Rob van der Willigen http://~robvdw/cnpa04/coll1/AudPerc_2007_P7.ppt Auditory Perception.

Rob van der WilligenRob van der Willigenhttp://~robvdw/cnpa04/coll1/AudPerc_2007_P7.ppthttp://~robvdw/cnpa04/coll1/AudPerc_2007_P7.ppt

Auditory PerceptionAuditory Perception

Q ui ckTi me™ and a TI FF (LZW) decompressor are needed to see thi s pi cture. Today’s goalToday’s goal

Understanding masking:

Critical bands in masking

Power spectrum model of masking

Measurement of masking

Q ui ckTi me™ and a TI FF (LZW) decompressor are needed to see thi s pi cture.

Psychoacoustics

SPL is not a measure of Perceived Loudness

Defined as the attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud.

Two sounds with the same sound pressure level may not have the same (perceived)loudness

A difference of 6 dB between two sounds does not equal a 2x increase in loudness

Loudness of a broad-band sound is usually greater than that of a narrow-band sound with the same (physical) power (energy content)

Recapitulation last weeks’ lecture


Psychoacoustics

Perceived Loudness: phone

A unit of LOUDNESS LEVEL (L) of a given sound or noise.

Derived from indirect loudness measurements

If SPL at reference frequency of 1kHz is X dB the corresponding equal loudness contour is the X phon line.

Phon units can’t be added, subtracted, divided or multiplied.60 phons is not 3 times louder than 20 phons!

The sensitivity to different frequencies is more

pronounced at lower sound levels than at higher.

For example: a 50 Hz tone must be 15 dB higher

than a 1 kHz tone at a level of 70 dB



Loudness Scaling: Magnitude of perceptual change

Psychoacoustics

Fechner predicted that a JND for a faint background produces the same difference in sensation as does the JND for a loud stimulus.

Thus, a scale of S (Loudness) should be derivable by counting intensity jnds

dS dI /I dS dI

II 0

I

0

s

Measure of loudness: sensation intensity (S) in JND units

Weber :I kI


dS dI

II 0

I

0

s

S 1

kln(I /I0)


Loudness Scaling: Magnitude of perceptual change

Psychoacoustics

Consequences of a logarithmic Loudness function:

Changes from 15 to 30 dB should be the same as the change from 30 to 60 dB.

If loudness additivity holds, two tones at 70 dB should sound as loud as one tone at 140 dB

What if the jnd does not represent a constant change in loudness? How could this be?

The jnd is determined by two things:

1) Perceptual distance (change in loudness)

2) Internal noise Fechner assumed (incorrectly) that internal noise

is constant.

Measure of loudness: sensation intensity (S) in JND units


dS dI

II 0

I

0

s

S 1

kln(I /I0)


Loudness Scaling: Stevens’ Power law

Psychoacoustics

Another function relating Loudness S is Stevens’ power law:

The exponent m describes whether sensation is an expansive or compressive function of stimulus intensity.

The coefficient a simply adjusts for the size of the unit of measurement for stimulus intensity threshold above the 1-unit stimulus.

maIS

=0.3



Scaling: Stevens’ Power law

Psychoacoustics

maIS

ln(S) ln(aIm ) ln(S) ln(a) mln(I)


Loudness Scaling: sone vs. phon

Psychoacoustics

SONE: a unit to describe the comparative loudness between two or more sounds.

One SONE has been fixed at 40 phons at any frequency (40 phon curve).

2 sones describes sound two times LOUDER than 1 sone sound.

A difference of 10 phons is sufficient to produce the impression of doubling loudness, so 2 sones are 50 phons. 4 sones are twice as loud again, viz. 60 phons.

6.00 )( ppkL

p is the base pressure of a sinusoidal stimulus, po is its absolute threshold.


Loudness Scaling

Psychoacoustics

Depends on:

Number of excited hair cells (hence bandwidth of sound)

Excitation of each cell (energy in each auditory filter)


Measuring Sound: Frequency Domain

Psychoacoustics


The Intensity Density Level of three types of NOISES:

Psychoacoustics

Physical parameters of sound waves: Power Spectrum Density

WHITHE NOISE BROWN (RED) NOISE GRAY NOISE

Intensity density

level [dB]

Log Frequency [Hz]


Psychoacoustics

Measuring Sound: Filter Characteristics

( )H f

Frequency

High Pass

( )H f

Frequency

Low Pass

( )H f

Frequency

Band Pass

Frequency

Band Reject

( )H f


Acoustic Filtering of the Auditory system: A-weighting

The shapes of equal-loudness contours have been used to design sound level meters (audiometer).

