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sources travel f & a resonance timbre

Welcome to

Sound is all around us. It’s everywhere!

As you learned earlier, all waves begin with a vibration.

Well, this goes for sound waves too!

All sound begins with something vibrating.

Guitar strings vibrate to create sound waves.


Any material object can vibrate – so just about anything can create a sound.

Here is a video clip of stiff metal vibrating to create a sound.


Leaves vibrate in trees from the wind . . .

Your vocal cords vibrate when you speak . . .

Even a column of air can vibrate to make a sound – like in a trumpet.

Click on the trumpet to hear the air and metal vibrating.


The vibrating object vibrates air molecules around it. These vibrations create a longitudinal wave of energy in the air . . . and as you know waves carry energy from one place to another – so the sound energy travels as a

wave through the air – from the object to your ears.

vibrating object

air molecules

detector (like your

ears)


Here’s a video clip that shows the process of sound travel


Since a sound wave is typically a longitudinal wave of air molecules, you might think that there must be air for sound to travel – and you’d be right.

Although, sound can travel through other material as well – water, steel, walls.

The vibrating source must push some material (medium)

for the sound wave to travel.

So, does sound travel in empty space (where there’s no material)?


Here’s a video clip that shows how sound does not travel through a vacuum.


Sounds are created by:

waves

molecules

strings

vibrations


Sounds travel as:

vibrations

longitudinal waves

transverse waves

energy


Earlier you learned about the frequency and amplitude of waves.

Frequency and amplitude are objective, measurable, physical quantities of any wave.

The frequency and amplitude of sound waves have subjective qualities:

Frequency relates to the pitch of a sound while amplitude relates to the loudness.

frequency ~ pitch

amplitude ~ loudness


This means: the more frequent the vibrations, the higher the frequency of the wave the higher pitch the sound.

Here are two different piano sounds of different frequencies, therefore different pitches.

Check them out!

high frequency

high pitchlow frequency

low pitch


Although sound waves are longitudinal waves, we can represent them as transverse waves. Here’s how: where there’s a compression it’s high pressure, where there’s a

rarefaction it’s low pressure. The graph below will help you see this connection.


Here’s a video clip showing the relationship of frequency of a sound wave to what you hear. Notice the sound waves are represented by transverse waves.


Humans can hear sounds generally in the range of 20 Hz to 20,000 Hz.

Sounds with higher frequencies than 20,000 Hz are call ultrasonic. Here’s an example:


Bats and dolphins use ultrasonic sounds to locate things. They sound out the sound waves and receive them after they reflect back off of objects. The time it

takes the wave to go and come back helps them interpret how far away the object is.


Humans also use ultrasound to see things they normally wouldn’t be able to. It’s used very often in medicine.

Here’s a web-site about ultrasound.

The amplitude of a sound wave relates to it’s loudness or volume. The bigger the amplitude, the louder the sound.


Here are some sound waves of different amplitudes.

Check them out!

small amplitude

low volumelarge amplitude

high volume


High volume sound waves have:

big molecules

high frequency

high amplitude

big wavelengths


Low frequency sound waves are:

low pitched

low volume

low speed

low to the ground


All material objects can vibrate to make a sound. The neat thing is all objects will naturally vibrate at a certain particular frequency. This is called the natural

frequency. An example would be a big thick guitar string would vibrate naturally at a low frequency. A smaller, thin guitar string would vibrate at a high frequency.

Usually (not always) larger and longer objects have lower natural frequencies, while smaller and shorter objects have higher natural frequencies.

All objects have a natural frequency at which they will vibrate.


If you push something to vibrate at it’s natural frequency, and keep pushing it at just that right rhythm, you can get it to vibrate like crazy – it will make a louder and louder sound. You can also get it to vibrate so

much it breaks.

When you push something at it’s natural frequency it’s called resonance.

Resonance involves matching frequencies.

Let’s say you’re playing some music on a stereo and occasionally something else in the room vibrates – like a vase – this is an example of resonance. At a given moment frequencies of the sound waves in the

music matched the natural frequency of the vase – and the vase resonated, that is it vibrated with an increased amplitude.


Resonance is like pushing someone on a swing – if you push at just the right rhythm you get them going really high. For sound, this means bigger sound.

Here’s a good video clip on resonance


Finally, if frequency determines the pitch of sound, and amplitude determines the volume, why is it that two same pitched and volume sounds can sound different?

That is, one can play the same note at the same volume on a piano and on a violin and almost anyone can tell the difference between the instruments – even though

the notes have the same frequency & amplitude.

So, what is the physical difference between the same note on different instruments?

or


The answer to this question is something called “timbre”.Timbre has to do with shape of the sound wave – for any given sound wave can have almost any given shape – and it’s this shape that determines the

quality or unique characteristics of the sound.

The best way to see this is by examples. (by the way, this could get very mathematical – but we won’t go there at this

point)

timbre ~ quality of sound


Here are the transverse wave representations of two different sounds.

Look and listen. Notice the difference in the quality of the sound and the complexity of the wave.

flute, simple wavedijerdoo, not as simple wave


Or here is an even more extreme example.

Again, look at the wave forms and listen to the difference in the sounds.

cello, fairly simple wave synthesizer, complex wave


The timbre of a sound wave is what makes your voice different from someone else’s. It’s like you have your own voice print (kind of like finger

prints). Here are two different voices.

male voice female voice



A trumpet sounds different than a clarinet

because

different frequencies

different amplitudes

different timbres

different wavelengths


Resonance happens when you:

make big sound waves

make high frequency sound waves

match amplitudes

match natural frequency


You’ve reached the end of our tutorial on sound waves!



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