10. What are reflection, reverb, and standing waves?

What is reflection?

Sound waves reflect off of objects the same way billiard balls bounce off the bumpers of a pool table—the angle of incidence equals the angle of reflection. A sound wave hitting a flat wall at 45° will reflect off it at 45°. The reflected wave can interfere with the incident (original) wave, producing the constructive and destructive interference mentioned above—it can increase its amplitude or, with phase cancellation, decrease its amplitude. In a typical listening environment, we are hearing sounds that have reflected off numerous objects and surfaces, with the reflections themselves interfering with other reflections. Just as color is determined by which frequencies of light are reflected or not, the ‘color’ or acoustic characteristics of a particular listening environment is determined by the angles and materials the sound may reflect off. Different materials reflect some frequencies more efficiently than others, due to their roughness or absobancy characteristics. The acoustic foam at the end of our studio, for example, does not reflect higher frequencies very efficiently, but has little effect on particularly low frequencies (we need a ‘bass trap’ for that). The distance both the incident sound and the reflected sounds must travel is another key element in the characterstics of an acoustic environment, since the incident sound typically reflects off many surfaces at differing distances from the listener, thereby striking the ears at differing times.

Diffusion, often a setting on reverb units, refers to higher frequencies spreading and dying out more quickly than lower frequencies, something we use as an aural cue to the size of a space. A football field will have a higher degree of diffusion than a small studio. Humidity plays a large factor in diffusion as well.

What is reverberation (reverb)?

The sound wave that reaches the listener's ear directly from the sound source is often referred to as the direct sound. These waves reach the listeners ears first in most acoustic environments. The first reflected sounds to reach a listener's ears are called early reflections. Since they travel a longer path, the amount of time it takes the first reflected sounds to reach our ears give us clues as to the size and nature of the listening environment. Because the reflected sound may continue to bounce off of many surfaces, a continuous stream of sound fuses into a single entity, which continues after the original sound ceases. The stream of continuing sound is call reverberation. The rate of build-up of echo density is proportional to the square root of the volume of the room.

The time-domain and frequency-domain reverb characteristics of an enviroment can be represented by its impulse response, which is equivalent to subtracting the original sound from its reverb and storing it. Combining digital sound files with an impulse response file in a process call convolution will result in something equivalent to playing the sound in that hall. Many high-end digital reverb units have stored impulse responses from famous concert halls. Later, we will see how digital filters are also measured by their impulse responses.

Because of the inverse-square law described above, reverberated sounds will eventually lose enough energy and drop below the level of perception. The amount of time a sound takes to die away is called the reverb time. A standard measurement of an environment's reverb time is the amount of time required for a sound to fade to -60 dB. Concert halls will normally have much longer reverb times than small rooms, but maybe not as much as tunnels. Rooms with lots of reverb are called 'wet' and those without are called 'dry.' The nature of both reverb time and characteristics of which frequencies will die away before others keeps acoustic designers up at night. Special chambers for acoustic research and recording of special sound examples are called anechoic chambers (an=no echoic=echoes), which should have reverb times of 0.

Almost all studios have either hardware or software reverb units. The controls are usually reverb time (how long will it take the reflections to completely die away, a pre-delay, which determine when the first early reflection will be heard, filters which allow the user to tune the acoustic characteristicss of the imaginary environment and diffusion, which determines how much more quickly the higher frequencies will die away.

Reflections from surfaces that stand out from normal reverberation levels are called echoes. An echo of prominent amplitude, close in time to the original sound may be referred to as a slapback echo. Concert halls with a focusing flat back stage wall may produce slapback echoes with sharp loud sounds, such as percussion.


What is a standing wave?

A particular pattern of constructive and destructive interference is called a standing wave, which is essential to the way string instruments produce sound, but very undesirable in the listening environment of an electronic studio.

The characteristic mode of vibration of a string with one fixed end is the standing wave pattern. In a normal reflection of a sound wave from a hard surface, the phase of the reflected wave is not changed. With a wave induced in a string with a fixed end, the wave reflects from the fixed end out of phase with the incident wave, creating patterns of constructive interfere at certain resonant frequencies. Nodes and antinodes on the resultant string correspond the points of minimum (node) and maximum (antinode) vibrations. Air columns in both closed and open tubes also exhibit standing wave properties. In this case, the nodes and antinodes refer to the minimum (node) and maximum (antinode) pressure in the tube. Woodwind instruments are examples of half- or quarter wave resonators that produce multiple standing waves—the differences are whether the tube is open at both ends (flute, including the embouchure hole) or closed at one end, such as the oboe or clarinet.


Above is a picture of a closed tube with a loudspeaker at one end. The pellets in the tube form the nodes and antinodes of a standing wave from a sine wave played through the speaker. Exhibit built and designed by the WonderLab Exhibit Team led by Don Marvel, Bloomington, Indiana.

Standing waves in rooms can cause certain resonant frequencies to either be unduly enhanced (nodes) or completely disappear (antinodes). For that reason, it is always a good idea to listen to your work from a few different spots in the studio, since we have not completely eliminated our standing wave potential.

For a very detailed discussion of the physics of standing waves (with excellent graphics) see:
http://hyperphysics.phy-astr.gsu.edu/hbase/waves/standw.html#c4

For further study, see Hyperphysics->Reflection, Hyperphysics->Auditorium Acoustics




An Acoustics Primer, Chapter 10
URL: www.indiana.edu/~emusic/acoustics/reverb.htm
Copyright 2003 Prof. Jeffrey Hass
Center for Electronic and Computer Music, School of Music
Indiana University, Bloomington, Indiana