Straw Oboe Experiment: Cut It Shorter, Hear It Rise

The moment this straw oboe experiment earns its place is when you snip the end off while your child is still blowing. The note jumps up mid-squawk, which is about as direct a link between length and pitch as physics will ever hand you. Keep snipping and it climbs again. Then you measure it, plot it, and discover that the straw has just told you the speed of sound to within about 15%, which is not bad for a piece of rubbish you found in a drawer.

Key Takeaways

• You’ll flatten and cut a straw into a double reed, get it sounding, then cut it progressively shorter and record the pitch each time.
• It teaches the relationship between tube length and pitch, and for older kids it yields an actual measurement of the speed of sound.
• Best for ages 5–12. Five-year-olds will need an adult to make the reed and may take a few tries to get a sound.
• The usual failure: biting too hard. The lips rest on the reed, they do not clamp it.
• Straws are single-use here. One straw per person, no sharing, and pieces go in the bin straight away.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Wide plastic drinking straws	4–6 per person	Supermarket, takeaway cup lids	Paper straws work but go soggy in a minute. A bubble-tea straw is ideal because it is wide
Scissors	1 pair	Kitchen drawer	Nail scissors cut the reed more neatly
Ruler	1	Kitchen drawer	A printed strip of paper marked in cm
Free tuner or spectrogram app	1	Any app store, or an online tuner in a browser	A piano app to match the note by ear, less precise
Paper for a results table	1 sheet	Anywhere	Graph paper is better
Hole punch	1, optional	Stationery drawer	Skip the finger-hole variation
Bin or cup for offcuts	1	Anywhere	Keep offcuts off the floor, away from toddlers

Time: prep 5 min, experiment 25 min, cleanup 2 min Cost: a handful of straws Mess level: low. Small plastic offcuts, which need collecting

Do the Experiment: A Straw Oboe Experiment in Six Cuts

Step 1 — Flatten about 2 cm of one end

Press the last 2 cm of the straw flat, hard, between a thumbnail and a finger. Run the nail along it several times. You want it properly creased, not just squeezed. Biting it flat also works and is what every child will do anyway.

Step 2 — Cut the flattened end into a point

Snip the two corners off at roughly 45 degrees so the flat end becomes a narrow triangle. The two flaps should almost touch at the tip. This is the double reed, and the gap between the flaps is the only thing that matters about it.

Step 3 — Blow, and expect it to take a few tries

Put about 1 cm of the pointed end inside your mouth. Rest your lips lightly on the flat part. Blow steadily and reasonably hard. A buzzy squawk arrives within a few tries. Almost everyone fails first by biting, which clamps the reed shut. Loosen the lips and blow again.

Step 4 — Snip the far end while it is sounding

With the straw sounding, cut 3 cm off the open end. The pitch leaps upward. Cut again. Again. This is the hook, and it is worth doing before any measuring begins.

Step 5 — Now do it properly and measure

Make a fresh straw. Cut it to exactly 20 cm, sound it, and record the pitch from the tuner app. Then cut to 18, 16, 14, 12, 10, 8 cm, measuring the pitch at each length. Keep the reed untouched throughout; only the far end changes.

Step 6 — Plot pitch against one divided by length

Make a table with length in metres, one divided by length, and frequency in hertz. Plot frequency on the vertical axis against one-over-length on the horizontal. It should come out as a straight line through the origin. Multiply the slope by four and you have an estimate of the speed of sound in air.

What you should see: The reed squawks rather than sings. It is nasal and buzzy, closer to a duck than a flute, and that is correct. A 20 cm straw usually lands somewhere around 350 to 420 Hz, which is near the F or G above middle C. Halving the length to 10 cm raises the pitch by roughly an octave. The measured frequencies come out consistently a little below the simple prediction, typically by 5 to 15%, and that gap is a real physical effect rather than sloppy work. Blow much harder and the note may suddenly jump to a much higher squeal, which is the tube switching to its next available mode.

What’s Actually Going On

The straw is a wind instrument with three parts: a reed, an air column and an open end. Blowing forces the two flaps of the flattened end towards each other until they nearly close, which chokes the airflow and drops the pressure, which lets them spring open again. That cycle repeats hundreds of times a second. The reed is the oscillator, but it is not what sets the pitch. The air column is.

