String Telephone Experiment: Sound Through Solids

The moment that makes a string telephone experiment worth doing is not the first whisper. It is when somebody reaches out and pinches the string halfway along, and the voice vanishes mid-word. Let go and it comes back. That single gesture proves the sound is travelling through the string and not through the air, which is the one thing a child cannot learn from being told. Then you turn it into a real test by swapping the string and counting how many whispered words actually get through.

Key Takeaways

• You’ll build a cup-and-string telephone, then compare four string materials by scoring how many whispered words come through correctly.
• It teaches that sound travels through solids, and more importantly that a guided wave loses far less energy than one spreading out through air.
• Best for ages 5–11. The scoring game is what keeps older kids interested.
• The usual failure: a slack string. It must be pulled genuinely taut, and it must not touch anything.
• A long loose string around small children needs supervising. See the parent table.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Paper or thin plastic cups	8 (two per telephone)	Kitchen, party supplies	Clean yoghurt pots, or tin cans if an adult files the rims
Cotton kitchen twine	10 m	Supermarket, hardware shop	Parcel string
Nylon fishing line, 0.3–0.5 mm	10 m	Fishing or hardware shop, pocket change	Thin nylon cord, or strong sewing thread
Wool yarn	10 m	Craft drawer	Any fuzzy, loosely twisted string
Thin wire, such as florist’s wire	10 m	Craft or garden shop	Picture-hanging wire; an adult handles the cut ends
Sharp pencil or skewer	1	Kitchen drawer	An adult makes the holes
Paper clips or buttons	8	Stationery drawer	Tie a large knot instead
Ruler or tape measure	1	Toolbox	Count paces and keep them consistent
List of 10 test words	1 sheet	Write them out	Use numbers from one to ten
Scissors	1 pair	Kitchen drawer	An adult cuts the wire

Time: prep 20 min, experiment 30 min, cleanup 5 min Cost: everything is probably in your house except the fishing line Mess level: low, but you need a corridor or a garden about 10 m long

Do the Experiment: A String Telephone Experiment Worth Scoring

Step 1 — Make the holes and fit the first string

An adult pokes a small hole in the centre of each cup’s base. Thread the cotton twine through from the outside, tie it round a paper clip or a button inside so it cannot pull back out, and repeat at the other cup. Cut the string so the two cups are 8 m apart.

Step 2 — Pull it properly taut

Two people, one cup each, walking apart until the string is tight enough to twang. It must not touch a wall, a door frame, a shoulder or the floor. This is the step that decides whether the whole thing works.

Step 3 — Write the test list and set the rules

Ten short words that do not rhyme: cat, boat, fork, drum, leaf, rope, mouse, chair, cloud, spoon. The speaker whispers one at random. The listener says what they heard. Score it out of ten. No repeats, no lip reading, and the listener faces away.

Step 4 — Score all four strings over the same 8 metres

Swap in the nylon, then the wool, then the wire, rebuilding with fresh cups each time or reusing the same pair. Same distance, same whisper volume, same ten words, same two people. Write down four scores out of ten.

Step 5 — Do the pinch test

With a string that works well, have a third person gently pinch it at the halfway point while the speaker talks. The sound stops. Release and it returns. Try pinching lightly, then firmly, and notice the difference.

Step 6 — Find the breaking distance

Take the best string and keep lengthening it: 8 m, 12 m, 16 m, as far as you can manage. Score at each length. Find the distance where the score drops below about five out of ten.

What you should see: Cotton twine and nylon line usually score eight to ten out of ten at 8 metres, clearly and surprisingly loudly. Wool typically scores two to five and sounds muffled, because fuzzy twisted fibres rub against each other and eat the wave. Thin wire often works extremely well and sounds oddly bright. The pinch test kills the sound almost completely with even light contact, which startles most children. Slack string gives nothing at all, no matter how loudly anyone shouts.

What’s Actually Going On

Your voice makes pressure waves in air. Those waves hit the cup’s base and push it in and out a tiny amount, which makes the cup a diaphragm: it converts pressure in air into mechanical motion of a solid. That motion tugs on the string, which carries the vibration along its length, and at the far end the second cup’s base does the same job in reverse, pushing air back into the listener’s ear. Two transducers and a transmission line. That is also, in outline, how a landline telephone works.

