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Discussion Starter #1
EN ISO 18233:2006 - Does REW comply?

Annex B of the above standard stipulates that, during swept sine measurements of room acoustic parameters, "a number of precautions shall be taken in the deconvolution process."

Does anyone here know whether REW automates these precautions and thus satisfies the standard?

Thank you!

11 Posts
Discussion Starter #3
Re: EN ISO 18233:2006 - Does REW comply?

A number of precautions shall be taken in the deconvolution process. If a Fast Fourier Transformation is used,
precautions against circular convolutions shall be taken. Furthermore, the spectral division may include
frequencies in the denominator with very little energy and precautions shall be taken in order not to enhance
the extraneous noise accompanying the measured response at these frequencies. This will often be the case
close to the boundary of the sweep range.
As the technique described here makes use of a non-periodic excitation signal, the most appropriate way to
obtain the impulse response is a linear (i.e. non-circular) deconvolution. The linear deconvolution can be
accomplished most simply by direct deconvolution or, if using spectral division, by extending the excitation
signal and the recorded response with zeros to double their previous size (zero-padding).
When the excitation is a sweep from lower to higher frequencies, the response to harmonic components will
appear before the main excitation at the same frequency. After the linear deconvolution, the responses to
harmonic components in the excitation will appear at negative time and may easily be removed (see
Reference [14] for further information).
When spectral division is applied, the excitation and the response are submitted to an FFT and the spectrum
of the response is then divided by the spectrum of the excitation signal. An IFFT yields the desired impulse
response in which the second half, corresponding to negative arrival times, can be disregarded. As described
in Reference [8], this method may also be used to remove the effects of harmonic distortion in the excitation
Alternatively to the linear deconvolution, a circular deconvolution using an FFT size equal to the acquisition
time may be employed. In this case, however, the distortion products could smear into the decay of the
impulse response. This means that the length of the excitation signal shall be chosen sufficiently longer than
the decay time. The distortion products will then appear in the noise floor where they can be safely discarded
by windowing without affecting the reverberant tail.
There is an important difference concerning the noise floor in the impulse responses obtained by linear and
circular deconvolution. Use of a circular deconvolution results in a noise floor that is basically constant, up to
the point where the first distortion products appear. The linear deconvolution, however, yields a decaying
noise tail that is increasingly low-pass filtered towards its end. This stems from the fact that this last part of the
deconvolution result originates from steady noise convolved with a sweep in reverse order (i.e. from high to
low frequencies). The user shall be aware of this affect so as not to confuse the decreasing noise floor with
the reverberant tail of the room.

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Re: EN ISO 18233:2006 - Does REW comply?

All pretty basic stuff that REW handles automatically.
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