Lecture 19

Lecture 19

November 3, 2004

ITEM DATE WEIGHT (%)

Exam #1 Friday, 9/24 15%

Exam #2 Friday, 10/22 15%

Exam #3 Monday, 11/22 15%

OP Questions Daily 25%

Final Exam Dec. 6th 30%

SCHEDULE REMAINING

ITEM DATE WEIGHT (%)

Exam #1 Friday, 9/24 15%Exam #2 Friday, 10/22 15%Exam #3 Monday, 11/22 15%

OP Questions Daily 25%Final Exam Dec. 6th 30%

SCHEDULE REMAINING

Last Time We discussed the loudness of sounds

Loudness is subjective … in our heads. Loudness is not LINEAR so we introduced

the logarithmic DECIBEL scale (dB or db) We looked at the way our brains add

loudness on the db scale. We discussed the frequency

dependence of our hearing. We discussed how very loud sound can

damage our ears.

The Cochlea Schematic

Rubber Membrane

Low Frequency High Frequency

Frequency Info

Resonance in the Basilar Membrane(Computed)

Simplified Version

Resonance !!

The Hair Cells

Damage from very LOUD noises.

Extreme Acoustic Trauma

Control, not Control, not exposedexposed

After After ExposureExposure

Guinea Pig StereociliaGuinea Pig Stereocilia damage (120 dB damage (120 dB sound)sound)

The Overall Hearing Process

Sound is created at the source. It travels through the air. It is collected by various parts of the ear

(semi-resonance). The tympanic membrane moves with the

pressure variations. The inner ear filters/amplifies the sound.

Hearing Continued

The sound hits the membrane at the entrance to the cochlea.

The pressure on the basilar membrane causes it to mive up and down.

The resonant frequency of the membrane varies with position so that for each frequency only one place on the membrane is resonating.

Some more on hearing There are hair cells along the basilar

membrane which move with the membrane. The motion of the hair cells creates an

electrical (ionic) disturbance which is wired to the brain.

The disturbance is in the form of pulses. The brain somehow relates the number of

pulse firings per second to tone and .. Wallah … music!

The source/brain connection

SoundSource

dB? ?

Signal Source IssuesRoom Acoustics

How the BrainReacts to

certain sounds

In what follows keep the following in mind:

The wavespreads

out.

Sounds do funny things

Experiment (1 signal from speaker) Class:

Hold your head still. Move it side to side by about a foot or so. What happens???

Sound Spreads Out

We get more than we bargained for.

Different Distances

Or, sorta like this …

Bounce off wall so travels farther and more waves before getting to ear!

Direct

Bounce off of wall.

Add ‘em up!

DEMO

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14

In Phase --- Add up nicely

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Out of Phase --- Don’t Add Up

One Wave Out

INTERFERENCE

Wave Interference CONSTRUCTIVE

The waves ADD together. DESTRUCTIVE

The waves are “out of phase” and cancel each other out.

Let the beat go on …..

Consider TWO sourced of sound. perhaps two speakers

Both are emitting sound.The two frequencies are NOT

the same.

Two Waves … different frequencies

f1

f2

Beat Frequency

12 fffbeat

We will return to this when we discuss consonance.

next topic = diffraction and room acoustics