Sound Insulation

Sound Insulation is a measure of how good a building element is at reducing sound as it passes through that element. Materials that are good for thermal insulation are not good (on their own) for sound insulation -see the insulation/absorption section of this site.

In many countries the sound insulation performance of building elements is specified by law. In the UK the performance of floors and walls separating dwellings in new build and conversions is set out in Approved Document E of The Building Regulations; this document has been revised; the document itself can be read elsewhere.

The testing method was BS.2750 but is now ISO-140; however they are essentially similar, [pay attention now] for airborne sound tests (relevant to speech and television noise) -

set up a loud sound source (or two such sources) on one side of the building element to be tested.

over a range of 16 frequencies (1/3 octaves from 100 Hz to 3150Hz) measure how loud it is in the source room.

over the same range of 16 frequencies measure how loud it is on the other side of the building element to be tested - the receiver room.

if the background level of sound is high in the receiver room, it could affect the readings of sound coming from the source room; therefore measurements are made of background sound with the sound source off; again this is over the range of 16 frequencies.

similarly if the receiver room is very echoy (reverberant) the incoming noise would be louder than if the room were acoustically dead; therefore the reverberation time is measured over the same range of frequencies, and the receiver room levels are standardised to an arbitrary reverberation time of 0.5 seconds over all frequencies.

the sound insulation at each individual frequency is then obtained by subtracting the corrected receiver room level from the source level.

Using BS.5821 or ISO 717, to obtain a single figure rating [DnT’w for airborne or LnTw for impact] the curve of the individual values is compared against a reference curve in a particular manner, and the reference curve is re-plotted so that the deviation from the measurement curve is within given limits. The single figure rating is the value of the re-plotted reference curve at a given frequency.

The new Building Regulations require tests to be carried out using the above procedure, but the end results for airborne tests are subject to a spectrum adaptation term (Ctr) which is almost always negative. The spectrum adaptation term weights the results towards better low frequency performance, i.e. structures that are better at low frequencies, e.g. heavy masonry, tend to produce better results. The single figure rating is expressed as DnTw+Ctr.

For impact sound (footfalls) a specific design of machine taps or raps on the floor (the machine is unimaginatively known as a tapping or rapping machine). Measurements are only made in the receiver room but the procedure is very similar to above except for the need to set the machine up in a number of positions on each floor.

The procedures are "horribly" complex. If you need these tests, you need to have them made by a competent noise consultant [see directory].

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