# Can sound do what quantum particles do?

> - Sound bends, spreads and interferes like any wave, and you can do the double slit with two speakers.
> - So the double slit tests whether something is a wave, not whether it is quantum.
> - The quantum part is random single hits, probability amplitudes, collapse when you measure, and entanglement.
> - Sound only gets quantum as phonons in cold solids, and classical waves can't do quantum computing.

By Emin Mahrt. Published 2026-10-13. Updated 2026-10-13.
Canonical: https://emin.de/posts/can-sound-do-what-quantum-particles-do
Category: science. Format: explainer. Language: en.
Provenance: mixed. AI-assisted: true. Reviewed by a human: false.

## The question I kept coming back to

I had a simple question in my head for a while. Sound is a wave. Light is a wave, and electrons behave like waves too. Everyone knows the double slit experiment, where electrons make stripes on a screen and the stripes go away as soon as you check which slit they took. So can sound do the same thing? Can sound waves show the same behaviour as quantum objects, with interference and with this strange effect of observation?

I went deep into it, and the short answer is yes and no. Sound does a lot of what quantum waves do. But the part that makes quantum physics quantum is exactly the part sound can't do. Here is where that line is.

**Basically,** I wanted to know if sound waves behave like quantum particles, and the answer is yes and no.

## What sound is

Sound is a mechanical wave. It needs something to travel through, like air or water or a solid, and that's why there is no sound in a vacuum. A speaker cone or a clap pushes the air molecules next to it together, then they spread apart again, and this pattern of squeezed and thinned air moves outward. The molecules themselves don't fly to your ear. They just swing back and forth around their place, and only the disturbance travels.

A sound wave has a wavelength, the distance from one squeeze to the next. It has a frequency, how many of those pass by every second, and we hear that as pitch. Humans hear roughly from 20 Hz up to 20,000 Hz. It has an amplitude, how strong the push is, and we hear that as loudness. And it has a speed that depends on the medium. In air at room temperature sound moves at about 343 metres per second, in seawater at about 1,531.

The important thing for my question is that all of this is classical physics. If you know the source and the medium, you can calculate exactly what the wave will do next. Nothing in it is random.

**Basically,** Sound is a pressure wave in a medium, and classical physics predicts exactly what it does next.

## Sound does everything a wave does

Sound reflects. That's an echo, and it's how sonar finds things under water. Sound refracts, so it bends when the air gets warmer or colder or when the wind changes, and a voice can carry further with the wind. Sound diffracts, it bends around corners and spreads out after a doorway, and that's why you can hear someone in the next room without seeing them. Low sounds bend more easily than high ones, which is why distant thunder is only a low rumble.

And sound interferes. When two sound waves meet and their peaks line up, they add up and it gets louder. When the peak of one meets the valley of the other, they cancel and it gets quieter, in theory down to silence. Two tones with almost the same pitch get louder and quieter in a slow rhythm, and that's called beats.

So sound has the whole set, and old classical wave physics explains every part of it.

**Basically,** Sound reflects, bends, spreads around corners and interferes, so it has every classical wave behaviour.

## Why size matters

What you see depends a lot on the size of the wave. A 100 Hz tone in air has a wavelength of about 3.4 metres, a 10 kHz tone about 3.4 centimetres. That's the size of doors, rooms, furniture and people. So sound bends around our everyday world all the time, and we hear things we can't see.

Visible light has wavelengths of about 400 to 700 nanometres. To make light bend and interfere you need tiny slits made with a lot of precision, and that's why light doesn't go around corners in daily life. Electrons have a wavelength too, and in the usual experiments it's about the size of the spacing between atoms in a crystal.

So the same wave behaviour shows up at completely different scales. Sound does it in a living room, electrons do it at the scale of atoms. That alone tells you nothing about quantum or not quantum. It's just wavelength.

**Basically,** Sound bends around doors because its wavelength is metres long, while light and electrons need tiny slits.

## The double slit with two speakers

You can really do the double slit with sound, and it's a classic classroom demonstration. One way is a wall with two openings. The easier way is two speakers driven by the same signal generator, so both play the same tone in step. A typical setup puts them about half a metre apart and plays something around 2,000 Hz.

Then you walk slowly across the room in front of them. You hear loud spots and quiet spots, one after the other. Those are the same bright and dark stripes Thomas Young saw with light, and people call them Young's fringes for sound. With a microphone and a meter you can map the pattern exactly. Right in the middle, at the same distance from both speakers, it's loud.

In a normal room it gets messy, because the walls reflect the sound and add more waves on top. Outside or in a dead room it works best. And you almost never get perfect silence in the quiet spots, because the two waves don't arrive with exactly the same strength.

**Basically,** Two speakers playing the same tone make loud and quiet spots in a room, the double slit pattern in sound.

## The double slit with electrons

Now the quantum version. A source sends single photons or single electrons, one at a time, toward a wall with two narrow slits. Behind it is a screen that records where each one lands.

