Advancing with Advancing Physics 2003 Ultrasound, not lasers 1
th
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Chris Shepherd, Teacher Support Manager, Institute of Physics, 2002
Updated June 2003 and delivered by David Huss, Dr Challoner’s Grammar
School
Plan
15.15-
15.30
15.30-
15.55
Plenary
Experimental Session
1
Introductions, and our experience of teaching waves by experimental means.
The advantages of ultrasound, tips, and safety measures.
Wavelength Measurement
Phase, Lissajous, Square Wave (short experiments)
Amplitude vs Distance (in free space)
Extension- Lloyd’s Mirror
15.55-
16.05
Feedback 1 Comments, classroom management, hazards, results, extensions
16.05-
16.35
Experimental Session
2
Lloyd’s Mirror
Extensions- Young’s Slits, Diffraction, Diffraction
Grating
16.35-
16.45
Feedback 2
Introductions
Comments, hazards, results, extensions, data for further research
Who we are and what we all do.
Issues associated with the experimental Teaching of Waves
Advantages of Ultrasound
Ultrasound has some distinct advantages for any demonstration and particularly group or class experiments on waves.
 It only requires a cheap transmitter and receiver, about £1.50 each, and standard equipment which you hopefully already have
A signal generator capable of producing 40kHz
A (set of) cheap (£16) digital voltmeters
Brief access to a reasonable CRO, real or computer based
 It has a manageable wavelength (mm-cm), as you will discover.
 It has a reasonable range, of about 2m
 It is safe, the exposure being below the recommended 110dB if you observe simple precautions (see Appendix)

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 You can have more than one experiment comfortably running at a time, unlike sound based experiments, so you can conduct class experiments.
 It lends itself to various projects and investigations
 You can run the experiment in full daylight, unlike some optics experiments
 You can perform a variety of wave experiments- interference, diffraction
Feel free to do concentrate on one experiment, or to move more swiftly through all the experiments!
A hard one for pupils- you may wish to coach a group through or demonstrate, but it cultivates useful skills in Years 12 & 13.
The wavelength obtained will be crucial for other experiments.
Oscilloscope, triggered off signal generator
Receiver
Transmitter
Move receiver steadily backwards
Ruler
N.B. Angle oscilloscope etc to avoid standing waves created by reflections off flat surfaces.
Coaxial lead
Red lead
Black lead- often earthed, and connected to the black terminal of the ultrasonic transmitter or receiver
Requirements
One double beam oscilloscope
2 coaxial leads
Signal generator
Ultrasonic transmitter
Ultrasonic receiver
Meter ruler
Signal generator
40 kHz

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Pupil Type Instructions
(Feel free to copy or amend for classroom use)
Safety
Avoid the transmitter being closer to your ears than 30 cm.
(Although there are no known risks from ultrasound, this keeps you below the limits imposed for industrial workers using ultrasound continuously.)
Apparatus Checks
Ask your teacher for help if necessary. Persevere- you are acquiring valuable skills!
1.
Check the signal generator works on a sine wave at 6V, using a multimeter on the a.c. voltage range, or using a 6V bulb.
2.
Check the oscilloscope works by connecting it directly to the signal generator at 40 kHz, using a coaxial lead. Take care to connect the black lead of the oscilloscope’s coaxial cable to the earth terminal of the signal generator, to avoid a short circuit. Use channel1 or X on the oscilloscope.
3.
Set the triggering of the oscilloscope to channel 1 or X, and make the trace occupy the upper half of the screen
4.
Connect the transmitter to the signal generator, in parallel to the oscilloscope.
5.
Connect the receiver to channel Y or 2 of the oscilloscope.
6.
Set the Source on the oscilloscope to Dual, so you can see 2 traces clearly, and make the second trace occupy the lower half of the screen.
7.
Face the transmitter and receiver toward each other, a small distance apart.
8.
Alter the signal generator frequency until you get the largest trace, at about
40 kHz.
The measurements
 Move the receiver slowly back next to the ruler, and you will see the lower trace move relative to the top trace.
As you move the receiver backward the waves take longer to reach the receiver.
Every time the bottom trace moves one complete screen wave, the receiver has moved back one wavelength.
 Measure the distance occupied by 10 wavelengths, and divided by 10 to get an estimate of the wavelength
Confirmation
If the speed of 40 kHz ultrasound is about the same at normal sound, i.e. 340 m/s, use the frequency and v = f  to generate an approximate check on your value of wavelength.

