Soundproofing FAQ'S - Common Questions About ...

What is a decibel (db)?

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A decibel is one unit on the decibel scale, which is a logarithmic scale. The name means one-tenth of a bel, a bel being an eponymous unit named after Alexander Graham Bell and used to compare power in electrical communication, voltage, or intensity of sound. The abbreviation of bel is B and decibel, dB. 10 dB = 1 B
Eighty-five decibels is the threshold for the possibility of noise-related hearing loss, and this guideline is intended to prevent such hearing loss. This figure suggests that many people who do not currently use ear protection should consider it. The following chart reveals that a great deal of the sound we&#;re exposed to is above that 85-decibel threshold. Because conditions may vary and distances are not specified, these figures are approximate.

 Decibels  Sound Source  0  low threshold of hearing &#; softest sound you can hear  10  leaves rustling in the breeze; quiet whisper  20  average whisper  20-50  quiet conversation  40-45  conversation between acts at a theatre; hotel lobby conversation  50  rainfall  50-65  loudish conversation  65-70  moderate traffic; hair dryer  65-90  train  75-80  factory (medium) &#; washing machine  90  heavy traffic &#; power lawn mower &#; busy city walk  90-100  thunder &#; walkman &#; tractor  100  boom box with volume turned high &#; chain saw &#; leaf blower  110  shouting; synphony concert  115  rock concert  120  ambulance siren  130  threshold of pain &#; loud fireworks &#; gunshot  140  airplane takeoff from short distance away  140-190  space rocket takeoff  170  shotgun

Acousticians use the dB scale for the following reasons:

1. Quantities of interest often exhibit such huge ranges of variation that a dB scale is more convenient than a linear scale. For example, sound pressure radiated by a submarine may vary by eight orders of magnitude depending on direction; expression in linear units carries with it the confusion of the location of the decimal point. Decibels values are characteristically between only -999 to +999.
2. The human ear interprets loudness more easily represented with a logarithmic scale than with a linear scale.

Amplitude measures how forceful the wave is. It is measured in decibels or dBA of sound pressure. 0 dBA is the softest level that a person can hear. Normal speaking voices are around 65 dBA. A rock concert can be about 120 dBA.
Sounds that are 85 dBA or above can permanently damage your ears. The more sound pressure a sound has, the less time it takes to cause damage. For example, a sound at 85 dBA may take as long at 8 hours to cause permanent damage, while a sound at 100 dBA can start damaging hair cells after only 30 minutes of listening.
Frequency is measured in the number of sound vibrations in one second. A healthy ear can hear sounds of very low frequency, 20 Hertz (or 20 cycles per second), to a very high frequency of 20,000 Hertz. The lowest A key on the piano is 27 Hertz. The middle C key on a piano creates a 262 Hertz tone. The highest key on the piano is Hertz.

As noise propagates (travels) from source to receiver, both its level (in dB) and its frequency content change.

There are a number of factors that affect the degree of change including distance, ground absorption, meteorological effects, and the extent of natural or constructed screening between the source and receiver.

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Distance

As a general rule, road noise levels tend to decrease by about 3 dB for each doubling of distance from the road (and increases by 3 dB for each halving of distance). In other words, the relationship between noise level and distance is non-linear. For example, moving from 10m to 40m from the road the reduction is approximately 6 dB, but if moving from 110m to 140m from the road (that is, also by 30m) the reduction is only 1 dB.

Ground absorption

The type of ground cover between a noise source and a receiver can have an additional large effect on the noise level. Acoustically &#;hard&#; surfaces, such as car parks or lakes, reflect sound and will not normally reduce the noise level. By contrast, acoustically &#;soft&#; surfaces, such as loose dirt, grassland, or bush, can provide a quite a large noise level reduction (in the region of -5 dB over 50m).

Meteorological conditions

Meteorological conditions (wind, temperature, humidity and rain) can either increase or decrease the propagation of noise. These conditions have little influence close to the road, but become very significant over long propagation distances (say beyond 100 metres). Downwind propagation of noise and temperature inversions can improve propagation, and increase noise levels. Being upwind from a road can reduce the noise level. Given the lack of control over meteorological conditions, most noise models assume moderately enhanced propagation (eg downwind) as the basis for prediction. NZS : provides further detail on this subject.

NZS : Acoustics &#; measurement of environmental sound(external link)

Screening

The topography and natural terrain between a source and receiver can act as a shield or screen. This reduces noise levels by restricting the direct transmission of noise and by absorbing some of the noise. Features such as buildings, barriers, walls, embankments and bunds can also provide screening from noise in the same way as natural features. Depending on the alignment of a road and the site geometry, the first row of houses or buildings next to a road may offer screening from noise to second and subsequent rows.

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