Lecture 9

Experiments in Psychoacoustics

Martin Giese

What you should remember

1. Perceptual threshold, JND, PSE

2. Psychometric function (PMF) / psychophysical function

3. Classical methods of psychophysics

4. 2AFC

5. Signal Detection Theory

6. Scaling methods

What you should learn today

1. Components of the auditory system

2. Basics on the psychophysics of hearing

3. Some tips for giving good presentations

Hearing

• Important sense for humans: Deafness more impairing than blindness

• High relevance for communication (speech) • Contrary to vision:

– covers whole environment (warning !)– cannot be deactivated by attention

Sound

Sound = mechanical pressure waves

• Frequency: 10 Hz … > 109 Hz Hz (Hertz) = oscillations / sec

• Speed: 343 m/s in air; 1484 m/s in water• Wave length: 34 cm (1kHz); 3.4 cm (10 kHz)

wave length

Sound

Frequency: low high

low

high

Amplitude:

Sound

Pure tone: only one frequency

Complex sound: multiple frequencies

White noise: all frequencies with equal amplitude

Sound

Frequency regimes:

Infrasound: < 10 Hz

Normal sound: 10 – 18 kHz (perceived)

Ultrasound: > 18 kHz

Sound

Amplitude: (logarithmic measure)

Sound pressure level (SPL)

SPL [dB] = 20 log(p/p0)

p0: reference pressure (20 pa, 1 pa = 1 N / m2)

p: amplitude

(From Sekuler & Blake, 1994)

Sound

dB value -- p / p0

3 dB – 1.414:1

6 dB – 2:1

20 dB – 10:1

40 dB – 100:1

120 dB – 1,000,000 :1

Hearing in the regime 0…160 dB = 1…108 !!!

Compression by

logarithmic scale !

The Auditory System

Ear: Overview

(From Kandel & Schwartz & Jessel, 2000)

Pinna

Ear: Overview

(From Sekuler & Blake, 1994)

Outer Ear

• Auditory canal 2.5 cm long, 7 mm wide• “directional microphone”• Resonant frequency ~ 3000 Hz• Displacement of tympanic membrane:

10-3 … 10-8 mm

diameter of H-atom: 10-8 mm

Middle Ear

Mechanical “impedance converter”:

Ear drum: Stirrup:

small force large force

large area small area ( 9 mm) 20 times smaller

Adjacent medium:

Air Liquid

Force outer ear /

force inner ear 1:90 (Sekuler & Blake, 1994)

Middle Ear

Acoustic reflex:

Small muscles (stapedius and tensor tympani) contract in presence of loud sounds

Function:• Adaptation for loud stimuli• Sensitivity reduction during

speaking and chewing

Inner Ear: Cochlea

Cochlea = “snail”• 2 ½ coils

• Length (extended): 34 mm, Ø <9 mm

• 3 channels (scala vestibuli scala media/ scala tympani)

• Filled with liquid (perilymph, endolymph) no vessels !

• Basilar membrane contains receptors (hair cells)

• Oval and round window

(Scala media)

(From Sekuler & Blake, 1994)

Organ of Corti

• Basilar membrane (BM)with hair cells

• Tectorial membrane

– arches over hair cells

– Makes contact with cilia

– Fixed only at one side

• During movements of BM ciliae are sheared

• Otoacoustical emissions

(From Kandel & Schwartz & Jessel, 2000)

Hair cells

Inner hair cells: Outer hair cells:

3500 12,000 / earSurrounding tissue Embedded in fluid

95 % 5 % AN fibers connect

Transduction Amplification (?)

(From Sekuler & Blake, 1994)

Theories about Cochlea Function

Frequency Theory: (E. Rutherford 1886):• Basilar membrane moves as whole

(“telephone hypothesis”)

• Neurons fire with same frequency as acoustic stimulus

Place theory: (H. von Helmholtz 1877):• Sites of the BM resonate for different

frequencies, like strings of a piano

Theories about Cochlea Function

Traveling wave theory: (Bekesy 1928):

• Movements of stapes induce traveling waves on the inhomo-geneous basilar membrane

• Site with maximum amplitude depends on frequency

• Tonotopic organization(approx. with log of f)

Georg von Bekesy

Nobel price, 1961

Theories about Cochlea Function

Traveling waves:

Tonotopic organization:

(From Sekuler & Blake, 1994)

Theories about Cochlea Function

Conclusion:

“Ohm’s Law of Acoustics”

The ear decomposes complex sounds in tones. It acts like

a Fourier analyzer.

