UNIVERSITY OF SURREY DEPARTMENT OF … lab scripts/exp_2K...tones that vary between purely periodic and purely aperiodic. As shown in Figure 2, As shown in Figure 2, vowel sounds (a,

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UNIVERSITY OF SURREY DEPARTMENT OF PHYSICS

Level 1: Experiment 2K

AUDIOMETRY and PERCEPTION of SOUND

1 INTRODUCTION and BACKGROUND 1.1 Aim In this series of experiments you will measure your own and your partner’s hearing ability and you will investigate human perception of sound. You will learn the methods by which hearing ability is measured. 1.2 Skills The particular skills you will start to acquire by performing this experiment are:

l Careful measurements involving the co-operation of another human being.

l Error analysis and the human factor. l Use of a range of auidiometry equipment. l Graph plotting and data interpretation.

Before commencing this experiment ensure that you have carefully read the Ethics Guidelines given in Appendix A. Then sign and date the Medical Physics Option form given in Appendix E and hand to Liz Griffiths before starting the experiment. 1.3 Background Concepts

Audiology is the study of hearing and hearing disorders [1]. The identification and measurement of hearing loss is performed by an audiologists using the techniques of audiometry.

Most of the sound that we hear has travelled through the air, although sound can also travel through solids. In air, sound travels in a standard atmosphere with a speed of approximately 341 m/s, which is much slower than the speed of light, which is 3 x 108 m/s. The difference between the speeds of sound and light is apparent to anyone in a thunderstorm. There is generally a noticeable time delay between when a flash of lightning is observed and a clap of thunder is heard [1].

The normal human ear of a young adult can detect sound with frequencies ranging from 20 Hz to 20,000 Hz. Higher frequencies are called ultrasonic, and lower frequencies are called subsonic. The greatest sensitivity for humans is found between frequencies of 500 through 8000 Hz [1]. Note that “middle C” for musical instruments is defined as having a frequency of 256 Hz. Thus it is obvious that sounds outside of the most sensitive range can nevertheless be heard if they are loud enough. In music, each octave above middle C corresponds to a doubling of the frequency, whereas each halving of the frequency corresponds to a lowering by one octave. How many octaves of sound are humans with normal hearing able to hear? (Show your calculation in your lab diary.)

Experiment 2K - Audiometry

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Consider a source of sound that produces a noise with an energy of E in a time, t. The power will be E/t, of course, and will have units of watts. What is the intensity of sound that will be heard by a listener? The answer depends on the distance away from the source, r. Assuming that the sound travels in all direction, it will be uniformly distributed over all points at a distance r; these points define the surface of a sphere with an area of πr2. The intensity, I, is defined as the power emitted by the source divided by the area of a sphere of radius, r. I follows the so-called inverse-square law because it varies as r-2. SI units for sound intensity are Wm-2.

Humans can generally hear sounds over a range of intensity spanning several orders of magnitude. A typical range is from 10-12 Wm-2 to 102 Wm-2 [1]. Below the smaller intensity, sound cannot be detected by the normal human ear. Above the higher intensity, it becomes uncomfortable to hear, and the “pain threshold” is reached. A “bel” represents an increase in tensity by a factor of 10 (i.e. one order of magnitude). Thus, the range of intensity for human hearing covers a range of 14 bels. As the unit of bels is quite large, a less coarse unit is often used. It is defined as one-tenth of a bel and is called the decibel. Hence, humans hear over a range of 140 decibels. When human hearing is tested, the lowest sound that the subject can hear is compared to what is defined as normal for a person with good hearing. If a subject can only hear a sound greater than an intensity of Is, but the normal lower limit of intensity is Io, then the sound must be louder by N decibels in order to be heard. N is given by N = 10 log(Is/Io).

When testing human hearing, so-called pure tones are often used. A pure tone is one that has a single frequency. The sound wave can be represented on a plot of amplitude versus time on which there is a single wavelength. In Figure 1, three pure tones are represented in waves (a), (b) and (c). The frequencies of the waves are integer multiples of each other. Can you see why? If these three sounds are played simultaneously, the complex waveform shown in (d) is the result. You can easily see that the complex waveform is not sinusoidal, but it is still periodic. As the frequencies are harmonically related, there is some tonality in the sound of waveform d. A waveform that cannot be described by a tonal spectrum of frequencies that are harmonically related is called aperiodic. Such a sound would consists of many frequencies. Aperiodic sounds are called white noise, in analogy to white light, which also consists of many frequencies.


