Lecture 16 - Sound & Perception

Question 1

Two definitions of “sound”

Answer 1

Perceptual definition

Physical definition

Question 2

Perceptual definition

Answer 2

sound is just what we experience when we hear things

is a sound experiences as loud or quiet? pleasant or unpleasant?

Question 3

physical definition

Answer 3

• sound as pressure changes in the air (or other medium).

– In the air, we are concerned with the relative density of air
molecules (how close or far apart the molecules are). Sound waves manipulate that density. Higher levels of local air pressure are called condensation, while decreased regions of air pressure are known as rarefaction.

– Importantly, this difference in pressure is a mechanical process. Force is applied to local regions of air molecules and this is transmitted to neighboring regions (translated through the atmosphere until it gets to a receiving organ). The pattern of changes in pressure across a space is a sound wave.

Question 4

condensation

Answer 4

pushing air molecules together

higher levels of local air pressure

Question 5

rarefaction

Answer 5

decreased regions of air pressure

Question 6

sound wave

Answer 6

the pattern of changes in pressure across a space

are longitudinal (forward and back)

condensation, rarefaction, condensation, rarefaction

Question 7

water waves are….

Answer 7

transverse (up and down)

Question 8

any sound is the

Answer 8

translation of the force from the stimulus going through space and then pushing in and out on the eardrum

Question 9

how speakers work

Answer 9

A speaker will compress air as the cone moves out. When the cone retracts, it leaves lower (rarefied) pressure areas. This pressure oscillation travels as a sound wave to your ear.

Question 10

The sound stimulus has three main physical dimensions

that we will examine:

Answer 10

• amplitude (intensity of physical stimulus): going to relate to the perceptual experience of loudness (greater the amplitude, greater the loudness)
• frequency: how often that pattern is repeated (related to pitch)
• complexity: tells us how many diff signals are arriving at once (related to timbre)

Question 11

sound and time

Answer 11

sound coming at different times

to make sense of the signal the ear has to have an exquisite sense of time: hearing monitors time (critical in understanding how hearing works)

Question 12

We study sound first as pure tones.

Answer 12

instead of rich complex tones, we start with the simplest

• Characterized by a sine wave.
• It has a smooth (one frequency), repetitive oscillation. Gunna repeat the same timing throughout the repeition
• Relatively rare in nature, but useful for describing basic features of sound (because they’re a nice simple stimuli to test, and we also have receptors for them).
• Also very useful for characterizing speech elements (speech recognition).

Question 13

amplitude

Answer 13

measuring how much condensation and rarefaction you have in the sound stimulus

have some measure of pressure and want to look at the change of pressure over one wavelength of a stimulus

max amplitude is condensation

min amplitude is rarefaction (negative pressure)

perceive these waves as the amount of loudness (intensity)

measured thorough Decibels (measure of pressure)

Difference in pressure (air molecule density) between high
and low peaks of wave

• Decibel (dB) is used as a measure of loudness, but is
actually a measure of pressure intensity.

Question 14

when you have Decibels you’re measuring

Answer 14

measuring the pressure level at a purely physical level

Question 15

decibel scale

Answer 15

relates the amplitude (physical pressure) of
the stimulus with the psychological experience of loudness.

dB = 20 x logarithm(p/Po)

p = sound pressured measured in the environment

Po = reference value - really near the average threshold

Question 16

Quietest place on Earth:

Answer 16

you can have a decibel level of less than zero, and that doesn’t mean there’s negative sound

the amplitude is negative relative to what we should be able to hear

if you have to have a physical amplitude of 1 micropascal anything less can’t be detected

Anechoic chamber at Orfield Labs in Minneapolis,Minnesota.

• Measured at -9.4 dB (Po = 1, below hearing threshold)
• negative pressure relative to the threshold of hearing
• it’s dampening sound, killing sound: the air pressure waves can’t be transmitted, they get broken up on the sound baffles on the side
• we need sound to keep balance: people can’t last in this room more than 30 minutes: can hear the blood in your ears, your heat beating, then they get vertigo cause they don’t get any sound cues to tell them where they are in space

Question 17

frequency

Answer 17

– Number of cycles within a given time period

– Measured in Hertz (Hz): 1 Hz is one cycle per second

– Perception of pitch is related to frequency.
– Tone height is the perceptual 
experience of increasing pitch 
that happens when frequency 
is increased.