Every single electron arrives as one dot at one place. But when you send many of them, keep both slits open and don't check which slit they used, the dots slowly build up stripes. It's the same kind of interference pattern as with sound. And they can't be interfering with each other, because they come one by one. Each electron somehow goes through both paths as a wave and lands as a particle.

Then you put a detector at the slits to find out which way each electron took. The stripes are gone. You get two plain bands behind the two slits, like you would get by shooting little balls through them.

**Basically,** Single electrons land as dots, build up stripes over time, and the stripes vanish when you check the path.

## Two kinds of wave

This is where I understood that the double slit is a test for waves and not a test for quantum. Young did it with light in the early 1800s to show that light is a wave, long before anyone talked about quantum physics. Sound, water and light all make that stripe pattern. If you see stripes, you learned that something is wavy, and that's all you learned.

The real difference is what is waving. In the sound experiment it's air pressure, something physical you can measure at every point. The loudness at a spot is just the sum of the two waves. In the quantum experiment the wave is the wave function, and it's not a physical thing moving through a medium. It's a probability amplitude. It tells you how likely it is to find the electron at a certain spot. The rule for that is called the Born rule, and it says you take the size of the wave at a spot and square it, and that gives you the probability. The stripes come from the amplitudes of the two paths adding up, and they only show up after many single hits.

The wave function itself changes smoothly and predictably as long as nobody measures, following the Schrödinger equation. But where one single electron lands is random. Experiments that break the so called Bell inequalities suggest this randomness is real and not just missing knowledge, so there are no hidden local details we just haven't found yet.

**Basically,** A sound wave is air pressure, a quantum wave is a probability amplitude, and only that makes single hits random.

## Looking changes an electron, not a sound

Say you try to find out which slit the sound went through. You put a microphone close to one slit. The microphone takes some energy from the wave and scatters a bit of it, and the pattern behind it changes a little. But the sound is still a sound wave. Further away it still interferes, and nothing about its nature changed.

It's like checking the pressure of a tyre. Some air always escapes, so you change what you measure. That's also an observer effect, but a boring classical one. You can calculate it, and with a better instrument you can make it smaller.

With the electron it's different. As soon as you get the information which path it took, the stripes disappear completely. This is not a clumsy detector that pushes the electron too hard. It's tied to the information itself. Heisenberg's uncertainty principle puts a hard limit on how precisely you can know some pairs of things at the same time, like position and momentum. In quantum physics there is a minimum disturbance you can't engineer away.

**Basically,** Measuring a sound wave nudges it a little, but checking an electron's path wipes out the interference.

## The measurement problem

Quantum physics has two rules for how a state changes, and they don't fit together. When nobody measures, the wave function evolves smoothly, you could run it backwards, and superpositions stay superpositions. When you measure, you get one result, picked at random with the Born rule probabilities, and the state jumps to match that result. People call that the collapse. It's sudden and it can't be undone.

The Schrödinger equation doesn't contain the collapse, and the collapse breaks its rules. The theory also doesn't say what counts as a measurement, or where the quantum system ends and the measuring device starts. That gap is the measurement problem, and physicists still argue about it. The Copenhagen view takes the collapse as a rule. Many Worlds says there is no collapse at all. Bohmian mechanics adds hidden variables. Objective collapse theories like GRW make the collapse a real physical process.

Decoherence explains part of it. A quantum system touches its environment all the time, air molecules and light and everything else, and gets entangled with it. Seen from the system alone, the interference fades out fast and it starts to look classical. That's a big part of why we never see big things in two places at once. But decoherence doesn't explain why you get this one result and not the other one.

Sound has nothing like this. A sound wave has definite properties all the time, and measuring only reveals them, maybe with a small disturbance.

**Basically,** Quantum physics can't explain how a measurement picks one result, and classical sound never has that problem.

## The observer is not a mind

The word observer confuses a lot of people. Popular stories turned it into the idea that consciousness creates reality, that the electron decides because a human looks. Some early discussions around von Neumann and Wigner pushed in that direction. But the physics doesn't need a person. The observer is any physical system that interacts with the electron and takes out the which path information. A detector in an empty room does it, and the environment does it too.

I think measurement interaction would be a better word than observer. It's a physical process, not a mental one, and the old word keeps feeding a lot of esoteric nonsense about quantum physics.

**Basically,** The observer in quantum physics is any detector or environment that takes path information, not a human mind.

## Phonons, the quantum of sound

There is a place where sound really becomes quantum, and that's inside solids. In a crystal the atoms sit in a grid and vibrate together. Quantum physics says this vibration energy comes in small fixed packets, and one packet is called a phonon. The name comes from the Greek word for sound. A phonon is to sound in a crystal what a photon is to light. It's not a basic particle like an electron, it's a quasiparticle, a shared vibration of many atoms. Long phonon waves are just the ordinary sound waves in that solid.

