Noise

Noise is our enemy. It is simply unacceptable to design a lifestyle automation system where cabinets buzz, equipment fans' scream and heating pipes crack. We follow some simple rules to avoid noise. We talk to manufacturers and distributors about noise levels. We try to certify or test equipment before we install it. When noise is unavoidable, we work with our clients to choose equipment locations and design sound insulation so impact in living areas is minimised. We check noise levels once equipment is in place and installed. We return equipment which does not meet the noise levels that we demanded. This section will help you to minimise noise, but please feel free to contact us if you have any questions.

What is a decibel?

A decibel (dB) is the measure of the level of noise (or sound) by reference to an arbitrary level of "silence" and, because of the dynamic range of the sound and the way we perceive sound, this ratio is log'd and multiplied by 10:

Sound Pressure Level (SPL) in decibels (dB)= 10 x log (E_i/E_o)

where E_i is the power level of the sound and E_o the power level of the "silence". So a noise level 10 times the level of silence has a value of 10db. A sound 100 times the level of silence has a dB value of 20 and so on.

The level of silence is defined as the minimum audible level of a 1000Hz tone (audible sound is between 20Hz and 20,000Hz with maximum human sensitivity at 1000Hz) and corresponds to a root-mean-square pressure of 2 x 10^-4 µbar or 2 x 10^-5 N/m².

We can't measure the sound level directly or its proxy, average power levels (E), so instead we measure the average pressure applied on a surface as a result of the sound's pressure waves (actually the equipment measure the maximum pressure and divides it by the square root of 2). This should give a good reading if the measurement is taken in the plane of the wave (ie. with the sensor pointing to the source).

Sound Pressure Level (SPL) = 10 x log (E_i/E_o) ~ 20 x log (P_i/P_o) = 20 x log (P_max in µbar) + 71

where P_i is the root-mean-square pressure being measured (= P_max/sqrt(2) ).

In practice, this means that a SPL of 0 dB you are in total silence (technically, at a level of noise you cannot hear!). Broadcasting studios work at 10-20 dB of background noise. A level of continuous noise of 90 dB is about the maximum one can stand. At about 120 dB you will be in pain.

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What do we mean by quiet spaces?

Broch⁽¹⁾ suggest the following noise ranges for various living spaces:

In practice you will find annoying any background noise level over 35 dB. Every 10db doubles the perceived sound level.

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How do i know if the equipment is "quiet"?

Actual noise is not just made up of a single frequency but a range and the human ear does not respond equally to all frequencies. To correct for this, sound pressure meters have the option to weight the different frequencies based on (generally) three different standards — referred as A, B or C-weighting.

For noise measurements, A-weighted scale is generally used (note that for hi-fi speaker calibration some people use the C-weighted setting). To measure the level of background noise in the room itself (before the equipment is installed) no weight is typically used.

Measurements are thus expressed as 60dB(A), 60dB(B) or 60dB(C) - meaning they used the A-weighted scale (or B or C) - or 60dB(lin) - meaning linear, not weighted. So a full measurement of the noise level in a room generated by a piece of equipment could read something like: "30dB(A) over 18dB(Lin) background" meaning a noise level of 30db using A-weighting in a room with a background noise of 18db measured with no weight.

Manufacturers of equipment (well only the ones that care!) may rate their equipment either as "Acoustic power" or "Acoustic pressure at 1m". Acoustic power is a measure of the total noise produced by the equipment (so it is independent of distance or position). Acoustic pressure is the SPL measured at an angle giving the lowest noise (!) at 1 meter. Because the later gives a much lower number the former is rarely found. Sometimes 1 meter is not appropriate (for instance for PC's which may be closer in normal use) and "operator position" measurements are used.

We normally measure A-weighted noise level in equipment at the closest of 1m or operator position at whatever angle we feel is noisiest and reject anything that gives a rating of over 35dB (± 2dB) — unless it is planned that will be enclosed or placed in its own closet.

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Noise reduction techniques

Using absorptive materials (carpets, rugs, curtains) reduce the level of sound reflection and the overall level of noise in a room. On the other hand, reflective materials (tiles, concrete, glass, painted walls) may increase the level of noise in a room.

If all else fails, try sound insulation. Various techniques are available depending on the source of noise and the extend of the problem. A noisy disc drive in a PC may be insulated. A noisy projector may be placed inside a sound proof cabinet. Entire rooms can be insulated using rockwool or specialised insulation materials. Sound proof doors can be installed. In some cases double doors or double ceilings/floors need to be employed.

Note that low frequency (bass) noise is nearly impossible to eliminate in its totality.

If you are trying to keep the noise out (rather than keeping the noise in!) similar techniques apply.

Notes:
(1) J T Broch; Acoustic Noise Measurement, 1971.

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