stant in book and it must be sufficient here to state that an octave, the eight level, or a worker in an industrial environment may not stay in the white notes on a keyboard from say, C to C, contains in fact twelve same noise conditions for very long, moving to other areas on the equally spaced notes. The frequency ratio 2:1 is an octave, as for factory floor, for example. This has led to the concept of an equival- example a to A (220Hz to 440Hz). The frequencies of two adjacent ent noise, or personal noise dose. The terms L for Equivalent noise notes, semitones in the table above, are in the ratio 1/12 or 1.05946. Level, or L for Equivalent Personal Daily exposure have been It should be noted that to a trained musical ear the mathematical adopted. The term L is being generally discarded in favour of relationship stated here is not quite correct, SO that, for instance, a ep,d' at least in health and safety contexts. Briefly, an L of n dBA is semitone above F, written as F# is different from a semitone below the equivalent in noise dosage to a steady sound of n dBA, although G, written Gb. the actual sound levels may have varied widely. It should be empha- Timbre (pronounced tarmbre) is the term used to describe the sised that L is not a simple average of individual noise levels. quality of a sound i.e. that which distinguishes the sound of, for Computation from dBA readings and their durations is possible but instance, a trumpet from a clarinet. There are two principal factors suitable measuring instruments are usually to be preferred. which determine the timbre of a sound: the harmonic content and Other commonly encountered units are: the starting transients. Figure 8.5 Portion of a keyboard Speech and music 8/9 Table 8.6 Frequencies corresponding to musical notes away in about 25mS. The ear/brain attaches great importance to the starting transients and if they are missing or distorted accurate identification of the source, say a musical instrument, may be difficult or even impossible. Frequency (Hz) 8.4.2 220.