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Introduction to resonance and tone quality perception

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A spectragraph is a picture that tells us about the frequency and intensity of the different parts of a sound.

Here is one which contains two vocal sounds graphed, a 'vocal hum' in blue, and a 'vocal buzz' in white. Both sounds were produced on the same pitch, with the mouth closed. (These are from a demo from Robert Lunte. My thanks to him)

Robertshumbuzz2.jpg?width=300

If you click on the picture, you will get a bigger one.

In my last blog post, I introduced you to the idea that there are multiple sounds in a sung tone, and that the resonances determine what vowel we perceive. Even sounds which are not vowels (this hum and buzz) have resonances which we interpret as tone quality.

Lets learn a bit about how to read a spectragraph, so we can discuss the physics of vocal resonance.

On this picture, all of the sound energy from 0 cycles per second up to 5000 cycles per second is graphed. Left=low frequency, right=high; Up=louder sounds, down=softer sounds. The scale is even left-to-right, much like the inches or centimetres on a ruler are all the same size.

When you see a peak up fairly high on the picture, then that means that a sound of a particular frequency is quite loud. Go ahead and count the white ones you can see. You should get more than 20.

Just for fun, count the blue ones, and notice if there are any that go higher than the white ones.

The note that the singer (Robert) is producing is the very lowest (leftmost) peak. In musical acoustics terms, this is the called the 'fundamental', or the first harmonic. All of the peaks to the right of that are the 'overtones', or the 2nd through the 25th harmonics. The relative strength of these harmonics is what we percieve as tone quality - the way our mind differentiates sounds for us.

Here is a different spectragraph, this time of two vowels, ee and ay. ee is in blue, ay is in white.

eeandayspectragraph.jpg?width=300

As I mentioned in my prior post on Vowels, the two lowest resonances are the ones primarily responsible for the perception of a particular vowel. Let's find them. Starting from the left, look for the highest blue peak. Its the 2nd one. Find the highest white peak. Its the 3rd one. This shows us that the lowest resonance for ee and ay are not the same. Ee's is lower than ay's.

Now, lets find the 2nd resonances. Moving to the right, find the next place a blue one sticks up fairly high above the white. This is the 2nd resonance for ee. Just to the left of it is the 2nd resonance for ay. The 2nd resonance for ee is _higher_ than the 2nd one for ay. When compared in combination with the lower resonances, the ones for ee are farther apart than the ones for ay. Between the two resonances, there is a fairly deep 'valley' where the harmonics are much softer.

In a given voice, each distinguishable vowel has particular spacings for these two resonances, and that is how we tell the vowels apart as listeners.

More next time!

Best Regards,

Steven Fraser

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