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Sound Intensity Level – Learning Outcomes

Learn about sound intensity, threshold of hearing, dB(A) scale, and more. Practice solving problems and explore hearing protection methods.

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Sound Intensity Level – Learning Outcomes

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  1. Sound Intensity Level – Learning Outcomes • HL: Define sound intensity and give its unit. • HL: Solve problems about sound intensity. • Define threshold of hearing and frequency response of the human ear. • Give some examples of sound intensity level. • HL: Convert changes in sound intensity to changes in sound intensity level. • Discuss hearing impairment and ear protection. • Describe the dB(A) scale. • Use a sound level meter.

  2. HL: Sound Intensity • The sound intensity at a point is the rate at which sound energy is passing through a unit area at right angles to the direction in which the sound is travelling. • Formula: • It is measured in watts per square metre (W m-2). • Sound spreads out in a sphere, so the total area at a given distance is (surface area of a sphere). • This also implies that sound intensity has an inverse square relationship with distance:

  3. HL: Sound Intensity • e.g. What is the sound intensity of a 50 W speaker at a distance of 10 m? • e.g. A foghorn has a sound intensity of 0.5 W m-2 at a distance of 100 m. What is its power rating? • e.g. The sound intensity of a wind turbine 150 m away is about 310-8 W m-2. What is its sound intensity at 75 m?

  4. Frequency Response of the Ear • The ear is not like a microphone – it picks up some sounds easier than others. • Certain frequencies sound louder than others at the same sound intensity. • The ear is particularly sensitive to sounds in the 2000 to 4000 Hz frequency range. by Lindosland – public domain

  5. Threshold of Hearing • Human ears cannot detect all sound. • A minimum sound intensity is required. • Due to the frequency response of the ear, this minimum intensity varies with frequency. • The threshold of hearing is the smallest sound intensity detectable by the average human ear at a frequency of 1 kHz. Its value is 110-12 W m-2.

  6. Sound Intensity Level • Our ears do not detect differences in sound intensity linearly (e.g. a sound which doubles in intensity does not sound twice as loud). • The highest sound intensity we can safely hear is about times as intense as the lowest. • Thus, we often use a logarithmic scale to represent sound intensity, called sound intensity level. • Logarithmic scales represent factors as linear differences. • In the case of sound intensity, a factor of 10 W m-2 is represented as a difference of 10 deciBels (dB).

  7. Sound Intensity Level

  8. HL: Sound Intensity Level • A doubling in sound intensity corresponds to a sound intensity level increase of 3 dB. Likewise, halving the sound intensity reduces the sound intensity level by 3 dB. • e.g. the sound intensity of a loudspeaker doubles as a person approaches it. What is the increase in sound intensity level? • e.g. the sound intensity level at a concert increases from 85 dB to 94 dB when the concert begins. By what factor has the sound intensity increased?

  9. Hearing Impairment • We have previously discussed noise pollution, but we need to look at hearing impairment in detail. • Exposure to sounds above a certain level can permanently damage hearing over time. • Health and safety regulations usually require a maximum exposure to sound levels without ear protection. • Ear protection is required for longer periods.

  10. dB(A) Scale • Due to the frequency response of the human ear, the dB scale does not accurately reflect how loud sounds are. • e.g. a 50 dB sound will appear louder at 2 kHz than at 50 Hz, because our ears are more sensitive to 2 kHz sounds. • The dB(A) scale takes this into account so that 50 dB has the same loudness to our ears regardless of the frequency.

  11. dB(A) Scale by Lindosland – public domain

  12. Sound Level Meter • Sound level meters are devices used to measure sound intensity level. • They use a microphone to pick up sound, then display a deciBel value, usually on the dB(A) scale. • Thus, they take the frequency response of the ear into account. by Harke – public domain

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