Georg Simon Ohm(1787-1854)

Auditory Pathway

• Different pathways to analyze:

– Structure of sound

– Localization of sound

• Important structures:– Auditory nerve (50,000 fibres)

– Cochlear nuclei (monaural)

– Relay nuclei (olive) in the brain stem (localization)

– Medial geniculate nucleus

– Auditory cortex (tonotopy !)

(From Sekuler & Blake, 1994)

Auditory Nerve

Dependence on SPL: Frequency tuning:

Different neurons responsible for different SPL regimes

(From Sekuler & Blake, 1994)

Hearing

Audibility Function (AF)

• Threshold SPL depends on frequency

• 0 dBSPL defined as smallest threshold for 2500 Hz

• Limited frequency range

(From Sekuler & Blake, 1994)

Audibility Function

Audible frequency ranges for different species

(From Sekuler & Blake, 1994)

Loudness Perception

(From Sekuler & Blake, 1994)

Loudness: subjective perceptual experience SPL: physical stimulus strength

• Measurement:

– Magnitude estimation

– Loudness matching Equal loudness contours

• Power law:

L ~ SPL0.67 • Unit: Phone

1 phone = 1dBSPL for f = 1000 Hz

Loudness Perception

Equal Loudness Contours (Isophones)

(From Sekuler & Blake, 1994)

Hearing regime:

Threshold to limit of pain

Speech area:

200 Hz – 5kHz, 50-80 Phone

Equalizer:

Masking

• Loudness perception and sound detection reduced in presence of background sounds

• Types of noise: broad band narrow band

• Noise reduces sensitivity only within limited frequency range

Measure for tuning width of auditory neurons

E

f

E

f

fc: center frequency

B: band width

(From Sekuler & Blake, 1994)

Masking

Egan & Hake (1950)• Band pass noise as masking

stimulus

• Measured: change of threshold SPLs

• Results:– Maximum effect near center

frequency

– effect over broad frequency range

– Asymmetry ! reflects asymmetry of

traveling wave on BM(From Goldstein, 1996)

Masking

Psychophysical tuning function (Zwicker, 1974)• Test tone with fixed frequency

• Vary mid frequency of masking stimulus (narrow band)

• Increase amplitude of mask until perception of test tone ceases

(From Goldstein, 1996)

Clinical Relevance

Hearing loss• >20 million Americans

• Reasons:

– Conduction loss

– Sensory / neural loss

• Test: bone conduction

• Presbycusis (loss of high frequency sensitivity

• Damage by (chronic) exposure to noise

• Drugs (aspirin) (From Sekuler & Blake, 1994)

Clinical Relevance

• “loudness recruitment”:rapid increase of loudness with intensity

(From Goldstein, 1996)

Clinical Relevance

Presbycusis

( presbys = “old”)

Difficulties dependent on degree of hearing loss

(From Goldstein, 1996)

Things that We did not Treat

• Pitch perception• Perception of timbre (sound characteristics)• Sound localization (binaural hearing)• Speech perception

Giving Good Presentations

Important Things

• Prepare audio-visual equipment before

• Clear logical structure, e.g.– Intro (Motivation / conditions)

– Description of experiment

– Results

– Conclusion

• Focus on a few important points

• Tell a story

Recommended

+ Use colors, illustrations

+ attract attention (i.p. begin and end, humor, …)

+ Face the audience, eye contact

+ open stance, clear natural gestures

+ Practice (“test talks”, get feed-back from friends)

+ Have a back-up plan

Things to Avoid

– Speaking not loud enough

– Getting nervous (Others don’t notice most errors !)

– Reading from script

– Too many details on a sheet (7 items ideal)

– Abstract general expressions

– Long sentences

– Bad timing

Suggested readings:

Sekuler, R., Blake, R. (1994). Perception. McGraw-Hill, New York. Chapters 9 + 10.

Elmes, D.G., Kantowitz, B.H., Roediger III, H.L. (1999). Research Methods in Psychology. Brooks/Cole Publishing, Pacific Grove. Chapter 14.

Additional Literature:

Goldstein, E.B. (1996). Sensation and Perception. Brooks/Cole Publishing Company, Pacific Grove. Chapters 8 + 9 + 13.

Kandel, E.C., Schwartz, J.H., Jessell, T.M. (2000). Principles of Neural Science. Mc Graw-Hill, New York. Chapter 30.

Literature