Lecture October 2
As you get older your sensitivity to frequency is worse. When you're born you can
hear so much (25-30hz). As you get older you lose the ability to perceive high
frequencies. It's okay because theres not much going on at this high of frequencies.
Frequency and the Psychological response to it (pitch)
The words you hear in conversation are different because of pitch. You need pitch to
understand meaningful auditory stimuli. Helmholtz suggested that for each different
frequency theres a spot on the basilar membrane that is maximally activated (for
1000hz theres a spot that's activated which codes for a 1000hz signal for what
you're listening to- Place Theory). The basilar membrane doesn’t respond at a
particular place it responds with a travelling wave - the wave has a peak though, a
spot of maximum movement. The place theory: theres a spot of maximum
stimulation, not a place or spot. Frequency theory: the basilar membrane vibrates at
the frequency of the tones coming in (1000hz tone, vibrating 1000 times per
second), if you measure that activity of the auditory nerve (the signal going up to the
brain) it is mimicking the frequency. Any nerve cannot fire more than 5000 times a
second, so you have a limitation. Place theory has a limitation too because the higher
frequency tones have very sharp point (the maximal stimuli point), but the low
frequency tones don't, they don't have a point of maximal stimulation that is easily
determined because it's more of a crest not a point, so it's not the best signal to the
brain - limitation says we can't perceive low frequency sounds well. Frequency
theory: limits higher frequencies, place theory: limits lower frequencies. Maybe both
of these things are at work and working together. Higher frequencies- place
information is useful frequency information is not, lower frequencies - frequency
information is useful place information is not.
You want to run an experiment where people only have place information, or
frequency information, and asking; "can they perceive the appropriate pitch?"
White noise: a sound that has all the audible noise frequencies in it, if you remove
some you wont notice though. Between radio stations that's what you get. White
noise is relaxing- it will relax you.
White noise is what we use for the experiments we mentioned above. Present white
noise to a person, all the frequencies so it stimulates all the frequencies on the
basilar membrane, so there's equal stimulation everywhere. You create frequency
information by pulsing it, say 1000 pulses per second of white noises, each pulse
covers the entire basilar membrane, but the basilar membrane itself is being
stimulated 1000 times a second (frequency information with a stimulus that doesn’t
give place information). If you pulse it 1000 times a second you should hear a
1000hz tone- this is what happens.
Place theory test: present white noise to someone but leave a gap- don't present any
frequencies between 980 and 1020 hz. The person can't tell the difference. Then you
stop the stimulus, you got the membrane vibrating fairly intensely and not it's tired.
All the places that have been stimulated for some period of time are tired and they
go into rest/ recovery state. That little range (980-1020) isn't tired cause they weren’t stimulated ad will continue it's normal level than its spontaneous activity,
which is now higher than everywhere else- you get an auditory after-image. The
spontaneous activity of everywhere is lesser because all the other places are tired.
This is place information, not frequency because at this point there is no stimulus.
We now have place information for around 1000hz (because this is in that interval).
The brain interprets a sound at 1000hz - this is the after image.
One more thing about timbre: (1)
The two curves add
If you take a stimulus that have a particular frequency, and another that has a very
close frequency (1000 and 1001 for example), if you dr