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Use the X-Y button on the CRO to examine the phase differences as you repeat the experiment.
Worth mentioning with pupils?
Both transmitter and receiver are sharply tuned to respond to just one frequency, i.e. are highly resonant devices. If you feed a square wave into the transmitter, given that the square wave contains 40 kHz plus a large number of higher frequencies, how might the transmitter respond?
What will the receiver pick up?
Try changing the signal generator from a sine wave to a square wave and note the appearance of the 2 traces on the oscilloscope, one direct from the signal generator, the other from the receiver.
Hypothesis- given an inverse square law for intensity, what would you expect for amplitude?
See how your results compare!
Extension- make a start on (5)!
Comments, classroom management, hazards, results, extensions?
a.c.
Volts
O
L
Transmitter d
Virtual source by reflection
Signal generator
40 kHz

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Y
It is best if X is between 0.5 -1.5 metres
X
Requirements
One a.c. voltmeter capable of resolving 10 mV
Lines of ultrasound maxima
Signal generator
Ultrasonic transmitter
Ultrasonic receiver
2 meter rulers
1 retort stand and clamp (not shown)
Smooth bench top surface
Instructions- Teacher version!
A smooth bench allows reflections to occur which create a virtual source that is exactly in phase with the original. This creates a vertical 2 source pattern, whose maxima can be mapped out as X and Y co-ordinates on graph paper.
The receiver is best clamped in position while X and Y co-ordinates are obtained.
The receiver needs to be angled towards the transmitter.
The height of the transmitter from the bench is, you will remember, d/2.
This map can then be used to test out the formulae n  = d sin  , or x =  L/d . out the maxima and test out the formula, which is valid if L is large compared to d and  and Y is not too large compared to those quantities.
Class Experiments
A class experiment is possible in a manner that is impossible with audible sound, and provides an alternative or a complementary approach to Young’s
Slits with light.
Groups of pupils can work on adjacent benches to each other without difficulty, unlike the situation with sound.
You can “pipe” the 40 kHz signal around the classroom using a cable with connectors in parallel, so you only need one signal generator.
Multimeters with sensitive a.c. voltage ranges are now cheap (£16 each), so a class set is affordable.
Extensions
By now you will have a flavour of what is possible.
You may wish to try the following:

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6) Young’s Slits
Use a second transmitter, but avoid bench reflections. What are the advantages and disadvantages over Lloyd’s mirror? Bear in mind the need to find a matched pair of transmitters, and the sharpness of the resonant peak, and any drift in the signal generator.
7) Diffraction- use one of the broader sealed sources, and see the angular variation of amplitude at fixed radius.
8) Diffraction Grating- borrow the cardboard mask- it works best if placed centrally between transmitter and receiver, 1 m from each.
9) Resonance- the current and voltage go through an interesting phase relationship near the 40kHz resonance - see the website
10) Summer project- Build a phased array!
Comments, hazards, results, extensions, data for further research.

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Have a look at the amplitude against angle plots on the website, for various widths of transducers, and a host of other interesting stuff. http://www.prowave.com.tw/p1.htm
The transmitter is 400ST160, and the receiver 400SR160
(400- corresponds to 40.0 kHz, T for transmitter, 160 for 16 mm external diameter)
The broader sealed sources are 400ET180, and 400ER180 (E for enclosed)
(I doubt the validity of their absolute measurements of intensity though- see
Safety.)
These transducers are made by Pro-Wave in Taiwan, and available in this country via Farnell, Maplin, Rapid, and Pro-Wave’s agents, Midas
Components.
Rapid and Midas currently sell them at the cheapest rates.
(Please note that this information supplied by me as an individual, and is not an endorsement by the IOP)
Rapid Education catalogue (April 2003-March 2004) page 329, in the PA audio/video section
Phone 01206 751166 www.rapidelectronics.co.uk
Open sources
Sealed sources
Order Code
*35-0175 transmitter
35-0182 tx
1+
1.80
*35-0180 receiver 1.80
3.40
25+
1.50
1.50
2.98
100+
1.35
1.35
2.66
(broader)
35-0184 rx 3.40 2.98 2.66
Rapid have a very fast turn around time.*The starred components are the ones used in this session.