At low sound levels, low-frequency components

contribute little to the total loudness of a complex sound.

Thus an A weighting is used to reduces the contribution of low-frequencies.


Acoustic Filtering of the Auditory system:Audiograms of non-humans also shows weighting


Psychoacoustics

Measuring Sound: Filter boundaries


Psychoacoustics

What is Masking?

“The process by which the threshold of audibility for one sound is raised by the presence of another (masking) sound.”

(American Standards Association, 1960)

How can masking occur?

1) Excitation: Swamping of neural activity due to masker.

2) Suppression: Reduction of response to target due to presence of masker.


Psychoacoustics

What is Masking? Simultaneous / Time Shifted

The presence of one sound masks (hides) the presence of another

A loud sound will mask a quieter sound (even if presentedbefore (forward masking) or after (backward masking) thequieter sound)

e.g. Given a masking tone of 400 Hz 70dB of SIL, a 600 Hz has to be >100 dB SILthan its minimal threshold level (i.e., threshold in quiet) in order to become audible in presence of the 400 Hz masker tone.


Psychoacoustics

Temporal aspects of Masking

(1) Post-stimulus/Forward/Post-masking: 1st Masker 2nd test tone

(2) Pre-Stimulus/Backward/Pre-masking: 1st test tone 2nd Masker

(3) Simultaneous Masking: Test tone and Masker together


Psychoacoustics

Two Definitions of Masking

The process by which the threshold of audibility for one sound is raised by the presence of another (masking) sound.

The amount by which the threshold is raised by the masker (in dB).


Fletcher (1940) conducted an simultaneous masking experiment in which there was band-pass noise and a single sine wave.

The frequency of the sine wave was always at the center frequency of the noise, and the power density of the noise was fixed.

The bandwidth of the noise was varied, and for each bandwidth the minimum intensity at which the sine wave could be perceived was determined.

With increasing bandwidth, the total energy of the noise increased.

Psychoacoustics

Critical bands in Masking


Psychoacoustics


Handbook of Psychology By Irving B. Weiner, Donald K. Freedheim, John A. Schinka, Wayne F. Velicer, Alan M. Goldstein

http://books.google.com/books?id=fErelr18MEUC&pg=PA87&lpg=PA87&dq=%22Fletcher+(1940)+%22+http://books.google.com/books?id=fErelr18MEUC&pg=PA87&lpg=PA87&dq=%22Fletcher+(1940)+%22++masking+experiment&source=web&ots=vz3C3Mzhgb&sig=EgANuNFgxcVLWlmnj9oWQYIDD9I#PPA88,M1+masking+experiment&source=web&ots=vz3C3Mzhgb&sig=EgANuNFgxcVLWlmnj9oWQYIDD9I#PPA88,M1


Psychoacoustics



A sine (signal) in the presence of noise that has a band width (in frequency) centered around the signal.

The wider the noise bandwidth the more the signal (sine wave) is masked.

critical band

Psychoacoustics


Past a particular (frequency) band-width beyond which the threshold doesn’t increase.


Psychoacoustics

Critical band: ERB

The transition point of the auditory filter is known as the Critical Band.

This has also been termed the Equivalent Rectangular Bandwidth (ERB).

SP

L (d

B)

Frequency (Hz)2000 Hz

150 Hz300 Hz450 Hz600 Hz

Physical bandwidth

Critical band

400 HzAuditory filter bandwidth

The critical band is thepoint at which thresholdsno longer increase.

Conceptually verypowerful, but not muchuse in providing anaccurate estimate of filter bandwidth.

Not possible to discernfilter shape from results.


Psychoacoustics

Critical band versus Critical Ratio

SP

L (

dB

)

Frequency (Hz)2000 Hz

Critical band = ERB

400 HzAuditory filter bandwidth


Psychoacoustics

Masking Curves versus ISO-L curves

(left Column)

Probe threshold, Lp, or Masker threshold, Lm, plotted with fp, as independent variable, will be referred to as "masking curves."

(right column)

Curves for a fixed probe frequency and with fm as the independent variable will be referred to as "iso-Lp curves" when the masker level Lm, (at probe threshold) is plotted as a function of fm.

For plots of the probe level Lp as a function of the masker frequency we will use the term "iso-Lm curves."


Experimental procedure:

The procedure for a masking experiment.

(a) The threshold is determined across a range of frequencies. Each arrow indicates a frequency where the threshold is measured.

