Phasing

Also called flanging or flangeing. An effect created by adding together two identical signals separated by a very short time delay (less than 25 ms, but strongest below 10 ms). These short delays are within the audio wavelength range, and the combination of the two signals affects the frequency spectrum of the composite sound.

Because different frequencies have unique wavelengths, there is a different time or phase delay for different frequencies. For example, a time delay of 1 millisecond causes a 360° phase difference in a 1 kHz wave, but only a 180° phase difference for a 500 Hz wave. Thus cancellation will occur at 500 Hz. In practice, cancellation is not complete but results in a drop of about 20 dB.

See: Interference, Phase-Shift, Pinna.

For two signals of amplitude A and frequency f, the resulting combined amplitude A_r, where the time delay between them is t, is given by:

A_r = A . /2 cos (2πft)/

The resulting amplitude will be at its lowest when ft = n/2, n = 1,3,5,7,...

For example, with t = 1 ms, cancellation in a complex spectrum will occur at 500, 1500, 2500, 3500,..... Hz. The effect is similar to that of a comb filter at these frequencies. The resulting colouration is described as a 'swishing' or 'jet-like' sound, and is often heard on commercial recordings. The time delay, however, is seldom constant in practice, and so there will seem to be a rising and falling pitch as well. The pitch heard corresponds to a frequency whose period equals the time delay. It is more pronounced when the reflected sound is stronger or there are multiple reflections with the same delay.

The effect may be heard environmentally when a broad band noise is combined with its reflected sound (as with a passing car or plane). The resultant filtering is heard as a change in timbre because the time delay is changing as a result of the speed of movement. See: Ground Effect.

For long time delays, the ear can distinguish the two signals, as in (tape) echo. As the delay becomes shorter, the two signals are indistinguishable and a simple type of reverberation effect is perceived. Phasing occurs with still shorter time delays.

Tape recorded sounds can be phased by recording and combining the playback of two versions of the same material on identical tape decks with close but not exact synchronisation. In the past, a common means of obtaining this effect with tape, called flanging, referred to a manual pressure exerted on the flanges of the tape reel causing it to run at a slower speed. This method introduced a fluctuating time delay, loosely referred to as flanging, whereas it is more properly termed phasing, with flanging being the method used to obtain it. Today, phasing is most commonly produced with digital delay units which allow very precise delays to be specified and modulated.

Compare: Beats, Binaural Hearing, Distortion, Doppler Shift, Feedback, Precedence Effect, Sound-on-Sound.

Phasing also refers to a matching of phase, as with loudspeakers, which normally should be in phase with each other. See: Quadraphonic.