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Midas Components sell a more extensive range
01493 602602. *The starred components are the ones used in this session.
Look at their website http://www.midascomponents.co.uk/Prowave/prowave.html
Air US Tranducers Price £ Each Price £ Each Price £ Each
Part Number 1 to 99 100 to 499 500 to 999
*400ST160 pop
*400SR160 pop
400ST120
400SR120
400ST100 pop
400SR100 pop
328ST180
328SR180
328ST160
328SR160
400PT160 pop
400PT120
400ET250
400ER250
400ET180 pop
400ER180 pop
400ET080 pop
400ER080 pop
400EP25B
400EP18A pop
500MB120
1.65
1.65
2.75
2.75
1.65
1.65
3.00
3.00
3.00
3.00
2.10
3.15
5.50
5.50
3.30
3.30
3.30
3.30
8.25
5.35
1.30
1.30
1.90
1.90
1.30
1.30
2.00
2.00
1.96
1.96
1.75
2.06
3.56
3.56
3.05
3.05
3.05
3.05
5.50
4.85
1.25
1.25
1.55
1.55
1.25
1.25
1.65
1.65
1.59
1.59
1.51
1.67
2.85
2.85
2.65
2.65
2.65
2.65
4.45
3.99
8.25 4.80 4.05
210AC180
235AC130
68.50
41.10
26.05
21.25
23.05
17.65
125AC250 75.35 28.77 25.05
RS catalogue number 184-5220 size 7 cable cleat, and M5 nut (45mm long), bolt and washers. www.rswww.com

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Contact with CLEAPSS revealed no previous enquiries on the subject, and the relevant Noise at Work Regulations only deal with audible sound.
The existing Health and Safety Executive directive refers to 110 dB, but this is for ongoing work situations, not short duration classroom experiments.
The literature conforms that 40 kHz ultrasound is considerably less hazardous than normal sound, the limit there for occupational exposure being 85dB.
A recent authoritative survey of the existing literature done for the HSE reveals how little has been done on the subject even for occupational hazards.
The number of workers exposed to ultrasound is small, none worked for longer than five years, and the effects are difficult to separate from those due to other background sound.
“For ultrasonic components above 20 kHz, the limits were set to avoid hearing damage in the audible (lower) frequencies. One-third-octave band levels of
105-115 dB were observed to produce no temporary hearing loss, and were therefore judged non-hazardous in respect of permanent hearing damage.” http://www.hse.gov.uk/research/crr_pdf/2001/crr01343.pdf
No formal UK guidelines yet exist that correlate duration with the effects, but the report cited non-UK regulations that permitted allowed increases of 3-12 dB above the recommended 110 dB for exposures of an hour, for workers exposed every day to ultrasound.
The latest figures from the manufacturer stated the output as 126 dB for a 10 V rms input at 30 cm from the source. A calculation with measured input power and generous coupling efficiencies (10%) shows that this is unlikely to be the case. The lack of easily available precision calibrated ultrasound meters would incline me to think that the manufacturer may have exaggerated the absolute magnitudes.
Erring on the side of caution
However so, it would be worth trying to get down to below 110 dB. On my calculations this is achieved by enforcing
 a maximum of say 6 V rms,
 cautioning pupils against having their ears closer than 30 cm from the source
 and limiting the exposure time by turning off the ultrasound between measurements
At any distance the intensity would of course drop rapidly. This limits the problems of multiple sources.
Anecdotally, the experiment has been conducted with ultrasound by staff members for 15 years with no problems, or even transitory effects even after several hours. At no time have pupils or staff reported problems.

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Conclusion
An exposure of 110 dB for an hour or less would therefore seem to be very well within the acceptable limits for 1 or 2 experiments.
Remember the 110 dB limit is for continuous, air-carried occupational exposure.
However erring on the side of caution it would be worth trying to get down to below 110 dB, by enforcing
 a maximum of say 6 V rms,
 cautioning pupils against having their ears closer than 30 cm from the source
 limiting the exposure time by turning off the ultrasound between experiments.
Thanks to Chris Clare, now Deputy Headteacher, Dr Challoner’s Grammar
School, Amersham for the original idea of using “raw” transducers, and Chris
Parkinson, David Fricker, Maurice Botley and Mike Ellis, current or former technicians there, for contributing ideas for the construction, and making up the apparatus.