Measure Sound?

Introduction This booklet gives answers to some of the basic questions asked by the newcomer to a noise measuring programme. It gives a brief explanation to questions like: What is sound? Why do we measure sound? What units do we use? How do we hear? What instruments do we use for measurement? What is a weighting network? What is frequency analysis? What is noise dose? How does sound propagate? Where should we make our measurements? How does the environment influence measurements? How should the microphone be positioned in the sound field? How do we make a measurement report? What do we do when levels are too high? Revision September 1984 1 Sound and the Human Being Sound is such a common part of everyday life that we rarely appreciate all of its functions. It provides enjoy- able experiences such as listening to music or to the singing of birds. It enables spoken communication with family and friends. It can alert or warn us – for example with the ringing of a telephone, or a wailing siren. Sound also permits us to make quality evaluations and diagnoses – the chattering valves of a car, a squeak- ing wheel, or a heart murmur. Yet, too often in our modern society, sound annoys us. Many sounds are unpleasant or unwanted – these are called noise. However, the level of annoyance depends not only on the quality of the sound, but also our atti- tude towards it. The sound of his new jet aircraft tak- ing off may be music to the ears of the design engi- neer, but will be ear-splitting agony for the people liv- ing near the end of the runway. But sound doesn’t need to be loud to annoy. A creaking floor, a scratch on a record, or a dripping tap can be just as annoying as loud thunder. Worst of all, sound can damage and destroy. A sonic boom can shatter windows and shake plaster off walls. But the most unfortunate case is when sound damages the delicate mechanism designed to receive it – the human ear. 2 Why Measure Sound? Measurements provide definite quantities which describe and rate sounds. These measurements can provide benefits such as improved building acoustics and loudspeakers, thus increasing our enjoyment of music, both in the concert hall and at home. Sound measurements also permit precise, scientific analysis of annoying sounds. However, we must remem- ber that due to the physiological and psychological dif- ferences between individuals, the degree of annoyance cannot be scientifically measured for a given person. But the measurements do give us an objective means of comparing annoying sounds under different conditions. Sound measurements also give a clear indication of when a sound may cause damage to hearing and permit corrective measures to be taken. The degree of hearing damage can be determined by audiometry which measures a person’s hearing sensitivity. Thus, sound measurements are a vital part of hearing conservation programmes. Finally, measurement and analysis of sound is a power- ful diagnostic tool in noise reduction programmes – from airports, to factories, highways, homes and re- cording studios. It is a tool which can help to improve the quality of our lives. 3 Just What is Sound? Sound may be defined as any pressure variation (in air, water or other medium) that the human ear can detect. The most familiar instrument for measuring pressure variations in air is the barometer. However, the pressure variations which occur with changing weather conditions are much too slow for the human ear to detect – and hence do not meet our definition of sound. But, if variations in atmospheric pressure occur more rapidly – at least 20 times a second – they can be heard and hence are called sound. (A barometer cannot respond quickly enough and therefore cannot be used to measure sound). The number of pressure variations per second is called the frequency of the sound, and is measured in Hertz (Hz). The frequency of a sound produces its distinctive tone. Thus, the rumble of distant thunder has a low frequency, while a whistle has a high frequency. The normal range of hearing for a healthy young person ex- tends from approximately 20 Hz up to 20,000 Hz (or 20 kHz) while the range from the lowest to highest note of a piano is 27.5 Hz to 4186 Hz. These pressure variations travel through any elastic medium (such as air) from the source of the sound to the listener’s ears. You probably already have some idea of the speed of sound from the familiar rule for determin- ing how far away a thunder storm is: count 3 seconds per kilometre or 5 seconds per mile from the time you see the lightning until you hear the thunder. This time interval corresponds to a speed of sound in air of 1238 km/hour or 770 miles per hour. For acoustic and sound measurement purposes, this speed is expressed 4 as 344 metres per second at room temperature. Knowing the speed and frequency of a sound, we wavelengths and low frequency sounds have long wave- can calculate the wavelength – that is, the distance lengths. from one wave top or pressure peak to the next. A sound which has only one frequency is known as a pure tone. In practice pure tones are seldom encoun- Wavelength (λ) = Speed of sound tered and most sounds are made up of different fre- Frequency quencies. Even a single note on a piano has a complex waveform. Most industrial noise consists of a wide mix- From this equation we can work out the wavelength at ture of frequencies known as broadband noise. If the different frequencies. For example at 20 Hz one wave- noise has frequencies evenly distributed throughout the length is just over 17 metres, while at 20 kHz, it is only audible range it is known as white noise and it sounds 1.7cm. Thus, we see high frequency sounds have short rather like rushing water. 5 The dB The second main quantity used to describe a sound is the size or amplitude of the pressure fluctuations. The weakest sound a healthy human ear can detect has an amplitude of 20 millionths of a Pascal (20 µPa) – some 500,000,0000 times less than normal atmospheric pressure. A pressure change of 20 µPa is so small that it causes the eardrum to deflect a distance less than the diameter of a single hydrogen molecule. Amazingly, the ear can tolerate sound pressures more than a million times higher. Thus, if we measured sound in Pa, we would end up with some quite large, unmanageable numbers. To avoid this, another scale is used – the decibel or dB scale. The decibel is not an absolute unit of measurement. It is a ratio between a measured quantity and an agreed reference level. The dB scale is logarithmic and uses the hearing threshold of 20 µPa as the reference level. This is defined as 0 dB. When we multiply the sound pressure in Pa by 10, we add 20 dB to the dB level. So 200 µPa corresponds to 20 dB (re 20 µPa), 2000 µPa to 40 dB and so on. Thus, the dB scale compresses a range of a million into a range of only 120 dB. The sound pressure levels (SPL) in dB and Pa of various familiar sounds are shown in the figure. One useful aspect of the decibel scale is that it gives a much better approximation to the human perception of relative loudness than the Pascal scale. This is because the ear reacts to a logarithmic change in level, which corresponds to the decibel scale where 1 dB is the same relative change everywhere on the scale. 6 What do we Hear? We have already defined sound as any pressure variation which can be heard by a human ear. This means a range of frequencies from 20 Hz to 20 kHz for a young, healthy human ear. In terms of sound pressure level, audible sounds range from the threshold of hearing at 0 dB to the threshold of pain which can be over 130 dB. Although an increase of 6 dB represents a doubling of the sound pressure, an increase of about 10 dB is required before the sound subjectively appears to be twice as loud. (The smallest change we can hear is about 3 dB). The subjective or perceived loudness of a sound is determined by several complex factors. One such factor is that the human ear is not equally sensitive at all frequencies. It is most sensitive to sounds between 2 kHz and 5 kHz, and less sensitive at higher and lower frequencies. 7 To complicate things even more, this difference in sensitivity to different frequencies is more pronounced at low SPLs than at high SPLs. This can be seen in the figure, which shows a family of equal loudness contours. These indicate the sound pressure level required at any frequency in order to give the same apparent loudness as a 1 kHz tone. For example, a 50 Hz tone must be 15 dB higher than a 1 kHz tone at a level of 70 dB in order to give the same subjective loudness. Impulse sounds present another problem in loudness evaluation. If a sound is of short duration, that is, less than one second, it is termed an impulsive or impulse sound. Practical examples of impulse sounds are type- writer and hammering noises. Because of the short duration of such sounds, the ear is less sensitive perceiv- ing its loudness.

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