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 2½ 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 20°C 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