Anechoic Wedges Research Articles

Rapid impedance tube determination of fabric transparency

Acousticians are continually being asked to verify fabric transparency for applications with absorptive and diffusive surfaces, as well as in sound reinforcement. Standard reverberation chamber methods can be used, but require large fabric and fiberglass samples. A quick and simple impedance tube method has been developed requiring only a 160 mm x 160 mm sample. Two measurements are made. One with an anechoic wedge termination and another with a rigid termination in which the fabric is applied to a 50 mm 6pcf fiberglass substrate. Four microphones are placed at a quarter and three quarters of the square tube’s width and height and the signal is summed. This unique microphone placement minimizes the 1st, 2nd, and 3rd harmonics resulting in a fourfold increase in the upper frequency limit. Impulse response measurements are made at three distances from the sample to calculate the reflection factor, impedance and normal incidence absorption coefficient from 63 Hz to 4000 Hz in one measurement. The small fabric sample is large enough to minimize non-homogeneous effects. A study of fabrics with a wide range of transparencies reveals how both acoustically transparent and backed fabrics can be used as sonic equalizers depending on the application.

Applications of the Dual Porosity Theory to Irregularly Shaped Porous Materials

Summary Non planar acoustic materials may be used in building and environmental acoustics to achieve as ignificant absorption at lo wf requencies. Examples of these materials are anechoic wedges or advanced design noise barriers. The shape of these materials is mainly based on empirical knowledge because afi ne numerical modeling (e.g. FEM, BEM )r equires large computational costs. Therefore, the optimisation of the general form and of the material used to realise these absorbing systems is limited. The purpose of this paper is to propose an original alternative to these limitations. The work basis relies on the theory for the acoustics of multi-scale porous materials, and in particular on double porosity materials, which has been initiated by Oln ya nd Boutin (J. Acoust. Soc. Am. 2003, 114(1)). It is shown in this paper that this theory could be successfully applied to the modeling of non planar sound absorbing materials. Examples are give nf or multi-layer systems involving perforated panels, material samples having an irregular surface and anechoic wedges. The discussion is based on comparisons between analytical simulations and measurements.

Prediction of the sound field in anechoic rooms : comparison of two different approaches

Despite the numerical advances, predicting the sound field in anechoic rooms is still challenging because it requires a fine modeling of the wall surface, acoustic properties and geometry, and the computation of the room sound field. It is proposed here to compare two innovative approaches to predict the performance of anechoic wedges, in particular to quantify the influence of porous frame motion, and to propose a numerically efficient model for the prediction of the sound field in anechoic room sounds at low frequencies. The first approach relies on the theory for modeling the sound propagation in double porosity media first presented by Olny & Boutin [J. Acoust. Soc. Am. 114 (1), 2003]. It has been shown that this approach could accurately predict the cut-off frequency of anechoic wedges. The interest is that this approach is numerically costless and that complex geometry could be considered. The second approach is based on the combination of FEM and BEM. It allows to predict the sound field in anechoic rooms. Compared to the previous approach, the latter one has the advantage of being able to identify structural effects inside the wedges and to propose an accurate prediction of the sound field in the room.

Low-frequency control options in surround-sound critical listening rooms

Acoustically small rooms, which are typically used for critical listening, be it mono or surround, can potentially overwhelm the spatial and spectral textures that contribute to an enjoyable listening experience. The options to control potential modal and speaker boundary interference in rooms include optimal dimensional room ratios, optimal speaker/listener placement, optimal multiple subplacement and dedicated low-frequency absorbers, including quarter-wave traps, anechoic wedges, Helmholtz and membrane resonators, and a relatively new and potentially significant addition, the metal plate resonator, which absorbs efficiently down to 50 Hz with a thickness of only 4 in. The design theory, proof-of-performance evaluation testing methods, and optimal placement guidelines for all of these tools will be reviewed.

