UNIVERSITY OF BATH DEPARTMENT OF ARCHITECTURE AND CIVIL ENGINEERING

Building Environment 1. Acoustics in buildings basics

Architectural acoustics is concerned with how buildings influence sound we hear with our ears. It involves an understanding of sound waves, the way these interact with the elements of buildings and the way our ears interpret the sound that falls on them.

Acoustics is often considered difficult but the difficulties are not due to mathematics but that the concepts initially seem to be rather foreign. Part of the problem comes from the sophistication of our hearing system that isolates what it requires, sometimes concealing the simpler realities.

Acoustics is particularly important in the following situations:

Acoustics within auditoria: music spaces, drama theatres, communal halls, lecture rooms etc.

Acoustics within large spaces: enclosed public spaces, sports halls, swimming pools etc.

Acoustics of schools, both within and between teaching spaces

Ingress of sound into residences from transport routes, especially road traffic noise

Noise travelling between adjacent dwellings, namely via party walls and floors

Selection of partitions within larger buildings such as offices to give adequate privacy

Architectural acoustics can be divided into two parts: the first 2 points above fall under the category of internal acoustics of rooms, whereas the remainder constitute noise control. This brief course touches on both these parts.

Acoustics basics

Sound is a wave motion which involves movement of air particles. Other examples of waves are on the surface of liquids and electro-magnetic waves, which includes light. An intriguing aspect of waves is that they exhibit similar characteristics. Waves involve energy travelling through a medium which returns to its original condition after the wave has passed. Sound is an example of a longitudinal wave motion because the air particles move in the same direction as the wave.

We are all familiar with frequency: high pitched sounds have high frequencies, bass sounds low frequencies. The frequency range we can hear is between 20Hz and 20,000Hz (Hz is short of Hertz, which is the same as cycles/second). In practice we can generally concern ourselves with a much smaller frequency range, typically 50 7,000 Hz.

Nearly all sounds we hear are a mixture of many different frequencies. Traffic noise is a good example, you cannot sing a note to most traffic noise, it is a mixture of all audible frequencies, with a slight bias towards low frequencies. It also has the complication that it fluctuates with time; in practice this is treated by simply averaging traffic noise over time. A neutral noise containing all frequencies is often called white or pink noise, which has a virtually constant loudness; falling water makes a noise like this.

The fundamental frequency interval is an octave (8 notes on a piano). If a man and a woman sing the same tune, they usually sing an octave apart. An octave interval is a

ratio of frequencies of 2:1. Because the fundamental step is a ratio, we are in fact dealing with a logarithmic frequency scale (as opposed to a linear one).

To present the frequency content of a sound, what is known as the

spectrum, we can divide up the full frequency range into octaves. Octave centre frequencies are based around 1000 Hz, as follows: 125, 250, 500, 1k, 2k, 4kHz etc. The 1000 Hz octave extends

between 707 and 1414 Hz (between 1/2 and 2 times the centre frequency). See a measured octave spectrum of traffic noise:

The speed of sound is constant, influenced slightly by temperature. At 20C the speed is 343m/s.

All types of wave are also described by wavelength, related directly to frequency and the speed of sound (Speed = Frequency x wavelength). The wavelengths of audible sound are 17m to 17mm. These values are similar to sizes of walls and objects in buildings, which means that sound can travel easily round corners. Wavelengths for light are very short, so acoustic behaviour operates on a much larger scale, as mentioned later for acoustic reflectors.

important range

2

The easiest way to quantify the magnitude of sound is in terms of the acoustic pressure, this is the size of pressure fluctuations about atmospheric pressure. Our ears can hear pressure fluctuations between 2.10

-5

and 20 N/m2, which is a vast range (one to a million) that would be inconvenient to have to use all the time.

In practice, our ears interpret this range logarithmically, which leads to the decibel (dB). The above pressure range corresponds to 0 to 120 dB, a much more manageable set of numbers. The magnitude of sound in

decibels is called the sound pressure level.

Sound pressure levels of common noises

dBA THRESHOLD OF PAIN 120 Disco noise 105 Full orchestra in concert hall, loud passage 95 Busy city road traffic noise 75 Loud speech at 1m 70 Conversational speech at 1m 50 Attentive theatre audience 40 Faint whisper at 1m 30

THRESHOLD OF HEARING (at 1kHz) 0dB

In fact, the decibel can be used in two ways: above it is as a measure of magnitude, but it can also be used for a difference, as occurs for instance when sound passes through a partition (as discussed under sound insulation in this course). Another example of the decibel used for difference measurements would be for the efficiency of a car silencer.

The quietest sound we can hear is known as the

threshold of hearing. This threshold significantly varies with frequency; we are much less sensitive to low frequencies. This makes sense because otherwise we would be able to hear our heart beat and blood flow in our heads. At the other extreme, at around 120 dB we feel pain in the ears; sound levels as high as this can cause permanent damage to the ears.

We use the word loudness to represent how loud we hear sound to be. We want a number to

represent how loud we judge a particular sound, but this needs to take account of our different sensitivity to different frequencies. This quantity is similar to the simple sound pressure level in

decibels, but includes an A-weighting filter to give a unit dBA. Note that in the table above, values are actually in dBA, except for the threshold of hearing which is quoted for a single frequency.

We can also derive the dBA for an octave spectrum using standardised correction figures for each octave.

Frequency (Hz): 63 125 250 500 1000 2000 4000 Hz

True spectrum 78 69 59 58 59 55 48

dBA correction -26 -16 -9 -3 0 +1 +1

dBA spectrum 52 53 50 55 59 56 49

To complete the calculation, we need to add together the

contributions of each octave. This requires decibel addition, for which the figure to the right is used. Decibels do not add linearly, two sounds of 60 dB together do not give a 120 dB sound (see table above!). In fact, two sounds of 60 dB give only 63 dB.

For the dBA value, we use the figure to add together pairs of dB values. Starting with the smallest:

(49) + (50) = 52.5 dB and repeating the process until we

have a single number, the loudness for this traffic noise is 63 dBA.

Procedure for combining two sound levels

0 2 4 6 8 10

+3

+2

+1

0

Difference between the two levels in dB

Add to the higher level dB