Home Soundproof Garage - Garage Conversion Stage 1 Soundproof a Room Stage 1 Part 6
Soundproof a Room Stage 1 Part 6

Reverberation Time

As was mentioned above, one of the key properties of a room's acoustical performance is the length of time it takes for the sound energy to die away. We need to add some more detail here; namely, how do we decide when the sound has "died away"? Is it half the volume it started at, or a third or a quarter?

The answer here is that the sound level must drop by a predetermined amount of 60 decibels (dB). The time taken for the sound to die away by this much is known as the reverberation time for 60dB or simply the RT60. This is the commonest designation for RT, but you may also find RT25 figures quoted. So when you see an RT60 time quoted, you know that the room has sufficient absorbency in it to remove 60dB of sound energy in the stated time. Remember however, that a material's absorbency is frequency specific. Therefore, because absorbency of a room's surfaces controls the reverberation time, that reverberation time is also frequency specific. This means that when you calculate the RT60 for a room, you will end up with a table of values similar to the one shown above: except that it will quote times and not absorption values.

Since a room will have a range of reverberation times, a convention has developed in which the room's overall reverberation time is taken to be represented by the RT60 value at 500Hz.

For example, the reverberation time for a small music practice room of say, 2.5m x 3.5m x 2m (W x L x H) should be between 0.2 and 0.3 seconds. This range of RT60 values will vary depending on the instrument being used. The range quoted here is appropriate for drums and brass, but if you play a stringed instrument, you will probably want a longer RT60 time of say 0.5 to 0.7 seconds. (These figures are only suggested values!)

In order to achieve the required RT60 times, absorption should be applied to the inner walls of the practice room. A method for doing this will be demonstrated in the subsequent pages.

If you'd like to investigate the reverberation time of your practice room more thoroughly, then you should download my Control Room Calculator spreadsheet. Please read the instructions on the download page carefully.

A technical quirk with absorbency

If you start looking at the absorbency figures quoted by manufacturers, you may well see absorbency values greater than 1! Many people ask "How is it possible for a material to absorb more energy than is present in the sound wave?".

Good question.

The first thing to point out is that absorption figures are usually derived from tests performed in reverberation chambers. A standard sized sample of the material is mounted according to particular criteria to the floor of the reverberation chamber, and then the absorption tests are carried out. The sample is not mounted flush to the floor, so therefore, the edges of the sample exert an effect on the test results.

The reason people think that absorption values greater than 1 are nonsensical comes from the assumption that sound waves loose energy due to absorption alone. However, energy can be lost from other effects such as diffraction (which is also known as the "edge effect").

Diffraction occurs either when a sound wave is travelling through an area of restricted width and then encounters an area of expanded width (such as travelling through a doorway from one room to another); or, when sound encounters the outside edge or corner of an object. Either way, diffraction causes the path of the sound wave to change direction, and in the process absorbs some energy.

Any time a moving object (in this case, an air particle) changes direction, then a force must be exerted upon that object. The change of direction that occurs during diffraction requires a change of energy, and that energy comes from the sound wave itself. So it appears that the material has actually soaked more energy than can be accounted for simply by absorption alone.

Opinion is divided as to how absorption values >1 should be handled. Some manufacturers print the values exactly as measured from the test chamber (which could result in values greater than 1); and other manufacturers choose to round absorbency values greater than 1 down to 1.

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