Absorption classes A to E are described in EN ISO 11654. The classes are determined by comparing the weighted ‘αw’ values to a series of fixed reference curves. So an A class provides the best absorption ability and an E class provides the worst. The absorption materials used as well as the cavity behind the product can affect the overall result.
Materials and structures with the ability to absorb sound energy and convert it into other forms of energy. They improve room acoustics by removing sound reflections and therefore reducing the background noise and the reverberation time.
Airborne sound arises when sound waves from television, talking, music or traffic pass through the air and then vibrate the wall, floor or ceiling so that they pass through to the other side.
Airborne Sound Insulation
Airborne sound insulation refers to the ability of a building structure to reduce the airborne sound transmission through it. The sound insulation is measured at different frequencies, normally 100-3150 Hertz. Airborne sound insulation is expressed by a single value in decibels – DnT,w or Rw. Airborne noise can only be significantly reduced by the addition of a dense product with high mass, but it must also be soft. The elements must also be well sealed.
- DnT,w – The Weighted Standardised Field Level Difference
This is a single number measure of the airborne sound insulation performance of a building element so the higher the decibel number, the better the insulation performance. It is characterised by the difference in noise level on each side of a wall, floor or ceiling. Because it is an on-site test it will be subject to flanking transmission. The equivalent test carried out in a laboratory is denoted by Rw.
- DnT,w + Ctr
This measurement adjusts the DnT,w figure by including a low frequency Correction Factor (Ctr). Low frequency noise such as bass music or traffic carries more energy and so is more difficult to reduce. The Ctr figure is a negative number so the DnT,w + Ctr figure will always be lower than the DnT,w figure. The closer the Ctr number is to zero, the better the product is at reducing low frequency noise. Approved Document E in England and Wales requires the inclusion of the Low Frequency Correction Factor.
- Rw – The Weighted Sound Reduction Index
This is a single number, expressed in decibels, which refers to how well a building element such as a wall or floor provides airborne sound insulation. An Rw result means that the acoustic test was conducted in an acoustic testing laboratory. The higher the number, the better the airborne sound insulation.
- Rw + Ctr
This measurement adjusts the Rw figure by including a low frequency Correction Factor (Ctr). This test is carried out in the same way as DnT,w + Ctr above except that it takes places in a laboratory rather than on-site.
Attenuation occurs when the energy in the sound wave weakens as it passes through a material due to the combined effect of scattering and absorption. This reduces the noise.
The decibel (dB) is the unit used to measure the intensity of a sound. The decibel scale is so vast that it is a logarithmic scale. Our ears can hear everything from our fingertips brushing lightly together to a loud jet engine. In terms of power, the sound of the jet engine is about 1 trillion times more powerful than the smallest audible sound.
On the decibel scale, the smallest audible sound which is near total silence is 0 dB. A sound 10 times more powerful is 10 dB. A sound 1,000,000 times more powerful than near total silence is 60 dB – normal conversation sound. This acoustic terms glossary gives some common sounds and their decibel ratings:
- Near total silence – 0 dB
- A low whisper – 15 dB
- Normal conversation – 60 dB
- A motorbike – 100 dB
- A rock concert or a jet engine – 120 dB
Perception of Increases in Decibel Level
- 1 dB – Imperceptible Change
- 3 dB – Barely Perceptible Change
- 5 dB – Clearly Noticeable Change
- 10 dB – About Twice as Loud
- 20 dB – About Four Times as Loud
Flanking transmission is the transfer of sound through paths around a building element, rather than through the building element itself e.g. noise can travel down a common wall in an apartment block as well as through the separating floor. Flanking noise can occur through indirect paths due to pipework, ducting or ceiling and wall cavities, as well as due to incorrectly sealed junctions or gaps in the building structure. Flanking sound is always present but it must be considered at design stage to reduce it significantly.
Occurs when noise bounces between parallel surfaces in a room.
Frequency – Hz
The frequency is the number of sound waves to pass a point in one second and it is expressed in Hertz (Hz). Screeching or birds chirping are examples of high frequency sound and music subwoofers or traffic noise are examples of low frequency sound. Low frequency noise is more difficult to reduce than high frequency noise as it carries more energy.
Impact noise is caused by impact or collision on the floors or walls of a connecting room. Typical sources of impact noise are footsteps, dropped items or dragging furniture on the floor above a dwelling.
Impact Sound Insulation
Impact sound insulation refers to the ability of a product or building floor structure to reduce the impact or footstep sound transmission through it. Impact sound insulation is expressed by a single value in decibels – L’ nT,w or Ln,w.
- L’nT,w – Weighted Standardised Field Impact Sound Pressure Level.
This is a measure of the impact sound performance of a floor. It is an on-site or field test that measures how much impact sound reaches the receiving room below through the floor when a tapping machine is used on the floor above. Because it is an on-site test it will be subject to flanking transmission. The lower the number, the better the performance.
- Ln,w – Weighted Normalised Impact Sound Pressure Level.
This is the equivalent test to that above except that is undertaken in a laboratory and so will not be subject to flanking transmission.
Noise is unwanted sound. Noise can often be the individual perception of a particular sound, e.g. a background noise.
Reverberation Time – T or RT
Reverberation Time is the length of time it takes for a sound event to become inaudible and it is measured in seconds. Measuring the reverberation time allows us to calculate the total sound absorption. The reverberation time varies according to the frequency.
Sound Transmission Class – STC
An American system giving a single number rating of a partition’s isolation value based on a laboratory measurement. Results may not be compatible with Rw as a different range of frequencies is used.