The Ear Audible Sound

122 13 NOISE

13.1 Background

It has become common practice to define noise, as unwanted sound and it has been known for many years as a cause of hearing loss in industry. So what exactly is sound and how do we hear it? Sound is the sensation that is perceived by the human or animal brain as a result of longitudinal vibrations of molecules of the air impinging on the ear. Sounds are actually pressure waves caused by a vibrating body, which radiate from the source. The human ear can sense and perceive small and rapid pressure waves as sound noise and convey information on their size amplitude and frequency to the brain.

13.2 The Ear

Source: Wikimedia Commons Figure 13.1 - A simplified diagram of the human ear 123 The external ear, i.e. the part we can see, receives the pressure waves and passes them along the auditory canal to a membrane - the eardrum, which is situated just inside the skull for protection. The eardrum vibrates in response to the sound pressure waves and this vibration is transmitted through the 3 small bones of the middle ear malleus, incus and stapes hammer, anvil and stirrup to another membrane, the oval window of the inner ear. The middle ear also contains the eustachian tube, which provides an opening to the throat and so maintains the middle ear at atmospheric pressure. This pressure equalisation is necessary because the eardrum is required to respond to rapid, small fluctuations in pressure, not to absolute pressure. The oval window in turn passes the vibrations on to the cochlea, a snail shaped organ containing liquid and some 25,000 receptive cells nerve endings. The vibrations generate pressure waves in the liquid of the cochlea, and these stimulate the nerve endings which transmit corresponding electrical signals to the brain. Each receptive cell has its own pitch response and thus is able to analyse and separate out a mixture of incoming signals into their individual frequency components. This facility enables the human ear to identify individual notes amongst the incoming volley of sound.

13.3 Audible Sound

Two key features of sound are frequency and intensity. The number of pressure wavesvibrations per second is known as the frequency, and is expressed in the unit Hertz Hz, the more fluctuations per second the higher the pitch of the sound. The frequency range of the human ear is normally quoted as being between 20 Hz and 20,000Hz 20 KHz. Middle C in music is at approximately 260 Hz musicians opinions vary between 255 – 278 Hz, and doubling the frequency raises the pitch one octave, hence the octave above middle C 260 Hz has a frequency of 520 Hz. By intensity I we mean the amplitude size of the pressure waves and is defined as the average amount of energy passing through a unit area in unit time and is expressed in watts per metre squared Wm 2 . 124 It becomes very complicated to quote noise levels in measurements of sound pressure Pascals or intensity Wattsmetre 2 , as the numbers are very unwieldy. We therefore relate them to a reference level in this case, the threshold of hearing and using a log scale for the result, a much more manageable figure can be produced. This is called the decibel which is one tenth of a Bel. The decibel dB has no dimensions as such; it is just a unit of comparison arranged in a logarithmic scale, so that increasing the number corresponds to a multiplication of intensity. The loudness of noise is a function of both the intensity and the frequency. Source: Canadian Centre for Occupational Health and Safety

13.4 Health Effects of Excessive Noise