The waves travelling along a taut string here are mainly longitudinal: tiny stretches and compressions running along the string’s length, not the visible side-to-side wiggles you get from plucking it. Longitudinal wave speed in a solid depends on how stiff the material is divided by how dense it is. The published figures make the point: sound travels at about 343 metres per second in air at 20 °C, around 1,493 in water, roughly 3,560 in copper and about 5,130 in iron.

But speed is not why the telephone works, and this is where most explanations stop too early. Shout across 8 metres of open air and your voice still arrives. The reason the string version sounds so much clearer is that the energy is confined. In open air, sound spreads out over an expanding sphere, so the intensity falls off with the square of distance: double the distance and you have a quarter of the intensity. Along a string the wave is guided in one dimension and cannot spread at all, so the only losses are internal friction in the material and whatever leaks out where the string is held. A guided wave is the whole reason for telephone cables, optical fibres and stethoscopes.

That also explains the pinch test. Your finger is a lossy, heavy boundary. Touching the string absorbs the wave and reflects part of it back, so almost nothing gets past. The same logic explains the wool: a fuzzy, loosely twisted yarn has thousands of internal rubbing surfaces, and each one converts a little vibration into heat. Nylon monofilament has almost none, which is why it beats wool so decisively.

Tension matters for a related reason. A slack string sags, and the sag means the cup’s small motion just takes up slack rather than stretching the string. No stretch, no longitudinal wave. The string has to be under enough tension that a push at one end is immediately felt as a stretch along the whole length.

How to Explain It to Your Kid

Ages 5–8: the string carries the voice like a tunnel

Let them do the pinching. Then say it plainly: your voice shakes the bottom of the cup, the cup shakes the string, the string carries the shake all the way down like water down a hosepipe, and the other cup shakes the air into your friend’s ear. Pinching the string is like standing on the hosepipe. Ask them to predict what happens if the string touches the door frame.

Ages 9–12: sound spreads in air but not in a string

Introduce the idea of spreading. Shouting sends your voice out in every direction, so most of it goes somewhere useless. The string sends all of it to one place. That is why a whisper works on a string and a whisper does not work across a room. Have them compare their four scores and rank the materials, then explain the wool result in terms of fibres rubbing.

Ages 13+: guided waves, impedance and the inverse square law

Give them the full picture: intensity falling as one over distance squared for a spherical wave, versus a one-dimensional guided wave whose losses are dominated by material damping. Introduce impedance matching at the cup and ask why a stiff, light diaphragm couples better than a floppy heavy one. The real investigation: measure score against length for two materials and compare the rate at which each decays.

The question to ask: “If we taped the middle of the string to a tabletop, would that be more like pinching it or more like leaving it alone, and why?”

Why Teaching This Early Pays Off

The spreading idea is the valuable one. Intensity falling off with distance squared explains why you move closer to a campfire, why phone signal drops away from a mast, why a camera flash is useless at the back of a stadium and why radiation dose falls so fast with distance. Children who meet it as a physical experience rather than a formula tend to keep it.

The second payoff is the transducer idea. A cup base that turns air pressure into motion is the same device as a microphone, a loudspeaker, an eardrum and a stethoscope diaphragm. Spotting the same component in four unrelated places is how engineering thinking starts.

The evidence on this kind of activity deserves honesty. Alfieri and colleagues found in 2011 that guided discovery beats unguided, which in practice means asking for the prediction before each pinch and each string swap rather than leaving a child to fiddle. The scoring game is doing something specific: it converts “it sort of works” into a number, which is what the National Research Council’s framework means by analysing and interpreting data. No single afternoon changes academic outcomes, and claiming otherwise would be overselling a very good twenty minutes.

