Correlation Detection of Early Reflections pt. 3

Whenever evaluating the sound quality of a musical playback system, it is important to remember that we do this because we love… music! Test signals are great, but real instruments are what brought most of us into this lifestyle to begin with.

In that spirit, we present the final segment of this classic paper that discusses the effect of certain types of reflections on one of our favorite sounds – a plucked guitar string.

From a presentation by Arthur M Noxon, PE, at the 11th AES Conference in 1992.  Read the full PDF on our website.

Incoherent Reflections and Perception

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.

Attack Transient Fidelity

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)

fig. 18 signal from a plucked guitar string

fig 19 Spectrum for a plucked guitar string

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.

fig 20 attack transient 125hz, 40ms

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.

fig 21 attack transient 125hz, 40ms

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.

fig 22 attack transient 400hz, 40ms

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

Conclusion

The quality of sound is effected by the type of nearby reflecting surfaces both for listening and for microphone recording. The existence of incoherent resonant reflections and their effect on perception has been qualified. The work to follow should be to quantify the dependance of this masking effect on time delay and signal strength. There are three periods of perception to be explored. Very early reflections affect imaging and localization. Beyond that, up to 35 ms belong to musical quality and tone recognition. Outside of this is the region of echoes and ambience. The attack transient is the area in which we as listeners are phase sensitive. Resonant reflections produce phase scrambled sounds and our suscepti­bility to this type of masking may well only belong to attack transient domain. Coherent and incoher­ent reflections affect differently the perception of sound depending on how soon following the direct sound the reflection is received. The slotted well type diffraction grating produces incoherent reflections when it is involved in its lower octave, resonant mode of energy storage and discharge. The resulting direct signal masking effect needs to be taken into consideration whenev­er resonant discharge reflections are used near a microphone, speaker or a listener. The coherent quality of a reflected signal is a controlling variable in perception of the richness of musical harmonic structure.

Be sure to look closely at figures 20-22 and observe the differences in transient shape and the overall waveform. They may appear subtle at first glance, but once you see the distortion and then conceive how this may manifest with a full band, orchestra, choir, or even electronically created music, you can appreciate how much this effect is changing the sound.

Back to the original point – we are audiophiles or recording engineers because we love music. Respect for the artists and creators – and ourselves – means not stepping on the musical product that was made with so much care.