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Figure 1. Three harmonic waves (a,b and c) and their sum (d). From reference [1].

Figure 2. Waveforms for the vowel sound “Ah” and for the consonant sound “Sh”.

Persons who are hard of hearing sometimes find it difficult to recognise human speech. Testing of a person’s ability to hear pure tones does not directly measure how well he or she can understand speech. The reason why is that speech consists of tones that vary between purely periodic and purely aperiodic. As shown in Figure 2, vowel sounds (a, e, i, o and u) tend to be periodic, whereas consonants are more aperiodic. In a pure-tone audiogram, the minimum intensity of sound that can be heard by the subject is determined for various frequencies. The results are shown as the difference in intensity between a normal subject and the test subject. Thus, if the test subject has perfectly normal hearing, the intensity will be 0 dB at all frequencies. If the hearing is better than normal, a negative value of dB is measured. A person with a hearing disorders might be able to hear well at some frequencies but not at others. A good example is a person who has lost hearing as a result of chronic exposure to excessive noise. Over time, the ability to hear at higher frequencies, centred around 4000 Hz, is gradually lost, as is shown in Figure 3. Human speech is based on tones with frequencies nearer to 2000 Hz. As a result, persons having noise-induced hearing loss often do not notice it until the loss becomes more severe and they have difficulty hearing in the frequency range of speech. By then, their hearing loss at higher frequencies is very severe. In industries in which workers are exposed to excessive noise, frequent checks are often made to detect the onset of any hearing loss. If you have ever been to a concert at which the music was excessively loud you might have noticed some temporary hearing loss afterwards. It has been found that the tiny hairs within the ear can be “fatigued”, but they are able to recover within a day or so. With exposure day after day, however, damage becomes permanent.


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Figure 3. Audiograms for a subject exposed to prolonged noise exposure. With increased exposure times (as shown on the plots), hearing loss is seen at higher frequencies. From reference [1].


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2. EXPERIMENT A: PURE-TONE AUDIOMETRY Before you start your measurements, you should calculate the relevant wavelengths of sound used in the experiments. Make a table showing the wavelengths of sound corresponding to the range of detectable frequencies and to the range of frequencies to which humans are most sensitive. In this experiment you will measure each other’s hearing and plot graphs to show the response of your ears to airborne sound. Because of the need for a silent background against which the ear’s response can be measured, the experiment is performed in the Acoustic booth. First decide who is going to be tested first, hereafter known as the subject, and who is going to do the testing. Then read through the whole of these instructions before commencing the experiment. Switch on the light and ventilating fan in the booth and try sitting inside it on a stool for a few moments with the doors closed to familiarise yourself with the feel of the booth and to make sure that you do not suffer from claustrophobia. Next plug the Audiometer and signal light unit into the mains and switch on the Audiometer. (The switch is under the lid.) Connect the DIN lead between the phone socket on the Audiometer and the DIN 4 input on the booth. The headphones plug into the DIN 4 socket inside the booth. Also plug the signal light to the stereo socket outside the booth and the hand controlled light to the similar socket inside. Set the controls on the front panel of the Audiometer as follows: Tone/Speech to Tone Normal +10dB to Normal Coupling to OUT Tone switch to OFF Mask to OFF Selector to S Red With this set up pressing the switch bar at the bottom of the panel produces in the red (right) earpiece of the earphones a tone whose frequency and intensity are selectable by the appropriate knobs, and which lasts as long as the bar is held down. 2.1 Practice Run The subject should sit in the booth with the doors open and put on the headphones (RED to RIGHT ear.) These headphones should be adjusted for comfort as they will be worn for some time. The subject should turn away from the tester so that the Audiometer can not be seen from inside the booth. The tester selects 1000Hz, 60dB and presses the bar 3 or 4 times at random intervals for a second or so each time. The subject should signal that the tone is being heard by flashing the signal light each time, trying to match the duration of the flash with that of the tone.


