Relative Noise

When does noise become a problem for sound systems? Is it when any unrelated sound is faintly audible, or when it is loud enough to impair the message conveyed by the sound system? Is a hiss worse than a hum? Is the buzz of lighting dimmers worse than the sudden breakthrough of a taxi dispatcher? Like so many audio issues, the answer is relative. However, it is not just analogue equipment that generates noise, interconnections between components and even the most sophisticated digital processors generate noise.

Noise is a fundamental part of all audio signals; even the highest quality sound system ever built included a noise component (and not just one component). A high quality audio system will simply have lower noise levels relative to the program (desired) audio. For clarity, hereafter, I will refer to the desired content as the program audio.

Noise is produced simply by using electrical signals to carry audio and it is in the background of every place a microphone has even been positioned to pickup sound. In fact, we have become used to the ambient noise that exists throughout the urban environment and this noise is used in television to mask the sounds of the studio crew. This is typical of the difference between noise that is acceptable (urban traffic during a street scene) and noise that is objectionable (murmur of production staff and sliding of camera cables on the sound stage). Ironically, the quietest rooms being built (such as concert halls and recording studios) often have problems with very tiny noises becoming objectionable, simply because they can be heard when all other noise is removed.

There are three ways of looking at the seriousness of a noise problem: the level of the noise relative to the level of the audio signal; the spectral distribution of the noise relative to the spectrum of the program audio; and the (temporal) occurrence of the noise. A noise that is loud enough will impair the quality of the program audio. A noise that has energy in narrow bands (tones) will become objectionable at a lower relative level than a noise which is spread evenly across the audio spectrum. Noises that occur sporadically or rhythmically are more noticeable than a constant and unchanging noise.

The noise produced by most active electronics is reduced to a single number for specifications. This is usually the ratio of the nominal program level to the noise level (signal-to-noise ratio). There may be a weighting curve applied to the noise level (A-Weighted or band-limited) to indicate the spectrum of noise that the manufacturer envisions as most appropriate for the intended use of the device. However, this is not enough information to determine if the noise generated by the device may have a spectral or temporal component that could make the noise more objectionable than the simple dB ratio would indicate.

When judging the quality of the noise produced by a sound system, a high-resolution spectrum analyzer is very useful. A 1/3-octave RTA can be used but the most useful device is a narrow-band FFT-based analyzer. These units are available in both hardware and software versions. Even some low-cost audio editing software can provide this function. The FFT analysis will provide a much clearer view of the noise distribution across the audio spectrum and will assist in identification of narrow-band tone-like noise and broadband noise. Those narrow-band noises can be audible even when they are lower in level than the broadband noise.

Noises that occur cyclically or sporadically will also become objectionable at lower levels because, like the tone-like noise, they call attention to their presence. A noise that is constant can become part of the overall background of noise in our environment but when the noise stops and starts it attracts the listener’s attention and becomes an annoyance. Many of these temporal noises are induced in interconnecting cables or during changes in preset settings within the sound system. Other systems in the building, from power to security can cause these intermittent noises to become a problem.

Noise, of any type, will not be an issue in a sound system, if it remains relatively low in level. This is best achieved during installation and configuration of the system, and not by follow-up remedial work. Good cable management practices during installation, that separate potential noise inducing signal cables and provide high-quality terminations, must be combined with developing the optimal gain structure that is specifically suited to the way the system will be used. This will separate the desirable sounds from their noisy relatives.

United Entertainment Media Inc.
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New York, NY, 10016
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