Because the reed end is almost closed and the far end is wide open, the straw is what acoustics calls a quarter-wave resonator: the standing wave inside fits a quarter of a wavelength into the tube’s length. HyperPhysics and The Physics Classroom both lay out the two cases, open-open and closed-open, and the straw is firmly the second. That gives frequency equals the speed of sound divided by four times the length. For a 20 cm straw and a speed of sound of 343 metres per second at 20 °C, that predicts 429 Hz.

Your measurement will come in lower, and there are two honest reasons. First, the open end does not behave as if the tube stops exactly at the cut; the air just outside gets dragged along, which adds an effective extra length of roughly 0.3 times the tube radius. Second, the reed itself is springy and the small volume inside your mouth couples to the tube, both of which make the instrument behave as though it were longer than it is. Neither of these is an error in your experiment. They are the corrections that real instrument makers spend their lives on.

There is a further detail worth telling an older child. A cylindrical tube driven by a reed at one end supports only the odd multiples of its fundamental: one, three, five times. That is why a clarinet, which is cylindrical, has its distinctive hollow tone, and why your straw sounds buzzy and jumps a twelfth rather than an octave when overblown. A real oboe is conical, not cylindrical, and a cone behaves much more like an open pipe with all harmonics present. So “straw oboe” is a nickname: acoustically your straw is a tiny clarinet.

The speed of sound you extract from the slope is temperature-dependent, which is a nice closing fact. Owen Cramer’s 1993 measurements in the Journal of the Acoustical Society of America pinned down how sound speed in air varies with temperature, pressure, humidity and carbon dioxide, and the headline is that it rises by about 0.6 metres per second for every degree Celsius. Breath is warm and humid, which nudges the straw’s pitch up slightly as you keep playing.

How to Explain It to Your Kid

Ages 5–8: short tubes squeak, long tubes honk

Let them do the cutting with you holding the straw. Keep it physical: the air inside the straw is wobbling, and a short bit of air can wobble faster than a long bit of air, the way a short skipping rope flicks faster than a long one. Then ask them to predict, before the next cut, whether the sound will go up or down. Being right three times in a row is the whole lesson at this age.

Ages 9–12: the pitch doubles when the length halves

Introduce frequency in hertz and have them read the numbers off the app. Point out the pair they care about: 20 cm and 10 cm. If the second number is roughly twice the first, they have found an octave, and an octave is a doubling. Have them check 16 cm against 8 cm too. Two confirmations of a numerical rule is a genuinely satisfying result for a ten-year-old.

Ages 13+: quarter-wave resonance and an end correction

Give them the formula, the data and the discrepancy. Ask them to compute the speed of sound from their slope and compare it with 343 metres per second, then explain why their answer is low. The extension that makes it a real investigation: add a measured end correction to every length and see whether the fit improves. They are now doing what acousticians do.

The question to ask: “If we filled the straw with helium instead of air, where sound travels nearly three times faster, would the pitch go up, go down, or stay the same?”

Why Teaching This Early Pays Off

Pitch and length is the first quantitative relationship most children ever meet in physics, and it arrives through their hands and ears at the same time. That combination is rare. It pre-loads the waves unit that turns up in middle school, and it gives a concrete grip on the idea of inverse proportion, which is one of the genuinely hard ideas in school mathematics: as one thing goes up, the other goes down, but not by a fixed amount.

It also kills a specific misconception. Children commonly believe the reed, the mouthpiece or the blowing makes the note, because that is where the effort is. It does not. The tube makes the note, and the proof is that you can change the pitch without changing anything you are doing with your mouth. Once that lands, trombones, recorders, bottles and organ pipes all make sense at once.

Alfieri and colleagues’ 2011 meta-analysis found guided discovery outperformed unassisted exploration, and the guidance here is simply asking for the prediction before each cut. Honesty about scale matters: this does not improve anyone’s test scores, and no study claims it does. What it does is give a child a physical memory attached to a formula, which is a reasonable thing to hope for from twenty-five minutes. The National Research Council’s framework calls the plotting step “using mathematics and computational thinking”, and it is unusual to get that practice out of a drinking straw.

