Hi there everyone, welcome back to Cody's Lab.
So today, I have a dog whistle and a sound spectrum analyzer here loaded up onto my phone.
So we've got frequency along this (horizontal) axis, intensity on this (vertical) axis.
And what I want to do today is...
Blow some different types of gas(es) through the dog whistle and see what effect it has on the sound that it puts out.
But of course, to compare, we're gonna start with air.
Fair warning, if you're sensitive to high-pitched noises...
You might want to turn down the sound or take your headphones out, whatever you need to do.
So, here we go. I'm just gonna blow with my breath for now.
Okay, so it looked like the peak sound from that was at about nine thousand, eight or nine thousand Hertz.
Which is well within the range of human hearing.
You know that's kind of interesting, because you know, aren't dog whistles supposed to be so high-pitched you can't hear?
Well with this particular whistle, that is not the case.
This is just a cheap one that I bought from the pet store for about seven bucks. So it's...
It's not like it's a high-end silent whistle or anything.
But that really doesn't matter for what I'm trying to teach today.
So, first thing I want to try doing is changing this to longer length.
So that those sound wiggles don't go away quite as fast.
So now we can...
Yeah, see now it stays there for a little while longer so we can see a more of an average of what this sound is being put out.
Let's actually put a piece of tape right there.
So I can write on my screen now.
Now let's actually...
Hook this up to a balloon, to eliminate any sort of bias based on my breath.
Okay, so I've got this balloon filled with air it's just hooked up to the whistle.
Let's turn this towards the speaker, okay.
Let that graph settle down, and then I'll just release the air.
Okay, we got a peak right there. Alright. Now we're gonna try it with helium.
I got some helium in the balloon now.
The peak is right over there.
As you can see, the helium was much higher-pitched in the air.
Looks like around 16,000 Hertz.
That's pretty significant boost there.
So I went from eight or nine to 16, that's almost twice as high a pitch.
If you've ever taken helium and inhaled it.
Well now that it makes your voice high and squeaky.
The same thing that the helium is doing to my voice, is what it's doing to the whistle.
The speed of sound, in helium, is faster than it is through air.
So inside the body of the whistle, the sound waves are bouncing back and forth in here.
In the air, it happens about 9,000 times a second. In helium, it was happening around 16,000 times a second.
Because it takes a smaller amount of time for the sound wave to get from one end to the other, resulting in a higher pitch.
The helium molecules weigh only about four atomic mass units, whereas the average in the air is right around 30.
I won't get into it very much here,
but you know take the root-mean-square of the mass and the temperature...
The helium particles for the same temperature and energy,
move about three times faster than the air molecules.
This means if you have a bunch of them you give them a push, the helium is gonna transmit that push much faster than the air.
Here's a table of the actual speed of sound through different gases in meters per second.
As you can see, helium, it's over a 1000 (m/s), oxygen it's about 320 (m/s).
Now you might ask...
Why did I only see a doubling in frequency, rather than a tripling in frequency, if the speed of sound through helium is three times faster.
Well, my current hypothesis on that is that my source of helium is contaminated with oxygen.
Which makes it, so that it overall is a heavier gas than I should be expecting.
So now let's try it with a gas that is heavier than air.
Heavier molecules travel slow, so the speed of sound is slower, the sound should be on a lower pitch.
So now I have some xenon, a noble gas, which is about five times heavier than air.
You also notice that the balloon sinks a lot slower when it's got the high molecular weight gases.
Because the diffusion rate is slower.
That really shouldn't affect anything, but the sound intensity.
Let's actually give that a test real quick.
I'll squeeze the balloon to increase the pressure and therefore increase the diffusion rate.
That's interesting...
Kind of made two peaks, one was about here (lower frequency).
Okay, I think I can see what's happening here.
The whistle is actually putting out multiple frequencies.
And the relative strengths of the different frequencies does seem to change depending on the pressure that is being put on it.
See, you get multiple peaks if you blow through it really hard.
Since I marked the position of the first peak with the air,
makes sense that I should mark the first peak for the xenon as well.
This means that the xenon did produce a frequency about half of what the air did.
As you'd expect from the speed of sound.
Now to be clear, this doesn't just work with the dog-whistle. This will work with other whistles as well.
And mostly I didn't use this one because it takes a lot more gas to make it work and the xenon rather expensive.
But let's try it with air and helium.
Okay, that's air to about 4,000 Hertz.
And now for helium.
Hmm so not quite as dramatic.
But it was a higher pitch.
About the same ratio of pitch change in fact, I think.
Hope I haven't annoys the neighbor's dogs too terribly.
Yeah, I haven't heard him barking so I think we're good.
And hope you guys enjoyed. I'll see you next time.