Sound Waves 101 What are they? (then new section)How best to limit their transmission?

Like the ripples formed when a rock is thrown into a pond, sound travels in waves in all directions from a point of origin…the distance between the peaks in the ripples relate to the frequency of a sound – the larger the distance, the lower the frequency of the sound.

The sound that travels through the air is a pressure wave that bumps the air molecules in an expanding sphere (moving outward like ripples on a pond) until it encounters a surface. If that surface is your eardrum, the sound wave excites the eardrum which eventually the ear sends signals to the brain that allows us to perceive the sound.

In architecture, when the sound encounters a wall or other surface it may be reflected, absorbed, and/or transmitted. That is, some energy comes back (reflected), some is turned into heat (absorbed), and some continues through (transmitted). See Figure x1.

Different types of sounds each contain a different combination of frequencies – i.e. speech, traffic, equipment fans and motors, aircraft, music, etc.  Each has its own frequency signature; in the previous section note that in figure x2 the male and female voice have different “signatures” that we can perceive with our own ears–not only can we usually tell between male and female, but sometimes we know who it actually is!

Impact sound is a special case where the waves or vibrations originate from physical impact with a surface like a floor or a wall (footsteps, moving furniture, etc.). The vibrating wall becomes a loudspeaker that broadcasts the impact as audible sound.

new section: how to limit transfer

Controlling each type of sound (frequency signature) requires a special strategy.

Transmission of unwanted sounds through an assembly can be reduced with appropriate  floor / ceiling and wall details. They are designed to dissipate and / or  limit the passage of airborne or impact sound energy by using (for instance) additional mass, staggering double wall studs or adding attenuation blankets (for these and other examples see section “Simple Initial Mitigation Strategies).

Reflection and absorption of sound energy inside of a room is determined by room finishes; absorption utilizing acoustical finishes (i.e. acoustical tile or wall panels) can reduce the sound level in a room before it can be transmitted to adjacent spaces.

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