SECOND YEAR COURSE AUTUMN

PERCEPTION
Hearing Lecture notes (3): Introductory psychoacoustics

1. BASIC TERMS

There is an importnat distinction between terms used to describe physical properties and those used to describe psychological properties. Psychological properties are usually influenced by many physical ones.

Physical        Psychological   
Intensity Level     	Loudness        
Frequency       		Pitch           
Spectrum        		Timbre

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1.1. Absolute threshold

Human listeners are most sensitive to sounds around 2-3kHz. Absolute threshold at these frequencies for normal young adults is around 0 dB Sound Pressure Level (SPL - level relative to 0.0002 dyne/cm2). Thresholds increase to about 50 dB SPL at 100 Hz and 10 dB SPL at 10 kHz. A normal young adult's absolute threshold for a pure tone defines 0 dB Hearing Level (HL) at that frequency. An audiogram measures an individual's threshold at different frequencies relative to 0dB HL. Normal ageing progressively increases thresholds at high frequencies (presbyacusis). A noisy environment will lead to a more rapid hearing loss (40 dB loss at 4kHz for a factory worker at age 35, compared with 20 dB for an office worker). The term Sensation Level (SL) gives the number of dB that a sound is above its absolute threshold for a particular individual.

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2. FREQUENCY RESOLUTION AND MASKING

Ohm's Acoustic Law states that we can perceive the individual Fourier components of a complex sound. It is only partly true since the ear has a limited ability to resolve different frequencies. Our ability to separate different frequencies in the ear depends on the sharpness of our auditory filters. The physiology underlying auditory filters is described in the previous Notes. The bandwidth of human auditory filters at different frequencies can be measured psychoacoustically in masking experiments (see below). The older literature refers to the width of an auditory filter at a particular frequency as theCritical Band . Sounds can be separated by the ear when they fall into different Critical Bands, but they mix together when they fall into the same Critical Band. For example, only harmonics that are separated by more than a critical band can be heard out from a mixture; only noise that is within a critical band contributes to the masking of a tone. A simple demonstration of the bandwidth of noise that contributes to the masking of a tone is in the following band-limiting demonstration which is Demonstration 2 in the ASA "Auditory Demonstrations" CD.

* In silence, you can hear all ten 5dB steps of the 2000Hz tone.

* In wide-band noise you can only hear about five because of masking.

* As the bandwidth of the noise is decreased to 1000 Hz and then to 250 Hz there is no change, because your auditory bandwidth is narrower than these values.

* When the bandwidth of the noise is decreased to 10 Hz, you hear more tone steps because the noise bandwidth is now narrower than the auditory filter and so less noise gets into the auditory filter to mask the tone.



The masked threshold of a tone is its level when it is just detectable in the presence of some other sound. It will of course vary with the masking sound. The amount of masking is the difference between the masked threshold and the abolute threshold. Generally, individuals with broader auditory filters (as a result of SNHL) show more masking. In Simultaneous masking the two sounds are presented at the same time. In Forward masking the masking sound is presented just before the test tone. It gives slightly different results from simultaneous masking because of non-linearities in the auditory system.

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2.1.Psychophysical Tuning Curves

A psychophysical method can be used to generate an analogy to the physiological frequency threshold curve for a single auditory fiber. A narrowband noise of variable center frequency is the masker, and a fixed frequency and fixed level pure tone at about 20 dB HL is the target. The level of masker is found that just masks the tone for different masker frequencies. Compare the following diagram with the FTC in the previous Notes.



Using these techniques (and other similar ones) we can estimate the shape and bandwidth of human auditory filters at different (target) frequencies. The bandwidth values are shown in the next diagram. At 1kHz the bandwidth is about 130; at 5kHz about 650 Hz.



Psychophysical tuning curves measured in people with SNHL often show increased auditory bandwidths at those frequencies where they have a hearing loss.

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2.2. Excitation pattern

Using the filter shapes and bandwidths derived from masking experiments we can produce the excitation pattern produced by a sound. The excitation pattern shows how much energy comes through each filter in a bank of auditory filters. It is analogous to the pattern of vibration on the basilar membrane. For a 1000 Hz pure tone the excitation pattern for a normal and for a SNHL listener look like this:



The excitation pattern to a complex tone is simply the sum of the patterns to the sine waves that make up the complex tone (since the model is a linear one). We can hear out a tone at a particular frequency in a mixture if there is a clear peak in the excitation pattern at that frequency.

Since people suffering from SNHL have broader auditory filters their excitation patterns do not have such clear peaks. Sounds mask each other more, and so they have difficulty hearing sounds (such as speech) in noise.

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3. NON-LINEARITIES

To a first approximation the cochlea acts like a row of linear overlapping band-pass filters. But there is clear evidence that the cochlea is in fact inherently non-linear (ie its non-linearity is not just a result of over-loading it at high signal levels). In a non-linear system the output to (a+b) is not the same as the output to (a) plus the output to (b).

3.1. Combination tones

If two tones at frequencies f1 and f2 are played to the same ear simultaneously, a third tone is heard at a frequency (2f1 -f2 ) provided that f1 and f2 are close in frequency (f2 /f1 < 1.2) and at similar levels. Combination tones are often absent in Sensori-Neural Hearing Loss.
First listen to a 1000 Hz pure tone
Now listen to a tone that changes in frequency between about 1100 and 1700 Hz
Now listen to the two added together when the moving tone is near the bottom of its range you should be able to hear another, lower tone come in which is in fact moving in the opposite direction. This is the 2f1 - f2 combination tone (also known as the Cubic Difference Tone).
You can only hear it when the higher tone is sufficently close to the steady 1000-Hz tone, because the excitation patterns produced on the basilar membrane from the two tones have to overlap in order for the combination tone to be generated.

3.2. Two-tone suppression



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. Two-tone suppression is often absent in SNHL.

4. WHAT YOU SHOULD KNOW

You should understand:

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