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Hi, now that we have dealt with standing wave and resonance, we can apply
that understanding to explain many different types of musical instruments
as you see this orchestra in front of you, we can explain a good 80-90% of these instruments
just using our 1D standing wave
the drums and the percussion are not going to quite work, but
the string section here, that's all "wave on a string" that you have analysed through your lab
back here, you have your flute, french horn and other brass instruments
and woodwinds, those are all air columns which is going to be our next lab
but at the same time, they can be roughly model as a 1D air column, therefore a 1D standing wave
First, when we talk about music
we are going to be dealing with different medium, so I am going to remind you what happens when
you vibration is moving across different media. So in this case, I have a thicker blue rope on this side
attached to a lighter orange rope on the other side, we are not going to care so much about the reflection
but what I want you to see is: sure, on this side, because the rope is lighter
your phase speed is going to be higher, so you wavelength is longer
but the time it takes for 1 hump to move across
on this side is the same as the amount of time it takes to create a wave on the transmitted side, so that's why
as the wave moves through
the wavelength can change but the frequency stays the same. So across different medium, whenever you deal with more than one
media, the frequency is going to stay the same, it is the wavelength that change
so in terms of talking about sound and instruments, we are moving across many different types of media
we have vibration on the string or whatever instrument you have, that gets amplified and transmitted through the air
which then vibrates our ear drum, then vibrates the different bones in our middle ear, and the fluid in your cochlea
and then the little hair inside your cochlea that's going to ultimate create the nerve signal in your brain
so through all these different medium, the only thing that is preserved is the frequency
so that's why it makes the most sense to talk about sound in terms of frequency
With these frequencies, you notice
sound frequencies would be usually in the range of hundreds or thousands of Hertz
that's the human hearing range is from 20 Hz - 20,000 Hz if you are young and not an old person like me
However,
very rarely do we shake an instrument at that speed, our hands aren't that fast, or our lungs
aren't able to change the modulation of the air that quickly. We don't make sound by
breathing in and out 100's of times a second, we don't create music by shaking a string at 100's of Hertz a second as well
That's why there is usually some kind of frequency selection mechanism
i.e. resonance that happens
in a musical instruments and that's usually done through creating standing waves
and so that's kind of a central part of most musical instruments is this frequency selection
where we can select certain frequencies and therefore certain pitches and tones and notes that we play
and that's often done through, again, resonance and standing waves
so before that, another important part is the excitation, you have to
put energy into the instrument so it can start to vibrate, and this excitation usually hits all different frequencies
so it could something like a constant air pressure that you apply through your lung
or just plucking the string on a guitar
or rubbing string using friction to get the string vibrating
this excites all the different frequencies
which then gets selected through the standing wave/resonance mechanism
and then finally, we need to amplify this sound and this amplication happens once again
at many frequencies, so that when you play different notes, the same amplication mechanism/device still works
and that's the three key parts of musical instruments
you need something to excite it, something to select the frequencies, and something to do the amplication
a little more concretely, say, for a cello
for instance
you still have these 3 parts, the first part, the excitation
is when you rub the bow on the string, that's going to start the string vibrating at all kinds of frequencies
now only certain of those frequencies survive, because
you have standing wave on a string, you have analysed that extensively
depending on the tension and the length, you can select certain frequencies
and then finally the amplication is done through the body of the cello which is basically a big box of air
which then allows you to vibrate a lot more air than otherwise, so you can project your music a lot better
I will refer you to the next video, which is linked to a neat little project that I discovered online
but you can hopefully see how just by focusing on these three basic parts
we can still create an instrument which is very similar to a conventional one