6. HARMONIC CONTENT

Have you ever wondered why 2 people singing exactly the same note sound different - the quality of each vocalist's voice is unique to them. Also if you ask a violin player & an oboe player to play the same note, you hear exactly the same note but the "timbre" or "character" of the sound from the 2 instruments is different. What is going on?

The most simple kind of sound is a sine wave - like the kind you were listening to in the previous video on phase. Let's look at the same note A 440 Hz using a spectrum analyser which shows us what frequencies are being produced. Let's then look at some other instruments playing the same note. What do you notice?

The video makes it clear what is going on. The first sound - the sine wave - produces just one frequency which is 440 Hz, this is called the fundamental. All of the other instruments as well as producing the same fundamental of 440 Hz also produce lots of other frequencies, these are called harmonics. We hear the same note being played because the fundamental is the loudest frequency being produced, but all of the other frequencies "colour" the different sounds giving them their own unique sonic fingerprint.

So, if someone were to compare two different instruments playing the same note & were to comment that one sounded "brighter" than the other; you would probably now assume that this was because the "brighter" instrument was producing more higher frequency harmonics.

7. ACOUSTIC ENVELOPE

This simply describes how a sound, starts, progresses & then finishes. Imagine the sound of a guitar string being plucked. It starts relatively loudly & then the volume dies away to silence as vibrations of the string become weaker & weaker. A piano is similar if the sustain pedal is held down. A hammer strikes a string & it vibrates, if the sustain pedal is released, the note will be silenced. A snare drum when hit, starts very loudly but its sound does not sustain & it stops very shortly after it has begun.

Many of the more expressive instruments can have their acoustic envelope controlled at any time. For instance a violin player can control the volume & duration of a note by altering the pressure of the bow on the string. Similarly a clarinet player can do the same by altering the amount of air they are blowing into their instrument. (Note however that a clarinet player can only make a note last as long as they can blow out!) The diagram below should help to give you an idea of acoustic envelopes.

ACEnvelope

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