level tends to concentrate in the low to mid frequency range, typically 125Hz to 2,000Hz. In addition, there is low frequency structure vibration related noise.
The ratings for classifying and comparing the various sound-proofing materials are:
- NRC, the noise reduction coefficient, which is for absorbers. The NRC rating is between 0 and 1, and its an average of how absorptive a material can be at these four frequencies: 250, 500, 1000 and 2000. NRC measures how well materials stop sound from reflecting, as a percentage of sound that a surface absorbs.
- STC, the sound transmission
class, which is for blockers. STC is a measure of how well a material blocks sound. The higher the rating the better. You can improve the STC of a wall by building it from a more dense material (sound insulation improves by about 5 decibels for every doubling of mass), by adding an air gap, or by adding sound absorbing material.
- IIC: In addition to the STC rating, which concerns airborne sound, floor and ceiling assemblies are also rated for impact sound resistance. Impact sound is structure-borne sound transmitted, when one body strikes another, such as in the case of footsteps and falling objects.
SRI: In countries outside the North America, SRI (Sound Reduction Index), describing an improvement of so many decibels (dB) sound reduction, is a more common measurement.
See also “Taming Engine Room Noise”
~ By Darrell Nicholson in ‘Practical Sailor’