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Reduce the intensity in steps of 10dB until the sound is no longer heard by the subject. Finally adjust the intensity up or down in 5dB steps to determine the threshold level, which is defined as the level at which the subject responds to the tone correctly at least 50% of the time. (For instance, 2 out of 3 times and 2 out of 4 times are both sufficient to conclude that the threshold has not yet been obtained.) Practice this until you both get the hang of it and can get a reproducible result. Note that the intensity control range extends below 0dB, although you are unlikely to reach these levels with the booth doors open. When you are both happy with the technique proceed to the test itself. 2.2 Test Procedure The subject now sits in the closed booth while the above test is applied at a range of frequencies from 125Hz to 8000Hz, the tester recording the threshold level each time. It will usually be adequate to start at the 40dB or even the 30dB level. The Selector switch is then turned to S Blue and the whole test is repeated on the other ear. Avoid spending a great deal of time on this test - the beginner often makes the mistake of taking too long on each frequency, in an effort to guarantee the best possible threshold measurement. Prolonging the test in this way leads to problems, since the subject tires quickly, particularly when listening to tones close to the threshold. It is preferable to proceed through the test rather quickly - no more than 10-15 minutes should be required to test both ears. The subject’s audio response can then be plotted on the chart provided in Appendix B, on which 0 dB corresponds to the standard of “normal” hearing, although the statistical scatter on this figure is appreciable. Now the tester and subject should change places and the whole experiment repeated. Thus, you and your partner will produce four different audiograms. Examine the hearing charts you have produced. Are there any appreciable differences in response between the right and left ears? Is there any significant drop in response at the higher frequencies? If so, does the subject often attend discos or play a Walkman very loudly? Either of these can result is loss of response, and it would be very worthwhile consulting your own doctor in these circumstances while avoiding further exposure to high sound levels. Look up in a standard textbook[1] the topic of “noise-induced hearing impairment”, and explain why such impairment commonly shows up at the higher frequencies first. Explain also why brief exposures to intense noise can produce a temporary threshold shift. Next repeat at least one of your measurements (i.e. left or right ear for one of the subjects). Evaluate the level of reproducibility that you observe. Make a list of factors that can limit the amount of reproducibility that you can achieve in such experiments.


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The above measurements were obtained in a very quiet environment. To examine the effect of background noise on hearing response repeat the whole tests with the Mask control set to 50dB. This injects a random noise signal into the opposite ear during the tests. Plot the results on the same charts as before and comment on any differences. 3. EXPERIMENT B: MEASUREMENT OF A SPEECH RECOGNITION

CURVE This experiment is concerned with measuring an individual’s ability to recognise speech using formal speech audiometric procedures. Pure-tone audiometry, as you performed in Experiment A, provides valuable data for diagnostic purposes but the correlation between a pure-tone audiogram and a person’s ability to understand speech is variable. Consequently, when it is necessary to assess a person’s speech perception abilities, a quantitative measure is required. The principles behind standardised speech audiometry are the following: (1) The speech test material has been validated and normalised on appropriate

groups of subjects with normal hearing. (2) The test material is recorded in an agreed manner with relevant calibration

tones. (3) The test subject is instructed properly as to how to respond to the test signals

and is able to respond accordingly. (4) The test material is appropriate to the test subject, e.g., he knows the language

and the vocabulary being used in the tests. (5) The audiometer being used is properly calibrated. (6) The environment for testing is appropriate, e.g. without visual or auditory

distraction. You should make sure that these principles hold for the tests that you conduct in the lab. 3.1 Test Apparatus and Audiometer Set-up The test apparatus consists of a pure tone audiometer with a speech channel into which is connected a CD player. The basic set-up of this apparatus is similar to the one used in Experiment A. On the front of audiometer, you should be able to locate the following control knobs: Hearing Level (dB); Masking (on/off); Output (Left, Right, Bone); Tone Switch; Function (Manual, Auto, Speech, Balance); and Frequency (Hz). In this experiment, you will be adjusting the sound level in increments of 5 or 10 dB. The masking should be "off". The output will be directed into either the left ear or the right ear. You will be using the Cd player, and so you will not need the "Tone Switch". Set the Function to "Tape". Then connect the CD player output to the audiometer. You can do this by plugging one end of the cable into "Tape line in/out" on the Cd player and the other end into the "Tape Input" on the audiometer. An input level control is adjacent to the input socket. The CD contains 15 lists of words, known as Boothroyd word lists. The word lists are written out on the score sheets in Appendix C. The words are each one syllable.