And phonons do the real quantum things. Researchers made single phonons, put them in superposition, showed interference between phonon paths, entangled phonons and even ran a quantum eraser with them. For that they couple the phonons to superconducting qubits on a chip and cool it all down to cryogenic temperatures, so random heat vibrations don't wash the effect out.

So yes, sound can be quantum. But only as phonons, in a solid, in a lab, in extreme cold. The sound in the air of your living room is a classical pressure wave, and classical physics describes it almost perfectly. Phonons might still matter later, as a way to store quantum information and move it between superconducting circuits and light.

**Basically,** Phonons are quantised sound in solids and show real quantum behaviour, but only in cold, controlled labs.

## Why speakers can't build a quantum computer

The next thing I wondered was why nobody builds a computer out of speakers, if a quantum computer runs on superposition and interference and sound has both.

The answer is the size of the space the machine works in. With n qubits a quantum computer works in a space of 2 to the power of n states, so every extra qubit doubles it. Superposition lets it use that whole space, and entanglement links the qubits to each other. Many quantum algorithms need that link.

Classical waves can add up, and you can mix many frequencies in one sound. But to get exponentially many states out of a classical wave system you would need exponentially more of something physical, like higher and higher frequencies or more and more energy. Adding speakers only grows it slowly. Classical waves also have no entanglement between separate parts and no interference of probability amplitudes. And quantum error correction depends on quantum properties and measurements that a classical sound system just doesn't have.

**Basically,** Sound can mix waves, but without entanglement it can't reach the huge state space a quantum computer uses.

## What I take from it

Sound is a great way to feel what interference is. You can walk through a room and hear the stripes. But the double slit with sound only proves that sound is a wave. The quantum part is something else. It's that a single electron interferes with itself, that the pattern builds up from random single hits, that checking the path kills the pattern, and that nobody can fully explain how a measurement picks one result.

| | Sound in air | Electron or photon |
|---|---|---|
| What is waving | air pressure | probability amplitude |
| Needs a medium | yes | no |
| Interference | yes | yes |
| Arrives as single dots | no | yes |
| Where one hit lands | predictable | random |
| Checking the path | small disturbance | pattern disappears |
| Entanglement | no | yes |

So my honest answer to my own question is that sound waves can copy the look of quantum physics, the stripes and the waves, but not what happens inside. Where sound really becomes quantum, as phonons in a cold crystal, it follows all the strange quantum rules, the measurement problem included. The math of waves is a shared language for both, but it describes two very different things.

**Basically,** Sound copies the stripes of the double slit, but randomness, collapse and entanglement stay quantum only.

## Sources

- [Young's fringes with sound waves (IOPSpark)](https://spark.iop.org/youngs-fringes-sound-waves) The two-speaker double slit as a classroom experiment.
- [Diffraction through slits (Salford Acoustics)](http://salfordacoustics.co.uk/sound-waves/diffraction/diffraction-through-slits) Sound through two slits and Young's fringes for sound.
- [The Nature of Sound (The Physics Hypertextbook)](https://physics.info/sound/)
- [Why does a double slit with sound never give complete destructive interference?](https://physics.stackexchange.com/questions/183399/why-does-a-double-slit-experiment-with-sound-waves-never-result-in-complete-dest)
- [What is the difference between quantum and classical interference?](https://physics.stackexchange.com/questions/225797/what-is-the-difference-between-quantum-and-classical-interference)
- [The Measurement Problem (John D. Norton, University of Pittsburgh)](https://sites.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/quantum_theory_measurement/index.html)
- [Philosophical Issues in Quantum Theory (Stanford Encyclopedia of Philosophy)](https://plato.stanford.edu/entries/qt-issues/)
- [The Role of Decoherence in Quantum Mechanics (Stanford Encyclopedia of Philosophy)](https://plato.stanford.edu/entries/qm-decoherence/)
- [Observer effect (physics), Wikipedia](https://en.wikipedia.org/wiki/Observer_effect_(physics)) Why the observer is a physical interaction, not a mind.
- [Phonon (Britannica)](https://www.britannica.com/science/phonon)
- [Quantum Erasure Using Entangled Surface Acoustic Phonons (Phys. Rev. X)](https://link.aps.org/doi/10.1103/PhysRevX.10.021055) Entangled phonons and a quantum eraser on a chip.
- [Quantum Acoustics with Multimode Surface Acoustic Wave Cavities (JILA thesis)](https://jila.colorado.edu/sites/default/files/2024-05/SlettenThesis.pdf)
- [New device opens door to storing quantum information as sound waves (Caltech)](https://www.caltech.edu/about/news/new-device-opens-door-to-storing-quantum-information-as-sound-waves)
- [The Classical-Quantum Limit (PRX Quantum)](https://link.aps.org/doi/10.1103/PRXQuantum.5.020331)
- [If quantum speed-up is due to the wave-like nature of quantum mechanics, why not just use regular waves?](https://quantumcomputing.stackexchange.com/questions/2595/if-quantum-speed-up-is-due-to-the-wave-like-nature-of-quantum-mechanics-why-not)