I’m perverse! Most people want to eliminate audio defects, whereas I need to create one, specifically the sound of an amplifier that is close to self-oscillation or ‘taking off’.
This effect was quite common on long-distance telephone calls until about twenty years ago.
This is what a Bell System technical manual said:
Near-singing distortion of transmitted speech signals occurs when losses in the singing path approach but do not equal the value required for sustained oscillation. Distortion may result in two ways. Losses in the circulating path may be sufficiently low to cause an appreciable time interval to elapse before the circulating signals die away, thus causing speech transmitted through the network to sound hollow or reverberant. Also, successive trips of the multireflected signals around the circulating path may phase in and out with the impressed signal at various frequencies in the passband of the network causing successive peaks and valleys in the attenuation/frequency characteristic. As the singing margin is reduced, the peaks become higher and the valleys lower making speech sound more hollow.
Another technical paper says:
Listener echo occurs at the far-end by circulating voice energy. This type of echo is also called ‘Singing’ or ‘Near-Singing’. Singing occurs when gain mismatches allow the energy returned from the listener echo path to be greater than the original energy, and when the waveforms are in-phase. This creates an ‘oscillation’ effect. Near-singing occurs at a point just prior to singing. Near-singing conditions usually create ‘hollow’ or ‘rain barrel’ effects.
In real life the effect was a breathy sound with a higher background noise level, a slight echo and whispery noises almost like thin ice cracking. Quite eerie in fact. Thanks to digital technology and the greater stability of modern amplifiers you never hear this sound nowadays.
Can any expert suggest how this effect might be recreated in Audacity please?