarization-maintaining fiber and the laser-diode interferometer was demonstrated. The advantages of this interferometer are the ability to use various laser beams in the 0.4-1.2- mu m wavelength region by varying only the acoustic frequency of the OTF, and the ability to use remote measuring equipment for the sound pressure by using a polarization-maintaining fiber. >

Jaw movement and bone-conduction in normal listeners and a unilateral hemi-mandibulectomee.

Relative movement between the mandible and skull may compress the auditory meatus and produce an excess sound in the ear canal. To test this, measurements of sound pressure in the ear canal, and movement of the upper and lower jaw are reported on normal subjects and a unilateral hemi-mandibulectomee. On the side the unilateral hemi-mandibulectomee has a jaw and in the subjects with normal jaws, the data confirm that mandible-movement relative to the skull produces sound in the ear canal. The excess sound in the ear canal when the jaw is free to move independently of the skull can be predicted from the relative movement between them.

Middle Ear Vibration and Sound Pressure Measurements in the Isolated Cochlea Preparation

A wide variety of auditory functions and responses are measured in terms of the sound pressure at some reference location in the ear canal. Another common measure that has been used to specify the mechanical input to the ear is the vibration of the malleus at the umbo

Initiation and suppression of combustion instabilities by active control

An active instability control (AIC) system is used to suppress low-frequency combustion instabilities in a non-premixed multiple-flame combustor and to study their initiation. The AIC device is based on a feedback control loop: The signal provided by a microphone is filtered, processed and sent to driver units plugged on the air duct. It allows the suppression of all unstable modes of the combustor and also provides a new and powerful method to study the instability initiation. Starting from a controlled flame with AIC, the control system is switched off and the growth of the instability is analysed through high speed Schlieren cinematography and sound pressure and reaction rate measurements. Nonlinear spectral analysis is used to extract spectral information during the oscillation growth from the initiation to the limit cycle. Results reveal that five different phases take place between stable combustion and fully established instability. The source of the first flame movements is the flow excitation by longitudinal acoustic waves. When the flow oscillation amplitude is high enough to allow interactions between flame sheets, combustion becomes periodically enhanced. This coupling leads to a fast instability growth until a limit cycle is reached.

On the relationship between sound intensity, pressure, particle velocity, and measurement accuracy

Sound intensity measurements are sensitive to instrumentation inaccuracies in a manner that varies with the sound field in which the meaurements are made. For this reason a number of “quality indicators” have been proposed to allow evaluation of the uncertainty of a measured intensity, the most widely used indicator being the “pressure‐intensity index.” Intensity is usually calculated from measurements of sound pressure and either pressure gradient (i.e., two‐microphone, p‐p devices) or particle velocity (p‐u devices). The interpretation and use of the pressure‐intensity index is explored for such devices, with emphasis on practical use. It is shown that this indicator is a “phase index” for p‐p devices, and an “impedance index” for p‐u devices. It is shown also that proper interpretation of the indicator varies with the manner in which the measurement device is realized. For example, p‐p devices that sum and difference before filtering require interpretation different from those that filter first. It is concluded that universal limits to ensure a given measurement accuracy cannot be established independent of the measuring device.

Active Noise Control to Reduce the Blade Tone Noise of Centrifugal Fans

This paper describes an active noise control method to suppress the blade tones of centrifugal fans. Two secondary sound sources are mounted into the cutoff region of the fan casing. These sources are driven with electrical signals that are synchronized with the rotation of the impeller, and their amplitudes and phase are adjusted to give maximum reduction for the blade tone levels in the inlet and outlet duct of the fan. With this design, the sound emitted by the secondary sources is introduced into the interior of the casing near the source region where the blade tone is generated, i.e., the cutoff. The present experiments were concentrated on the reduction of the fundamental of the blade tone for centrifugal fan with impeller diameters between 280 mm (11 in.) and 710 mm (28 in.). Two different designs of secondary sources were investigated. In the first, two loudspeakers are contained within an enclosure which has an open end made of a curved perforated plate which replaces part of the original cutoff. The second design incorporates two vibrating plates which replace portions of the outlet duct side and the volute side of the cutoff. Reductions in tone sound pressure level of up to 23 dB have been observed for a variety of aerodynamic loading conditions and fan inlet geometries. To obtain a better understanding of the physical mechanism of this active noise control method, sound pressure measurements were also made on the inner surface of the fan casing along the volute. Both amplitude and phase of the blade passing frequency component were measured relative to a reference signal derived from the impeller rotation. The result of this experiment is that the sound field inside the casing is dominated by the pressure pattern rotating together with the impeller. Since the impeller tip Mach number is well below sonic speed, however, the radiation efficiency of the rotating pressures is very low. The blade tone noise measured in the far-field is generated by the unsteady pressures at the cutoff which in turn are produced by the flow leaving the impeller. This aerodynamic noise generating mechanism is modified by the active sources located in the cutoff.

