A Numerical Investigation Of Standard Condenser Microphones

Roisin

2000

A new method of measuring spatial correlation functions in reverberant sound fields is presented. It is shown that coherence functions determined with appropriate spectral resolution contain the same information as the corresponding correlation functions, and that measuring such coherence functions is a far more efficient way of obtaining this information. The technique is then used to verify theoretical predictions of the spatial correlation between various components of the particle velocity in a diffuse sound field. Other possible applications of the technique are discussed and illustrated with experimental results obtained in an ordinary room.

Section: Experimental Results In a Reverberation Roommentioning

confidence: 90%

The coherence of reverberant sound fields

Roisin

2000

“…Figure 12 shows the average free-field correction of the three microphones calibrated and values given in Ref. 22 and the standard. 20 The results shown in Fig.…”

Section: Resultsmentioning

confidence: 99%

A time-selective technique for free-field reciprocity calibration of condenser microphones

Barrera‐Figueroa

Rasmussen

2003

In normal practice, microphones are calibrated in a closed coupler where the sound pressure is uniformly distributed over the diaphragm. Alternatively, microphones can be placed in a free field, although in that case the distribution of sound pressure over the diaphragm will change as a result of the diffraction of the body of the microphone, and thus, its sensitivity will change. In the two cases, a technique based on the reciprocity theorem can be applied for obtaining the absolute sensitivity either under uniform pressure or free-field conditions. In this paper, signal-processing techniques are considered that improve the accuracy of the free-field calibration method. In particular, a fast Fourier transform ͑FFT͒-based time-selective technique for removing undesired reflections from the walls of the measurement chamber has been developed and applied to the electric transfer impedance function between two microphones. The acoustic centers of the microphones have been determined from the ''cleaned'' transfer impedance values. Then, the complex free-field sensitivities of the microphones have been calculated. The resulting complex sensitivities and acoustic centers have proved to be in good agreement with previously published data, and this confirms the reliability of the time-selective technique, even in nonanechoic environments. Furthermore, the obtained results give a new reference for further comparisons, because they cover a frequency range with an accuracy that has not been obtained by previously published data.

“…Details of the formulation can be found elsewhere, 1 but summarizing, such a formulation relates a possible incident wave p I and the pressure p and velocity v at the point Q on the surface S of a body to the pressure at the external point P, where G͑R͒ = e −jkR / R is the free space Green's function, R is the distance between the points P and Q, z 0 is the characteristic impedance of air ͑c͒, and C͑P͒ is the solid angle seen from P. In the axi-symmetric case, the velocity and pressure are independent of the rotation angle, . However, it is possible to introduce non-axi-symmetric boundary conditions if the pressure and velocity are expanded in cosine series.…”

Section: Bem Modelingmentioning

confidence: 99%

“…In this case, a lumped-parameter approximation of the acoustic impedance was used. 1 This approximation is only accurate at frequencies below the resonance frequency, and therefore it is expected that the hybrid method should give less accurate results at frequencies near and above the resonance. Besides, as mentioned above, at frequencies above 15 kHz the pressure sensitivity determined from reciprocity, and thus the free-field correction, is no longer reliable due to longitudinal and radial resonances in the couplers.…”

Section: B Pressure Sensitivitymentioning

confidence: 99%

“…Furthermore, numerical calculations are sometimes used to complement experimental results at frequencies where the experimental methods might yield unreliable results. [1][2][3][4][5] However, the numerical calculations are usually carried out under a number of assumptions that are not necessarily completely realistic. Several attempts to develop a complete coupled model of a condenser microphone numerically are described in the literature.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Hybrid method for determining the parameters of condenser microphones from measured membrane velocities and numerical calculations

Barrera‐Figueroa¹,

Rasmussen²,

2009

Typically, numerical calculations of the pressure, free-field, and random-incidence response of a condenser microphone are carried out on the basis of an assumed displacement distribution of the diaphragm of the microphone; the conventional assumption is that the displacement follows a Bessel function. This assumption is probably valid at frequencies below the resonance frequency. However, at higher frequencies the movement of the membrane is heavily coupled with the damping of the air film between membrane and backplate and with resonances in the back chamber of the microphone. A solution to this problem is to measure the velocity distribution of the membrane by means of a non-contact method, such as laser vibrometry. The measured velocity distribution can be used together with a numerical formulation such as the boundary element method for estimating the microphone response and other parameters, e.g., the acoustic center. In this work, such a hybrid method is presented and examined. The velocity distributions of a number of condenser microphones have been determined using a laser vibrometer, and these measured velocity distributions have been used for estimating microphone responses and other parameters. The agreement with experimental data is generally good. The method can be used as an alternative for validating the parameters of the microphones determined by classical calibration techniques.