When you play recorder or whistle, the pitch depends on how hard you blow into the tube. E.g. when you blow a whistle, initially the pitch is slightly lower when there is less air flow. This seems counter intuitive since the airflow should only affect the amplitude of the sound waves (like in many other instruments and tubes) and the frequencies which the resonating cavity choose to amplify should depend only on its length, which is constant. So why would the dominant sound we hear be affected by the air speed?
[Physics] Why does pitch increase when you blow harder into a whistle
acousticsflowfluid dynamicsoscillatorsresonance
Related Solutions
The adhesive on tape is a viscoelastic material. I think popular brands like Sellotape use a low molecular weight synthetic rubber like polyisobutylene.
Anyhow, as you peel the tape away from the roll the adhesive stretches until the tension in it is greater than adhesion to the top of the tape below it, then it breaks free. As it breaks away, the stretched adhesive relaxes and the tension is released. As you continue pulling, the next bit of adhesive starts stretching until it too suddenly pulls free, and the cycle is repeated. This means that as you pull the tape off the roll it comes free from the roll in a series of jerks. If the frequency of the jerks is in the audio range, say 50 times a second or greater, you'll hear it as an audible tone.
The faster you pull then tape off the roll the higher the frequency of jerks will be. This is partly because the adhesive will only stretch a certain distance before pulling free, so a higher velocity of tape removal requires more jerks per second. However it's also because the elastic properties of the tape are frequency dependant and it becomes less elastic at higher peeling speeds.
If you're interested in pursuing this further I found a paper analysing the dynamics of tape peeling here.
In order to properly understand this without any unnecessary "controversy", let's break the whole process of sound generation and perception into 5 important, but completely separate parts. We'll then proceed to explain each part using a few different examples and pieces of derivative logic:
- Vibration of the vocal folds
- Transmission of energy from vocal folds to air in the vocal tract
- Resonance and Attenuation in the vocal tract
- Transmission of energy from the end of the vocal tract (mouth) to the surrounding medium
- Reception and perception of sound by another human.
Now:
1. The frequency generated by the vocal folds depends on the tension exerted on them and surrounding muscles. This is a neuromuscular process and is NOT affected by Helium or any other gas (at least in the short term).
So our vocal folds continue to vibrate at the same frequency in helium as in normal air.
2. Sound is produced by the transmission of the vibrations produced in the vocal fold, to the air in the vocal tract. This "transmission" doesn't occur by any magic. The vocal folds - as they vibrate - push and pull columns of air in their immediate vicinity, not very different from the way you may push a child on a swing at specific intervals, so as to produce sustained oscillations, and brief enjoyment. (The "pull" in this analogy though, is provided by gravity).
The point is, the child oscillates at the same frequency at which you are pushing the swing. i.e. If you are pushing the swing once every N seconds, the child also completes a swing once every N seconds. This is true regardless of the weight of the child, correct?
Similarly, the air in the vocal tract, also vibrates at the same frequency as the vocal chords. This fact, is also true regardless of the mass of the air particles.
In other words, the frequency of sound does not change, regardless of the medium in which it is transmitted.
Time-Out
The last one was a doozy. Frequency of sound does not change? Then why on earth does helium sound different from normal air?
While the frequency of sound does not change, the SPEED of sound does. Why? Consider this old classical physics equation:
Kinetic Energy = 0.5 * m * v^2,where m = mass and v = speed (let's not say 'velocity' for now)
Now the vocal chords, vibrate with the same Force N at the same frequency T. Thus the energy it conveys must be the same in ALL media.In other words, for a given constant value of Kinetic Energy, v^2 is inversely proportional to the mass of the particles.
This naturally means sound travels faster in Helium than in air.
Now, we know the other old equation:
Speed = Wavelength x Frequency
Now since we know that the FREQUENCY of sound is the same in Helium and in Air, and the speed of sound is greater in Helium, it follows that the Wavelength of sound is greater in Helium than in Air. This is a very important conclusion, that bears directly on our next deduction.
3. Now, we have a very important conclusion in our kitty - "Wavelength of sound is greater in Helium than in Air".
Remember that the vocal tract is often modelled (simplistically) as an open or closed tube. To refresh why that's important, see Wikipedia.
The vocal tract is actually not really a cylinder, but a fairly complex shape. This means it has areas of constriction and expansion that change depending on the position of your tongue, tension in the tract, and several other factors.
So in a sense, in these complex configurations, the vocal tract can be modelled as a series of tubes of varying diameters and varying levels of "closure" of either openings.
Now this means, that different parts of the vocal tract, depending on their geometrical configuration and their material characteristics, resonate with different WAVELENGTHS of sound.
Notice I said WAVELENGTHS and not FREQUENCIES. In common parlance, "Frequencies" is often used since W and F are directly inter-related in a common medium. However, even if we change the medium through which sound is being propagated, the interaction of sound waves with open and closed tubes depends strictly on its wavelength and not its frequency.
Now would be a good point to return to the marquee conclusion we drew from point 2 - "Wavelength of sound is greater in Helium than in Air".
This leads us to the following KEY/FINAL CONCLUSIONS:
In a vocal tract filled with Helium:
1. The frequencies of sound do not change
2. The wavelengths of sound DO change
3. Because the wavelengths have changed, the portions of the sound spectrum produced by the vocal chords that are attenuated and resonated by different portions of the vocal tract, also change.