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Don’t look at the lists now, or it will spoil the test for you if you are the subject! Earphones will be used to deliver speech signals to the subject. It is also possible to perform the tests when delivering speech by means of a bone vibrator (in contact with the skull behind the ear) or free field, i.e. in the air from a loud speaker. Decide who will be the subject and who will be the tester. The CD player should be placed so that the the tester can hear the CD playing, but the subject cannot hear the test material while wearing the headphones. The volume control on the CD player does not affect the output to the audiometer. Play the CD and adjust the level of the calibration tone to read zero on the level meter on the audiometer. The level control should now be kept constant for the rest of the test. 3.2 Performing the test Instructing the subject The tester should advise the subject which ear will be tested. The tester should instruct the subject that he will be asked to listen for a word and then say that word out loud, as they have heard it. If the subject is not sure of the word, he should guess it. If only a part of a word is heard, then the subject should only say that part. The subject should respond as quickly as possible and give only a single response. The tester should inform the subject that he will hear simple monosyllabic words, such as “dog” and “cat”. At the beginning of the test, the words will be easy to hear, but they will then get quieter in later tests. Speech detection threshold The tester should present the connected speech to the subject at a level where it can be easily heard. The tester should then reduce the level in 5 dB steps until the subject can no longer hear it. The tester should then raise the level in 5 dB steps until the subject can hear it once again. Finally, the tester reduces the level again until the subject can no longer hear it. The tester should record the level at which speech was last heard. This level should be compared to the level of the subject’s pure-tone audiogram at 500 Hz. Are they similar? Comfortable listening level for continuous speech The tester should tell the subject that a test is being done to find the speech level that is most comfortable. The connected speech should be played again starting at a level of about 50 dB. The subject should say whether this is too loud, too quiet or just right. If the sound is too quiet or just right, the tester should increase the level in 10 dB steps. When the subject says the level is too loud, the tester should reduce it in 5 dB steps, asking after each adjustment if it is too loud, too quiet or just right. This procedure is continued until the subject is happy that the right level has been found so that he can listen to the CD for a long period of time with no discomfort but while also being able to hear it clearly. Obtaining a speech recognition curve It is now time to start the speech recognition tests using the Boothroyd word lists.


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The first list should be played for the subject at the comfortable listening level that was determined in the previous section. The method for scoring is described in the next section. The tester should reduce the level by 10 dB and then play the next word list for the subject. Tests should be continued at successively lower 10 dB levels. The tester should continue until within 10 dB of the speech detection level for the subject, determined previously. The tester should plot the results on a speech audiogram (see Appendix D). An example of a typical speech audiogram is shown in Figure 4. How does the speech level where a 50% score is obtained compare to the levels of the pure tone audiogram?

Figure 4. A typical speech audiogram for monosyllabic words and for words with two syllables (spondees). The success rate is higher for spondees, because the

subject can use a knowledge of the language to guess some of the sounds correctly, even when they have not been heard. From reference [1].

The other ear of the subject should be similarly tested, making sure that new word lists are used. How do the two ears compare? A person with a hearing condition known as sensori-neural hearing loss will not be able to understand all words even when the sound level is high. An example of a speech audiogram for such as subject is shown in Figure 5.


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Figure 5. A speech audiogram for a subject who has sensori-neural hearing loss.

From reference [1]. Scoring the test Each word in a list consists of three phonemes: a consonant, then a vowel, and then a final consonant. Three points are assigned to each word in the Boothroyd lists, one for each phoneme that is correctly heard. The first two words are for practice, so that the perfect score for the remaining 10 words is 30 points. If a subject correctly hears only one phoneme (such as the vowel), then only one point is awarded for that word. If just one consonant is wrong, the score would be two points. For each test, a percentage score is obtained by considering the points awarded out of total of 30 points. An example of how to score a test is given below.

Test word Subject’s Response Score Cat cat 3 Dog fog 2 Hat -a- 1 Rug rug 3 Heap heat 2 Thumb sum 2 Goat gate 2 Dope dope 2 Five live 2 Vet --- 0 Total 20 Score = (20/30)x100% = 66%


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4. EXPERIMENT C: PSYCHOPHYSICAL PERCEPTION OF

SOUND INTENSITY Psychophysics concerns itself with the direct measurement of sensation, and the relationship between the strength of an external stimulus and one’s perception of its subjective intensity[2]. Early experiments in psychophysics were able to do no more than to suggest that the relationship between these two quantities was non-linear. Later investigations showed that in order to produce a just detectable change in perception a fixed percentage must be added to the strength of the stimulus (rather than a fixed increment.) In 1850 G.T.Fechner conceived the idea that a doubling of the stimulus might produce a constant increment in the perceived sensation, and this concept became the basis for the formulation of a power law of the form:

