dBmeterHow do you compare how quiet your product is against those of your competitors and use it as a marketing advantage ?

How can you advertise a number that characterises the quietness of your product?

If you have a sound level meter you can measure the sound pressure level one metre from your product and apply an A-weighting to the sound meter output so that the measured sound is weighted according to a standard weighting. This will account the different relative loudness that the human ear hears sounds of different frequencies.

The question then is: Is that A-weighted sound pressure level you just recorded sufficient to characterise the quietness of your product. The answer is a very loud NO !

The quantity you need to measure to characterise the sound output for a product is A-weighted sound power level and not A-weighted sound pressure level. What is the difference between the two quantities? Sound pressure level corresponds roughly to the loudness we hear. Sound power in contrast is the amount of sound energy per unit time emanating from a source.

Sound pressure level (how loud we hear sound) depends on several factors; how far away we are from a sound source, how much of the sound is reflected or absorbed by the surrounding environment and in which direction from we stand from the source if the noise source has a directional bias.

Sound power is a property of a system or product. In other words, for particular climatic conditions and performance settings, the sound power emanating from your product should be the same no matter where it is used and from what distances the sound power is measured….provided it is measured correctly. Therefore the sound power of a product is the quantity that should be used to compare the performance of your product with that of your competitors.

If the sound pressure level of a product is measured from one metre away and your competitor takes the same reading of their product from two metres away, their product may come out looking quieter … even if it’s really not! Further, if both measurements are taken from the same distance away, the readings will be skewed by how reflective or absorbent of sound is the environment in which the measurements are taken.

Both sound pressure level and sound power level are typically given in decibels (dB) and if they are both A-weighted, both are given in dBA. However, the decibels for sound power and sound pressure (and remembering that decibels are a multiple of the log of a ratio between the raw quantity and a reference number) are given against two completely different reference numbers. For sound power, the decibel readings are given against a reference of one picowatt – that is a measure of power. Sound pressure level reading in decibels is given against a reference of 20 microPascals – a measure of pressure.

The difference between sound power level and sound pressure level is analogous to the difference between the power of a heater and the temperature of a room. The temperature depends on not only the power of the heater but how far away you are away from it, the thermal properties of the room, the direction you are from the heater and on the background ambient temperature as well.

It is a lot easier to measure sound pressure level than sound power as sound power cannot be simply read off from a meter. Sound power needs to be calculated from several sound pressure level readings taken around your product in a systematic manner and then corrected for the effects of both background noise and the sound reflection properties of the test environment. Consequently organisations use sound pressure level as a measure of the noisiness/quietness of their products.

Austest Laboratories has an acoustic measurement facility, software, tools and expertise that can effectively measure sound power levels. Measurements are taken in a dual EMC/Acoustic semi-anechoic chamber. The chamber has been characterised with suitable acoustic properties allowing for appropriate calculation factors to correctly measurement not only for background noise but for room effects. Our work is performed in strict accordance with authoritative standards for determining airborne acoustical noise such as IEC 60704 and ISO 3744.