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Linear microphone array for acoustic impulse response measurements using reciprocal method: Effects of microphone holder
Abstract
This article describes a linear microphone array used for measuring head-related impulse responses simultaneously at various radial distances using the reciprocal method. The microphone array consists of miniature 5.8 mm diameter electret condenser microphones (ECMs) arranged on a boom, using a 3D printed microphone holder with pillars. The frequency response of the ECM with the 1 mm thick band holder increased monotonically above 5 kHz and was 2 dB higher at 20 kHz compared to the frequency response of the bare ECM. When multiple ECMs were arranged on a boom at 200 mm intervals using microphone holders with a 3 mm diameter pillar, reflections from the pillar and boom were negligible, regardless of whether the height of the ECM from the boom was 30 mm or 60 mm.
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Sensing of high-definition three-dimensional (3D) sound-space information is of crucial importance for realizing total 3D spatial sound technology. We have proposed a sensing method for 3D sound-space information using symmetrically and densely arranged microphones. This method is called SENZI (Symmetrical object with ENchased Zillion microphones). In the SENZI method, signals recorded by the microphones are simply weighted and summed to synthesize a listener's headrelated transfer functions (HRTFs), reflecting the direction in which the listener is facing even after recording. The SENZI method is being developed as a real-time system using a spherical microphone array and field-programmable gate arrays (FPGAs). In the SENZI system, 252 electric condenser microphones (ECMs) were almost uniformly distributed on a rigid sphere. The deviations of the microphone frequency responses were compensated for using the transfer function of the rigid sphere. To avoid the degradation of the accuracy of the synthesized sound space by microphone internal noise, particularly in the low-frequency region, we analyzed the effect of the signal-to-noise ratio (SNR) of microphones on the accuracy of synthesized sound-space information by controlling condition numbers of matrix constructed from transfer functions. On the basis of the results of these analyses, a compact SENZI system was implemented. Results of experiments indicated that 3D sound-space information was well expressed using the system.
The harmonic distortion on impulse response measurement, with logarithmic time stretched pulse (TSP) was studied. The measurement of acoustic impulse response with a low signal output level causes measurement error due to a low signal-to-noise ratio. The impulse response of a loudspeaker was measured with varying signal level in an anechoic room. The results show that the measurement error of impulse response caused by distortion could be reduced by approximately 10dB with log-TSP.
A study was conducted to measure the head-related transfer functions (HRTF) with a dummy head by the reciprocal and direct methods. The reciprocal HRTF measurement system consisted of a microphone array with fixed microphone sockets 30 degrees apart and an earplug speaker inserted into the dummy's ear canal. The microphone array consisted of a circular aluminum rod with a radius of 1.05m and 12 microphone plastic sockets. The microphones were mounted at the sockets and arranged in a circle with a 1-m radius. A closed-type loudspeaker was used for the direct method and a small electret condenser microphone embedded in a silicon impression material was used as an earplug microphone. Results indicated that the contour pattern of HRTFR was similar to that of HRTF D in the mid frequency range. The first peak and notch patterns were the same between them and the spectral distance between these contour patterns in the mid-frequency range was 3.1 dB.
An efficient method for head-related transfer function (HRTF) measurement is presented. By applying the acoustical principle of reciprocity, one can swap the speaker and the microphone positions in the traditional (direct) HRTF measurement setup, that is, insert a microspeaker into the subject's ear and position several microphones around the subject, enabling simultaneous HRTF acquisition at all microphone positions. The setup used for reciprocal HRTF measurement is described, and the obtained HRTFs are compared with the analytical solution for a sound-hard sphere and with KEMAR manikin HRTF obtained by the direct method. The reciprocally measured sphere HRTF agrees well with the analytical solution. The reciprocally measured and the directly measured KEMAR HRTFs are not exactly identical but agree well in spectrum shape and feature positions. To evaluate if the observed differences are significant, an auditory localization model based on work by J. C. Middlebrooks [J. Acoust. Soc. Am. 92, 2607-2624 (1992)] was used to predict where a virtual sound source synthesized with the reciprocally measured HRTF would be localized if the directly measured HRTF were used for the localization. It was found that the predicted localization direction generally lies close to the measurement direction, indicating that the HRTFs obtained via the two methods are in good agreement.
A theoretical foundation is developed for horizontal holographic surround sound systems based on the two-dimensional Fourier transform. It is shown how the theory leads to the recording and reproduction of sound fields using circular arrays of microphones and loudspeakers. DFT processing of circular microphone arrays produces the spherical harmonics of the sound field, and is a generalization of certain higher order multipole microphones. The performance of the array is shown to be improved by using directional microphone elements. The angular sinc panning functions arise naturally from the reproduction theory, and a mode-matching approach further verifies that they are optimum at low frequencies. General windowing is reviewed for the control of artifacts at high frequencies and the radial error is proposed as a useful parameter for designing windows. Sound intensity theory is examined for visualizing surround sound fields, and a complex instantaneous velocity is introduced, which is easy to generate and equivalent to previous definitions for the case of monochromatic sound fields. The reproduction performance of a five-loudspeaker surround system is examined, and its performance over wider reproduction areas is also considered.
Measurement of head-related impulse response using maximum amplitude optimized TSP signal
- M Terashima
- D Morikawa
- P Mokhtari
- T Hirahara