xkβα = (2) where α represents the magnitude of the perceived sensation and β the magnitude of the stimulus. The value of the constant k depends on the units of measurement, but the exponent x depends on the type of sensation being explored, such as reaction to light and sound, warmth and cold, vibration and shock. Work carried out after Fechner showed that a power law of the above form held for a wide variety of stimuli and responses. For example, although the value of x for visual perception of length was 1 (i.e. that α was directly proportional to β, which is just as well when you think about it), it could be as high as 3.5 for response to electric shock. (Doubling the strength of a shock increases the perceived sensation by 23.5 or about 11 times.) β is as small as 0.33 for perception of visual brightness. In these experiments you will investigate the validity of this power law and attempt to determine x for the auditory perception of sound level. 4.1 Auditory sensation. In this investigation the audiometer is used to produce stimuli in the form of 3000 Hz signals of various intensities in the range 40 to 110 dB. One experimenter (A) will control the audiometer while the other (B) records their perception of the loudness of the signal. A Experimenter A should write out a list of audiometer loudness settings

(which should not be shown to B at this stage) in 10 dB steps from 40 to 110 dB random order, but starting with a signal strength of 50, 60, or 70 dB. B should prepare a table in which the first column is simply numbered from 1 to 8.


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B B should now sit in the acoustic booth and put on the headphones. A should set the audiometer frequency to 3000 Hz and check that the other settings are unchanged from those used in the earlier audiometer measurements - note particularly that masking must be turned off. The signal may be fed to either of B’s ears, unless the earlier investigations have revealed a very marked difference in their sensitivity.

C A should now press the bar on the audiometer and deliver about a 1 second

burst of sound at the first selected loudness, which B should record as level 10 (as in A of the previous experiment). Repeat this for the remaining seven sound levels, leaving a few seconds gap between each, while B assigns a value to the perceived loudness of each sound in turn.

D Now construct a table of perceived loudness α, log α and log [sound level].

Note carefully that the dB scale is already a log scale since

sound level [dB] = 10 log β (4)

from the definition of the decibel. Plot log α against sound level. Is this a linear relationship? What is the gradient and hence the value of x? Note: gradient = x / 10.

E A and B should now interchange roles and repeat steps A to D. Note: In both these experiments there is no objection to your carrying out several

runs if you wish. Also, if you prefer, you may use the audiometer that was used for the Boothroyd speech recognition test.

4.2 Analysis Until carrying out their own experiments has persuaded them, many scientists have rejected psychophysical measurements completely. Are you convinced as a result of your investigations that the power law has any scientific basis? Psychophysical literature maintains that the value of the exponent x is about 2/3 for auditory perception. Do your results support this? To a physicist, who is used to working with basic scales, such as length, weight, etc., in which the value of the exponent x is 1 it will appear odd at first sight that scales exist with widely different values of this exponent. It is worth pointing out that the scale of perceived brightness (apparent magnitudes) of stars, first devised empirically by Hipparchus 2000 years ago, and which you may encounter in the Hubble experiment, is a logarithmic one. This was later refined into Pogson’s Law, which suggest that the perceived brightness m is proportional to the log of the light energy received from the star, f. Psychophysical experiments differ considerably from the great majority of those that you will have done in the past. List some of these differences and suggest reasons for them.


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5. EXPERIMENT D: CONDUCTION OF SOUND THROUGH BONE It is well known that sound can travel through solids as well as through air. But the speed of sound is slower in solids. The velocity v is a function of the compressive

modulus of the solid, K, and its density, ρ, as: ρν

K=

People with normal hearing perceive most sounds through conduction by air. The one major exception is the sound of one's own voice. In that case, vibrations from the throat travel through the skull and are detected by inner ear. At virtually the same time, sound is emitted into the air and detected that way. Consequently, most people are very surprised when they hear the sound of their own voice in a tape recording. It sounds different to what they normally hear, because they are not sensing much sound through bone. In cases in which a person's ear drum has been damaged, they can still hear through bone vibrations. In this experiment, you will measure a subject's ability to hear pure tones via vibrations in the skull. Use the free-standing audiometer that you have used for the speech recognition tests. Set the Output to "Bone" and set the Function to "Manual". This function allows the user to determine the time intervals between the tones by pushing the manual tone switch. Position the head set so that the vibrator fits snugly, and comfortably. The vibrating pad should be positioned on the skull behind the ear. Follow a testing procedure similar to that used in Experiment A. Prepare a plot of your results and compare to the pure-tone results for the same subject when the sound was transmitted through air by the headphones. Is there a noticeable difference between the two types of audiograms? 6. REFERENCES [1] H.A. Newby, Audiology, (Library Cat.No. 616.28). [2] S.S. Stevens, Psychophysics, J. Wiley & Sons (1975).


