How Do Vibrations Make Sound?

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How Do Vibrations Make Sound?

This seems like a straightforward question with  straightforward answer, but all may not be what it seems.

That said, the science is straightforward:

A sound wave is a transfer of energy as it travels away from a vibrating source. Sound waves are formed when a vibrating object causes the surrounding medium to vibrate. A medium is a material (solid, liquid or gas) which a wave travels through. As sound waves move through a medium the particles vibrate forwards and backwards. A sound’s volume, how loud or soft it is, depends on the sound wave. The more energy put into making a sound or a sound wave, the louder the volume will be. The farther a sound wave travels, the more it spreads, this makes it more difficult for us to hear a sound. So the nearer you are to a sound the louder it sounds to us. A sound wave enters the ear and is changed into nerve signals, which are interpreted by the brain.


As a given object vibrates or oscillates in air, for example, the air molecules near or around the medium will be moved back and forth in relation to the frequency and force of the vibration.

This energy then moves through the air as a pressure wave.

If the vibration is strong and slow, like bass drum hit with a soft mallet, the sound will be loud and low in pitch, if the vibration is weak but fast, like a pin falling on a wooden floor, the sound will be soft and high pitched.

The air molecules themselves don’t move across the room, much like a water molecules on one side of a swimming pool do not move to the other side in a wave, it’s the energy that moves through the medium.

This is why there is no sound in space, there is no medium for the sound wave to move through.

Think about it, you cannot make water waves in an empty swimming pool.

Here’s where it gets interesting. Once these vibrations are propagating through the air, can it be called sound?

After all it is just energy moving through a given medium, so when does sound become sound?

From one perspective the only thing that makes sound exist are ears, human or otherwise, and nervous systems and brains (or ganglia) to interpret and decode those vibrations into meaningful data – i.e., sound.

So, without ears does sound exist?

It is an interesting idea to think about. Some argue that sound is sound is sound. The vibrations are measurable and quantifiable events, they exist regardless of who or what hears them, but that opens up a whole philosophical can of beans — do things or events exist or happen outside of something or someone experiencing them?

Of course this is where the (in)famous “If tree falls in a forest and no one’s there to hear it fall, does it make a sound?” question comes up.

A forest is a busy place, with many creatures in it, so even if there are no humans around, some kind of animal or bug will experience the vibrations and use that data to run away from, or investigate the noise, i.e., the wave energy is converted into sensory data.

Arguably, it is only then that the tree makes a sound.

But if it just so happens that there are no creatures nearby to catch those waves with their auditory senses, and the energy merely propagates into nothing, does it make a sound at all? Even more interesting is; if nothing is around to hear the tree fall, can we go one step further and say no tree fell at all if nothing experiences it?

What about if a man is standing in the forest near the tree when it falls, but is wearing very good hearing protection with 100% noise suppression, and he takes a physical measurement of the sound wave’s energy without feeling, recording or hearing it, does the measurement alone qualify the wave energy as sound?

In Scientific American Volume 50 Number 14 (April 1884) the question was asked “If a tree were to fall on an uninhabited island, would there be any sound?” And gave a more technical answer, “Sound is vibration, transmitted to our senses through the mechanism of the ear, and recognized as sound only at our nerve centers. The falling of the tree or any other disturbance will produce vibration of the air. If there be no ears to hear, there will be no sound.”


So, to finally answer the question; how do vibrations make sound?

Well, they make sound by transferring energy away from a vibrating source, propagating through any given medium, solid gas or liquid, to our our ears and brain, where the information stored within the waveforms is decoded and interpreted.

If you think about it further this means that sound only really exists in our minds. It is our brain that decodes the energy wave, it is our brain that separates noise from meaningful input, for pleasure or survival, and without us to hear all these waveforms, what do those noises become, and do they even exist at all?