A brief discussion

CHAPTER 7 – Measured Tones

ear

WHY do we need to hearhear?

WHY did earsears evolve in most (not all) species??

Must hang on the head in a place where it usually won’t be tightly covered.iPods etc. excluded

It should be very sexy. It can’t be too heavy. It should be sensitive. It should be able to sense direction of

the sound’s origin. (Why?) It should be able to distinguish a

range of frequencies. (Why?)

A sound “collector” to channel as much of the sound as possible to the actual collector.

A sensor that collects the sound and converts it into something that moves. A membrane at the end of the collector would

work because it will move in response to the pressure changes in front of it as long as the pressure at the back of the membrane remains the same.

A converter to adjust the amplitude to the desired strength of the signal. Transmits forward into the process.

A frequency sensor/discriminator An “electrical signal” producer A “wire” or connection to the brain. A Brain A program in the brain’s computer (cortex)

that converts the signal into something perceived as sound.

The Ear

collection

sensorad

just

men

t

freq. separate

wire

BrainBalance

In the beginning … Paramecia patrol the ooze of early life

A spot detects levels of light (not yet an eye) She chemically senses her environment (not yet a

nose) She can sense collisions with objects (but not a sense

of touch) During first 80 % of the 3.5 billion years that

evolution proceeded Flatworms, mites and rotifers (whatever they may be) eyes, sense of touch, chemical receptors (taste?) No sound at all.

200,000,000 years later, the ear began to emerge.

canal to inner ear Deflects sound in

1 in 4 have a bump here

Pinna amplifies (resonance) the higher frequencies – consonants

Catches, reflects snd.

Ear canal resonance

MinimalAmplification

1.0

100Later!

Pinna

Ear C

anal

Ear Drum

Eustachian Tume

Atm. Pressure

Cochlea

We have collected the sound and applies a bit or resonance via the pinna folds.

We sent the sound down the ear canal. We have moved the eardrum with the sound. We connected the eardrum to the cochlea.

It is NOT a dirty word!

Balance

Frequency&

Transmissionto the brain.

AS COILED IN THE EAR

High

response

Low

high frequency low frequency

crosssection

high frequency (Stiff) low frequency

What would a wave traveling down the basilar membranedo to the shape of the membrane??

f

RESO

NAN

CE

Incompressible

compressible

Inne

rOut

er

Brain

Many fire at the same time

Neurons can spontaneously fire at a rate of a few tens of Hz.

It is the CHANGE in rate that IS the neural message.

The collector nerves are often in contact with more than one hair cell.

If a cell fires, there is a time before it can fire again.

Long exposure to loud (later) music can literally fracture the hair cells.Those cells no longer respond to sound.This can lead to deafness.There is a “Society for Deaf Musicians” that

is a result of this. Modern bands usually take precautions.

Very loud noise can shatter an eardrumMay recover, may not.

Nerves can die. Bad news.

Age 40Ear is only 10% of what it was when you

were younger. Age 80

You have lost most of your high frequency hearing so the “quality” of what you hear changes significantly.

Consonants sound in the high frequency region which explains why older people have difficulties hearing (without any pathology).

You lose ½ Hz. per day in high frequency hearing.20,000 Hz. 10,000 Hz. 7000 Hz.

Dead Most music is in the <4000 Hz. range so

it is ok to put a stereo in your casket. As you age, you lose sensitivity to the

higher harmonics of some instruments.

Move to

find dire

ction

Slight delay due to different distances.

Pinna “bouncing” leads to different quality that the brain can sense

Can’t do this for long wavelengths

Distance Effect Phase Amplitude

Pinna Effect Large

Amplitude and Phase differences are minimal Localization is difficult.

Very Low frequencies, hearing doesn’t work too well (next slide) soDANGER!!!

Loudness Recruitment Cells lose sensitivity to low or moderate sound

intensities but respond normally at high intensities.

Beethoven had this problem!

loudness (soon)

HearingNormal Response

Log of a number (for our purposes) is the number of zeros that it containswhen written as an integer.

  Number of   dB

X Zeros Log(X) 10 x Log(X)

10 1 1 10

100 2 2 20

1000 3 3 30

10000 4 4 40

100000 5 5 50

1000000 6 6 60

0

log10I

Idb

Let I0 be the lowest intensity (defined next time … use your imagination for now) of sound that you can hear. db or decibel definition: