Compressors are electronic-filtering devices that boost or limit sound to a predetermined volume level within a set frequency. A multiband compressor takes the concept a step further by offering an audio engineer a choice of frequencies to which compression could be added. This type of compressor is used primarily in audio mastering — the final stages of the music and film-recording process — but also can be used by musicians during live performances.
Mastering of recorded music typically involves careful analyzation and dissection of all parts of a recording. Since sound encompasses every frequency within audible range, a single-band compressor is not always adequate for optimal sound quality. With the ability to isolate groups of frequencies in parts of the audible spectrum, a multiband compressor is capable of fine-tuning volume levels within preset ranges.
With the exception of frequency isolation, a multiband compressor operates much like a standard compressor. An adjustable threshold control is used to limit the volume level while attack and release controls are used to tell the compressor when to act and when to let go of the audio signal. Mathematical ratios are used to determine the minimum and maximum threshold. Attack and release functions are typically based on an engineer's experience and the amount of audio signal present.
Aside from the obvious benefits of frequency isolation, multiband compressors can boost or reduce the volume of whole groups of frequencies if necessary. For bass-heavy hip-hop or rock music, the lower frequencies are set at a higher threshold level. For mid- and high-range dominant music, such as classical or folk music, the bass frequency threshold may be reduced, thereby allowing the other frequencies to be favored.
The sound of a multiband compressor is usually transparent, much like single-band compressors. Since compressors add no frequency alteration or sound effects, the only way to audibly discern compressed audio is to compare the raw audio signal with the compressed signal in a studio setting. Engineers also rely on sound meters to monitor volume levels and to assist in determining compression ratios. To the casual listener, the processed sound of a multiband compressor typically does not sound processed at all.
Compression levels in the audio and broadcast industries are somewhat standardized, but this typically depends on the platform. A person might notice that most commercially produced musical recordings play at similar volume levels. In contrast, television commercials are sometimes noticeably louder than regular television programs, and radio listeners may also notice a volume difference when changing stations. All of these audio signals usually make use of a multiband compressor and single-band compressor combination, but the standard average volume level differs from industry to industry.