Audio: echo (0:03)
echo plays three pairs of sounds in succession. Each individual sound has a very short duration of 10ms. The echo figure shows the composite waveform.
- The first pair of sounds are 100ms apart. You should hear a discrete sound followed by its discrete echo. This is known as a true echo.
- The second pair of sounds are 50ms apart. You should just perceive them as discrete sounds although it is at this level of delay that they start to become indistinguishable.
- The final pair of sounds are 10ms apart. Although the two sounds are discrete, most listeners will perceive them as one sound.
An echo is a reflection of a sound.
A true echo is a single reflection of a sound. In a true echo the original sound and its delayed echo are heard as two discrete sounds. When the time delay between a sound and its echo is small, 50ms or less, the true echo effect begins to disappear. Around 20ms, the two sounds are perceived as one. This remarkable fact underpins all delay techniques. The phenomenon is known as the precedence effect or Haas effect.
The effect in echo simulates the sound of a ball bouncing off a surface in a computer game. It uses a triangle wave with a fundamental of 200Hz enclosed in a 2-segment exponential envelope. The sound of the echo can be subtly altered by choosing another type of waveform. A sine, saw or pulse wave all give equally good results as does white, pink and red noise. Changing the waveform in this way enables you to build a small library of sample echoes for use in computer games and video.