The Delay Spectrum and Audio Production

Audio delays are all around us. Sound bounces of surfaces and reaches our ears at different times – which we can often perceive. While recording it’s important to understand the role of the delay spectrum and the impact this can have. A slight delay can influence the sounds we are trying to produce – some of this we can use to our benefit – some we try to avoid.

Audio Delays

In audio production delay is a concept where the original audio signal is followed closely by a delayed repeat. The delay time can be as short as a few milliseconds or as long as several seconds. A delay effect can include a single delay or multiple delays. Delay can be processed – either by the musician or within the DAW. In music and audio production delay forms the basis of effects such as reverb, chorus, phasing and flanging.

Comb Filtering

Short delays take the form of a deep notch in the frequency spectrum. When this type of delay is increased (by milliseconds) we will get a series of harmonically related notches. This is known as comb filtering. Comb filtering can cause problems when we are recording. When you set up a microphone you need to be aware that the sound will come from the instrument/voice – but it will also be reflected off nearby surfaces before hitting the mic – causing delayed signals along with the original signal. Comb filtering can also occur when you set up 2 or more mics to record from one source if they are located at different distances from the source. For example this can be an issue when recording drums with multiple mics.

If multiple versions of a signal are recorded with a slight delay, certain frequencies will be added and others will be lost. A frequency-response plot would show a sequence of peaks and dips extending up the audio spectrum, their position depending on the time difference between the two waveforms.

Comb Filtering

A small delay can have a serious impact on recording. It’s important to give a lot of consideration to mic placement and nearby flat surfaces to reduce the possibility of comb filtering. When the delay time is increased to around ten milliseconds the sound starts to buzz and pitch can be observed. By 35 milliseconds or above two separate sounds (echo) can be observed.

Creating Pitches with Delay

When you have short delays and feedback introduced then this can result in an audible pitch. If you increase the delay time with feedback this will result in the frequency/pitch decreasing until it is no longer heard and it evolves into two separate sounds.

We observe that delay and frequency are related. This theoretical relationship enters reality when we introduce audio effects – chorus, phasers, flangers, short delays, long delays, reverbs and filters into the mix.

Slapback Delay

Slapback delay occurs when the delay time is lengthened to the 40-120 millisecond range and mixed against the dry signal. This is a medium, static delay and helps to add a sense of space to the signal – the equivalent to playing in a room. It is often used as a quieter alternative to reverb.

An example would be to set a low delay time to around 110 milliseconds and a dry/wet output to about 20% which will result in an a slapback version of the audio signal.

This type of delay was used extensively in the 1950s specifically in pop, surf and rockabilly recordings.

Synchronized Long Delays

When we hear long delays we experience a distinct echo in the sound – like being in a large room or a canyon. These types of delays are used to put an emphasis on a particular vocal or instrument. Often they are used in conjunction with the tempo of the music. To do this we don’t use delay time such as milliseconds – rather musical units such as quarter notes and eighth notes. This keeps the delay time working with the music.

The stereo spectrum is important and the delay does not have to be the same as the original dry signal. There can be a distinction between the original dry signal and the delayed wet signal by setting filter effects to the looped signals.  The delayed version may be panned to the sides of the recording. An example for this is the ping pong delay which bounces the sound from left to right and left to right.

One issue with long delays is that they can clash with the harmony of the piece of music. So it should be used wisely and with consideration for the piece of music. This type of delay can be heard in psychedelic, ambient and electronic music.

The Delay Spectrum and Audio Production

We have looked at the delay spectrum and specifically comb filtering, creating pitches with delays, slapback delays, and synchronized long delays. We have observed that delays can be a very useful tool for audio producers especially in the mixing process. We do have to be aware that there are dangers associated with delay – in particular comb filtering. As music producers we should strive to use these types of effects to add to the music we are producing – rather than using them because we can.