Line Microphone Research Articles

SHAPE MEMORY ALLOY ACTUATORS ARE USED TO CREATE A VARIABLE AREA JET NOZZLE FOR NOISE REDUCTION: A REVIEW

The real study details the design and implementation of Shape memory alloy actuators to produce a noise-reducing variable area jet nozzle. A subscale design specification of a changeable area jet nozzle was created using SMA actuators in an asymmetrical design. Commercial transportation planes must be quieter, cleaner, and more efficient, according to the international community. The aviation industry is reacting by developing new technology in order to achieve those objectives. Changing the area of a commercial jet engine's fan nozzle can result in substantial noise reduction and reduced fuel economy. At takeoff and approach, a bigger diameter reduces jet velocity, which reduces noise. In cruise, adjusting the diameter to account for variable Mach numbers, altitude, and other factors helps optimise fan loading and minimise fuel consumption and emissions. Boeing has tested a 20 percent area change scaled variable area jet nozzle. At the nozzle exit, Shape Memory Alloy actuators were utilised to place 12 interlocking panels. To maintain a range of consistent diameters with variable flow circumstances and to alter the diameter under constant flow conditions, a closed loop control system was utilised. At each condition, acoustic data was gathered using side line microphones, and flow field measurements were taken using PIV at various crosssections. The design of a variable area nozzle is explained in this work. The diameter of the nozzle and its influence on acoustic performance are discussed. The effects of the joints between the interlocking panels are seen in the flow field data.

Influence of microphone position error on generating spatial null sensitivity point by line microphone array

We developed a method that picked up a sound of a target source located beyond the noise source [1]. It used an end-fire line microphone array whose direction was toward the noise source and created a spatial null sensitivity point at the noise source position. This method relied on the assumption that the precise distance between microphones was known so that all microphone outputs from the noise source became in phase by using the delay calculated by the distance. In this study, we analyzed the influence of the error of microphone distance on the performance of the method. As a result, if the real distance was larger than that of the system’s knowledge, the dip of sensitivity at the noise position was separated along the direction normal to that of the line of the microphones. On the other hand, it the distance was smaller than that of the system’s knowledge, the dip did not separate and just became shallower. Based on these results, we will propose how to compensate the microphone position error. [1] A. Ando, et al., 3pSP3, Acoustics ’17 Boston, 2017.We developed a method that picked up a sound of a target source located beyond the noise source [1]. It used an end-fire line microphone array whose direction was toward the noise source and created a spatial null sensitivity point at the noise source position. This method relied on the assumption that the precise distance between microphones was known so that all microphone outputs from the noise source became in phase by using the delay calculated by the distance. In this study, we analyzed the influence of the error of microphone distance on the performance of the method. As a result, if the real distance was larger than that of the system’s knowledge, the dip of sensitivity at the noise position was separated along the direction normal to that of the line of the microphones. On the other hand, it the distance was smaller than that of the system’s knowledge, the dip did not separate and just became shallower. Based on these results, we will propose how to compensate the microphone position error. [1] A...

Wavenumber–domain separation of rail contribution to pass-by noise

In order to counteract the problem of railway noise and its environmental impact, passing trains in Europe must be tested in accordance to a noise legislation that demands the quantification of the noise generated by the vehicle alone. However, for frequencies between about 500Hz and 1600Hz, it has been found that a significant part of the measured noise is generated by the rail, which behaves like a distributed source and radiates plane waves as a result of the contact with the train's wheels. Thus the need arises for separating the rail contribution to the pass-by noise in that particular frequency range. To this end, the present paper introduces a wavenumber–domain filtering technique, referred to as wave signature extraction, which requires a line microphone array parallel to the rail, and two accelerometers on the rail in the vertical and lateral direction. The novel contributions of this research are: (i) the introduction and application of wavenumber (or plane–wave) filters to pass-by data measured with a microphone array located in the near-field of the rail, and (ii) the design of such filters without prior information of the structural properties of the rail. The latter is achieved by recording the array pressure, as well as the rail vibrations with the accelerometers, before and after the train pass-by. The performance of the proposed method is investigated with a set of pass-by measurements performed in Germany. The results seem to be promising when compared to reference data from TWINS, and the largest discrepancies occur above 1600Hz and are attributed to plane waves radiated by the rail that so far have not been accounted for in the design of the filters.

