Transfer-function measurements using sweeps as excitation signals rather than pseudonoise signals show significantly higher immunity against distortion and time variance. Capturing binaural room impulse responses for high-quality auralization purposes requires a signal-to-noise ratio (SNR) greater than 90 dB, which is unattainable with maximum-length sequence (MLS) measurements because of loudspeaker nonlinearity, but it is fairly easy to reach with sweeps due to the possibility of complete rejection of harmonic distortion. Before investigating the differences and practical problems of measurements with MLS and sweeps and arguing why sweeps are the preferable choice for the majority of measurement tasks, the existing methods of obtaining transfer functions are reviewed. The continual need to use preemphasized excitation signals in acoustical measurements is also addressed. A method to create sweeps with arbitrary spectral contents, but constant or prescribed frequency-dependent temporal envelope, is presented. Finally, the possibility of simultaneously analyzing transfer functions and harmonics is investigated.

Transfer-Function Measurement with Sweeps

The impulse response of an acoustical space or transducer is one of its most important characterizations. In order to perform the measurement of their impulse responses, four of the most suitable methods are compared: MLS (maximum-length sequence), IRS (inverse repeated sequence), time-stretched pulses, and SineSweep. These methods have already been described in the literature. Nevertheless, the choice of one of them depending on the measurement conditions is critical. Therefore an extensive comparison has been realized. This comparison was done through the implementation and realization of a complete, fast, reliable, and cheap measurement system. Finally, a conclusion for the use of each method according to the principal measurement conditions is presented. It is shown that in the presence of nonwhite noise, the MLS and IRS techniques seem to be more accurate. On the contrary, in quiet environments the logarithmic SineSweep method seems to be the most appropriate.

Comparison of different impulse response measurement techniques

An international review of laser Doppler vibrometry: Making light work of vibration measurement

It is a government funded initiative to significantly raise the quality and availability of resources for the computational processing of Portuguese After an initial plan for discussion by the community (white paper) a network was launched, headed by a small group (Linguateca’s Oslo node) at SINTEF ICT (formerly SINTEF Tele og Data) This network had as main goal to guarantee that Information was provided and gathered at one place on the Web Resources were made public, maintained, and further developed in connection with the scientific community Evaluation initiatives were launched

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Information and Communication Technologies

Conventional watermarking techniques based on echo hiding provide many benefits, but also have several disadvantages, for example, a lenient decoding process, weakness against multiple encoding attacks, etc. In this paper, to improve the weak points of conventional echo hiding, we propose a time-spread echo as an alternative to the single echo in conventional echo hiding. Spreading an echo in the time domain is achieved by using pseudonoise (PN) sequences. By spreading the echo, the amplitude of each echo can be reduced, i.e., the energy of each echo becomes small, so that the distortion induced by watermarking is imperceptible to humans while the decoding performance of the embedded watermarks is better maintained as compared with the case of conventional echo hiding, as shown by computer simulations, in which several parameters, such as the amplitude and length of PN sequences and analysis window length, were varied. Robustness against typical signal processing was also evaluated in these simulations and showed fair performance. Results of a listening test using some pieces of music showed good imperceptibility.

Time-spread echo method for digital audio watermarking

Transfer functions of acoustic systems often exhibit wide dynamic ranges and very long impulse responses. A ‘‘time‐stretched’’ pulse as proposed by Aoshima (ATSP), though originally given in a very specific form seems to be one of the most promising signals to measure transfer functions with characteristics of acoustic system mentioned as above. In this paper, this pulse (ATSP) is first generalized and then optimized for the measurement of long impulse responses. This optimized ATSP (OATSP) has an almost ideal characteristic to measure impulse responses shorter than its specific length N. Moreover, it is newly shown in this paper that OATSP has also a good characteristic to measure impulse responses longer than N. Discussion is presented on how to design OATSP suitable for a specific situation of measurement by analyzing errors, when the pulse is used to measure impulse responses longer than N.

An optimum computer-generated pulse signal suitable for the measurement of very long impulse responses

Measurement of resonance frequency and loss factor of a microphone diaphragm using a laser vibrometer

A laser pistonphone system capable of measuring the sensitivity modulus and phase of microphones is proposed. For the primary vibration calibration, the sine-approximation method has been applied which is used to calculate the complex sensitivity of accelerometers. This method is based on a laser interferometer with quadrature output instead of the fringe-counting method that is used in conventional laser pistonphones. Here, the laser pistonphone system based on the sine-approximation method with the quadrature signal is proposed and the measured results are compared to those obtained by the reciprocity method. The measured sensitivity with the proposed method agrees well with the result of the reciprocity method for the infrasound range. However, in case of the phase sensitivity, the discrepancy is relatively high in comparison to the sensitivity modulus. The results show that the proposed system is capable of calibrating the complex sensitivity of microphones. The improvement of the uncertainty and extension of the frequency range can be achieved by modification of the excitation system for obtaining the required stable excitation with long stroke.

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Development of a Laser Pistonphone System to Calibrate the Sensitivity Modulus and Phase of Microphones for Infrasonic Frequencies

Radiation sound field analysis and measurement of acoustic center of microspeaker for mobile phone

The transfer function of an acoustic system, in general, often ex­ hibits a wide dynamic range and a very long impulse response. The time-stretched pulse (TSP) proposed by Aoshima (ATSP) has a small peak・facto호 and is accordingly suitable for the measuring impulse responses. The pulse is not so suitable, however, for the measurement of impulse responses over a wide frequency range. In this paper, we try to generalize and optimize this method (OATSP). This makes the method applicable for measuring of impulse responses longer than the length of the TSP. An analysis of error in such a case is also shown. Finally, we discuss how to implement this technique in specific measurement conditions.

Hack-Yoon Kim

Papers

An optimum computer-generated pulse signal suitable for the measurement of very long impulse responses

Measurement of resonance frequency and loss factor of a microphone diaphragm using a laser vibrometer

Development of a Laser Pistonphone System to Calibrate the Sensitivity Modulus and Phase of Microphones for Infrasonic Frequencies

Radiation sound field analysis and measurement of acoustic center of microspeaker for mobile phone

Measurement of long impulse response by using a time-stretched pulse