Sound Power and Pressure Level Explained

The sound power level indicates the total acoustic energy that a machine, or piece of equipment, radiates to its environment. The sound pressure level is a measure for the effect of the energy of an acoustic source (or a collection of sources). It depends on the distance to the source(s) and acoustic properties of the surroundings of the source. You need to consider the sound pressure level when you are choosing your new air compressor.

Given a well-defined operation condition, the sound power level of a machine is a fixed value, where the sound pressure level always depends on position and environment.

In relation to the EU Machinery Directive we speak about the “emission sound pressure level”. This represents the sound pressure level an operator or bystander is exposed to, in a free field above a reflecting plane. If you need to keep noise to a minimum in your workplace, you can look at our range of silenced air compressors.

LpA, LwA

Levels are described in ISO standards, using the following abbreviations or descriptors:

Lp: sound pressure level, and
LW: sound power level

If A-weighting has been applied, then the descriptors above contain the character A at the end, for example: LpA

dB and dB(A)

dB (abbreviation for decibel) is the “unit” for both pressure and power level. It’s not a unit because it’s the logarithm of a quotient, that’s why Lp and LW are both expressed in dB, each with a different reference. The decibel is essentially unit-less!

The A in dB(A) is not supported by ISO standards. The reason is that the A must be contained in LpA or LWA, and with a proper description. That is what’s needed for a good specification, requirement or measurement result.

Safety Note: If you see db, DB or Db in relation to noise or sound levels you should not take the information at face value. It suggests that the author is unfamiliar with some important concepts. You should ask yourself if you can trust someone who does not know that dB must be used (e.g. knows were dB “comes from”) because it suggests that the person, institute or company lacks the most basic knowledge about sound levels and related subjects.

Sound power level
LWA dB(A)
Theoretical average sound pressure level
LpA dB(A)
L 1m 4m 15m
80 72 60 48.5
85 77 65 53.5
90 82 70 58.5
95 87 75 63.5
100 92 80 68.5
Typical reduction 8 20 31.5

The decibel which is normally abbreviated dB(A) is the unit used to measure the intensity of a sound. The human ear is incredibly sensitive. A person’s ears can distinguish between wide ranging sounds. You can hear everything from brushing your fingers on a notepad to the loudest explosion or jet aircraft. To put these differences into perspective a jet aircraft is 1,000,000,000,000 times louder than the quietest audible sound.

Looking on the decibel scale, the smallest audible sound which is near silence registers at 0 dB. An increase of 10 decibels equates to a 10 fold increase in noise to your ear.

Important – Air Compressors are measured and compared within the industry @ 4 meters. To comply with EEC regulations all air compressors have to be marked in LWA. This is a different measurement of sound which is taken from a different distance. For an example a typical 3hp piston compressor is rated at its lowest at 73 dB(A) @ 4 meters on the LWA scale this would equate to approximately 93 decibels. Please beware of this as all products have LWA marked on them with stickers similar to the image below.

Air compressor sound sticker

 

Here are some common sounds and their decibel ratings

140 dBA Aircraft carrier deck during take-off and landing
120 dBA Whistle at 1m distance, test run of a jet at 15m distance
Threshold of pain, above this fast-acting hearing damage in short action is possible
115 dBA Take-off sound of planes at 10m distance
110 dBA Siren at 10m distance, frequent sound level in discotheques and close to loudspeakers at rock concerts
105 dBA Chain saw at 1 m distance, banging car door at 1m distance (maximum level), racing car at 40m distance
100 dBA Frequent level with music via head phones, jack hammer at 10m distance
95 dBA Loud crying, hand circular saw at 1m distance
90 dBA Motorcycle at 6m distance
85 dBA 2-stroke chain-saw at 10m distance, loud WC flush at 1m distance
80 dBA Very loud traffic noise of passing lorries at 7.5m distance, high traffic on an expressway at 25m distance
75 dBA Passing car at 7.5m distance, un-silenced wood shredder at 10m distance
70 dBA Level close to a main road by day, quiet hair dryer at 1m distance to ear
65 dBA Normal Conversation
60 dBA Petrol Lawnmower
55 dBA Low volume of radio or TV at 1 m distance, noisy vacuum cleaner at 10 m distance
50 dBA Refrigerator at 1m distance, bird twitter outside at 15 m distance
45 dBA Noise of normal living; talking, or radio in the background
35 dBA Very quiet room fan at low speed at 1m distance
25 dBA Sound of breathing at 1m distance
0 dBA Auditory threshold