Before You Start: What Parents Should Know

Best age	5–12. Under 5 the reed is too fiddly and the mouth-in-tube combination is not worth it
Supervision	Close for the scissors, light after that
Real hazards	Scissors; short straw offcuts are a choking hazard for under-3s, so collect them immediately; blowing hard repeatedly can make a child lightheaded, so take breaks
Don’t	Don’t share straws between people, don’t put more than about 1 cm of the reed into the mouth, and never inhale through the straw

When It Doesn’t Work

• No sound at all, just rushing air. Nine times out of ten the lips are clamped. Rest them gently on the reed and blow harder, not tighter. Letting the straw sit a fraction further out of the mouth often fixes it instantly.
• It worked and then stopped. The reed flaps have stuck together with saliva. Pull them apart with a fingernail and re-crease the flat section.
• The pitch readings jump around wildly. Your tuner is catching a harmonic rather than the fundamental. A spectrogram app is better here because you can see the whole stack and pick the lowest line yourself.
• A paper straw collapsed. They always do. Use plastic for the measuring run, and use paper only for the first demonstration if that is all you have.
• The straw is too narrow to flatten properly. Thin coffee-stirrer straws rarely work. The wide bubble-tea kind are the most reliable, followed by standard fizzy-drink straws.

Take It Further

1. Add finger holes. Punch three holes along a 20 cm straw at 4 cm, 8 cm and 12 cm from the open end. Covering and uncovering them changes the effective length, so you can play a scale of sorts. Opening the hole nearest the reed has the biggest effect, and working out why is a good puzzle.
2. Change one variable: diameter. Make reeds on three straws of different widths but the same length. The pitch barely changes, which surprises almost everyone and proves that length, not thickness, sets the note.
3. Overblow on purpose. Take a 20 cm straw and blow progressively harder until the note jumps. Measure both frequencies. The ratio should be close to three, not two, which is the fingerprint of a cylindrical reed instrument.

For a different way of changing a resonator, where volume rather than length does the work, see a water xylophone tuned by volume. The broader story of making instruments from scratch is in our piece on building musical instruments and engineering, and if you want to see your straw’s harmonic stack as numbers, pair this with reading a real spectrogram.

What to Watch For Over the Next 3 Months

• Week 4: They blow across a bottle and comment on the water level changing the note, connecting it without being told.
• Month 2: The word frequency appears in the right place, or they talk about something being “longer so it’s lower”.
• Month 3: They ask about an instrument’s plumbing. “Why does a trombone have a slide?” or “Why is the big pipe at the back of the organ?” are the questions to encourage, because they already have the answer and just need permission to say it.

Frequently Asked Questions

My child cannot get any sound out of it. What now?

Loosen the lips. The instinct is to bite, and biting holds the reed shut so no air can make it oscillate. Have them rest their lips on the flat part with no pressure at all, then blow hard. If that fails, re-crease the flattened end with a fingernail; a reed that is not properly flattened will never start.

Is a straw oboe really an oboe?

Acoustically, no. A real oboe has a conical bore, which supports all harmonics. A straw is a cylinder driven by a reed, which supports only odd harmonics, so it is much closer to a tiny clarinet. The nickname is harmless, but the distinction explains the buzzy tone.

Why is our measured frequency lower than the formula says?

Two reasons, and both are real physics. The open end behaves as though the tube is slightly longer than it is, an effect called the end correction. And the reed plus the air in your mouth adds compliance, which also lowers the pitch. Expect to land 5 to 15% below the prediction.

Can we reuse the straws tomorrow?

Not really. The reed flaps get soft and stick, and a used straw is not something to hand to a sibling. Treat them as disposable, use one per person, and collect the offcuts. Four straws per child is plenty for a full session.

How accurate is the speed of sound we get from this?

Within roughly 10 to 20% if the measurements are careful, which is respectable. The systematic error all pushes one way, making your answer too low, and recognising that a consistent bias is different from random scatter is one of the more valuable things a teenager can take away from an afternoon like this.

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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.

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Sources

1. 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
2. Georgia State University, Department of Physics and Astronomy. “Open and Closed Pipes / Standing Waves in Air Columns.” HyperPhysics. https://hyperphysics.gsu.edu/hbase/Waves/opecol.html
3. The Physics Classroom. “Open-End Air Columns.” https://www.physicsclassroom.com/class/sound/Lesson-5/Open-End-Air-Columns
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