From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Explaining the principles of noise measurement, the decibel unit, and its relation to sound intensity.
Identifier | ExFiles\Box 138\1\ scan0107 | |
Date | 3rd July 1934 | |
X634 TO SC. LHS.{Lord Herbert Scott} ID. OK. BN.{W.O. Bentley / Mr Barrington} HN.{F. C. Honeyman - Retail orders} FROM EV.{Ivan Evernden - coachwork} RE NOISE. H.{Arthur M. Hanbury - Head Complaints} Smith COPY TO WCR. HS.{Lord Ernest Hives - Chair} S. Over 90% of the noise in a town is produced by road transport, and the amount of noise in general increases with the speed. As speeds go up so the average noise level becomes higher until it reaches the discomfort level. In America a very considerable amount of work has been done on the measurement of noise. Only in recent years have means been found of doing this. The new unit of noise, the "decibel" has come into use. It is a unit of loudness or change in sensation, but it must be stated straight away that it has no individual significance. It is merely a convenient method of expressing a ratio between two degrees of loudness. From a development of Weber's Law the decibels of loudness between two sound intensities P1 & P2 is expressed by the following equation. db{Donald Bastow - Suspensions} = 10 log P1/ P2 It will be apparent that there must be created an artificial horizon of zero noise, which has been called the "threshold of human hearing" or the base line from which to measure ratios. This has been discovered by testing a large number of people of apparently normal hearing in a soundproof cell. It has been fixed at 1 millibar sound pressure. Loudness appears to increase in arithmetical progression one noise being so many times as loud as the other, but the sound energy intensity increases in logarithmic progression. This is an important fact to appreciate in connection with the measurement of sound by means of the masking methods such as is used in the Standard Thelphone Co. Acoustic Meter used for the tests in the report attached. For example. For 20 db.{Donald Bastow - Suspensions} above threshold Intensity P1 is given as 20db.{Donald Bastow - Suspensions} = 10 log. P1/Po where Po is 1. = 10 log. P1 Therefore log P1 = 2. and P1 = 100 units. For 20db{Donald Bastow - Suspensions} above 20db.{Donald Bastow - Suspensions} or 40 db.{Donald Bastow - Suspensions} above threshold. | ||