What do we hear?

• Sound is a compression wave - it only “looks” like a wave if we plot air pressure against time

time ->

Air Pressure

Period - amount of time for one cycle

Frequency = number of cycles per second (1/Period)

Sensing Vibrations

• Inside the cochlea is the basilar membrane• Movement of the oval window causes ripples

on the basilar membrane

Hearing• Detection • Loudness • Localization• Scene Analysis • Music• Speech

Detection and Loudness

• Sound level is measured in decibels (dB) - a measure of the amplitude of air pressure fluctuations

Detection and Loudness

• Sound level is measured in decibels (dB) - a measure of the amplitude of air pressure fluctuations

• dB is a log scale - small increases in dB mean large increases in sound energy

Detection and Loudness

• Sound level is measured in decibels (dB) - a measure of the amplitude of air pressure fluctuations

• dB is a log scale - small increases in dB mean large increases in sound energy

• We have a dynamic range that is a factor of 7.5 million!

Detection and Loudness

• minimum sound level necessary to be heard is the detection threshold

Detection and Loudness

• detection threshold depends on frequency of sound:

• very high and very low frequencies must have more energy (higher dB) to be heard

• greatest sensitivity (lowest detection threshold) is between 1000 hz to 5000hz

Detection and Loudness

• Detection can be compromised by a masking sound

• even masking sounds that are not simultaneous with the target can cause masking (forward and backward masking)

Detection and Loudness

• Loudness is the subjective impression of sound level (and not identical to it!)

Detection and Loudness• For example,

tones of different frequencies that are judged to be equally loud have different SPLs (dB)

Detection and Loudness• Hearing loss due to exposure to high-intensity

sounds (greater than 100 dB) is frequency-specific and can last many hours

Detection and Loudness• Incidence of noise-related hearing loss is increasing dramatically• iPods and other “earbud” music players are thought to be partly

responsible• How loud is an iPod?

– maximum volume is approximate but is somewhere between 100 dB (hearing damage in about 2 hours) to 115 dB (hearing damage in about 15 minutes)

• Consequences: difficulty understanding speech, tinnitus, deafness • Your perception of loudness adapts so it’s hard to tell how loud your

iPod is - LOCK THE VOLUME ON YOUR iPOD!