Lesson 7 – Introduction

Many interesting sound processing effects can be achieved by simulating sound propagation behaviours. A classic example is the reverberation effect which is a complex rendering of the sound propagation phenomena called sound reflection.

In this perspective, they can be summarised to:

– reflection effects

– filtration effects

A sound must be considered as an energy propagating like a disturbance through the air. The easier way for us to visualise this is to refer to the sinusoidal wave motion. It consists of peaks and troughs, which represent areas of compression and rarefaction of the particles composing the air.

A wave motion behaves accordingly to certain rules. One is the superposition during propagation, which tells that a wave can cross over many other waves, without its propagation (speed and direction) being affected. Locally, yet, the amplitude is affected:

This local modification of amplitude is called interference, the second major rule. Interference can be constructive, where two peaks of the wave or two troughs add together creating a bigger one (constructive interference raises peaks and trough’s amplitude); or can be destructive, when a peak adds up to a trough and therefore their difference will create locally a smaller peak or trough.

The third essential rule is that sound propagates spherically, in three dimensions, and the bigger the sphere the larger the initial energy is spread. The sound’s energy will have a bigger reach, but will inversely proportionally loose power at each individual point of the wavefront. Imagine a dark red balloon, that through the inflation progressively loses its dark red colour and turns into a lighter red one.

One property of sound propagation is that when sounds encounter an obstacle it changes the direction of propagation. Generally speaking, although this is a simplification, if sounds bounce back or deviate with an angle related to the impact’s angle, it is a reflection. If sound changes direction of propagation by bending around an obstacle is diffraction and instead it is refraction if bends due to the change in the propagation medium (like sound passing from air to water, or from warm air to cold air areas).

Focussing on reflection, when a sound hits perpendicularly into an obstacle we hear it coming back, a time after the original sound. We could say that the propagating energy coming back makes us aware of the acoustic properties of the environment, its characteristic and qualities, giving also information on the sound. A sound without ‘coming back’ can only be achieved in an anechoic room or even almost when falling out from an aeroplane. When making sounds in those environments, our impression of the sound changes considerably compared when in a room or more familiar environments. The difference is in the absence/presence of reflections.

In presence of a sound reflection, a listener perceives the sound coming out of a source and then its reflection. The time difference between these two sound waves creates several well audible acoustic effects such as echoes, and faster ones, but still detectable by our ears, which are called early reflections. Early reflections have an important role in sound source localisation.

The fact that the original sound at some point might encounter the reflected sound, is the cause of the many interferences that create filtration effects. With the term filtration two effects on sound can be indicated: the enhancement and the cancellation of component frequencies, better known as resonance and damping.

A complex sound (composed by many harmonics/overtones) constructively and destructively interferes with its identical copy. Not just the main frequency, but any of its component frequencies are subject to interference rules. If that happens, timbral modifications occur. The further the delay between the original and its copy determine the size of the timbral modifications.

The effect of resonance occurs when a sound’s amplitude gets boosted due to constructive interference. The damping, even at the point of full cancellation, is caused by destructive interference. Therefore, a complex sound that is losing certain frequencies or having some harmonics amplitudes boosted due to interference, changes its internal balance, sounding slightly or even greatly different.

The objects explored in this section (tapin~ and tapout~) should help you investigate these phenomena, better know in computer music under the name of delay effects. They are in fact effects obtained by the simultaneous playback of a signal and its delayed copy (or many copies), at fixed or varied time intervals.