Capturing sound of target source located beyond noise source by line microphone array

We had developed a method that captured the target sound when the noise source was located in front of the target source. It used a line microphone array whose direction was toward the noise source and created a spatial null sensitivity point at the noise source position. Our previous method, however, could reduce the noise only in a narrow area around the null point. To extend the noise reduction area, we developed a new method that creates some adjacent null points with more than two microphones. The experimental result showed that, if the target source was located 8 m distant from the microphone, the new method with three microphones could create a -20 dB noise reduction area whose center was 6 m distant from the microphone with a radius of 0.5 m, and captured the target sound without changing its timbre.

Miniaturization of a Narrow-Angle Directional Microphone with Suppressed Rear Sensitivity

This paper describes a miniaturization of a narrow-angle directional microphone that enables rear sensitivity to be significantly suppressed. The microphone assembly comprises of a line microphone capsule and a second-order pressure gradient directional microphone. This combination successfully reduced the residual rear sensitivity of conventional line microphones. The previously proposed microphone is too long to set in a small space. Therefore, the microphone is miniaturized by using small electret condenser microphone capsules, which are stacked in the small case, and by modifying the line microphone capsule's directivity to hypercardioid. The proposed microphone successfully suppresses rear sensitivity by more than 20 dB compared to the frontal sensitivity in the frequency range from 130 Hz to 2.5 kHz. Furthermore, it can be driven by a normal 48 V phantom power supply. Finally, the microphone was tested in several on-the-spot broadcasts, and it proved to be effective for practical use.

The Acoustics of Pipe Arrays

Summary A theoretical analysis is given for the acoustical behaviour of the pipemicrobarograph systems used to detect acoustic gravity waves and other modes of infrasound. It is shown how to compute the response of the microbarograph to a fluctuating pressure at any one inlet port of the pipe and how the results of such computations may be used to calculate the response to a plane sound wave traversing the system. The analysis is illustrated by numerical examples obtained by means of a computer program. These examples confirm that the tapered tube modelled after Daniels’ line microphone has very good characteristics, but that good results may also be obtained using pipes of uniform bore. The work leans heavily on Benade’s calculations of sound propagation in a circular conduit. For the purpose of observing acoustic-gravity waves and other rapidly progating modes of infrasound (of period 1-1000 s) it is common practice to enhance the signal-to-noise ratio by attaching a pipe to the microbarograph. This pipe is often several hundred feet in length and perforated by small inlet ports spaced at intervals of a few feet along its length. At a given frequency the disturbances entering the various ports are added acoustically in the pipe and transmitted rapidly along the pipe to the microbarograph. The noise, mainly atmospheric turbulence advected with the wind, is partially incoherent at the different inlets, and moreover travels slowly compared with the acoustic signals we are trying to observe and slowly compared with the speed of sound inside the pipe. Thus the contributions from the fast-moving signals, at least at low frequencies, tend to be added in phase at the microbarograph whereas the contributions from the noise do not. Daniels (1949) suggested the use of a tapered pipe narrowing in steps toward the end far from the microbarograph. The acoustic impedance of an inlet was carefully matched to the change in characteristic impedance of the pipe at that inlet in such a way that waves travelling away from the microbarograph in the pipe were not reflected. It was realized, however, that in order for the pipe to act properly as an array many inlets were required, far more than a convenient number of steps in the pipe radius. Moreover, only certain gauges of pipe were available, so that considerable compromise was required in order to use Daniels’ design in the field. On top of this, when practical dimensions for pipes are taken into account it is found that at the frequencies of interest in geoacoustics the narrow tube approximation is more applicable than Daniels’ theory which, with its assumed high velocity of sound in the pipe, agrees more with the wide tube approximation. These considerations lead one to speculate on the true acoustic behaviour of the

