4000 Hz after 10 years' exposure correlates with TTS after a few days' exposure. Prevention of noise-induced hearing loss by noise reduction. OR sound may be used to mask noise e.g. musak, music, or sound conditioning. Care must be taken to ensure that the mounting and the machinery do not resonate. Sounding board modifications - if vibration cannot be isolated then board modifications must be made e.g. cut board into several pieces, each smaller than wavelength of most undesirable spectral component of the noise, coating the board with vibration absorbing material.
Sound energy is transformed into heat energy by passing waves through soft fibrous materials.
Energy is lost through the frictional interaction of fibres. The problem with absorption treatments is that the sound must pass through a great deal of such materials before significantly decreasing sound energy. Absorption treatments usually more effective against high frequency than low frequency. BUT in modern offices often have problems with false ceilings. Such barriers must be made of material thick enough to prevent any sound transmission through it.