Digital Audio Fundamentals

Information about digital audio production, processing and editing, as well as resources available at the Library.

Why process audio?

Audio production involves processing and editing recorded material to enhance its fidelity or optimise bandwidth to ensure that it is suitable for broadcast, or wider distribution. 

There are many different types of signal processing that can be applied to digital audio, but the most common forms include: EQ (or equalisation), compression, reverb and echo (delay), and filtering.

Equalisation (EQ)

Equalisation in sound production is the process of altering the balance of different frequency components in an audio signal. Human ears can detect a wide range of frequencies—roughly 20 Hz to 20 kHz. Each element of a sound mix has energy in different parts of that range. Equalisation (or EQ'ing) ensures that all the different frequencies within a mix are balanced and clear. The below diagram outlines the common areas of a mix and their respective frequencies.


When mixing together multiple pieces of audio it is important to ensure that the overall mix is consistent. Compression adjusts the overall dynamic range of a piece of audio by detecting when it exceeds a specified level, and then attenuating it by a specified amount. Simply put, it narrows the difference between the loudest and softest parts of a track so that it’s more consistent in level.

Compression is most often used when a mix contains dynamics such as quiet and loud parts - compression can help boost the softer moments while also taming the louder sections.

Most compressors share a common set of parameters:

Threshold: Threshold is the level at which a compressor will act on a piece of audio, expressed in decibels (dB). 

Ratio: When a signal has risen above the threshold, the ratio setting controls how much it is turned down. This is also known as "Gain reduction".

Attack and release: Attack is the amount of time it takes for a compressor to reach a specified amount of gain reduction once a signal has risen above the threshold. This is often expressed as either "fast" or "slow", meaning that compression will act on the signal either quickly or slowly.

In contrast, once a signal drops below the threshold, the release setting determines the speed at which a compressor returns the signal to its original uncompressed state. Like attack, this setting is often expressed as "fast" or "slow" release. However, it's worth pointing out that "fast" and "slow" in relation to attack and release are extremes on a spectrum, not a binary state.

Attack and release are measured in milliseconds (ms).

Knee: The knee determines the character of the gain reduction that’s applied to a signal. Using a soft knee will cause the compressor to gradually begin compressing a signal, increasing the ratio smoothly as it approaches the threshold, and in effect creating a more gentle transition and a more transparent sound. A hard knee means that this transition is more abrupt.

Make up gain: Once a signal has been compressed, its overall level will be quieter. Make up gain is a level control at the output of the compressor which allows for the compressed signal to be turned back up in order to compensate for the gain reduction caused by the compression.

Reverb and delay

Reverb and echo (also referred to as delay) are effects that can occur naturally when capturing audio recordings - many spaces have natural reverberation and/or echo. This can either be of benefit to the engineer or an issue impacting the clarity of a recording. In some cases, these effects are artificially applied to audio recordings to modify the timbre of instruments and vocals, or create a particular atmosphere or mood.

Here's a video that explains the difference between echo/delay and reverb:


An audio filter operates within the frequency range of 0 Hz to 20 kHz, responding to various frequencies in a circuit. Its functions include amplifying (boosting), allowing (passing), or reducing (attenuating) specific frequency ranges. Different filter types cater to various audio applications, such as hi-fi stereo systems, musical synthesizers, effects units, sound reinforcement systems, instrument amplifiers, and virtual reality systems.

Filter types:

Low-pass filters permit frequencies below their cutoff points to pass through, gradually diminishing frequencies above the cutoff. They find application in audio crossovers to eliminate high-frequency elements from signals directed to a low-frequency subwoofer system.

High-pass filters, conversely, enable the transmission of high frequencies surpassing the cutoff point while progressively reducing frequencies below the cutoff. In audio crossovers, high-pass filters are employed to eliminate low-frequency content from signals directed to a tweeter.

Band-pass filters facilitate the passage of frequencies within the range defined by their two cutoff frequencies, attenuating those outside this spectrum. On the other hand, band-reject filters attenuate frequencies between their two cutoff points, allowing those outside the 'reject' range to pass through.

All-pass filters allow the transmission of all frequencies but modify the phase of each sinusoidal  (i.e. a sine wave) component based on its frequency.



Elen, R. (1996). Sound processing. In Borwick, J. (Ed.) Sound recording practice (4th ed.) (pp.204-229). Oxford University Press.

Thompson, D. M. (2005). Understanding audio. Berklee Press.