glossary & definitions
- airborne noise
- decibel (db)
- impact noise
- sound levels & their perception
- sound transmission class (stc)
- weighted sound reduction index (rw)
The Association of Australasian Acoustical Consultants. The AAAC is the body representing acoustic professionals and sets the benchmark for high professional standards and acoustic performance through its complementary star-rating system.
Airborne noise comes from common sound sources such as voices, TVs and radios. The noise performance of a building structure is called the Sound Transmission Class (STC). The higher the STC the better the structure is at isolating airborne noise. An STC rating of 45 means that the sound passing through the building is reduced by 45dB.
The decibel measures sound pressure or electrical pressure (voltage) levels. It is a logarithmic unit that describes a ratio of two intensities, such as two different sound pressures, two different voltages, and so
A decibel (dB) is the unit used to measure how loud a sound is. It starts at 0 – almost total silence. A decibel scale isn’t linear however. Going from 50dB to 100dB is not twice as loud but 16 times as loud; and from 60dB to 30dB is not half as loud but one-eighth as loud. Sound becomes dangerous to our hearing when it reaches certain levels. Regular exposure for more than one minute to 110dB for instance, risks permanent hearing loss. And prolonged exposure to any noise at or above 85dB can cause gradual hearing loss.
Frequency is the pitch of a sound and it’s measured in a unit called Hertz. Humans have a frequency range of hearing from 20Hz (low) to 20,000 Hz (high). Frequencies beyond this range exist, but they are inaudible to us.
Impact noise is one of the most intrusive and frustrating types of noise. You’ll usually hear it from the floor above you. It could be footsteps and shoes, a chair sliding across a wood or tile floor, or something falling on the floor. Impact noise travels freely through a structure and through air pockets.
LnTw is the weighted, standardised impact sound pressure level of a floor/ceiling assembly. The lower the LnTw, the better the acoustic performance.
LnTw+Ci is the combined weighted, standard plus spectrum adaption term measures the impact sound insulation performance of floor and ceiling systems. The lower the LnTw+Ci, the better the acoustic performance.
Unwanted sound – what can be a pleasant sound to one person can be an unwanted noise to another.
Sound is energy made by vibrations, that travel through the air and can be heard when they reach the ear.
sound levels & their perception
Sound is perceived in different ways by every individual person, so what sounds loud to you may be quiet to others. Our perception of what constitutes as ‘noise’ is affected by subjective factors. These include the type of noise (one person’s music might be another person’s noise), our mood, the time of day, background noise levels and our expectations. Sudden noises such as a motorbike exhaust or screeching brakes can be more disturbing than steady or expected noises. Frequency of noise may also have different impacts.
sound transmission class (stc)
The noise performance of a building structure is called the Sound Transmission Class (STC). The higher the STC the better the structure is at isolating airborne noise. An STC rating of 45 means that the sound passing through the building is reduced by 45dB.
sound transmission loss
How much a sound reduces when it passes through a sound barrier, measured in decibels.
weighted sound reduction index (rw)
Rw is a number which measures the airborne sound insulation of a wall, partition or ceiling, adjusted and measured to reflect different noises such as human voice noise or low frequency road traffic noise. The weighted sound reduction index (Rw) is normally similar or slightly lower number than the sound transmission class (STC) rating value.