Article

# On the accuracy of the subtraction method for in-situ reflection and diffusion coefficient measurements.

Graduate Program in Architectural Acoust., Rensselaer Polytechnic Inst., 110 8th St., Troy, NY 12180.

The Journal of the Acoustical Society of America (Impact Factor: 1.65). 03/2010; 127(3):1752. DOI: 10.1121/1.3383680 Source: PubMed

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**ABSTRACT:**The problem of measuring the sound-absorption coefficient in situ is approached in a systematic way, accounting for parasitic reflections and background noise. The basic reflection method is improved by using pseudo-random binary sequences of maximum length as the test signal. The measurement procedure is orientated towards digital processing techniques and uses the advantages of modern instruments. The inherent cross-correlation process greatly improves the noise immunity. The relationship between the loudspeaker characteristics and the sample size is clarified. The use of a non-rectangular analysis window is suggested. The proposed method was tested in a normally furnished room using portable instruments; the normal-incidence results are in good agreement with values measured using the conventional Kundt's tube method.Applied Acoustics - APPL ACOUST. 01/1993; 39:119-139. - [Show abstract] [Hide abstract]

**ABSTRACT:**Conventional Schroeder diffusers have been successfully used for many years. However, their frequency range is limited by the flat plate effect that occurs when all the wells radiate in phase. This occurs at harmonics of p times the design frequency f(0), where p is the small prime that is used to generate the structure. A typical diffuser, using p=7 and f(0)=500 Hz, has an upper frequency limit of only 3.5 kHz. Achieving a first flat plate frequency above 20 kHz requires a prime equal to at least 41 and results in diffusers that are too big to be practical in most applications. This paper suggests an alternative approach using number theoretic sequences that, although short in length, are based on large integers. Two new sequences, Type-II Luke and power residue, have this desired characteristic. They are investigated using both simple models and the more exact boundary element method. The results show the flat plate effect is moved to much higher frequencies as expected. For Luke sequences at certain frequencies, redirection rather than dispersion is achieved. Modulation techniques can be used to mitigate these problems. Power residue sequences perform the best, providing good diffusion and a flat plate frequency outside the audible range.The Journal of the Acoustical Society of America 05/2008; 123(4):2035-42. · 1.65 Impact Factor -
##### Article: Angle-dependent in-situ measurements of reflection coefficients using a subtraction technique

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**ABSTRACT:**The paper describes an improved PC-based method of measuring the complex reflection coefficient in situ for perpendicular and oblique sound incidence. From the impulse response measured in front of the surface, the direct sound is cancelled by subtraction with a previously obtained pseudo-free-field response. The frequency response of the reflection coefficient is determined by a fast Fourier transform after gating out parasitic reflections. Additionally, the signal separation is improved using digitally preemphasized pseudonoise signals.Applied Acoustics. 01/1995;

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