Kundt's tube

Key takeaways

Kundt's tube belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Kundt's tube to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Kundt's tube from memory before moving on to harder problems.

Reference excerpt

Kundt's tube is an experimental acoustical apparatus invented in 1866 by German physicist August Kundt for the measurement of the speed of sound in a gas or a solid rod. The experiment is still taught today due to its ability to demonstrate longitudinal waves in a gas (which can often be difficult to visualise). It is used today only for demonstrating standing waves and acoustical forces.

How it works The tube is a transparent horizontal pipe which contains a small amount of a fine powder such as cork dust, talc or lycopodium. At one end of the tube is a source of sound at a single frequency (a pure tone). Kundt used a metal rod resonator that he caused to vibrate or 'ring' by rubbing it, but modern demonstrations usually use a loudspeaker attached to a signal generator producing a sine wave. The other end of the tube is blocked by a movable piston which can be used to adjust the length of the tube. The sound generator is turned on and the piston is adjusted until the sound from the tube suddenly gets much louder. This indicates that the tube is at resonance. This means the length of the round-trip path of the sound waves, from one end of the tube to the other and back again, is a multiple of the wavelength λ of the sound waves. Therefore, the length of the tube is a multiple of half a wavelength. At this point, the sound waves in the tube are in the form of standing waves, and the amplitude of vibrations of air is zero at equally spaced intervals along the tube, called the nodes. The powder is caught up in the moving air and settles in little piles or lines at these nodes, because the air is still and quiet there. The distance between the piles is one half wavelength λ/2 of the sound. By measuring the distance between the piles, the wavelength λ of the sound in air can be found. If the frequency f of the sound is known, multiplying it by the wavelength gives the speed of sound c in the air:

c = λ f {\displaystyle c=\lambda f\,}

The detailed motion of the powder is actually due to an effect called acoustic streaming caused by the interaction of the sound wave with the boundary layer of air at the surface of the tube.

Further experiments By filling the tube with other gases besides air, and partially evacuating it with a vacuum pump, Kundt was also able to calculate the speed of sound in different gases at different pressures. To create his vibrations, Kundt stopped the other end of the tube with a loose-fitting stopper attached to the end of a metal rod projecting into the tube, clamped at its center. When it was rubbed lengthwise with a piece of leather coated with rosin, the rod vibrated longitudinally at its fundamental frequency, giving out a high note. Once the speed of sound in the air was known, this allowed Kundt to calculate the speed of sound in the metal of the resonator rod. The length of the rod L was equal to a half wavelength of the sound in metal, and the distance between the piles of powder d was equal to a half wavelength of the sound in air. So the ratio of the two was equal to the ratio of the speed of sound in the two materials:

c metal c air = f λ metal f λ air = λ metal λ air = L d {\displaystyle {\frac {c_{\text{metal}}}{c_{\text{air}}}}={\frac {f\lambda _{\text{metal}}}{f\lambda _{\text{air}}}}={\frac {\lambda _{\text{metal}}}{\lambda _{\text{air}}}}={\frac {L}{d}}\,}

Reason for accuracy

A less accurate method of determining wavelength with a tube, used before Kundt, is simply to measure the length of the tube at resonance, which is approximately equal to a multiple of a half wavelength. The problem with this method is that when a tube of air is driven by a sound source, its length at resonance is not exactly equal to a multiple of the half-wavelength. Because the air at the source end of the tube, next to the speaker's diaphragm, is vibrating, it is not exactly at a node (point of zero amplitude) of the standing wave. The node actually occurs some distance beyond the end of the tube. Kundt's method allowed the actual locations of the nodes to be determined with great accuracy.

See also Chladni plates, another standing wave visualization technique. Rubens tube, demonstrates the relationship between standing sound waves and sound pressure.

References

Further reading Hortvet, J. (1902). A manual of elementary practical physics. Minneapolis: H.W. Wilson. Page 119+.

Illustrations

Worked examples

Example 1 — a first encounter with Kundt's tube

Start with the simplest possible case. Write down what Kundt's tube claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Kundt's tube before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Kundt's tube ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Kundt's tube

In research: Kundt's tube appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Kundt's tube in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Kundt's tube is common in secondary-school and first-year university syllabi. It links to neighbouring topics Acoustics, so understanding it makes those chapters shorter.
In everyday life: Look for Kundt's tube outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Kundt's tube in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Kundt's tube means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Kundt's tube out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Kundt's tube in simple terms?

Kundt's tube is an experimental acoustical apparatus invented in 1866 by German physicist August Kundt for the measurement of the speed of sound in a gas or a solid rod. The experiment is still taught today due to its ability to demonstrate longitudinal waves in a gas (which can often be difficult…

Why does Kundt's tube matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Kundt's tube?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Kundt's tube.