Anechoic wedge design and development/anechoic chamber qualification testing

The initial research conducted at Harvard University’s Electro Acoustic Laboratory for the development of the anechoic chamber will be reviewed in this presentation. These findings, which were originally presented in the Office of Scientific Research and Development National Defense Research Committee Report No. 4190 (OSRD, No. 4190), will be discussed. The research conducted by Leo L. Beranek and his colleagues during WWII established the basic design and construction for anechoic wedges and chambers for years to come. Contemporary design and materials used in anechoic wedge construction have evolved to encompass a variety of room acoustic treatments, to include fiberglass, foam, and metallic wedges. Design and construction of current commercially available chambers and wedges will be reviewed. The second half of the presentation will focus on the development of anechoic and hemianechoic chamber qualification procedures to current ISO and ANSI standards. The use of various sound sources will be discussed as well as the correlation between these standards and other test methods in determining chamber performance.

Development of a high-frequency precision anechoic test facility

Although anechoic wedges with absorption coefficients of 0.99 as tested in typical impedance tubes meet ISO inverse square-law standards, they may not be satisfactory for anechoic rooms used for measurements of microphone response characteristics. Due to size restrictions, impedance tube tests are limited to frequencies below about 250 Hz and there is no standard method to test wedges for absorption at higher frequencies. This paper describes how pulse signal and reverberation room tests were employed to rank the high-frequency performance of three types of anechoic wedges considered for a microphone test facility: Standard patented MetadyneTM (perforated metal) wedges—conventional fiberglass wedges and an improved high performance Metadyne wedge system. The results of laboratory tests and tests conducted in anechoic rooms are presented. In all cases the preferred system, high-performance Metadyne wedges, is shown to meet the stringent acoustic absorption requirements needed for this application. The paper indicates the need for new standards for laboratory testing of anechoic wedges at high frequencies and an alternate method to inverse square-law tests for evaluating anechoic room performance such as stepped frequency sweep tests. Such a new standard will be proposed in the presentation.

Comments on ‘‘A different look at the anechoic wedge’’ [J. Acoust. Soc. Am. 75, 855–858 (1984)

We present measurements to show that the reversed wedge design has limitations for anechoic room linings. Measurements of the low‐frequency performance differ significantly from Warnaka’s and we question the value of reverberation room measurements of the absorption coefficient of anechoic room linings.

A different look at the anechoic wedge

Anechoic wedges are conventionally installed with the tapered point of the wedge pointing toward the test space and with the flat base of the wedge either against a rigid wall or separated from the wall by an air gap. A comparative investigation has been carried out in which the wedges were first conventionally oriented and then turned 180° so that the points of the wedges faced the wall. These comparative tests were performed in reverberation rooms both here and in Europe and also in two different impedance tubes. The results are extremely interesting. They tend to indicate that a ‘‘reversed’’ anechoic wedge can give a lower cutoff frequency than when conventionally oriented. Some possible explanations are offered for the measured performance.

A different look at the anechoic wedge

Anechoic wedges are conventionally installed with the tapered point of the wedge pointing toward the test space and with the flat base of the wedge either against a rigid wall or separated from the wall by an air gap. A comparative investigation has been carried out in which the wedges were first conventionally oriented and then turned 180° so that the points of the wedges faced the wall. These comparative tests were performed in reverberation rooms both here and in Europe and also in two different impedance tubes. The results are extremely interesting. They tend to indicate that a “reversed” anechoic wedge can give a lower cutoff frequency than when conventionally oriented. Some possible explanations are offered for the measured performance.

Reduction of fan noise in an anechoic chamber by reducing chamber wall induced inlet flow disturbances

The difference between flight and ground static noise data has arisen in the past few years as a significant problem in fan jet engine noise testing. The additional noise for static testing has been attributed to inlet flow disturbances or turbulence interacting with the fan rotor. In an attempt to determine a possible source of inflow disturbances entering fans tested in the Lewis Research Center anechoic chamber the inflow field was studied using potential flow analyses. These potential flow calculations indicated that there was substantial flow over the wall directly behind the fan inlet. This flow near the wall anechoic wedges could produce significant inflow disturbances. Fan noise tests were run with various extensions added to the fan inlet ducting to move the inlet away from this back wall and thereby reduce the inlet flow disturbances. Significant noise reductions were observed with increased inlet length. Over 5‐dB reduction of the blade passage tone sound power level was observed between the shortest and longest inlet at 90% fan speed and the first overtone was reduced 9 dB. High‐frequency reductions in the broadband noise were also observed.

