Building Acoustics – theory

Sound in a room

Assume that we put our sound level meter in a fixed position in a room and that we start a sound source. The sound source could be a machine or a combination of a noise generator, a power amplifier and a loudspeaker or anything else making enough noise to make useful measurements.

What we will observe now is that the sound pressure level in the room will not rise indefinitely as the sound source continues to “pour” noise energy into the room. Instead, the sound level stabilises. Why is that?

Simply because the rate of sound energy input to the room is exactly compensated by the rate of energy absorbed in the room.

To be accurate, the absorbed energy consists of two parts; the energy actually absorbed by the room boundaries (the walls, ceiling and floor) and the energy transmitted through the boundaries into other rooms or out in the open.

Consequently,the sound level in the room will be reduced if more absorbing materials are brought into the room or more energy is transmitted through the boundaries.

We can increase the amount of absorption by bringing in carpets, soft furniture, curtains etc.

A very effective way of increasing the amount of transmission through the boundaries is simply to open a window!

Sound insulation

In building acoustics where we concentrate on the noise level due to what goes on in adjacent rooms (or outside) an important parameter is the sound insulation.

Values in building acoustics should be normalised and objective, i.e. independent of the current conditions (amount of absorption) of the room. Primarily we want to characterise a wall’s ability to insulate rather than the current sound level in the receiving room. This is very much because most countries have regulations on how good the insulation at least should be.

We are here talking about the source room – which is where the noise originates – and the receiving room – where we measure the amount of noise coming through the wall.

By correcting the measured level difference in a certain way, we compensate for the effect that the reverberation time has on the sound level in the receiving room.

Note that many buildings have homogenous structures of low loss factors, typically solid concrete walls. In such constructions sound energy is transmitted with very little attenuation. This calls for impact sound insulation measurements in addition to the airborne sound insulation.

Level difference measurements

When measuring airborne sound insulation between two rooms, we normally do as follows:

One of the rooms is defined as the source room. In this room we put a signal generator/power amplifier/loudspeaker combination and at least one microphone connected to our analysing system.
If we use one microphone only, we must make several level measurements at different positions about the room to calculate a spatially averaged sound pressure level. The loudspeaker must also be used in at least two positions otherwise we cannot guarantee that the measured sending room level is representative for the sending room. The other room is defined as the receiving room. In here we put at least one microphone which we move about the room to calculate a spatially averaged level even here. The excitation signal used is normally white or pink noise, used broad band or in octaves/third-octaves. Pink noise is a broadband noise with a spectrum whose level changes by -3dB/octave as the frequency increases. To explain why we use pink noise let us first look at White noise which is the term for a broadband noise signal whose spectral desity does not vary with the frequency. It contains all frequencies, like light reflected from a white coloured surface.

White noise is said to have a flat spectrum. Since the bandwidth of an octave (or third octave) becomes progressively wider by doubling the width for each octave, the use of white noise will cause the total energy content of a given octave to be the double of the energy content of the octave preceding it.

If we instead use pink noise we reduce the energy by a factor 2 for each octave. This is because -3dB corresponds to halving the energy. The amount of energy per octave will then remain constant since 2×0.5=1.

Red noise has a spectrum falling off by -6dB/octave as the frequency increases. The pink noise is called pink because it represents a spectrum between white noise.

In the receiving room we measure the sound pressure level in octaves or third-octaves, depending on the Standards applicable to our measurement.
Sometimes, the wall has so good insulation properties that the sound pressure level measured in the receiving room remains buried in the background noise of the receiving room. In these cases the source room level should be increased.
However, sometimes the level needed will be prohibitively high for practical loudspeakers. One way to circumvent this is by exciting the sending room with bandpass-filtered noise instead of broadband noise. The loudspeaker’s maximum permissive output level can then be spent on a single frequency band, which will result in an improvement of 10-15dB which may be what is needed to achieve successful results.

This will require instrumentation capable of making serial analysis. Needless to say, perhaps, that most Norsonic building acoustics instrumentation has been designed with this in mind.

The measured receiving room level is subtracted from the source room level and a correction for the receiving room reverberation time is then made. This latter correction is because we want to characterise the sound insulation between two rooms independent of the receiving room’s furniture condition.

ISO 18233 describes the requirements and guidelines for the use of new measurement methods in building acoustic measurements. We can learn more about that on the respective sections.