Introduction to psycho-acoustics:Some basic auditory attributes

For audio demonstrations, click on any loudspeaker icons you see ....

Terminology — objective vs. subjective terms

• Objective terms are physical, and describe the ‘outer’ world.– Intensity or amplitude (dB SPL)– Fundamental frequency (Hz)– Spectral shape (dB SPL)

• Subjective terms are psychological, and describe the ‘inner’ world.– Loudness– Melodic pitch– Timbre

• Psycho-acoustics is about the mapping between these two domains.

A caveat about the word ‘pitch’

• As a subjective descriptor, pitch is often used to describe aspects related to fundamental frequency (melodic pitch) as well as timbre (quality or colour).

• More confusingly, ‘pitch’ is also used as a physical descriptor.– Voice fundamental frequency (the vibration rate of the

vocal folds, is often referred to as voice pitch.

Thresholds and the auditory area —an audiogram (not the clinical type)

Thresholds and coding of loudness

• Much of the shape of the curve describing absolute threshold as a function of frequency can be accounted for by sound transmission into the inner ear.

• Important aspects of loudness coding appear to arise from properties of the basilar membrane and neural encoding of level.

• Logarithmic scales (like the decibel scale) are much more appropriate for describing our sensations of loudness than a linear scale.– For example, we can detect a change in intensity of

about 1 dB across a wide range of absolute levels.• Loudness appears to be related to the total

amount of firing in the auditory nerve.

Pitch perception for complex periodic signals

• Sinusoids (at least those below 4-5 kHz where our perception of pitch is strong) do not constrain theories of pitch perception very much, as there is information both in place (tonotopic mapping) and time (phase locking) codes.

• Focus here on complex periodic waveforms with rich harmonic spectra (for example, vowels). These always have a strong and clear pitch.

Hypothesis 1

• We ascertain the pitch of complex tones by listening only to the fundamental component in the complex (Helmholtz).

• Contradicted by ...

The case of the missing fundamental.

• We can use the fundamental if it is there, but we don’t need it.

The pitch of complexes with a missing fundamental

time

frequency

Masking spectral and virtual (the pitch we extract from complexes with a missing fundamental)

time

frequency

speech speech

What happens to a train of

pulses (a complex periodic

waveform with a rich

harmonic spectrum) when it is

analysed by the cochlea?

A similar outcome for a

more ‘ecologically

valid’ sound, a vowel.

Pitch perception for complex periodic signals

• Appears to rely on a spectro-temporal analysis.• At frequencies corresponding to the first few harmonics,

individual harmonics will be separately resolved by the cochlea. Therefore, information can be coded both in terms of place (changes in amount of activity across the auditory nerve fibre array) and time (synchrony of nerve firing to individual harmonics).

• At higher frequencies, individual harmonics cannot be resolved. The fundamental frequency is revealed as temporal modulations of the waveform amplitude envelope, which will result in synchronised neural activity.

Timbre

• A kind of ‘wastebasket’ category.

• Can include ….– amplitude envelope variations (attack and decay)– gradual changes both in spectral envelope and

fundamental frequency– the range between tonal and noise-like character– the effects of a prefix, an onset of a sound quite

dissimilar to the ensuing lasting vibration

A special (but very significant) case:Timbre for steady-state complex periodic signals

• That aspect which distinguishes two different sounds of the same perceived loudness, pitch and duration.

• Also referred to as colour (in vowels) or quality.• Variations in vowel colour are caused by variations in

what aspect of their spectra?

Their spectral shape

or spectral envelope.

A change in spectral envelope

How are changes in spectral envelope (and hence timbre) likely to be coded in the auditory nerve?

• By the place code.

• The excitation pattern of a vowel (on the basilar membrane and across the auditory nerve fibre array) will reflect, at least to some extent, its spectrum.

Temporal aspects of sounds are very important for timbre distinctions in general ...

piano played ...

forwards backwards

Full demonstration of the effects of tone envelope on timbre

You will hear a recording of a Bach chorale played on a piano (click on the loudspeaker at right) ...

Now the same chorale will be played backwards ....

Now the tape of the last recording is played backward so the chorale is heard forward again, but with an interesting difference ...

This is where the lecture finished, but there are a few further slides (and an audio demonstration) you

should have no trouble with.

(Well, maybe the audio demonstration is tough to follow, but it’s interesting to listen to.)

Psychoacoustic reflections of auditory frequency selectivity

( the fact that the peripheral auditory system does a kind of frequency analysis on all incoming signals )

A masked audiogramFor a fixed narrow-band masker, determine the change in

threshold for sinusoidal probes at a wide variety of frequencies.

Excitation pattern (spectrum) or tuning curve (frequency response)?

Psychophysical tuning curves Determine the minimum level of a narrow-band masker at a wide variety of frequencies that will just mask a fixed sinusoidal probe.

Excitation pattern (spectrum) or tuning curve (frequency response)?

Other auditory abilities also reflect frequency selectivity

The ability to hear out individual harmonics in a periodic complex sound.

speech

Really, the End!