Fundamentals of noise control

Measurement

Sound in air is measured using a microphone that measures sound pressure. This sound pressure measurement is presented as a sound level in decibels (dB).

The decibel scale is logarithmic; each time the sound pressure doubles, the level is increased by 6 dB. The decibel is a scale and to be used as a unit it needs a reference value.

For sound in air, we use 20 micropascals (µPa) as a reference to equal 0 dB. This value was chosen because it is the average threshold of human hearing at 1 kilohertz (kHz).

Humans are normally able to detect sound frequencies from about 16 to 20 kHz, with reduced sensitivities at the low- and high-frequency ends.

The A-weighting curve was introduced to try to take this frequency-dependent sensitivity into account. We thus get A-weighted sound levels, which are denoted by dB(A).

The sound level in a typical open-office environment is generally around 45 to 50 dB(A), speech heard from a distance of 1 m is 55 to 60 dB(A), a truck heard at 3 m is 85 dB(A) and the threshold of pain is at 120 to 130 dB(A). It takes a change of at least 3 dB(A) to be noticeable for the ordinary person, with a 10 dB(A) increase sounding twice as loud for most frequencies.

How humans perceive sound is complicated, because there is quite a variation in sensitivity in the general population. For example, the threshold of hearing for most people is about 0 dB(A) on average, with about 95 percent of the population falling within a range of ±10 dB(A).

Much progress has been made to quantify the perception of sound. Average perceived loudness, for example, can be calculated following an International Organization for Standardization (ISO) standard to produce a number in units of ‘Sones’ – a more accurate representation of perceived loudness than dB(A), with a doubling in Sones indicating a doubling in perceived loudness.

To undertake an acoustic design, noise must be evaluated in greater detail than at an overall level. Noise is generally broken into octave, or one-third octave frequency bands for this analysis, with 50 hertz (Hz) to 5 kHz being the normal range of interest. A frequency analysis is necessary because the acoustic properties of materials vary with frequency. As a rule, the lower the frequency the less sound reduction a material achieves.

Source: Timber Design Guide, 2007

Airborne sound

Sound transmission can be divided into two types of sources: airborne sound sources and impact sound sources.

Airborne sound sources are those that transmit sound energy through the air to a partition. Impact sound sources transmit sound energy through direct contact with a structure.

In both cases, after passing through a partition, the sound energy is radiated into the air to be heard. Sources of airborne sound include speech and music, and sources of impact sound include footsteps and door slams.

The insulation of noise generated by airborne sound sources is known as airborne sound insulation, and insulation of noise generated by impact sound sources is known as impact sound insulation.