A Narrow-Angle Directional Microphone With Suppressed Rear Sensitivity

A novel microphone that enables rear sensitivity to be significantly suppressed has been developed to improve open-air recording quality. Its assembly comprises a line microphone capsule and a second-order pressure gradient directional microphone. In conventional line microphones, residual rear sensitivity causes an influx of unexpected noise, especially at lower frequencies. Our microphone successfully suppresses rear sensitivity by more than 10 dB compared to conventional line microphones in the frequency range below 1 kHz in which major outdoor noise often occurs. Furthermore, it needs no complicated signal processing circuit and can be driven by a normal 48 V phantom power supply. Finally, our microphone was tested in on-the-spot broadcasts. Its rear sensitivity suppression proved to be effective for practical use, and its sound quality was found to be sufficient for use in TV programs. This paper describes the fundamental principle of the microphone's rear sensitivity suppression, the measurement results of its acoustic characteristics and field-test results obtained with it in on-the-spot broadcasts.

Variable area jet nozzle for noise reduction using shape memory alloy actuators

A significant reduction in noise and improved fuel consumption can be achieved by varying the area of a commercial jet engine's fan nozzle. A larger diameter at takeoff and approach can reduce jet velocity reducing noise. Adjusting the diameter in cruise, to account for varying Mach number, altitude, etc, can optimize fan loading and reduce fuel consumption. Boeing recently tested a scaled variable area jet nozzle capable of a 20% area change. Shape memory alloy actuators were used to position 12 interlocking panels at the nozzle exit. A closed loop control system was used to maintain a range of constant diameters with varying flow conditions and to vary the diameter under constant flow conditions. Acoustic data by side line microphones and flow field measurements at several cross-sections using PIV was collected at each condition. In this paper the variable area nozzle's design is described. The effect of the nozzle's diameter on its acoustic performance is presented for a range of Mach numbers and mass flow rates. Flow field data is shown including the effects of the joints between the interlocking panels.

Line microphone array system positioned close to a wind turbine wing for broadband noise source measurements

Line microphone array system positioned close to a wind turbine wing for broadband noise source measurements

大気中のインフラソニック波の研究

Continuous observation of the infrasonic wave has been recently started at Aichi Uni-versity of Education in Kariya City. Major interest is in the infrasounds frequently radiated into the atmosphere by the summit explosion of the volcano Sakura-jima.In detection of the infrasonic signal in the turbulent boundary layer, it is often indis-pensable to reduce the irregular noise caused by the turbulent fluctuation of the wind. In the present study, a new type of wind noise reducer has been designed. It is a multi-pipe line microphone, having a number of branches of pipes of equal diameter. The function of it is to take the equally-weighted spatial average of the pressure field along a straight line, and is essentially the same as the Daniels' line microphone (1959). However, the manufacturing of the multi-pipe line microphone is practically easier than the Daniels' type. Another merit of the new device is in that it is as easy to take the two-dimensional average if necessary.From several field tests, it has been shown that the wind noise is remarkably attenuated by the use of multi-pipe line microphone, although the measured noise attenuation has been smaller than the theoretical estimation based on the turbulence model given by Priestley(1965) by about 5dB in average.

Tubular Microphone Windscreen for In-Duct Fan Sound-Power Measurements

The design of a porous tabular windscreen for in-duct fan sound-power measurements is described. Such a windscreen tends to cancel out turbulent pressure fluctuations. The windscreen also modifies the response of a microphone to acoustic waves, since it effectively converts an omnidirectional microphone into a line microphone with a pronounced directivity. This directivity can be used with advantage in both the plane-wave and cross-mode frequency regions, since the microphone sensitivity to reflected waves is reduced. However, in the cross-mode region, this directivity is a complication, since the acoustic power carried depends upon the angles at which the cross modes propagate. The ideal directivity in this region for estimation of sound power from pressure measurements is (cosθ)12. The directivity has been made to approximate this. Experimental data of the frequency response, directivity, and signal-to-noise ratio are presented. [Work done partly at Herrick Laboratories, supported by Air Conditioning and Refrigeration Institute, and partly at Carrier Corporation.]