A New Anechoic Facility for Supersonic Hot Jet Noise Research at Lockheed-Georgia

Much confusion about jet noise has come about as a direct result of inadequate facilities and insufficient knowledge and control of test conditions, in many cases giving rise to completely erroneous conclusions. The facility described here has been carefully designed, accounting for the shortcomings of other facilities and being guided by the stringent demands of ongoing jet noise research at Lockheed. The design goal was the capability of testing model jets up to 2000°F at pressure ratios as high as 8, in a free-field environment anechoic at all frequencies above 200 Hz. A comprehensive series of flow visualization and temperature mapping experiments in a one-sixth scale-model anechoic room was conducted. The results dictated the design of the exhaust collector/muffler to provide entrainment and room cooling air in the quantities demanded by the jet operating conditions. In order to optimize the choice of material and anechoic wedge design to achieve the 200-Hz requirement, a special impedance tube was used extensively in performance evaluations. Some features of the facility are (1) an exhaust collector providing air in quantities dictated by the particular jet operating condition with no special forced-air injection or fan installation and (2) a “cherry-picker” crane used to gain access to instrumentation, etc., for maintenance, calibration, and setup, thus eliminating the need for access platforms, etc. The crane is stowed by remote control under an anechoic cover during all test operations. The superior performance of the room is described by intensity/distance plots obtained from a fixed loudspeaker source and moving microphone installation.

A Unique Method to Measure the Sound Absorption Coefficient for Oblique Incidence

The method requires subsequent accurate recording of the absolute value of the transfer function between omnidirectional sound source and microphone in an anechoic environment for three configurations: (1) without the sample or barrier, (2) without the sample but with a barrier blocking the line of sight between source and receiver, and (3) with both the barrier and the sample in place. The first two recordings “calibrate” the barrier, while the second and third provide an interference pattern between the sound refracted around the edge of the barrier and that reflected from the sample. The reflection factor given as |R(f)| = d3d1 |p2(f)||p1(f)| |1 − |p3(f)||p2(f)|| must be evaluated at the frequencies of the minima and maxima of the interference pattern. d3/d1 is the ratio of the path lengths of the reflected and direct signal, p1/p2 is the insertion loss of the barrier, and p3 and p2 are the signals recorded with and without the sample in place, respectively. Experimental results on scale-model anechoic wedge surfaces are presented. [This study was sponsored by the NASA-Langley Research Center.]

Quiet Wind-Tunnel Facility for Investigating Flow Noise

A low-noise, low-turbulence, subsonic wind tunnel was designed for investigation of both wide- and narrow-band flow-noise processes. The open-circuit design incorporates such innovations as interchangeable open-jet and closed-duct test sections within a sealed test chamber that can be either reverberant or anechoic by the use of removable wall treatment. Low noise level at the test section is achieved by using upstream treatments, such as anechoic wedges off the face of the inlet and a honeycomb of soda straws, and by using downstream treatments, such as a combination muffler-diffuser and another honeycomb of soda straws. The design concept, the aerodynamic performance, and the acoustical performance are presented.

