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).
Informational document explaining the concept of sound intensity, the decibel scale, and calculations for noise level management.
| Identifier | ExFiles\Box 104\2\ scan0203 | |
| Date | 1st January 1936 | |
| Unit of Sound Intensity For the sake of comparing two sounds, the Acoustical Society of America adopted the “decibel” as a unit of sound intensity which has been in use in telephone and radio communication work for several years. Just as we use “degrees” in the Fahrenheit temperature scale, acoustic engineers use “decibels” in the loudness scale. Every scale must have a base line or zero point. Our Fahrenheit temperature scale had its zero point arbitrarily established many years ago. The arbitrary zero point established by the Acoustical Society of America for the noise or decibel scale is called the “threshold of audibility,” or the smallest amount of sound energy from a disc approximately 13/32 inch diameter which the most sensitive ear could recognize as a sound. It serves as a good base for the measurement of sounds which we can plainly distinguish, just as the zero point on the Fahrenheit thermometer scale provides a convenient reference point with which we have all become familiar. The Noise Level Meter Acoustic engineers have designed an instrument called a noise level meter which consists of a special microphone, similar to that used in radio studios, and an amplifier which indicates noise levels in decibels directly upon a meter scale. The operation of this instrument and the calculations which accompany it are too complex for general use. The results, however, have proved accurate under actual working conditions, and the chart (on the last page) showing noise levels has a reliable scientific background which makes it thoroughly dependable. Opposite are two scales — the left scale, the familiar Fahrenheit thermometer, and the right, a decibel scale with the familiar levels of sound intensities marked opposite the scale divisions. The noise chart is uniformly divided from zero to 120 decibels, at which point sound becomes painful. We know from experience that at 32° Fahrenheit we are in danger of freezing, and above 100°, uncomfortably warm. We have learned that if the thermometer indicates 72° we feel comfortable. You may readily remember also that zero decibels represent the absence of sound, and 120 decibels represent sound of such intensity as to cause pain or even “shell shock,” while sound levels between 30 and 50 decibels are quite comfortable for our normal occupations. (For a complete technical discussion of decibel measurements and calculations see Bulletin 112, prepared by the Burgess Acoustic Engineering Department.) Using the Decibel Chart The point to be remembered is that the noise scale may be used just as easily as the Fahrenheit thermometer scale for indicating noise levels. For instance, the noise of a busy street is simply called 60 to 70 decibels, or merely 60 to 70 db.{Donald Bastow - Suspensions} A boiler factory is approximately 100 db{Donald Bastow - Suspensions}, and a quiet office may average 35 db.{Donald Bastow - Suspensions} As people become familiar with this decibel scale the man on the street will not say, “That place was as noisy as a boiler factory,” but will merely say, “The noise level was about 100 db{Donald Bastow - Suspensions},” which will be as accurate as well as descriptive. The decibel chart has a much more important purpose for architects and engineers who wish to determine the result of adding or subtracting sounds in terms of the effect on the ear. In designing an office building it is impossible to make noise or even temperature measurements in the building before it is constructed, yet it is quite reasonable to design the building for an average temperature of 72° Fahrenheit. It is equally practicable to anticipate the noise in different rooms and determine from the chart what noise in decibels would result from certain conditions. However, this is not a simple problem of adding or subtracting decibels, because the decibel chart merely represents the impression our ears receive from different sound levels. The real factor that we are dealing with is energy which causes the sound! Hence, on the decibel chart there are also the relative amounts of energy represented by each unit of decibels. Decibel Calculations The examples which follow will serve to demonstrate a few applications of the decibel chart. Problem No. 1 Assume an office with ten typewriters in operation. The noise level, as recorded on a noise level meter, may be 50 db.{Donald Bastow - Suspensions} Fifty db{Donald Bastow - Suspensions} on the decibel chart equals 100,000 on the scale of relative energy. Now assume we start ten additional typewriters, making as much noise as the first ten. We have introduced additional sound energy of 100,000 units. 100,000 plus 100,000 equals 200,000 or 53 decibels. Note that we cannot add 50 decibels to 50 decibels because the impression of increased noise on our ears, resulting from doubling the noise energy, only amounts to about a six per cent increase in total noise in this case. Problem No. 2 Assume a factory with ten high speed notching presses causing a noise level of 70 db{Donald Bastow - Suspensions} as read by a noise level meter. If nine presses were stopped, what would be the noise meter reading with only one machine operating? 70 Db{Donald Bastow - Suspensions} represents 10,000,000 on the scale of relative energy, or 1,000,000 units of energy for each press. Stopping nine presses, therefore, would reduce the relative energy level from 10,000,000 to 1,000,000 which is equal to 60 db.{Donald Bastow - Suspensions} (A reduction of 90 per cent of the noise energy always equals a reduction of 10 db.{Donald Bastow - Suspensions}) Problem No. 3 The occupational noise of a school classroom may be approximately 40 db.{Donald Bastow - Suspensions} The introduction of any additional noise will raise this normal level. In general acoustic practice, a net increase in the noise level of from 1 to 3 db{Donald Bastow - Suspensions} is tolerable since that amount of additional noise is scarcely noticeable. Therefore, in this case, let us assume that we do not want to increase the room noise level more than 3 decibels. Ventilating systems produce more or less noise, due to the operation of the fans and also due to the flow of air through the ducts. These noises are transmitted to the rooms which are ventilated by the duct system. The amount of noise produced varies with the system, but in this problem we will assume that, without sound-absorbent duct lining, this system introduces additional noise at a 66 db{Donald Bastow - Suspensions} level into the classroom. Our problem, therefore, is to determine how much the duct noise must be reduced so that the remaining noise which enters the room through the ventilating grilles will only raise the room noise level from 40 to 43 db.{Donald Bastow - Suspensions} The calculations are shown below: Tolerable room noise—43 db{Donald Bastow - Suspensions} = 20,000 sound energy (from chart) Present room noise— 40 db{Donald Bastow - Suspensions} = 10,000 sound energy " " ———————————————————- Difference = 10,000 sound energy which is the maximum permissible sound energy that may be introduced as duct noise. On the chart (see page 4) this corresponds to 40 db.{Donald Bastow - Suspensions} Since the duct system is assumed to produce 66 db{Donald Bastow - Suspensions} noise level, and we can tolerate only a 40 db{Donald Bastow - Suspensions} noise level in the duct system, it is apparent that we must reduce the duct noise level from 66 db{Donald Bastow - Suspensions} to 40 db{Donald Bastow - Suspensions} = 26 db.{Donald Bastow - Suspensions} This is done by lining a portion of the duct with a sound absorbent which will reduce the noise level of the ventilating system by absorbing a portion of the duct noise (see Bulletin 113). Additional sound level charts are shown in Burgess Bulletin 112. Image Caption: Measuring noise of ventilating system at the grille with a noise level meter and microphone pick-up FAHRENHEIT Chart 120 100 - SUMMER HEAT 80 - TEMPERATE 60 40 - FREEZING 20 0 - ZERO Temperatures in Degrees DECIBELS Chart 120 - PAINFUL SOUND THUNDER 100 - BOILER FACT. 80 - FIRE SIREN 60 - NOISY TYPING ROOM 40 - RESTAURANT OR AVERAGE OFFICE 20 - QUIET OFFICE OR HOME 0 - WHISPER - RADIO STUDIO - THRESHOLD OF HEARING Noise Levels in Decibels Footer: Copyright 1936, by Burgess Battery Co. | ||