There is a subjective aspect to incoherent reflections. The demonstration of this effect was first performed in a respected hi end audio manufacturer’s demo room at the 1988 CES, Las Vegas. The audio playback system had random depth well diffuser panels behind and between the speakers, set up to diffuse the front wall bounce. A CD track was played that had solo classical guitar work. The perceived musical quality of a plucked low E guitar string was radically affected by the reflections out of the random well depth diffuser. All 15 people in attendance simultaneously could repeatedly witness this effect. Its characteristic was identified as being a ‘colorless note’. The fundamental string tone was present but its expected rich harmonic structure seemed to be obscured. When the random depth well diffuser panel was covered over with a blanket and the musical section was replayed; the easily recognized and all so familiar sound of a plucked acoustic guitar string returned. High frequency string sounds did not seem to have this ‘colorless’ quality, only the lower frequencies, those with substantial transient partials in the middle octaves, 250 to 750 Hz.

Correlation of continuous sound is a straightforward statistical sampling process. Trying to do correlation on the attack transient of a plucked guitar string is more difficult because of the short time period of the attack and the long time period of the sustain. A study of the attack transient wave form itself does show the effects of different types of reflecting surfaces.

The signal out of a plucked electric guitar string is shown (Fig. 18) with rapid harmonic detail changes in the first 50 ms for a 125 Hz note. The overall long term spectrum for this pluck (Fig. 19) shows strong and regular upper partials. The signal was recorded and played back over a small, average speaker. Its sound was reflected off of the three types of surfaces and captured by a mic and storage scope. The first 40 ms of each bounce shows the evolution of the attack transient into the sustain wave form.

(note: in figures 20-22, the top trace shows a flat wall reflection, middle trace shows a specular/absorptive scattering reflection, and the bottom trace shows a random well depth diffuser reflection)

The first pluck (Fig. 20) series show a substantial transient difference during the initial 10 ms between the random well depth diffuser and the other two. Beyond the attack transient is seen a wave form change. With the random well depth diffuser there is strong third harmonic detail added to the positive peaks of the fundamental.

A second pluck at 125 Hz was recorded, this time with more harmonic detail due to a shifted finger position (Fig. 21). Again, the specular/absorptive reflection is very similar to the wall reflected signal except for reduced amplitude. The random depth well reflector shows again the first 10 ms attack transient distortion. It also shows harmonic distortion in the sustain particularly with accentuated rise times in the positive part of each fundamental peak.

The persistent upper partial distortion in the 3 to 400 Hz region out of the random depth diffuser led to another test, this time at 400 Hz. Again, serious distortion (Fig. 22) in the first 10 ms of the attack transient is observed. Also in the sustain is seen more than simple reduction of levels as with the specular/absorptive diffuser. Here, every other cycle is louder and sharper peaked while adjoining pulses are quieter and more grounded than with the other two reflecting surfaces.

In all three cases, the attack transient and the sus­tain were distorted by reflections off the random well depth diffuser.