Frequently Asked Questions about Acoustics

Acoustic treatment is the control of reverberation and echo while soundproofing is the prevention of sound travelling from one room to another.

Some examples of acoustic treatment include putting foam panels on the wall or installing acoustic plaster tiles for the ceiling.

Whereas examples of soundproofing include installing isolators between 2 walls, or changing out a normal window to an STC-rated window.

Porous materials are effective in absorbing sound waves. Mineral infills like rockwool, fibreglass or earthwool are commonly used to absorb mid range to high frequency sound waves. Polyurethane or melamine foam are used to absorb sound as well.

Perforations in wooden panels or metal panels can also function as resonant absorbers. These resonant absorbers have narrow-band absorption, which means they do not absorb well in a full-range of frequencies, but only at the intended frequency.

These are acoustic measurements used to determine the effectiveness of a material used in either soundproofing or acoustic treatment.

NRC (Noise Reduction Coefficient) is a number used to measure the effectiveness of a material to absorb sound. It is between 0 – 1. The higher the number, the better the material absorbs the sound. An NRC of 0 means the material does not absorb any sound, whereas and NRC of 1 means the material absorbs 100% of the sound, similar to an open window to an infinite space outside. NRC is commonly used as a measure for materials used in acoustic treatment.

STC (Sound Transmission Class) is a measure of how well a partition (window, door, or wall) can block out noise travelling from one space to the next. The higher the STC number, the better the ability of the partition to block out the sound. For example, a door with STC 38 is good for typical office meeting rooms, whereas a door rated STC 52 is for use in a recording studio. The STC is a number used for partitions in soundproofing.

RT60 (Reverberation Time 60) is a measurement of how long it would take a sound to decay 60dB. It is used to measure the amount of echo or reverberation in a room. The higher the RT60 number, the more reverberant the room. A non-treated hall can have an RT60 time of 4 seconds. After installing acoustic panels on all wall surfaces, the RT60 time can be brought down to a more comfortable 1.2 – 1.8 seconds. RT60 is a measurement used to determine the amount of reverberation in a space for acoustic treatment.

The quarter wavelength principle is a rule of thumb that acousticians use to decide the type of absorber to be used for absorbing an intended frequency. The quarter wavelength principle states that for an effective (100%) absorption of a particular frequency, the thickness of the substrate must be at least a a quarter of the wavelength of that frequency to be absorbed.

For example, to absorb 250 Hz, the thickness of the substrate used must be at least 300mm thick. The wavelength of 250 Hz is 1.2m. Take that divided by 4 and you get 0.3m or 300m. This sounds quite impractical as the thickness of a substrate required to absorb a low frequency can be as thick as 1m! Luckily, it is not imperative to have 100% absorption of the intended frequency for most acoustic treatment to be effective.

In the past, abestos was used for many insulation work in construction. Exposure to abestos causes scarring of the lungs, which leads to lung cancer. Today, it is banned in most parts of the world.

Luckily, the insulation used in present day is safe for health. Casual exposure to rockwool, fibreglass wool, and earth wool do not cause permanent health damage. However, workers who handle these insulation materials and have long-term exposure are still recommend to wear a mask and long sleeve clothing to prevent irritation to both the lungs and skin.