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COP2012 - Acoustics: SWL vs SPL

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daniel castro

on 12 November 2012

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Transcript of COP2012 - Acoustics: SWL vs SPL

Community of Practice 2012
Acoustic Department
Notes on the difference between SWL vs SPL Notes on SWL Why Decibels...? dB or dBA? Sound Power Level
Sound energy radiated per unit of time by a sound source,
obtained by integrating the sound intensity over an imaginary surface surrounding a source; measured in Watts (W) 79dB / 79 dBA SWL: Sound Power Level
SPL: Sound Pressure Level from wikipedia Daniel Castro
daniel.castro@wge.com.au Sound Pressure Level
Airborne sonic vibration create +/- variations in the atmospheric pressure. It is these fluctuations in pressure which are detected by the ear what we call Sound Pressure Level. Summary of quality of data
normally required by Acoustic Engineers SPL = 79dB / 79 dBA @ 10m SWL = 79dB / 79 dBA measured Bad Acceptable Good 63Hz 125Hz 50Hz 500Hz 1000Hz 2000Hz 4000Hz dB dBA
45 56 58 64 72 74 80 92 83 Excellent SWL dB re 1pW according to ISO ISO 9614 / 3741 / 42 / 43; ANSI S12.33 Noise Criteria (NC): Accepted noise level for a given space.
Defined for center band frequencies (Hz): 63 125 250 500 1000 2000 4000 8000 Interior design goals, ASHRAE, 1987 Noise Rating Curves (NR) are developed by the ISO to determine the acceptable indoor environment for hearing preservation, speech communication and annoyance. SWL is the cause, SPL is the effect Analogy 2: Lighting
The following comparison of sound and light may help illustrate the distinction between these terms. Think of sound power as the wattage rating of a light bulb; both measure a fixed amount of energy. Sound pressure corresponds to the brightness in a particular part of the room; both can be measured with a meter and the immediate surroundings influence the magnitude of each. In the case of light, brightness is more than a matter of bulb wattage. Asking for a 90 dBA motor is a lot like asking for a “light:” you don’t know what you are going to get. Most of us are much more familiar with light than sound. If someone says he has a 100-watt light bulb, you have some idea of the candlepower available, but if you want to read by the light, you want to know the light intensity level at the reading location. To determine the light intensity level you would need to know:
“How far away is the light?” If the light is a mile away, it is not much use. The analogous sound question is “How far away is the motor?”
“Is the light outdoors?” With no walls to reflect the light, all but the direct light radiates out into the free field of space. The analogous sound question is “Is the motor outdoors?”
“Are the room walls reflective if the light is not outdoors?” A room covered with black velvet would not reflect much light regardless of its size. The analogous sound question is “How reverberant are the walls?”
(From usmotors.com) Analogy 1: Heating
A sound source radiates power P and this result in a sound pressure p. Sound power is the cause – Sound pressure is the effect. Consider the following analogy. An electric heater radiates heat into a room and temperature is the effect. Temperature is also the physical quantity that makes us feel hot or cold. The temperature in the room is obviously dependent on the room itself, the insulation, and whether other sources of heat are present. But for the same electrical power input, the heater radiates the same power, practically independent of the environment. The relationship between sound power and sound pressure is similar. What we hear is sound pressure but it is caused by the sound power emitted from the source (here: vacuum cleaner).
Too high a sound pressure may cause hearing damage. So when trying to quantify human response to sound, such as noise annoyance or the risk of hearing loss, pressure is the obvious quantity to measure. It is also relatively easy to measure: The pressure variations on the eardrum we perceive as sound are the same pressure variations which are detected on the diaphragm of a condenser microphone. (SPL meter.)
The sound pressure that we hear, or measure with a micro-phone is dependent on the distance from the source and the acoustic environment or the sound field in which sound waves are present. This in turn depends on the size of the room and the sound absorption of the surfaces. So by measuring sound pressure we cannot necessarily quantify how much noise a machine makes. We have to find the sound power because this quantity is more or less independent of the environment and is the unique descriptor of the noisiness of a sound source. This is especially true for noise fighters, and less or not at all for audio engineers. (From Brüel & Kjær) Sound vs Noise
Amplitude / Level / Pa / dB
Frequency / Pitch / Hz
Human ear according to ISO ISO 9614 / 3741 / 42 / 43; ANSI S12.33 Noise ~ Sound
Level / Amplitude [Pa - dB]
Frequency / Pitch [Hz]
Octave bands
Human ear Notes on Sound Pressure Level 2 Let's measure some noise AS2107:2000
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