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Following Sound Transmission

In measuring what happens to sound, several key terms need to be understood. They are: reverberation, reflection, sound transmission class, noise reduction coefficient, Mass Law and Limp Mass Law.

Reverberation is the prolongation of a sound in an enclosed space caused by continued reflections of the sound after the source has stopped emitting energy. Different spaces have different reverberation times. A high reverberation time, such as that caused by highly reflective room surfaces, will cause a build-up of noise level in that space.

Reflection refers to sound that strikes a surface or several surfaces before reaching the receiver. The shape of a space and the material on the surfaces affect reflection. Reflective corners or peaked ceilings can cause annoying reflections and create loud spaces. Parallel surfaces can cause standing waves of sound between them. Domes and concave surfaces cause reflections to be focused rather than dispersed. Absorptive surfaces can help eliminate reverberation and reflection problems.

Sound Transmission Class (STC) is a single-number rating of the barrier effect of a material or its assembly. Higher STC values indicate more efficiency in reducing sound transmission. The rating assesses airborne sound transmission at a range consistent with the frequency range of speech and does not assess low frequency sound transfer. Because of this, special attention should be given to spaces where noise transfer concern is other than speech, such as mechanical equipment, music, and transportation equipment sounds.

Additionally, any penetration, air gap or flanking path can seriously degrade the sound abatement on walls with a high STC rating. Flanking paths are a means for sound to transfer from one space to another by traveling through something other than walls and floors, such as ductwork, plumbing or corridors.

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Noise Reduction Coefficient (NRC) is a single number index for rating the absorption level of a material. The standard is the average of the mid-frequency sound absorption coefficient and is unrelated to a material’s STC rating.

Mass Law is the principle of physics that explains transmission of noise from one area to another. It states that the more mass or weight a sound wave must move to create vibration transfer, the greater the noise energy.

Limp Mass Law states that higher density plus more flexibility equals greater reduction of sound transmission. The stiffness of a material can reduce the effectiveness of mass to slow down the transfer of sound, so very thin, stiff materials would have a very high vibration transfer. Thus, the ideal noise barrier would be heavy and flexible. Only following this law would lead one to believe glass is not the ideal material to insulate against sound because of its lighter weight and stiffness. However, one must also recognize that components of windows and doors, particularly the glass, can be modified to reduce sound transmission.