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APPENDIX A: Notes taken from the University of Surrey Ethical

Guidelines for Research and Teaching A.1 Teaching Experiments (i) Advisory Committee considers that it is ethically acceptable to request

an undergraduate or postgraduate student to participate in physiological experiments (e.g. swallowing a naso-gastric tube or using an exercise bicycle) or in experiments in the behavioural sciences as a normal part of his/her course on the understanding:

(a) that the supervisor ensures that all such studies conform with

the Committee’s Guidelines; (b) that the student has the right to decline a particular procedure

on religious, physiological grounds, etc; (c) that the student must be assured that, by declining to

participate in a particular procedure, his/her course marks will NOT be adversely affected;

(d) that undue academic pressure or financial inducement should

not be brought to bear on the student; (e) that the policy and procedures be observed (section 3.4 refers)

relating to students undertaking tests as a routine part of a programme of teaching or research, arising from unexpected results with possible health implications for the volunteers/participants might arise;

(f) that the onus is on the members of staff conducting the

experiment to take reasonable steps to ascertain that the student is in good health and knows of no reason why he/she should not participate.

A.2 Policy and procedures relating to students undertaking tests as

a routine part of a programme of teaching or research, arising from which unexpected results with possible health implications might arise

At the outset of appropriate projects/classes/experiments, it is the duty of the

academic supervisor to indicate to those concerned (volunteers/investigators) that some apparently untoward results may be obtained and to draw the student’s attention to the notes on the schedule referring to participation.

(i) In any practical teaching or research schedule in which ill-health in a

subject may be discovered incidentally, the following information should be included in writing or displayed:


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“Students will be asked to participate on the understanding that:

(a) the procedure is explained and understood to be entirely voluntary;

(b) the student has a right to decline to participate or, having

accepted, to withdraw at any time; (c) neither declining or accepting to participate will affect the

assessment of the course in any way; (d) the student is in good health and knows of no reason why he/she

should not participate.” In the event of untoward results being obtained, the following may be

helpful guidelines: (ii) Where the supervisor alone is the investigator, he/she should: (a) advise the student that there are wide variations between

individuals; (b) indicate that, it is possible that however unusual a result may

be at first sight, there may be several well-documented anomalies;

(c) avoid the concept of “normal/abnormal” but employ rather the

concept of “a range of reference values”; (d) cite, for example, the case of red hair - i.e. red hair is unusual in

Caucasian races but not unhealthy; (e) resist any attempt to interpret the results in the department -

particularly in terms of medical significance or diagnosis; (f) advise the subject to consult the Student Medical Officer in

confidence in the first instance. The onus will be on the subject to take or disregard the advice.

(iii) Where a student is acting as the investigator: (a) the procedures set out in section 3.4(i) above should be

explained to the student by the academic supervisor, including the requirement by the investigator to treat any results with the strictest confidence;

(b) where an untoward result is obtained, the investigator should

report the matter as soon as possible to his/her academic supervisor, who will then take appropriate action.


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APPENDIX B: Pure Tone Audiogram Data Sheet


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APPENDIX C: Boothroyd Word List


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APPENDIX D: Speech Audiogram Data Sheet


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APPPENDIX E: Medical Physics Option During this laboratory exercise, you will measure the hearing of your partner, and your own hearing will be tested. In carrying out these experiments you will not only learn some physics as applied to the human body, but also become familiar with some audiometry testing techniques To satisfy the requirements of the University Ethics Guidelines it is important that you understand that: (a) the procedure in each experiment is entirely voluntary; (b) you have the right to decline to participate or, having accepted, to

withdraw at any time; (c) neither declining or accepting to participate will affect the assessment

of coursework in any way; (d) you are at present in good health and that you know of no reason why

you should not participate. If you are in any doubt at all about (d) you are advised to seek the advice of a qualified medical practitioner before signing below and returning the slip to the academic demonstrator. I agree to participate in the audiometry experiments and am at present in good health. Signed: ..................................................... Date: ..............................

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UNIVERSITY OF SURREY DEPARTMENT OF … lab scripts/exp_2K...tones that vary between purely periodic and purely aperiodic. As shown in Figure 2, As shown in Figure 2, vowel sounds (a,