Physics of Sound Copy_DEL

Physics of Sound

Sound is the sensation produced in the ear when exposed to vibrations in the air and other media. Sound can also be perceived by direct contact with vibrating objects. Sound has three measurable qualities:

All three measurable qualities of sound play a role in hearing loss in the industry.
Frequency
Amplitude
Duration

Frequency

Frequency is the speed of the vibration and this determines the pitch of the sound (highness or lowness of the sound). Frequency is measured as the number of wave cycles that occur in one second. The unit of frequency measurement is Hertz (Hz).1

Although the frequency of 4000Hz is most commonly affected by industrial noise, it would also depend on the amplitude and duration of exposure when determining the degree of loss.

The  human ear is capable of hearing sound over a frequency range of 16-20000Hz. The frequencies that comprise speech are in the range of 250-4000Hz. We perceive frequency as pitch. The lower the frequency, the lower the pitch of the sound (bass). The higher the frequency the higher the pitch of the sound. Certain sounds are heard at certain frequencies.  For example, vowels are heard at low frequencies, while consonants are heard at higher frequencies. This becomes relevant when determining noise-induced hearing loss as the damage occurs in the higher frequencies.  

Figure 1.15: Visual image of Frequency.

https://study.com/academy/lesson/what-is-frequency-definition-spectrum-theory.html15

Figure 1.16: Graph of sound waves ¹⁴

Amplitude

The loudness or intensity (volume) of the sound is determined by the amplitude (height) of the sound wave and this is measured in decibels (dB).  Normal hearing ranges between 0 – 25dB and “loud” noise is any sound equal to or greater than 85dB.

Sound has both volume and pitch. Volume is seen as an increase in amplitude of the sound wave2.

Figure 1.17: Graph of sound waves representing Amplitude16

Duration

The length of time of noise exposure. This is an important factor when considering how much noise someone can be exposed to before hearing loss occurs. The longer the duration of exposure to noise in any one period the greater the damage caused. In industry, the dB reading is given in A – weighted averages, using the baseline of an 8 hours per day or 40 hours per week work schedule. The  “3dB exchange rate” formula is used to calculate noise limits and exposure times. This means that for every 3dB above 85dBA, the maximum exposure time must be cut in half.

WP DataTables

Figure 1.18:  Table Indicating Noise Limits According to NIOSH17

Figure 1.19: Graph: Average noise-induced permanent threshold shift (dBHL) at 1-2-3-4 kHz for a median individual in the adult population (≥18 years old) for a given 8-h equivalent exposure level (LEX8h) and duration (derived from ISO, 2013)18

The figure above displays noise-induced permanent threshold shift increasing sharply in the early years of exposure, the rate of loss slows after this. This demonstrates that the first 10 – 20 years of exposure are the most critical in terms of hearing loss, hearing loss continues to occur with exposure time.