(b) The threshold is re-determined at each frequency (small arrows) in the presence of a masking stimulus (large arrow)

Psychoacoustics

MASKING CURVE


Psychoacoustics

The Masking Curve

Shown is the hearing curve (red) and a single tone (sine-wave) with a frequency of 1kHz (black).

The green curve is the masking curve due to that tone. Indicates the amount that the threshold is raised in the presence of a masking noise centered

The band of noise in yellow at a centre frequency of about 1.5kHz cannot be perceived by the human ear because of the masking effect of the tone at 1kHz.


Experimental procedure:

First, a low level test tone is presented.

Then, masking tones are presented with frequencies above and below the test tone.

Measures are taken to determine the level of each masking tone needed to eliminate the perception of the test tone.

Assumption is that the masking tones must be causing activity at same location as test tone.

Psychoacoustics

ISO-Lp curves (Lm versus Fm): Psychophysical Tuning Curve


The procedure for measuring a psychophysical tuning curve.

(a) A 10-dB SPL test tone (blue arrow) is presented.

(b) Then a series of masking tones (red arrows) are presented at each frequency.

The psychophysical tuning curve is determined by measuring the sound pressure of each masking tone that reduces the perception of the test tone to threshold.

Psychoacoustics

ISO-Lp curves (Lm versus Fm): Psychophysical Tuning Curve


Psychophysical tuning curves for a number of test-tone frequencies (dots). Notice how the minimum masking intensities for the curves match the shape of the audibility curve (dashed line). (Based on Vogten, 1974).

Psychoacoustics

Psychophysical tuning curves: ISO-Lp curves (Lm versus Fm)

Psychophysical Tuning Curves (PTCs):

Fixed signal; masker level adjusted to just mask signal.

Advantages: Concept v. similar to neural tuning curves, allowing direct comparisons.

Potential problems: “Off-frequency listening” Detection of beats if using a sinusoidal masker.


The procedure for measuring a psychophysical tuning curve.

A10 dB test tone (black arrow) is presented and then a series of

masking tones (red arrows) are presented at the same time as

the test tone. The psychophysical tuning curve is generated by

determining the SPL threshold of the masking tones needed to

reduce the perception of the test tone to threshold

Psychoacoustics

Psychophysical tuning curves: ISO-Lp curves (Lm versus Fm)

Vibration patterns on the basilar membrane caused by 400, 800 and 1000 Hz tones

(a) Three human psychophysical tuning curves generated using the method described in right figure. The arrows show the frequency of three different test tones. You can see from the figure that when the masking tone is the same as, or close to, the test tone in frequency, the intensity of the masker needed to mask the test tone is low.

(b) Three neural tuning curves showing the stimulus intensity needed to generate a constant response (firing rate) in the nerve fiber of a cat. Each curve represents a different auditory nerve fiber


Frequency Tuning Curves (FTCs): measured by finding the pure tone amplitude that produces a criterion response in an 8th nerve fiber.

Psychoacoustics

Psychophysical tuning curves versus Frequency Tuning Curves

Psychophysical Tuning Curves (PTCs):

Fixed signal; masker level adjusted to just mask signal.


Resulting tuning curves show that the test tone

is affected by a narrow range of masking tones.

Psychophysical tuning curves (PTC) show the same pattern as neural tuning curves which reveals a close connection between perception and the firing of auditory fibers

Advantages: Concept v. similar to neural frequency tuning curves (FTC), allowing direct comparisons.

Potential problems: “Off-frequency listening”Detection of beats if using a sinusoidal masker.

Psychoacoustics

Summary ISO-Lp curves (Lm versus Fm) or PTC


Psychoacoustics

TWO_TONE SUPRESSION

In single auditory nerve recordings, the response to a just supra threshold tone at CF can be reduced by a second tone, even though the tone would - itself have increased the nerve's firing rate.

A similar effect is found in forward masking. The forward masking of tone a on tone c can be reduced if a is accompanied by a third tone b with a different frequency, even though b has no effect on c on its own.


Psychoacoustics

Iso-Lm curve (Lp versus Fp): Masked Audiogram

Lp as function of Fp Masking curves (masked audiograms) for a narrow band of noise centered at 1 kHz and bandwidth of 160 Hz. (Lm is constant)

Each curve shows the elevation in the threshold of sinusoidal signal as a function of signal frequency. That is: for a fixed narrowband masker, the change in threshold for a single-tone probe over a specific frequency range is determined.

The overall noise level of each curve is indicated in the figure.