Infrasonic waves from the marine storm of April 7, 1966

Nearly sinusoidal microoscillations of air pressure of the order of 1 to 10 microbars (dynes/cm²), which have been recorded on a Lamont tripartite array of line microphones, have been identified as infrasonic waves arising from an intense atmospheric low pressure system off Newfoundland. Although the nearly sinusoidal pressure variations show good coherence among the stations, there is enough wave breakdown among the stations to suggest interference from a widespread source. Other factors indicate this source to be ocean waves. It is concluded that the spectral character of both microbaroms and microseisms is a function of a common generating mechanism, but microbarom amplitude variations depend also on atmospheric conditions along the path.

A Century of Microphones

Of the various manifestations of a sound wave, the action of pressure on a diaphragm still is the universal means for detecting the presence of sound. The diaphragm actuates a transducer converting its motions into equivalent electrical waves. Innumerable transducers have been tried, but five are pre-eminent: 1) carbon, 2) condenser, 3) piezoelectric, 4) moving conductor, 5) moving armature. Important microphone improvements during the late twenties and the thirties have come about as a result of application of equivalent circuit analysis to acoustical structures. The principle of pressure microphones, pressure gradient microphones, combination microphones and phase shift microphones are described. Each of these has found an important niche in modem microphone applications. A small number of important applications require super-directional microphones. Here three approaches are used: 1) reflectors, refractors and diffractors, 2) line microphones, 3) higher-order combination microphones. In the future, improvements in design of directional microphones will continue. Wireless microphones are bound to increase in popularity. New methods of transduction based upon solid-state technology appear to be imminent. Unconventional methods of sound pickup may find wide use in space communication.

A new cardioid-line microphone

A new microphone, the Electro-Voice Model 642, is described. Designed particularly for television and motion picture applications where the usable microphone working distance must be a maximum, the model 642 has improved directivity and high sensitivity. The improved directivity is achieved by functioning as a first-order gradient microphone at bass frequencies and as a line microphone at higher frequencies. A detailed description of the directional characteristics is included.

Noise-Reducing Line Microphone for Frequencies below 1 cps

A novel type of line microphone is described that utilizes a distributed input primarily for the purpose of improving signal-to-wind-noise ratio. The input to the transducer takes place through a tapered pipe that is coupled to the atmosphere by means of acoustical resistances uniformly spaced along the length of the pipe. The relationship between the resistances of the openings and the longitudinal variation in the characteristic impedance of the pipe is so adjusted as to make the system nonreflecting. Microphones of this type have been constructed for use in the frequency range below 1 cps for the purpose of studying atmospheric pressure oscillations in this range. A prototype, 1980 ft long with 100 openings, which gives an improvement in the signal-to-noise ratio of as much as 20 db under severe wind conditions, is described.

Noise-Reducing Line Microphone for Frequencies below 1 cps

A novel type of line microphone is described that utilizes a distributed input primarily for the purpose of improving signal-to-wind-noise ratio. The input to the transducer takes place through a tapered pipe that is coupled to the atmosphere by means of acoustical resistances uniformly spaced along the length of the pipe. The relationship between the resistances of the openings and the longitudinal variation in the characteristic impedance of the pipe is so adjusted as to make the system nonreflecting. Microphones of this type have been constructed for use in the frequency range below 1 cps for the purpose of detecting atomic explosions at great distances. A prototype 1980 ft long with 100 openings is described, which gives an improvement in the signal-to-noise ratio of as much as 20 db under severe wind conditions.

Line Microphones

A line microphone with useful directivity normal to the line axis is described. Several types of line microphones with useful directivity along the line axis are described as follows: A simple line, a line with progressive delay, two simple lines and a pressure-gradient element, two lines with progressive delay, and a pressure-gradient element. An ultra-directional microphone employing line elements exhibits uniform directional characteristics over the range from 85 to 8000 cycles.