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Camberwell Library21 March 2013Appendix AAppendix A Glossary of Acoustic TermsA.1 DecibelA.1.1The ratio of sound pressures which we can hear is a ratio of 106:1 (one million:one). Forconvenience, therefore, a logarithmic measurement scale is used. The resulting parameter iscalled the ‘sound pressure level’ (Lp) and the associated measurement unit is the decibel (dB).As the decibel is a logarithmic ratio, the laws of logarithmic addition and subtraction apply.A.2 FrequencyA.2.1The repetition rate of a sound wave. The subjective equivalent in music is pitch. The unit offrequency is the Hertz (Hz), which is identical to cycles per second. A thousand hertz is oftendenoted kHz, e.g. 2 kHz = 2000 Hz. Human hearing ranges approximately from 20 Hz to 20kHz. For design purposes, the octave bands between 63 Hz to 8 kHz are generally used. Themost commonly used frequency bands are octave bands, in which the mid frequency of eachband is twice that of the band below it.A.3 Noise IndicesA.3.1Noise indices recorded during a survey that are of relevance to BB93 include the following:L Aeq,T The A-weighted equivalent continuous sound pressure level over a period oftime, T.L Amax,T(F,S) The A-weighted maximum sound pressure level over period of time T, with fastor slow time weighting.L A1,T The arithmetic mean of the A-weighted sound pressure level exceeded for 1% ofthe measurement period, T. Indicative of the maximum noise levels.L A10, T The arithmetic mean of the A-weighted sound pressure level exceeded for 10%of the measurement period, T. L A10 is the index generally adopted to assesstraffic noise.L A90,T The A-weighted sound pressure level exceeded for 90% of the measurementperiod, T. L A90 is widely accepted as indicative of the background noise level.1