Amplitude modulation (AM)
When a high frequency sinusoidal signal (carrier wave) is multiplied with audible frequency signals. The audible frequency signal will modulate the amplitude of the carrier signal. This is how AM radio works. Audible sound is transduced to an electrical signal that modulates a carrier frequency (e.g. AM radio carrier frequencies are 540 to 1600 kHz). The modulated carrier signal is sent through the air and detected by the radio receiver that demodulates it to recover the audible frequency signal. This is transduced by the speaker back into an acoustic signal.

Complex tone
A complex tone has other higher frequency fluctuations that are integer multiples of the fundamental frequency . The complex tone may or may not have a sinusoidal fluctuation at the fundamental frequency.
See and hear examples .

Consonance
Tones presented together with a minimum of roughness. A combination of tones regarded as pleasing.

Dissonance
Roughness that results when tones with appropriate frequency difference are presented simultaneously.

Frequency
Sound travels through the air as waves that produce alternating compressions and rarefractions (expansions) in the air at a fixed location. The compressed air is at a higher pressure and the expanded air is at a lower pressure. Our ear detects these changes in pressure. Similar to waves on water, the waves travel along at a certain speed but individual water or air particles for the most part just move up and down (for water) or back and forth (for air). For further explanation see introductory websites.
Sound waves travel through the air at about 340 meters per second, dependent on altitude. The rate at which the pressure fluctuates at a fixed location determines the frequency of the sound. Frequency is usually expressed in Hertz (Hz), which refers to the numbers of cycles per second of the alternating pressure. For example, a middle C (C4) pure tone on a tempered scale (see lesson 3) cycles the pressure 261.63 times per second, or at 261.63 Hz. A good human ear can detect sounds with frequencies that range from 20 to 20,000 Hz. This is the audible frequency range. Most sounds from musical instruments occur below a few thousand Hz. Sound above 20,000 Hz is called ultrasound Sound below 20 Hz is sometimes called infrasound

Frequency Modulation (FM)
When a high-frequency sinusoidal (carrier) wave has its frequency shifted as a function of time by audible frequency signals.
(FM radio carrier frequencies are 88 to 108 MHz).

Fourier Series
A method of approximating a periodic function by a series of sine and cosine functions.

Fundamental frequency
A steady sound has pressure fluctuations that oscillate, or repeat themselves, at a frequency known as the fundamental frequency. The pressure fluctuations may have other higher frequency fluctuations in them that are integer multiples of the fundamental frequency.
See and hear examples

Harmonic
Pertaining to a tone whose rate of vibration is an exact multiple of a given primary tone.

Hertz (Hz)
Number of cycles per second.

Inharmonic partial
A partial that is not a harmonic of the fundamental. Its frequency is not an integer multiple of the fundamental frequency.

Modal analysis
Some information about Modal analysis.

Mode shape
Some information about Mode shape.

Natural frequencies
Certan frequencies at which, in the abscence of damping, a structure, such as a musical instrument, will naturally vibrate once excited.

Octave
The basic unit in most musical scales. Notes judged an octave apart have frequencies nearly in the ratio 2:1.

Perfect fifth
Interval between the first and fifth degrees of a major scale, or interval between two pure tones whose frequencies are in the ratio 3:2, or 7 semitones.

Perfect fourth
Interval between the first and fourth degrees of a major scale, or interval between two pure tones whose frequencies are in the ratio 4:3, or 5 semitones.

Pure tone
If the oscillation is just a simple sinusoidal wave at that fundamental frequency without any other integer frequency multiples, then it is a pure tone.
See and hear examples

Resonant frequencies
Certan frequencies at which a structure, such as a musical instrument, will naturally vibrate once excited.

Spectrum
Refers to a "recipe" of the amplitude and frequency of each component of a complex tone.

Ultrasound
Sound having a frequency above the limits of audibility (20 kHz).