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Time to get back to some oldie-but-goodie technical concepts. If you’re not sure what the graph means, no problem. You’ll understand after reading this paper.

A common question from our audiophile customers is: where should I place my diffusers? Our typical reply is another question: what type of diffuser are we talking about?

Not all diffusers are created equal. Those that maintain coherence improve imaging and harmonic accuracy. Those scrambling the signal will color and mask the sound. Read on to learn more about this important acoustic insight, and then adjust your room treatment accordingly.

From a presentation by Arthur M Noxon, PE, at the 11th AES Conference in 1992. Enjoy this excerpt in classic technical paper style, and look for the conclusion in our next issue.

Abstract

Coherent and incoherent reflections are very different, both in physical and psychoacoustic properties. Perception effects such as imaging and musicality are very sensitive to the type and tuning of reflections off nearby surfaces. Coherent reflections can have strong correlation coefficients and add information to the direct signal. Incoherent reflections with random phase signals are weak in correlation and provide strong masking effects.

0-Introduction

Diffusion is the process of mixing up sound. In a 100% diffuse sound field, there is no sense of acoustic direction, sound comes equally from all directions. Diffusion may be at times a desirable condition for acoustic energy. It is created by a sequence of diffusing reflections. A sound reflection can be either coherent or incoherent. This quality is very important to be specified because the coherence of a reflection has a significant sound masking effect on perception.

1-Sound Diffusers

A device that helps to develop the state of diffusion by increasing the scattering of sound is called a sound diffuser. There are four types of sound diffusion mechanisms.

1. Diffraction (sound bends around corners)
2. Refraction (turns by changing wave speeds)
3. Reflection (changing direction upon impact)
4. Resonance (resonant storage and reradiation)

The first three sound tuning mechanisms are pretty well known. They change the direction of sound but not the time wise evolution of the waveform itself. The scattered sound has the same sonic signature as the incident sound, they are highly correlated and therefore a coherent diffusion process takes place.

The last process, resonance, is not usually considered to be a sound diffuser. Incident sound on a resonator will stimulate the build up and decay of sound in the resonator. Resonant discharges are often practically point sources and so the reradiated sound is well distributed in space. The sound of a ringing, resonant decay has its own time wise evolution. The incident wavetrain will have a pressure vs. time signature that is not followed by the sound of the ensuing resonant decay. Correlation between the incident waveform and the resonant discharge is very low. Resonance forms the basis for an incoherent class of sound reflections.

2-Time Delayed Reflections and Perception

There are distinct time periods that relate to the various properties of perception. Reflections within the first few milliseconds following the direct sound belong to localization, i.e. the perception as to where sound is coming from. Reflections within the next 30 to 50 ms belong to fusion, the development of sound tone recognition. Reflections outside of 60 ms develop the impression of echo and ambience. The coherency of reflections with respect to the direct signal may well affect the quality of perception differently in each of these three time regions. Once this relationship is known, it can be utilized by recording engineers and acoustic designers to better achieve desired performance.

Reflections of sound that follow the direct signal within 50 ms are not distinctly heard but are blended together, fused into a composite sound. If only one reflection is heard, the phase add and cancel comb filter coloration effects will be heard. If there are many reflections, randomly off set in time, the phase add effect averages out to zero and the composite sounds just like the direct signal. Whenever reflections do not sound like the direct signal, the composite also does not sound like the direct signal. The goal of this paper is to introduce and measure coherent and incoherent reflections and then to subjectively evaluate the impact of each when audited within the 50 ms sound fusion time window. It will be shown that incoherent reflections, which may be acceptable in the 60 ms plus time period as ambience or echo signals, are degrading to musical quality if perceived during the 50 ms sound fusion period.

This summation or coloration of signals smeared together within the 50 ms perception window is a distinct aspect of tone recognition but not the whole picture for listening. It does not account for the consequence of variations in the time ordered detail of the harmonic structure in the attack transient. The accuracy of musical quality belongs to the 20 ms attack transient. It is the only event in which the timing and the phase alignment of the overtones in complex signals is detectable. Over the last few years speaker manufacturers have recognized and accommodated both time and phase alignment in the design of speakers. It is no longer sufficient to know how the sound level of each of each partial varies with time, we must also have correct time alignment and phase of the partials. One technique that measures in this area of psychoacoustic perception is the correlation test.

3-Coherent Reflections

Correlation is a measure of how similar one signal is to another. If a direct signal (Figure l) causes a simple time delayed reflection (Figure Ib) the correlation factor between the two signals (Figure Ic) is zero everywhere in time except at the time delay, and then their correlation is 100%.

If the specular reflection is splintered, scattered out in time, correlation will still exist, but spread out over the range of time over which the multiple reflections take place. Two types of splintered reflection systems were tested and both show correlation to exist over a longer time period than that of the single flat wall bounce.

In Figure 2a is shown a reflection/absorption diffusion grid that is composed of alternating depth of reflecting surfaces interspersed with sound absorbing segments. In this system, every other reflector is curved to backscatter over a wider angle than the adjacent flat reflecting strips.