4. This results in the sound spectrum output by the combination of the vocal chords and vocal tract in Helium, being different from the sound spectrum output in normal Air.
5. This means, the net distribution of energies among high and low frequencies (or the timbre) changes with a change in sound medium. Whereas the fundamental frequency of the sound (closely related to pitch) does not change.
Let's look at the spectrogram of two sample sounds helpfully provided in the NSW article.
Unfortunately due to the experimental conditions the two sounds do not have the same content (different sentences are spoken) and therefore the spectrogram cannot be exactly relied upon. However, the fundamental frequency in both is roughly the same and therefore supports our conclusion that the pitch is the same. Since different words are used in either sound, a timbre comparison cannot be made (since the difference in energy distributions visible in the spectrogram can be attributed to the different words spoken).
Also, for simplicity and ease of understanding a "Melodic Spectrogram" has been used in favor of the raw, noisier spectrogram. It was generated using Sonic Visualizer.
We are not Done!
We started with the promise of explaining sound transmission and reception/perception in FIVE parts. We are done with only three. Let's get through the remaining parts very quickly.
4. Transmission of sound from mouth to air - As covered by point 2, with a change in medium, the sound frequency does not change, but the wavelength does. This means that the only effect of filling a room with helium as well (rather than just the vocal tract) is to increase the wavelength of the sound.
5. The above has no impact on sound perception. The ear and brain together are primarily a FREQUENCY receiver. The ear translates air pulsations into hair cell oscillations, which then translate to synchronous pulses on attached neurons. Since the timing of the pulses is correlated ONLY to frequency, and the timing of the pulses is what produces notions of pitch, timbre etc, we can safely assume that the ear transcribes sound to the brain faithfully based on frequency. Wavelength has no impact on this process.
However, the ear, just like the vocal tract, is non-linear. Which means that it too, is going to attenuate/resonate some sounds (the specific non-linear properties of the cochlea are still being studied). However, UNLIKE the vocal tract, the ear/cochlea is a sealed, fluid-filled chamber. The properties of the cochlea are not affected by surrounding air but only by the fluid, which of course could be affected by blood composition and other biological factors. But NOT the immediate environment.
Thus at the root of all the confusion around production and reception of sound in alternative media like Helium, is that the vocal tract's non-linear characteristics are affected by the surrounding medium, whereas the ear's are not. That's it.
Best Answer
I don't believe the other answers are correct. FGSUZ describes pushing air out of a tube, which sort of plays a little part, but not the whole story.
The way woodwind instruments produce sound, is they cause a column of air within the instrument to vibrate. This is done by splitting the air stream. Instruments such as the sax or clarinet use a reed to do this. A concert flute or a wine bottle blows air across a sharp edge, and a recorder or a whistle uses something called a fipple.
In any case, that splitting of the air causes a pressure differential in the stream. One side of the split goes into free air, the other side goes into the body of the instrument. Additionally, virtually all of the air you blow goes out into free air, very little goes into the body*. We know from Bernoulli's principle that the moving air is at a slightly lower pressure. In an attempt to equalize, the column of air in the body will begin to move to fill the low pressure zone. Because the air has some mass and momentum, it will overshoot, and a newly-created high pressure zone will push the column of air back the other way, and the process will repeat.
Pressing keys or (un)covering different holes will change the effective length of that air column, which you can think of as changing its mass**, which results in different pitches sounding.
So when you blow with a greater airspeed, you will create a slightly more intense pressure differential, and so will create a little bit more relative energy to oscillate the air column. Blow a little slower, and the pitch will go down a little bit. Smoothly alternate between and you may have a nice vibrato.
What's really important here, is it's not the volume of air that is important, but the air's speed.
This phenomenon is also why many wind instruments tend to sound sharp at high notes, and flat on low notes, and the player needs to correct by varying their airspeed, as the keys or holes on the instrument alone are not enough to get the right pitch.
In the case of a concert flute, which uses a sharp edge, rather than a fipple or reed, the player can aim their air, and directly control that pressure relationship, by varying the proportion of how much goes into the embouchure hole and how much goes over it. As a result, a skilled flutist can bend notes often more than a whole step up or down, based on air stream control alone, without changing anything about the flute itself, or without changing airstream velocity.
Lastly, if you produce enough power in your airstream, you can overblow and play 1 or more octaves above the note as fingered. When playing in the upper registers, the tendencies for the instruments to sound increasingly sharp as it goes higher becomes more dramatic.
Edit: I want to mention, but couldn't figure out where to work it into the answer above, but air speed is really important. Especially on the concert flute, it is important to the extent of massive frustration to newcomers. A fishing-line sized stream of air over the mouthpiece at the right speed will speak louder and clearer than 100 times more air if it's uncontrolled and slower. New flute players are often taught to think about "hot" vs "cold" air when learning to control their air stream. And, ultimately, when a player has attained sufficient skill, they can play quiet notes, by carefully blowing very small amounts of air, at very high speeds, and sound out even the highest notes quietly. If the physics of the instrument was about pushing air out of the body of the instrument, this would be impossible. It's not, because that tiny bit of air at the right speed is still enough to create that pressure differential, no matter how small.
*Not true for reed instruments; the air-splitting behavior is caused by the reed itself, but the rest of the concepts are still true.
**Massive oversimplification that borders on being completely wrong, but frankly it doesn't really matter.