Psychoacoustics

Shape of auditory filter: Excitation patterns

The shape of auditory filters as determinedfrom the shape of tuning curves in masking experiments


The auditory filters can be approximated by rectangular filters, but better determination of the filter shape is possible.

The critical bandwidth at a particular frequency can be estimated using the formula

where P is the intensity of the signal, N0 is the noise power over a 1-Hz range, K is the threshold of detectability (usually 0.4), and W is the critical band width (CB). N0 is independent of frequency.

For example, the CB at 1000 Hz is 160 Hz; however, in reality rectangular filters are not accurate; the shape changes with frequency and amplitude

Better approximations of the auditory filters look like this:

W P

(K N0)

Psychoacoustics

Shape of auditory filter: Power Spectrum Model

SP

L (d

B)

Frequency (Hz) 1000 Hz

Critical band

160 HzAuditory filter

bandwidth


Psychoacoustics

Power Spectrum model of Masking

Fletcher's experiment led to a model of masking known as the power-spectrum model that is based on the following assumptions:

1. The peripheral auditory system contains an array of linear overlapping band-pass filters.The non-linearity of the filters is now well known.

2. Listener detect signals by using just one filter with a center frequency close to that of the signal.

Listeners clearly combine information across filters

3. Only the components of the noise which pass through the filter have any effect in masking the signal.

Energy outside the filter can play an important role (see literature on informational masking and co-modulation masking release)

4. Detection threshold is determined by the amount of noise passing through the filter, calculated as the ratio of the long-term power spectra of signal and noise.

Fluctuations in the masker can play a strong role


Psychoacoustics

Shape of auditory filter: notched noise method

signal

maskermasker

Hypotheticalauditory filter


Psychoacoustics


The deviation from each of the noise edges to the signal frequency is denoted by delta f.

The measurement consists of determining the signal threshold for different notch widths, while maintaining the level of the noise masker constant.

Since the signal is symmetrically placed at the center of the notch, the method cannot reveal any filter asymmetries.

As the width of the notch is increased, less and less noise leaks through the filter skirts and the threshold is reduced.

The variation in threshold with notch width can be seen as a measure of thearea of the noise leaking through the filter skirts.

Then, assuming that threshold corresponds to a constant signal to masker ratio, the filter function can be obtained by differentiating the threshold function respect to delta f,given that the integral of a function between certain limits corresponds to the area under that function.

This is the basic idea that has been used to determine the filter shapes using this method.


Psychoacoustics


The "notch-noise method" involves the determination of the detection threshold for a sinusoid, centered in a spectral notch of a noise, as a function of the width of the notch.

On the basis of results obtained with this method, auditory frequency selectivity can be described in terms of an "equivalent rectangular bandwidth" (ERB) as a function of center frequency.

Both spectral and temporal analysis contribute to the detection of the sinusoid.

The CB and the ERB have been found to be proportional for center-frequencies above 500 Hz.

Advantages: No influence of beats. Allows accurate measurement of filter “tails” (remote regions). Analysis can take into account off-frequency listening.


Psychoacoustics


The shape of the auditory filter centered at 1 kHz, plotted for relative response of the filter in dB as a function of fequency.


Psychoacoustics

Iso-Lm curve (Lp versus Fm) or Masked Audiograms

Fixedmasker

Adjustable signal

‘upward spreadof masking’

High-intensity maskers spread their effect towards high frequencies. A response of the Basilar Membrane.


Psychoacoustics

Iso-Lm curve (Lp versus Fm): Beating

TONE-TONE masking causes BEATS:

The masking patterns from masked audiograms do not reflect the use of a single auditory filter. Rather, for each signal frequency the listener uses a filter centered close to the signal frequency. Thus the auditory filter is shifted as the signal frequency is altered.

Moreover, varying the frequency distance between two tones results in changed perception. A sense of roughness emerges for distances below a certain threshold (Rocchesso).

If the frequencies are far enough apart we perceive two tones.

As they close a sensation of roughness emerges, but the separate tones can still resolved.

As they get even closer, we stop perceiving two separate tones and hear a single tone that beats.


Psychoacoustics

Excitation patterns or Masked Audiograms

Moore and Glasberg(1983) presented a simple method for obtaining an estimate of the “internal representation”of any arbitrary signal.

To be precise, it is the output of the bank of auditory filters calculated by the Patterson(1976) method.

As a simplification, the tails of the filter are ignored, removing the parameter from the equation, leaving the following

This allows the output of a bank of filters to be easily calculated for any input.

