1. 1 Hearing Sound is created by vibrations from a source and is transmitted through a media (such as the atmosphere) to the ear. Sound has two main attributes: 1.Frequency of Sound Waves: When sound is generated, it causes vibration and makes the air molecules to be moved back and forth. This alternation creates corresponding increase and decreases in the air pressure.

2. 2 The vibration forms sinusoidal (sine) waves. The height of the wave above and below the midline represents the amount of above- normal and below-normal air pressure respectively. The waveform above the midline is the image of the waveform below the midline in a sine wave. The waveform repeats itself again and again in a sine wave.

3. 3 The number of cycles per second is called the frequency of the sound. Frequency is expressed in hertz (Hz) and is equivalent to cycles per second. The human ear is sensitive to frequencies in the range of 20 to 20,000 Hz (highest sensitivity between 1000 to 3000 Hz), but it is not equally sensitive to all frequencies. In addition, people differ in their relative sensitivities to various frequencies.

4. 4 2.Sound intensity: It is defined in terms of power per unit area. The Bel (B) is the basic unit for measuring sound. The most convenient measure is the decibel (dB), 1 dB=0.1B. (see figure 6.2)

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8. 8 Complex Sounds: Very few sounds are pure. Most complex sounds are non-harmonic. Masking: It is the condition when one component of the sound environment reduces the sensitivity of the ear to another component.

9. 9 AUDIOTORY DISPLAYS There are four types of human functions involved in the reception of auditory signals: 1.Detection 2.Relative discrimination (differentiating between two or more signals presented together) 3.Absolute identification (only one signal is present) 4.Localization (knowing the direction that the signal is coming from)

10. 10 Detection of signals  Signals can occur in peaceful surroundings or noisy surroundings.  The signal plus the noise (SN) should be distinct from the noise (N) itself.  If the above is not the case, the signal can not be detected in the presence of noise. The threshold of the detectability of the signal is elevated.  This threshold should be exceeded by the signal if it is to be detected accurately.  What Level? Annoyance?

11. 11 Relative Discrimination  Relative discrimination of signals on the basis of intensity and frequency depends in part on interactions between these two dimensions.  A common measure is the just-noticeable difference (JND).  JND is the smallest difference or change along a stimulus dimension that can just be detected 50% of the time by people.  The smaller the JND, the easier it is for people to detect differences on the dimension being changed.

12. 12 Absolute Identification  The number of levels along a continuum (range or scale) that can identified usually is quiet small. See table 6-1 (page 174)  It is better to use more dimensions with fewer steps or levels of each dimension than to use fewer dimensions and more levels of each.

13. 13 Localization  The ability to localize (guess the direction of sound) the direction from which the sound is coming from is called stereophony.  People mostly relay on intensity and phase of sound to determine the direction of sound.

14. 14 NOISE  Noise is referred to as unwanted sound.  In the context of information theory, noise is defined as “ that auditory stimulus of stimuli bearing no informational relationship to the presence or completion of the immediate task”.

15. 15 How loud is it?  Loudness depends on intensity and frequency.  A low frequency tone will not sound as loud as a high frequency sound of the same intensity.  One of the most important effects of noise is hearing loss.  Occupational hearing loss.  Temporary loss, permanent loss.  Continuous noise or noncontinuous noise.  Physiological effects of noise.

16. 16 TWA - Time Weighted Average Noise Levels - and Noise Dose The TWA shows a worker's daily exposure to occupational noise (normalised to an 8 hour day), taking into account the average levels of noise and the time spent in each area. This is the parameter that is used by the OSHA Regulations and is essential in assessing a workers exposure and what action should be taken. Working Out the Noise Dose and TWA Before working out the worker's TWA you have to measure the different high noise levels that the worker is subjected throughout a normal working day. The Time Weighted Average is calculated using these noise levels together with the amount of time that the worker is exposed to them.

17. 17 First calculate the Noise Dose as: Total Dose = 100 x (C1/T1 + C2/T2 + C3/T3 +... + Cn/Tn) where Cn = time spent at each noise level Tn= Permissible time (from table)

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19. 19 TWA= [16.61 X log(D/100)] + 90 The link below contains a noise calculator: http://www.noisemeters.com/apps/occ/twa-dose.asp