Before You Start: What Parents Should Know

Best age	5–11. Older children stay engaged if you run the scoring as a competition between strings
Supervision	Close for the string, light otherwise
Real hazards	A long loose cord is a strangulation and trip hazard for small children. Never let a string go around a neck, cut it to length rather than leaving metres trailing, and put it away when you finish. Thin wire has sharp ends, so an adult cuts it and folds the tips over
Don’t	Don’t use tin cans with children under 8 unless an adult has filed the cut rim smooth, and don’t run the string across a hallway where someone will walk into it

When It Doesn’t Work

• Nothing comes through at all. The string is slack. Nine times out of ten this is the answer. Walk further apart until it twangs.
• It works, then stops when someone moves. The string is brushing a wall, a chair or a sleeve. Clear a straight path with nothing in it.
• The sound is there but faint. Your cups are too floppy. A thin plastic cup or a yoghurt pot has a stiffer, lighter base than a thick paper one, and the difference is audible.
• The knot keeps pulling through the hole. Tie it round a paper clip, a button or a matchstick inside the cup. A bare knot in thin nylon will always pull through eventually.
• It only works one way. One cup’s knot is loose, or its hole has torn wider. Check both ends and remake the weaker one.

Take It Further

1. Change one variable: tension. Use the same cotton string at the same 8 m, but mark it so you can set three different tensions by walking a little further apart. Score at each. Tension helps up to a point and then stops mattering, and finding that plateau is a real result.
2. Test the wet-string claim. Plenty of sources say a damp cotton string works better. Test it properly: score dry, then soak the same string, wring it out and score again, same words, same distance. Water adds mass and damping but smooths down the fibres, so the result is not obvious. Report what you actually get.
3. Go round a corner. Add a third person who holds the string at a bend with a loop of smooth plastic tube or a drinking straw. See how much signal survives the turn. This is, in miniature, the problem every cable installer has.

The cup-as-diaphragm idea shows up again in our DIY stethoscope project, and for the groundwork on how vibration becomes sound in the first place, see how sound works with speakers and resonance. If the speed figures in this article interested your child, measuring the speed of sound with an echo lets them get the number themselves.

What to Watch For Over the Next 3 Months

• Week 4: They press an ear against a table or a railing to listen to a tap from the far end, unprompted.
• Month 2: The word vibration gets used correctly about something unexpected, such as a door, a wall or a radiator.
• Month 3: They explain why standing closer to a speaker is louder in terms of spreading rather than just “it’s nearer”. That is the inverse square idea arriving, and it is worth far more than the telephone itself.

Frequently Asked Questions

How long can a string telephone be?

Longer than most people expect. Ten to fifteen metres of taut nylon line usually carries a clear whisper, and twenty is achievable in a garden on a calm day. The limit is not the length itself but keeping that length taut and untouched.

Does the string have to be string, or will anything work?

Anything that can be pulled taut and has low internal friction. Nylon line and smooth cotton twine are excellent, thin wire is often the best of all, and fuzzy wool is the worst. That ranking is the experiment.

Why do tin cans get used in all the old pictures?

Because a thin metal base is stiff and light, which makes it an efficient diaphragm, and because cans were what people had. They work very well. They are also sharp when opened, so for young children a plastic cup is the sensible trade.

Is the sound really going through the string and not the air?

Yes, and the pinch test is the proof. If the sound were travelling through air, touching the string would make no difference at all. Having your child perform that test themselves is more convincing than any explanation.

Can three cups share one string?

You can tie a third cup to the middle and it will hear both ends, which children find delightful. But the extra cup is a loss point, and the two end cups will hear each other noticeably less well than before. That trade-off is worth pointing out.

---

About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

---

Sources

1. Georgia State University, Department of Physics and Astronomy. “Speed of Sound in Various Bulk Media.” HyperPhysics. https://hyperphysics.gsu.edu/hbase/Tables/Soundv.html
2. Cramer, O. (1993). “The variation of the specific heat ratio and the speed of sound in air with temperature, pressure, humidity, and CO2 concentration.” Journal of the Acoustical Society of America, 93(5), 2510–2516. https://doi.org/10.1121/1.405827
3. National Park Service. “Understanding Sound.” U.S. Department of the Interior. https://www.nps.gov/subjects/sound/understandingsound.htm
4. Acoustical Society of America. “Acoustics.org: public resources on sound and vibration.” https://acoustics.org/
5. Alfieri, L., Brooks, P. J., Aldrich, N. J., & Tenenbaum, H. R. (2011). “Does discovery-based instruction enhance learning?” Journal of Educational Psychology, 103(1), 1–18. https://doi.org/10.1037/a0021017
6. White, R. T., & Gunstone, R. F. (1992). Probing Understanding. Falmer Press, London.
7. National Research Council. (2012). A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas. National Academies Press. https://doi.org/10.17226/13165