We have used the ICRA babble noise to generate an 8-talker babble. As a background - babble noise is used as a substitute for competing talkers as babble noise lacks lexical content, but contains the same envelope and frequency content as 'real' speech. Babble noise is, therefore, less distracting for listeners and hence it's a more direct, 'peripheral' type of masker. Lexical masking occurs more centrally.
Anyway - we are interested in whether the masking effects we see are due to the slowly varying part of the babble noise (the envelope) or the frequency content.
Now, because we used an 8-talker babble using asynchronous, random starting points, the amount of modulation in the masker is less, as all the 8 babble noises are randomly added. Hence, it becomes more of a static type of noise, and listening in the gaps is hardly possible. The latter is especially important - are there gaps present in the noise, and if yes, how long are they? Likely there are no true gaps anymore, but only stretches of noise where relatively little noise is present, so shallow gaps in the noise.
Now, I wish to compare the characteristics of the single-talker original noise and our 8-talker babble in terms of:
- amount (number/density) of gaps present in the noise
- the gap duration
- and amplitude of the gaps (modulation depth)
I think a good starting point is to determine the envelope of the fluctuating noise using a Hilbert transform. But then, how do I go about analyzing the envelope in terms of the two parameters, namely gap-duration and amplitude? I have added the envelope of a shorty stretch of the original single-talker babble below (x-axis label is seconds):
- I'm used to do my signal analysis in Matlab;
- Noise files were software-corrected for rms for overall sound level;
- Noise was played back in a sound-proof booth and we will record the signals back with a KEMAR manikin to include the head shadow effect.