The brain is amazing.  You can tell two instruments apart when they are playing the same melody at the same time.  In fact, your brain takes all the overtones (harmonics) that make up the sound of one instrument playing one note and you just hear one trumpet or one piano sound.  You don't "hear" all the different frequencies in one instrument as your brain uses unconscious inference to group the similar objects together as one.  read more »

When you hear a musical sound, let's say at 500 Hz, you are also hearing related sounds at 1 kHz, 1.5 kHz, 2.0 kHz and so on until the energy level of the frequency drops off.  Understanding harmonics will help you identify related frequencies and control your sound system's output accordingly.  Find out how...  read more »

"Is there a difference between volume and decibel level?"

Volume is not a well-defined measure of sound.  Decibel is defined...as a ratio.

If you have two audio signals, and one is a ten times stronger than the other, you can say it is 10dB stronger. If it is 100x stronger it is 20dB, if it is 100x weaker, it is -20dB. It's a logarithmic formula.

Sound pressure levels (SPL) given in dB refer to 20microPascal at 0dB, which is actually around the threshold level of hearing anything at all.  read more »

Sound is what you hear. I hope you aren't satisfied with that answer. A sound engineer needs to understand not only what they hear, but what happens with sound.

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Sound comes from vibrations caused by fluctutations in the air pressure. In fact, that's how the ear drum works.  read more »