Laboratory calibration of standard and measurement condenser microphones

The reciprocity technique is the most highly developed and most widely used primary calibration method at the major national standards laboratories of the world. Critical measurements of sound pressure in air are usually traceable to such calibrations, which are briefly described with reference to specific apparatus and procedures at the NBS. Secondary methods, including those based upon reciprocity, can offer improved convenience with minor to significant sacrifices in accuracy and precision. Dynamic signal analyzers incorporating digital signal processing are now sufficiently accurate and precise to offer promise for improved and/or more readily automated measurements, especially secondary free-field calibrations. Specific examples from the NBS research and measurement services are discussed.

Mounting and selection of microphones for two-microphone impedance measurements

The determination of acoustic impedance as specified in ASTM E-1050 requires the measurement of sound pressure at two positions along a standing wave tube. The choice of microphones and their mounting can have a significant influence on the measurement results. A variety of microphone types (i.e., phase-matched condensers, probe tube types, condenser microphones fitted with phase correctors, etc.) will be considered and the results from these various types will be compared. The mounting of the microphones (i.e., flush with the tube walls, near the middle of the tube, etc.) will also be considered.

Measurement distance, direct field bias, and critical frequency in the measurement of reverberant sound fields

The determination of the sound power output of small sources in reverberation rooms and the transmission loss of building partitions require the measurement of space-averaged sound pressure squared. Such measurements are contaminated by a direct field bias error. This article reviews the underlying theory and presents a number of graphs for determining the direct field bias error as a function of measurement distance, source directivity factor, room volume, and reverberation time. Also, an in situ experimental method for determining the effective directivity factor of a source is presented in graphical form. Finally, the concepts of ‘‘modal overlap’’ and Schroeder frequency are explored in terms of an elementary electrical resonant circuit and the more descriptive term, number of modes per modal bandwidth, is proposed.

Study of acoustic response of anisotropic steel structures

The purpose of our experiment is to learn the influence of stiffening on the acoustic response of steel plates. The investigation is carried out on a rectangular plate that is clamped in an opening (1.20 × 2 m2) between a reverberant room and a test room. The sound transmission loss is determined by using two different methods: a method that is based on the sound pressure measurement and the two‐microphone intensity method. The mean radiation efficiency is obtained by measuring the panel vibration velocity with a regular mesh. The measurements are made over a frequency range from 250 to 10 000 Hz for a diffuse field excitation. The following series of samples is tested: a 1.5‐mm‐thick‐steel plate; a plate reinforced with uniformly spaced tubular stiffeners; a plate reinforced with uniformly spaced line stiffeners; and a corrugated panel (the space between the corrugations is the same as the space between the stiffeners). The acoustic effect of variation of bending stiffness is discussed, and all the exper...

On the formulation of the intensity method for determining sound reduction indices

The standard formulation of the sound intensity method for determining sound reduction indices is revised. In pursuance of the principle of Gaussian integration, the practical implications of the conditions relating to the shape and absorption contents of the measurement surface used in determining the transmitted power are investigated. An extended formula is presented, containing additional terms to account for the size of the measurement surface, boundary interference effects and calibration mismatch. It is shown that, since the intensity method employs both sound pressure and sound intensity measurement systems, it is more prone to calibration error than the classic two-room method involving the subtraction of two pressure levels. If the receiving room is reverberant, sound absorption by the test object will constitute a component of sound power in a direction opposite to that of the transmitted power. The net effect is a reduction in the apparent transmission, resulting in an overestimation of the sound reduction index. The absorption error increases with the flanking ratio and with the fraction of the total receiving room absorption located on the surface of the test object.

カナル補聴器のイヤシミュレータと実耳外耳道内音圧測定について

カナル補聴器のイヤシミュレータと実耳外耳道内音圧測定について

Demonstration of sound power determination using sound intensity measurements

Sound power determination using direct measurement of sound intensity is presently under study by ANSI and other standards organizations. When compared with the conventional methods of sound power determination that are based on sound pressure measurements (e.g., ANSI S1.31–S1.36), the intensity approach offers significant advantages. Precision sound power determinations can be made without a special test chamber, and the result can be as much as several orders of magnitude less sensitive to ambient noise. Also, measurements can be made over any convenient control surface in the nearfield or farfield. After a brief introduction, the authors will demonstrate these advantages by performing a series of sound power determinations on a known source using commercially available equipment.

Possible use of acoustic intensity measurement in standard tests of vehicle noise

Existing standard tests of motor vehicle noise, such as passby tests, are based on sound pressure measurements with single fixed‐location microphones. Recent developments in two‐microphone acoustic‐intensity techniques, and in instrumentation, permit the practical measurement of the sound power of vehicles and of components, such as engines. This can be performed in a normal test environment; a special acoustical facility such as an anechoic chamber or a reverberation room is not required. Sound power, which is a single‐number measure of the total output of a noise source, appears better suited to determining the overall effect of a vehicle on community noise. Also sound power is usually measured indoors, thereby avoiding scheduling delays due to bad weather and the inherent variability in the data associated with outdoor testing. Good repeatability (i.e., a standard deviation less than 5% or 0.2 dB) has been demonstrated in the measurement of the sound power of a medium‐duty truck, using acoustic intensity methods in an indoor test. Similar repeatability has been demonstrated in the measurement of the sound power of an engine for a heavy‐duty truck. These and other advantages and disadvantages of the use of acoustic intensity measurement methods for standard tests are discussed in the presentation.