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Introduction to Resonance and Tone Quality Perception

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

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 that are not vowels -- this hum and buzz -- have resonances, which we interpret as tone quality.

Let's learn a bit about how to read a spectrograph, 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 where:

Left = low frequency

Right = high

Up = louder sounds

Down = softer sounds

The scale is even left-to-right, much like the inches or centimeters 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 Lunte) 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 perceive as tone quality - the way our mind differentiates sounds for us.

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

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. It's the second one. Find the highest white peak. It's the third one. This shows us that the lowest resonance for ee and ay are not the same. Ee's are lower than ay's.

Now, let's find the second resonances. Moving to the right, find the next place a blue one sticks up fairly high above the white. This is the second resonance for ee. Just to the left of it is the second resonance for ay. The second 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.

This essay was first published November 23, 2008 on The Modern Vocalist.com the Internet's #1 community for vocal professionals, voice health practitioners and pro-audio companies worldwide since November 2008.


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