How I was listening to a sine sweep all day long

Impulse Response

How to measure a Room Response

Why don’t just measure an amplitude and phase characteristic

What methods can we actually use

  • MLS (Maximum Length Sequence) technique: a program playbacks a periodic pseudo-random signal, which is almost the same as white noise, but the program is able to correlate it’s acquired copy with original signal in order to recover room impulse response.
  • IRS (inverted repeated sequence): the sequence consists of MLS where every even sample is inverted.
  • Time-Stretched Pulses: “this method is based on a time expansion and compression technique of an impulsive signal. The aim of using an expansion process for the excitation signal is to increase the amount of sound power emitted for a fixed magnitude of this signal and therefore to increase the signal-to-noise ratio without increasing the nonlinearities introduced by the measurement system” [2]
  • Logarithmic SineSweep technique: probe signal is built with a sine-sweep with logarithmic changing frequency, so as to diminish harmonics influence on the result.
  • MLS 60.5 dB
  • IRS 63.2 dB
  • Time-Stretched Pulses 77.0 dB
  • SineSweep 80.1 dB

Farina’s method

Probe impulse

  • it has to contain all frequencies you want to measure,
  • it has to be self-orthogonal, so as to compress itself into Dirac’s impulse during auto-correlation.
eq. 1 Linear sweep chirp
eq.2 logarithmic chirp

How to build a correct reconstructing filter

The generation of the inverse filter is simply matter of time-reversing the excitation signal, and then applying to it (the reversed impulse) an amplitude envelope to reduce the level by 6 dB/octave, starting from 0 dB and ending to

eq3. Upper frequency boundary

The Code

How to use it on your own

  • Measured Room Response is the reason I did this piece of code,
  • Deconvoluted Room Response is the result of Wiener Deconvolution of room response with itself just for check,
  • Recorded log-sine sweep is the sound the microphone has just heard,
  • Spectrum of the record is the spectrum of the previous signal,
  • Deconvolved sine sweep is the result of Wiener Deconvolution of record with the measured room response just for check,
  • Spectrum of deconvolved sine sweep is the spectrum of the previous signal.

ARC

 by the author.

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Embedded Software Engineer and Digital Signal Processing specialist

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Mikhail Baranov

Mikhail Baranov

Embedded Software Engineer and Digital Signal Processing specialist

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