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Name: Sumit
Status: student
Grade: 9-12
Country: India
Date: April 4, 2011

If the photon is the smallest particle of light, then what is the smallest particle of sound?

Hi Sumit

Light is an electro-magnetic wave or particle (actually it can be both a wave and a particle) and its smallest particle manifestation (as far as we know today) is the photon. Sound (as the human ear hears it) is a pressure wave. Sound is not an electro-magnetic wave and therefore you cannot speak of sound in terms of particles. So when you speak of the components of sound you need to speak of the frequency of the sound wave which determines the pitch of the sound.

From: "the generally accepted standard range of (human) audible frequencies is 20 to 20,000 hertz(Hz)."

So there is no such thing as the smallest particle of sound. However, in computer networks, music can be digitally transmitted and in such a case, the "bit" could be called the smallest particle of sound, but I don't think that is what you are asking about.

Sincere regards, Mike Stewart

Sumit -

the "phonon" is very commonly discussed in solid-state physics. The phonon is a minimum quantum of sound-wave. It does act a bit like a particle even though it travels through a solid crystal lattice, and consists only of a vibration in that lattice. The carries some momentum, does collisions which conserve momentum, and when generated or absorbed it often appears or disappears all in one piece.

Although, it is possible for a photon interacting with an atom to change its wavelength by any percentage small or large. It is even more common for a phonon to be changed to higher or lower energy when it interacts with a excited crystal defect, or with a defect plus other phonons. There is a spectrum of possible phonons in each solid, just like there is a spectrum of photon wavelengths in vacuum. Like a photon, a phonon has zero rest-mass. Its mass is due only to the energy it carries. These kinds of things remind you that it is more like a quantized quantity of wave action, instead of a hard particle with a distinct type and mass like an electron.

Did you know that a silicon solar cell can't absorb a photon without the help of at least one phonon? This might not be true of blue light, but it is true for red and infrared light. Silicon is called an "indirect band gap" semiconductor, because absorbing a photon needs to make a phonon or it cannot happen. Something about conservation of momentum, I think. Others like gallium arsenide are "direct-gap" semiconductors.

Jim Swenson

The fundamental particle of sound (that is, the fundamental "particle" of vibration) is called a "phonon". See the reference: for more details.

Vince Calder

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