1. Hearing
and other senses

2. Sound Sound: sensed variations in air pressure
Frequency: number of peaks that pass a point per second (Hz)
Pitch

3. Sound Measurement Range: p < 0.001 bar (normal breathing)
p > 1000 bar (jet plane)

4. Sound Pressure Level (SPL)
SPL = Lp = 20 log10 ( p/pr)
p = RMS sound pressure of target sound
pr = RMS sound pressure of reference sound
(e.g bar)
SPL units: decibels (dB)

5. Sound Measurement Devices
The device on the left is a sound level meter and is primarily used for noise abatement activities and acoustical work such as determining noise control criteria for an occupancy or for ambient noise analysis and control. The device in the center is a sound level meter/noise dosimeter which accumulates, or logs noise exposure for an entire work shift. This instrument is primarily used for OSHA hearing conservation activities. The device on the right is a previous-generation sound level meter.

6. A, B, and C Scales

7. Hearing Anatomy & Physiology
Pinna
Auditory
Canal
Tympanic
Membrane
Ossicles
Malleus (hammer)
Incus (anvil)
Stapes (stirrup)
Ligaments
Muscles
Amplitude reduction
Pressure amplification
Attenuation reflex
(protection, low
frequency masking)
Oval
Window
Cochlea
Auditory
Nerve
Auditory
Cortex

8. Cochlea (“unrolled” from actual spiral shape)
Oval
Window
Scala Vestibuli
& Scala Media
Helicotrema
Stapes
Basilar Membrane
Organ of corti
Hair cells
Scala
Tympani
Round
Window
High Frequency
Low Frequency

9. Masking by Pure Tones

10. Hearing Loss

11. Noise - Induced Hearing Loss

12. OSHA Standards: Continuous Noise

13. OSHA Standards: Impulse Noise

14. Noise Dosage total (daily) dose = sum of partial doses
Requirement: total dose < 1.00

15. Example
Worker exposed to 90 dBA for 4 hours, 105 dBA for 30 minutes. Within dosage limits?
dBA = 4 / 8 = 0.5
dBA = 0.5 / 1 = 0.5
Total dosage = = 1.0
Since 1.0 < 1.0, dosage is OK

16. Noise Control Source Path Receiver Design Barriers Ear plugs
Maintenance Enclosures Ear muffs
Mountings Baffles
Mufflers

17. Hearing Protectors