Acoustic Flow Resistance Research Articles

Resonance mode and sound pressure produced by circular diaphragms of electrostatic and piezoelectric speakers

To investigate the frequency response characteristics of a push–pull electrostatic speaker (60-mm diameter) and free-edge-like piezoelectric speakers (68-mm and 73-mm diameters), this study was employed optical and acoustic measurements from 20Hz to 20kHz. The optical measurements of displacement amplitude and mode shape were used to predict sound pressure levels (SPLs). Comparisons with measured SPL values were used to verify the predictions. When using a mesh with acoustic flow resistance, i.e., unless damped, the electrostatic and piezoelectric speakers both produced numerous resonant frequencies. The two evaluation methods produced SPL values that were in good agreement. The piezoelectric speakers produced jagged SPL curves with peaks steeper than those of the electrostatic speaker. At the first axisymmetric mode, the electrostatic speaker was affected by acoustic resistance, which resulted in the following quality factor (Q) values: without mesh (Q=6.16) and using a mesh with specific acoustic resistance of 145 rayl (Q=1.74). By contrast, the piezoelectric speakers at modes (0,1)–(0,4) were unaffected by acoustic resistance, which resulted in Q values of 10–11, regardless of whether mesh was applied. Because higher Q performs larger acoustic response and lower Q presents wide broadband, this study concluded that the characteristics make electrostatic speakers suitable for headphones and piezoelectric speakers suitable for audio signaling devices.

Double-leaf microperforated panel space absorbers: A revised theory and detailed analysis

A double-leaf microperforated panel space absorber (DLMPP) is composed of two microperforated panels (MPPs) placed in parallel with an air-cavity in-between, without a back wall or any backing structure. This was proposed as a space sound absorber, which can be used for a sound absorbing screen or partition. A conventional MPP absorber with a rigid back wall is effective only around its resonance frequency, which is usually at middle frequencies, and not effective at low frequencies. However, a DLMPP can be effective also at low frequencies, because an additional sound absorption is produced by its acoustic flow resistance. In the authors’ previous work, theoretical analyses on the acoustic properties of a DLMPP were carried out using a simplified electro-acoustical equivalent circuit model. However, the equivalent circuit model includes an approximation, and more sophisticated theory is required for a better prediction and detailed discussion. In this paper, a revised theory for a DLMPP is presented: A Helmholtz integral formulation is employed to obtain a rigorous solution for more precise prediction of the absorptivity of a DLMPP. The result of the present revised theory is compared with that of the equivalent circuit model, and the difference between them is discussed. A parametric survey is made through numerical examples by the present revised theory to discuss its acoustic properties.

Noise in miniature microphones.

The internal noise spectrum in miniature electret microphones of the type used in the manufacture of hearing aids is measured. An analogous circuit model of the microphone is empirically fit to the measured data and used to determine the important sources of noise within the microphone. The dominant noise source is found to depend on the frequency. Below 40 Hz and above 9 kHz, the dominant source is electrical noise from the amplifier circuit needed to buffer the electrical signal from the microphone diaphragm. Between approximately 40 Hz and 1 kHz, the dominant source is thermal noise originating in the acoustic flow resistance of the small hole pierced in the diaphragm to equalize barometric pressure. Between approximately 1 kHz and 9 kHz, the noise originates in the acoustic flow resistances of sound entering the microphone and propagating to the diaphragm. To further reduce the microphone internal noise in the audio band requires attacking these sources. A prototype microphone having reduced acoustical noise is measured and discussed.

Standing wave apparatus for measuring fundamental properties of acoustic materials in air

A standing wave apparatus employing multiple microphones for measuring fundamental properties of acoustic materials is analyzed. The apparatus is basically a tube with a compression driver at one end with a rigid plug at the other and a finite length sample of acoustic material strategically located in between. A microphone array is then used in obtaining the pressure and velocity boundary conditions of the sample from which basic acoustic properties can be calculated. Such properties as internal propagation constant, characteristic admittance (inverse impedance), and bulk modulus are measured and data for a 100-pore-per-inch (ppi) open-cell acoustic foam is presented. A relatively new parameter called the complex flow impedance is measured under low- and high-intensity levels and is shown to exhibit finite amplitude properties. Of particular concern is how finite sample lengths effect the accuracy of the measurements. It is shown that in general a long sample length (on the order of a wavelength or more) is required for the accurate measurement of propagation constant, characteristic admittance, and bulk modulus measurements while a short length (much less than a wavelength) is required for good finite amplitude flow impedance measurements. Previous studies of flow impedance have been carried out using a similar but not identical apparatus [W. E. Zorumski and T. L. Parrott, ‘‘Nonlinear Acoustic Theory for Rigid Porous Materials,’’ U.S. NASA TN-6196, 1971; K. U. Ingard and T. A. Dear, ‘‘Measurements of the Acoustic Flow Resistance,’’ J. Sound Vib. 103, 567–572 (1985)]. However, the previous studies have lacked an analysis of the possible errors in the measurement methods that can be very significant for these types of flow impedance measurements.

Perforated sound absorbing panel

A perforated panel having a thickness of at least about 6 mm, for use as a sound absorbing element, especially at lacunarias. Each perforation is covered by a thin fiber cloth which is bonded to the edges of the perforation, rendering said panel element a specific acoustic flow resistance of between 1200 and 4000 Nsm-3.

Measurement of acoustic flow resistance

A method of measuring the acoustic flow resistance and reactance of porous materials is described, and some illustrative experimental data are presented and discussed. For example, data for stacks of wire mesh screens have been obtained and analyzed, with particular attention to the dependence of the flow resistance on frequency and wire diameter.

Theoretical and experimental investigations of microphone probes for sound measurements in turbulent flow

When sound measurements are made in flow ducts one has to provide for a sufficient signal-to-noise ratio between the desired sound waves and the undesirable turbulent pressure fluctuations, which the flow excites at the microphone. Theoretical and experimental investigations show that the turbulent noise signal can be reduced considerably by means of a cylindrical tube with an axial slit, which is covered with cloth. The signal-to-noise ratio between sound and flow noise increases with both frequency and tube length; it becomes smaller for higher flow velocities. One may vary the acoustic flow resistance of the slit covering within a wide range without diminishing the efficiency of the flow noise suppressor. When an additional pipe filled with damping material is inserted into the slittube a particularly smooth frequency response of the acoustic sensitivity is achieved.