Transcript Lecture 21 - UCF Physics

Lecture 21
Winding Down on the
Room-Head stuff
Last Time
Room
Acoustics
dB
Signal Source Issues
Room Acoustics
?
How the Brain
Reacts to
certain sounds
Topics

Person
– Instruments (strings and pipes)

Space

Loudness, Hearing, and the Ear
– decibels, the parts of the ear, the cochlea

To the Brain
– Hair cells to neurons

Processing
– Strange things in the mind

To Music
– Dum De Dum Dum! (Beethoven!)
DONE
– Reflection (and properties of walls), Diffraction or
Scattering, Interference, Reverberation
A Basic Musical
Question
WHAT MAKES TWO OR MORE TONES
SOUND WELL
TOGETHER?
One Observation

Two close frequencies sounding together
produce beats.
– Beat frequency is the difference between the
two frequencies
 fbeat=|f1-f2|
– We don’t seem to “like” beats.
– Beats exist in the sound itself, it is NOT an ear
phenomenon.
Two Tones f2=f1 (In Phase)
Two Tones f2=f1 (Out of Phase)
f1 and f2 close together
Consonance – PURE TONES
Octave
Think about the Cochlea

Different frequencies “excite” vibrations at
different resonance points along the basilar
membrane.
The Frequency Detail
Remember Resonance??
Cochlear Response
Close Frequency Effect
one tone sensation
What about a
COMPLEX
sound?
Single “tone” from a Guitar
(or any stringed instrument)
Doubling
LOG (Frequency)
OCTAVE
Two – REAL tones an octave apart
Octave
Two “unrelated” tones
Messy
The Fifth and its Harmonics
Not AS
Messy
Remember the Consonance
Experiment with two “single”
tones?
Consonance Experiment for two
complex tones (overtones)
The Strange World
of
Psychophysics
Strange things can happen in
the EAR
and
in
the
BRAIN!
Salvatore Dali’s Br
Munch, Edvard
The Scream
Strange Stuff
Play 2 tones (f1 and f2) at equal volumes.
 Start the two at the same frequency and
gradually sweep the f2 frequency up.
 A tone LOWER than f1 will be heard my
most people!

– This is a “difference tone” = |f2-f1|

As f2 approaches 2f1, a rough tone and
then something beat-like is heard.
– Beats of a “mistuned consonance”.
The “signal”
BEATS
MISTUNED
CONSONANCE
Two Tones, a Fifth apart
f0 = 1/t0=(1/2)f1
f0=(3/2)f1
Fundamental
Tracking
Subjective
Pitch
A Quick Trip
(no detail)
From the EAR
to the
Brain
The Ear
The Cochlea
Roll It Out
Cochlear Response
The Cochlear Model
Reality
The thickness of the membrane is also a
function of position.
 My previous model leaves this out.
 This actually reverses things a bit.
 Do not worry about this mere fact.

– Important point is that the cochlea has a
resonant frequency that is a function of
position along it.
Neuron Connection
Beethoven