# Make waves

22nd November 2002 at 00:00
Wendy Swarbrick demonstrates how experimental work in physics can bring an A-level lesson on sound to life.

This lesson combines all my favourite lesson items - plenty of varied experimental work that the students can be involved in, links to things that interest them, and room for differentiation. It is followed by a lesson using the Multimedia Sound CD-Rom (Cambridge Science Media, site licence pound;175), where students can reinforce all of the lesson ideas as they use the sampled sounds on this CD and record and synthesise their own.

Previously students have used a long helical spring and another software pack Waves and Vibrations (Anglia Multimedia, pound;57.58), to revise GCSE waves, and to understand superposition, node formation and stationary waves. We begin with a quick question and answer session. Then we move over to look at Melde's experiment, which is already set up to show large vibrations with about three or four nodes. I demonstrate the effect of changing the tension, and the frequency of the signal generator. Then I ask pairs of students to find a resonant frequency and measure the node to node distance, and then set up a results table on the whiteboard. Students add their results, and everyone calculates the wave speed for each set of readings. Hopefully this comes to a constant answer, with plenty of discussion about experimental uncertainty.

We move to a sonometer (see box). I strike a middle C tuning fork on a bung, and hold its base on the sonometer. I ask any guitar or violin-playing student to tune the wire to the same note by changing the separation of the knife-edges. This is hard, as the timbre is so different. Then I demonstrate how a small piece of paper will fly off the wire when the base of the sounding tuning fork is held on the wire at one knife edge. Pairs of students then find the lengths corresponding to the tuning fork frequencies, and fill in a results table on the board. It is much easier to go up in order, as even the tone deaf can then shorten the string by small increments, while watching the paper rider for vibrations. Meanwhile we discuss stringed instruments and their design. Students plot a graph of inverse length against frequency now or at home. I challenge them to find the velocity of the wave, and, if appropriate, give them the relevant equation so that they can find the mass per unit length of the wire.

I usually startle students at this point by blowing through a model organ pipe, and asking "what is going on here?": We look at Kundt's dust tube, and observe the swirling powder at the antinodes, but I do not attempt careful measurement, just draw out the similarities and differences between vibrating air columns and vibrating strings. We discuss open and closed tubes, and listen to the change in note as the end of the organ pipe is closed. The pipe is played to a microphone connected to a cathode ray oscilloscope. Students take readings to find the frequency of the note, and the wavelength. We multiply them together to see if we have a reasonable value for the speed of sound in air.

A quick reprise of the key ideas - and we are usually late finishing the lesson!

LESSON NOTES

Salters Horners advanced physics

This new approach to advanced-level physics is a context-led course where physics concepts are introduced from familiar or particularly interesting situations. "The Sound of Music" unit looks first at how CDs are read, simulating this with large-scale models read with microwaves. Other units look at sport, satellite design, making sweets, archaeology and spare-part surgery.

www2.york.ac.ukorgsegsaltersphysics Key ideas

* Reinforce ideas of the super-position of waves and formation of stationary waves, N-N = 1Z2.

* Understand stationary waves on a stretched string and see harmonics.

* Revise wave equation, v = fl.

* Be able to use relationship for the velocity of a transverse wave on a stretched string.

* Understand the formation of stationary waves in a column of air and see nodes.

* Learn the formulae for the fundamental frequency of a column of air (closed and open).

* Revise the use of the cathode ray tube and its calibration.

Definitions

Melde's experiment

A fine string about 150cm long is tied to a vibrator, connected to a signal generator. The vibrator is clamped about 30cm above the bench. The other end of the string goes over a pulley clamped at the same height, with a load of about 200 grams hanging from it. Both stands need to be G-clamped to the workbench.

Sonometer

A piano wire (or something similar) is stretched over knife edges, one of which is moveable, then passed over a pulley and weights are hung from the end. A load of about 5kg gives a reasonable length for middle C. A set of tuning forks and a metre rule is also required.

Kundt's dust tube

A long glass tube (measuring about 4cm in diameter and 1.5m in length) rests on three slotted bases. One end of the tube is closed with a rubber bung and Lycopodium powder is sprinkled down the tube. A small speaker (from a radio measuring about 4cm in diameter) is driven by a signal generator at the open end of the tube.

Organ pipe and cathode ray oscilloscope (CRO)

A recorder works as well as an organ pipe and has the advantage of variable length. The microphone is connected to the Y-plates of the CRO, and the time base and gain adjusted to display a clear waveform.

Two-dimensional vibrations

During longer lessons I would finish by demonstrating 2-D standing waves on metal plates clamped on top of the signal generator. I would use tea leaves (suggested by the Salters Horners advanced physics team) to show the nodes.

Safety issues

Sonometer

Put an upturned stool under the heavy weights. Use goggles in case the wire snaps.

Organ pipe

Take care when closing the end, so the players' teeth are not threatened.

Wendy Swarbrick is physics subject leader at Steyning Grammar School, which is a large, mixed comprehensive in West Sussex The lesson forms part of the Salters Horners AS physics course for Year 12 students

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