Hearing and other senses
Sound Sound: sensed variations in air pressure Frequency: number of peaks that pass a point per second (Hz) Pitch
Sound Measurement Range: p < 0.001 bar (normal breathing) p > 1000 bar (jet plane)
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. 0.0002 bar) SPL units: decibels (dB)
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.
A, B, and C Scales
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
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
Masking by Pure Tones
Hearing Loss
Noise - Induced Hearing Loss
OSHA Standards: Continuous Noise
OSHA Standards: Impulse Noise
Noise Dosage total (daily) dose = sum of partial doses Requirement: total dose < 1.00
Example Worker exposed to 90 dBA for 4 hours, 105 dBA for 30 minutes. Within dosage limits? 4 hr @ 90 dBA = 4 / 8 = 0.5 0.5 hr @ 105 dBA = 0.5 / 1 = 0.5 Total dosage = 0.5 + 0.5 = 1.0 Since 1.0 < 1.0, dosage is OK
Noise Control Source Path Receiver Design Barriers Ear plugs Maintenance Enclosures Ear muffs Mountings Baffles Mufflers
Hearing Protectors