842 Chapter 29
To many, the subject of studio acoustics is considered a black art, often surrounded by
considerable hype and incomprehensible terminology. However, this is no longer the
case. Today, there certainly is an element of art in achieving a desirable and a predictable
acoustic environment, but it is very much based on well-established scientifi c principles
and a comprehensive understanding of the underlying physics of room acoustics and
noise control.
Studios and control rooms create a number of acoustic problems that need to be
overcome. Essentially, they can be divided into two basic categories: noise control and
room acoustics, with the latter including the interfacing of the monitor loudspeakers to
the control room.
29.2 Noise Control ........................................................................................................
The increased dynamic range of the hi-fi medium has led to corresponding decreases in
the levels of permissible background noise in studios and control rooms.
A single fi gure rating (e.g., 25 dBA) is not generally used to describe the background
noise requirement as it is too loose a criterion. Instead a set of curves that take account
of the spectral (frequency) content of the noise are used. The curves are based on an
octave or 1/3 octave band analysis of the noise and also take account of the ear’s reduced
sensitivity to lower frequency sounds.
The curves used most frequently are the NC (noise criterion) and NR (noise rating)
criteria. Figures 29.1 and 29.2 graphically present the two sets of criteria in terms of the
octave band noise level and frequency. (Although as Figures 29.1 and 29.2 show, criteria
are not exactly the same; the corresponding target criteria, e.g., NC20 or NR20, are
frequently interchanged.)
The NC system is intended primarily for rating air conditioning noise, while the NR
system is more commonplace in Europe and can be used to rate noises other than air
conditioning. (An approximate idea of the equivalent dBA value can be obtained by
adding 5 to 7 dB to the NC/NR level).
Table 29.1 presents typical design targets for various studio and recording formats. Many
organizations, for example, the BBC, demand even more stringent criteria, particularly
at low frequencies. From Table 29.1 and curves shown in Figures 29.1 and 29.2 , it can
be seen that the background noise level requirements are pretty stringent, typically being