Audio: pan (0:06)

Figure: pan

pan plays a wailing siren moving from left to right. The pan figure shows the waveform in stereo.

Panning is sound in space.

The aim of panning is to move the location of sound. Panning cannot take place in mono because there is only one sound location. It can only occur in stereo, which has two sound locations, or surround sound, which has multiple sound locations.

Panning is achieved by lowering the amplitude of a sound in one speaker whilst raising the amplitude in another. A hard left pan in a stereo sound system raises the amplitude of the left speaker to maximum and reduces the amplitude of the right speaker to zero. A hard right pan does the converse.

Pan control in hardware is achieved by turning a knob, called pan or balance, from left to right or vice versa. The centre position distributes sound equally between the speakers. Pan control in software uses the same approach.

The methods for panning are the same as those for crossfading. pan uses the equal power method.

A siren is a loud noise sounding an alarm or a warning. There are three basic types:

  1. High-Low siren: two alternating notes.
  2. Yelp siren: a single sound rising rapidly in frequency.
  3. Wail siren: a single sound with variable frequency.

pan is a wailing siren. This is how it is created:

  • A siren is a harmonically rich noise. Any noise will do as a source, pan uses a square wave.
  • The frequency of a wailing siren rises and falls. This can be achieved by applying a chirp effect. The wail in pan consists of an up chirp from 400Hz to 1200Hz in one ninth of a second followed by a down chirp back to 400Hz in two ninths of a second. The wail thus repeats itself a fraction over three times a second. Generally, the shorter the repeat time of a siren the more urgent is the sound.
  • The amplitude of the complete sound in pan rises and falls using an envelope. It has a very short fade in from zero to 40%, then a rise to 100% at the midpoint, followed by a fall to 40%, and then finally a fade out to zero. This enhances the realism of the panning motion.
  • Outdoor sirens usually operate at lower frequencies than indoor ones to enable the sound to travel further. The siren in pan was not filtered, but an outdoor yelp siren, such as an airhorn or a foghorn, would benefit from a low pass filter.
  • The use of delay moves the location of a sound and pan uses a comb filter with a variable delay time. This simulates the Doppler effect produced by a siren moving towards then away from the listener. A small amount of reverb was added to simulate the sound of the siren bouncing of walls and surfaces.
  • Pan also moves the location of a sound. pan moves the sound from left to right in stereo using the equal power panning method.

Writing a moving siren effect is an excellent project to undertake. It is worth doing because there is a lot of pleasure in writing different types of sirens. It also uses all the techniques covered in this part of the guide. And it illustrates how to create motion in sound effects by using amplitude and frequency effects to move the sound up and down, phasing and delay effects to move the sound to and from, and panning to move a sound left and right. Three dimensional moving sound effects, the pinnacle of noise.