Economic Design and Construction of Anechoic Chambers

For the past 16 years, the design of anechoic chambers has been the subject of a good deal of acoustic research. Prior research has yielded a design employing a large number of light-density wedges (2.S lb/ft3) which had to be supported a fixed distance from the chamber wall. The design considerations which formed the nucleus of a research program by the Industrial Acoustics Company, Inc., were the following: (1) The wedge had to be made of a rigid material. (2) The over-all length of any anechoic wedge must be maintained as short as physically possible. (3) The wall area covered by a single wedge should be made as large as possible. The first series of investigations made was to determine whether or not the conventional air space could be eliminated. Two figures showing test results of two particular experiments indicate that, if the wedge sizes were maintained constant, there is no advantage in having the air space behind them. It was also observed that the low-frequency absorption could be matched to the high-frequency absorption by altering the wedge-shaped portion, or tapered length, of the anechoic wedge as well as the base depth. In order to maintain as small as physically possible the over-all length of our anechoic-chamber wedges, experiments were performed to show that the standard 4-in.-thick IAC Noise-Lock panel could be used as part of the wedge's length without affecting the acoustic property, in any manner, of either clement. A graph is presented showing the final design curve of the IAC wedge for use in obtaining the length of the anechoic wedge required for any given cutoff frequency, as well as actual test results of wedges constructed to check the IAC design curve. The test method used in the laboratory was to analyze a standing wave pattern produced by a sound source emitting a pure tone. The schematic drawings given show a typical wedge arrangement in the test duct. The method of test used to evaluate a completed installation is slightly different. It is accomplished by generating a pure tone at one point in the room, moving the microphone away from the sound source until the entire room dimension is traversed, and then by comparing the results with those predicted by the inverse-square law. The correlation between IAC laboratory tests and field installations shows excellent agreement, within ±1-db measurement accuracy. The elimination of the air space behind the wedge allows a more efficient use of wedge material. This advantage, plus the use of a more rigid glass-fiber material than previously used, enables a more pleasing appearance to be maintained in the anechoic chamber, as well as a less elaborate construction for movable portions of an IAC anechoic chamber (such as the wedge door or window-wedge plugs). All these features are important to the economic design and over-all performance of an anechoic chamber. The incorporation of a larger base area and a shorter wedge length, coupled with the use of prefabricated structures in which the additional design criteria of quiet intake and exhaust of ventilation air can be provided, as well as the support of the entire structure on suitable vibration eliminators, is strongly recommended. In order to show that a more economical structure can now be manufactured than heretofore possible, a comparison is made of prefabricated and concrete rooms utilizing a new or old wedge design.

Apparatus and Procedures for Predicting Ventilation System Noise

A method has been developed for measuring the acoustic power output in watts and the frequency spectra of ventilating fans operating into ducts. Both the inlet and the exhaust noise are determined as a function of back-pressure. The fan is ideally connected to a circular section of duct about equal in diameter to the fan diameter and five to ten diameters in length. Three to six diameters from the input end of this duct an air straightener is located. The acoustic measurements are made with a condenser microphone enclosed in a wind screen located four to eight diameters from the inlet. The outlet end of the duct couples through an exponential connector to an anechoic wedge structure two feet square. This is backed by a plenum chamber containing an array of holes that can be progressively closed to produce increased back-pressure. Sound pressure level measurements are made in frequency bands and converted to acoustic watts per band. The inlet noise is measured over a hemispherical surface outdoors to obtain the directivity pattern and the acoustic power for each frequency band. Engineering formulas and charts for applying the data to prediction of the noise produced in rooms are presented, including the effects of room characteristics, duct losses, and the distance from the ventilating grille in the room. Criteria for permissible noise levels in various types of rooms and auditoriums are presented.

Method for Measuring Noise of Ventillating Fans

A method has been developed for measuring the acoustic power output in watts and the frequency spectra of ventilating fans. Both the inlet and the exhaust noise are determined as a function of back-pressure. The fan is ideally connected to a circular section of duct about equal in diameter to the fan diameter and five to ten diameters in length. Three to six diameters from the input end of this duct an air straightener is located. The acoustic measurements are made with a condenser microphone enclosed in wind screen located four to eight diameters from the inlet. The outlet end of the duct couples through an exponential connector to an anechoic wedge structure two feet square. This is backed by a plenum chamber containing an array of holes that can be progressively closed to produce increased back-pressure. Sound pressure level measurements are made in frequency bands and converted to acoustic watts per band. The inlet noise is measured outdoors on a hemispherical surface to get the directivity pattern and the acoustic power for each frequency band. Engineering formulas for applying the data to prediction of the noise produced in rooms are presented, including the effects of duct bends, duct losses, and the distance from the ventilating grille in the room.