W (g) (1 r)(1 pg)e pg r

(1 pg)e pg


Psychoacoustics


Given auditory filter shapes, it is possible to derive masking patterns for any arbitrary stimulus.

Under the power spectrum model assumptions, a masking pattern is equivalent to an excitation pattern – the internal representation of a sound’s spectrum.

But does masking pattern = excitation pattern?


Psychoacoustics


The shapes of excitation patterns for narrowband stimuli as a function of level can be determined approximately from theirmasking patterns.

The patterns closely resemble masked audiograms at similar masker levels, showing the classic"upward spread of masking."

Moore and Glasberg (1987b) concluded that the critical variable determining the auditory filter shape is the input level to the filter.


20

80

70

60

50

40

30

20

10

3

0 2 3 4 5 6 7 8 9

Frequency (kHz)

10 11 121

40

BM

Ve

loci

ty(d

B r

e.

1µ/s

)

60

Cochlear nonlinearityCochlear nonlinearityActive processing of sound

The response of the BM at location most sensitive for ~ 9 KHz tone (CF).The level of the tone varied from 3 to 80 dB SPL (iso-intensity contours).

BM input-output function for a tone at CF (~9 kHz, solid line) and a tone one octave below (~4.5 kHz) taken from the iso-intensity contour plot.

INPUT level (dB SPL)OU

TP

UT

Res

pons

e in

dB

CF= 9 kHz

~4.5kHz

Frequency [kHz]

Res

pons

e in

dB


Psychoacoustics

Uses for Excitation patterns (Masked Audiograms)

Loudness: Transformed area under the excitation patternSuggested by Fletcher, formalized by Zwicker, refined by Moore.

Timbre: Centroid, or center-of-gravity of an excitation pattern.

Pitch: Positions of peaks within the excitation pattern or amplitudes.

Masking: Predicting the masking effectiveness of an arbitrary stimulus.

Used (with modifications) in audio coding, e.g., MP3.


Psychoacoustics

Limitations of Excitation patterns (Masked Audiograms)

Nonlinearities, such as suppression and distortion products, are not accounted for:

Can overestimate masking: Ignores temporal information (envelope or fine structure)

Beats Effects of masker modulation Detection of tones in roving-level narrowband noise

Can underestimate masking:

Stimulus uncertainty (e.g., Neff and Green, 1987) can produce large amounts of “informational” masking without any energy around the signal frequency.


Psychoacoustics

Filter Bandwidth as function of center frequency

"equivalent rectangular bandwidth" (ERB)

ERBN= 24.7•(4.37F+1) Hz

The value of ERBN is in Hz, but center frequency F is in kHz.

For lower frequencies, the ERB decreases with decreasing center-frequency, while the CB remains close to constant.

The discrepancy can be explained by the assumption that the temporal fine structure of the signal is not resolved in loudness summation, while it contributes substantially to frequency resolution for f < 500 Hz. Due to differences in bandwidth definition, the ERB is narrower than the classical critical band at all frequencies.


Psychoacoustics

Auditory filter shape as a function of BARK

A frequency scale on which equal distances correspond with perceptually equal distances.

1 bark = width of 1 critical band

Above about 500 Hz this scale is more or lessequal to a logarithmic frequency axis.

Below 500 Hz the Bark scale becomes more andmore linear.


The bandwidth of a filter is referred to as the “Critical Band” or “Equivalent Rectangular Bandwidth” (ERB).

ERB’s and Critical Bands (measured in units of “Barks”, after Barkhausen) are reported as slightly different.

ERB’s are narrower at all frequencies.

ERB’s are probably closer to the right bandwidths, note the narrowing of the filters on the “Bark” scale in the previous slide at high Bark’s (i.e. high frequencies).

The term “Critical Band” is less accurate than the ERB scale.

Bear in mind that both Critical Band(widths) and ERB’s are useful, valid measures, and that you may wish to use one or the other, depending on your task.

There is no established “ERB” scale to date, rather researchers disagree quite strongly, especially at low frequencies. It is likely that leading-edge effects as well as filter bandwidths lead to these differences.

The physics suggests that the lowest critical bands or ERB’s are not as narrow as the literature suggests.

Psychoacoustics

Summary: frequency selective masking

Rob van der Willigen http://~robvdw/cnpa04/coll1/AudPerc_2007_P7.ppt Auditory Perception.

Documents

Transcript of Rob van der Willigen http://~robvdw/cnpa04/coll1/AudPerc_2007_P7.ppt Auditory Perception.