RogerBW's Blog

The podcast for which I do sound tech (yes, all right, I'm also one of the speakers and writers) sometimes uses old recordings as interstitial music.

We do this because they're conveniently out of copyright, but they are very often monophonic. That's not a bad thing in itself, but cutting from a stereo recording to a mono sound effect and back again sounds odd, particularly if one's listening with headphones.

Fortunately there's a trick by which one can desynchronise the wavefronts of a mono recording enough that it doesn't cause this perceptual change: there's still a single sound source, but it's not quite as blatantly right in the middle of one's head. The technique takes advantage of the psychoacoustic Haas effect.

I do this in Audacity, so that's how I'm going to describe it.

1. Duplicate the mono track. Make sure the two are in perfect synch. One of the two tracks is going to get modified; I'll call that the "B" track.

2. Push the two tracks to the extreme edges of the stereo space.

3. Apply a high-pass filter to the "B" track. Default parameters are fine: 6dB per octave rolloff, 1kHz cutoff frequency.

4. Raise the amplitude of the "B" track by 10dB or so. (Tweak to taste.)

5. Slide the "B" track so that it occurs a few milliseconds earlier than the other. I usually use about ten. Some people like to go as high as thirty; it increases the effect but also makes things mushier. Again, tweak to taste.

6. Merge the two mono tracks together to make a single stereo track.