How to Record Audio
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Key Terms

In digital recording the sound wave is converted into an electric signal by a microphone. The physical properties of the sound wave are converted into digital information which can then be decoded for final reproduction. In digital audio, the value of the audio signal is sampled which produces numbers representing the value of each sample.

Sampled audio is usually displayed visually as a waveform. Most voice recording is done as a mono recording and therefore appears as a single sound track. The image on the track represents the waveform of the recorded speech.

Typical Audio Waveform

Typical Audio Waveform


In digital recording the intensity of the sound signal is measured at regular intervals, which is called sampling. At each sampling point, the audio signal is assigned values. The more samples per second the greater the accuracy and quality of the recording. For example, CD quality sound is sampled at 44,100 samples per second or kHz and this is considered the standard rate to use.
Sampling Rate in Audacity

Sampling Rate in Audacity

Using lower sampling rates can significantly reduce the sound quality. (Other commonly used sampling rates are 22.05 kHz and 11.025 kHz).

Measuring Sound

Sound levels are measured in decibels (dB) which is a unit for comparing the loudness of two different sounds. The usual basis of comparison is the normal minimum sound level that someone can hear, which is considered to be 0 dB. The sound levels of other things illustrate how much louder a sound is than this minimum. Ten decibels (10dB) is the level of sound made by rustling leaves. A car at 10 metres may be as loud as 80 dB.

When working with Digital Audio the zero decibels reference level is considered to be the maximum level for the sound and the signal should not exceed this. When it does, the peaks in the digital signal are cut off or "clipped" at the 0 dB (maximum) level. This clipping causes distortion and should be avoided.

In Audacity there are Input and Output meters that allow you monitor the levels of the audio as it is being recorded and played back. It's best to record at levels below the maximum to be safe. If you see your recording meter peaking you should reduce your recording volume.
Output and Input Meters

Typical Output and Input Meters


Bit-rate refers to how many bits (digital 1s and 0s) are used each second to represent the sound signal. The bit-rate for digital audio is represented in thousands of bits per second (kbps) and is related to file size and sound quality. Higher bit-rates result in larger file sizes but better sound quality. Lower bit-rates result in smaller files but poorer sound quality.

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