# How does the density of air affect the note you hear?

• B
• rafk_1
In summary, the conversation discusses the relationship between the density of air and the perceived note of a plastic bottle marimba. The tension in the bottle's skin, determined by the internal air pressure, determines the frequency of the note. The conversation also explores the effects of pressure on sound velocity and resonant frequencies, particularly in wind instruments. Ultimately, the participants agree that the perceived pitch of the plastic bottle marimba is not affected by the density of air, but rather the pressure.
rafk_1
TL;DR Summary
How does pressurized air in a bottle change the sound of the note being played (more air = higher note) but what is the explanation behind why this works?
I'm making an instrument for a high school physics project and I was wondering how the density of air changes the perceived note and the explanation behind it. I've managed a few possible theories but none that I was fully confident in. I'll reference a video which inspired my idea, adding more pressurized air to the bottle changes the pitch but I'm not exactly sure how this could work, does anyone have any answers? Thanks for the help.
Plastic Bottle Marimba

Welcome to PF.
You are making a drum. The skin of the bottle is the membrane. When the bottle is struck it deforms, then it springs back to the original shape at a rate determined by the tension in the skin. The tension in the skin is determined by the internal air pressure. So the frequency of the note rises as the air pressure is increased.
It is not the density of the air, but the pressure that makes the difference.
https://en.wikipedia.org/wiki/Drum

rafk_1 and nasu
Your ear measured frequency. It hears the same frequency as the source. The frequency of the source may or may not depend on the properties of air. A flue or organ pipe is different from an electronic loudspeaker.

rafk_1 and anorlunda
I know that wind instruments play higher pitch in less dense air. This can be a problem for recording studios at high altitudes.

The instrument described in the OP is not a wind instrument. It is a percussion instrument where the air pressure is used to adjust the tension in the membrane.

jbriggs444
Hornbein said:
I know that wind instruments play higher pitch in less dense air. This can be a problem for recording studios at high altitudes.
To first order, pressure does not affect sound velocity and, hence, the resonant frequency for cavities of a given length. The reduction in density due to pressure cancels with the corresponding reduction in bulk modulus for an ideal gas. Are there some second order factors at play here?

You get a higher tone in a flute in a helium-oxygen atmosphere compared to a nitrogen-oxygen atmosphere. You do not get a higher tone for a 1/2 bar atmosphere compared to a 1 bar atmosphere.

To be clear, this would be relevant if we were discussing wind instruments and resonances due to the speed of sound in air. But we are actually discussing a percussion instrument. The resonances are due to the vibration of the instrument. Like the resonance of a violin string as it is being tightened.

Last edited:
nasu
jbriggs444 said:
To first order, pressure does not affect sound velocity and, hence, the resonant frequency for cavities of a given length. The reduction in density due to pressure cancels with the corresponding reduction in bulk modulus for an ideal gas. Are there some second order factors at play here?

You get a higher tone in a flute in a helium-oxygen atmosphere compared to a nitrogen-oxygen atmosphere. You do not get a higher tone for a 1/2 bar atmosphere compared to a 1 bar atmosphere.

To be clear, this would be relevant if we were discussing wind instruments and resonances due to the speed of sound in air. But we are actually discussing a percussion instrument. The resonances are due to the vibration of the instrument. Like the resonance of a violin string as it is being tightened.
I say this because when Earth Wind and Fire recorded in Colorado they complained that the high altitude made their wind instruments go sharp.

1. Just the wind instruments? The Earth and Fire instruments were OK?
2. Bands say a lot of things on stage - "Hello Muncie! It's great to be back! One of our favorite cities!"

jbriggs444 and nasu

## 1. How does the density of air affect the note you hear?

The density of air can affect the note you hear in several ways. First, the density of air affects the speed of sound, which in turn affects the pitch of the note. When air is denser, sound travels faster, resulting in a higher pitch. Second, changes in air density can also affect the resonance of the instrument, altering the quality and tone of the note.

## 2. Does the density of air change the frequency of the note?

Yes, the density of air can change the frequency of the note. As mentioned earlier, when air is denser, sound travels faster, resulting in a higher frequency or pitch. This is why instruments produce different notes when played at high altitudes where the air is less dense.

## 3. How does temperature affect the density of air and the resulting note?

Temperature affects the density of air because hot air is less dense than cold air. This means that the speed of sound is faster in hot air, resulting in a higher pitch. Temperature can also affect the resonance of an instrument, changing the quality and tone of the note.

## 4. Can changes in air pressure affect the note produced by an instrument?

Yes, changes in air pressure can affect the note produced by an instrument. This is because air pressure is directly related to air density. When air pressure decreases, air density also decreases, resulting in a higher pitch. This is why instruments may sound different at high altitudes where the air pressure is lower.

## 5. How does the density of air affect the sound of wind instruments?

The density of air plays a crucial role in the sound produced by wind instruments. In wind instruments, air is blown into a chamber, creating vibrations that produce sound. The density of air affects the speed of these vibrations, resulting in changes in pitch and tone. Additionally, changes in air density can also affect the resonance of the instrument, altering the overall sound produced.

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