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Soundproof a Room Stage 1 Part 2

How sound-proof is "sound-proof"?

The amount of acoustic isolation required before a room can accurately be described as "sound-proof" has been the subject of much discussion. The original way of describing the acoustic isolation provided by a particular construction was to quote a table of transmission loss (TL) values (measured in decibels). These values describe the amount of sound energy lost during transmission at various frequencies. From a table of TL values, it is easy to see how well building materials will perform when trying to contain low, mid and high frequency sounds. The typical situation is that a material (or wall construction) may be very good at preventing the transmission of high frequency sound, but be poorer at preventing the transmission of low frequency sound.

Many manufacturers objected to printing tables of "scientific" information about the TL of their products, claiming that people would not understand this information. They wanted to reduce the table of TL values down to a single number, thereby making the information easier to understand. There is some truth to this argument, but unfortunately it's also a double-edged sword. If you simplify TL information down to a single figure, then it is also possible to hide the fact that materials may have widely differing performance at different frequencies (particularly low frequencies).

Nonetheless, an average TL value was developed known as the Sound Transmission Class (STC) which manufacturers now prefer to quote instead of printing tables of frequency specific TL values. This does simply the information, but it can also hide potential problems.

There are several important points about STC values that must be clearly understood:

  1. The motivation behind the design of the STC calculations was to provide a single value that represents the material's transmission loss. The STC calculation results in an average sound level drop in decibels taken across sixteen, one-third octave bands. The bigger the number, the greater the transmission loss, and therefore the better the wall is at blocking the transmission of sound.
    The only thing spoiling the party here is that a material's TL can vary greatly at different frequencies. Materials like plasterboard (drywall) are very good at preventing the transmission of high frequency sound, but are poorer at preventing the transmission of low frequency sound.
  2. Since we are talking about an average TL, it is often the case that the wall construction could perform acceptably well at one frequency, but unacceptably at another (typically lower) frequency.
    The practical realities of controlling sound transmission mean that you will want to block sounds of all frequencies from escaping from your room. Therefore, it must be clearly understood that just because a particular wall construction has a quoted STC value of 54dB, does not mean that you will achieve a 54dB TL for sound at both low frequency (e.g. 120Hz) and a higher frequency (e.g. 4KHz).
  3. In order to derive an STC value for a wall construction, a representative sample of the wall is constructed in a test chamber and measurements are taken to determine the amount of sound transmitted. The results of these tests vary greatly depending on the exact construction used, how the sample wall was mounted in the test chamber, and even the test chamber construction itself.

The point here is that you should not read too much into an STC value. You should regard an STC value like the picture on the front of a box of breakfast cereal. The picture presents the cereal in wonderfully glamorous surroundings, but underneath, in small text are written the words "Serving suggestion".

An STC value is much like that, a suggestion of the wall's transmission loss capabilities.

Probably the most rigorous set of testing ever performed on the transmission loss of gypsum clad wall constructions was done in 1998 by the Canadian Institute for Research in Construction. They have produced a large report that now acts as a master reference for TL data for 350 different types of wall construction. This paper can be downloaded from here

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