From Hydrogenaudio Knowledgebase
Frequency is defined as the number of cycles of a periodic motion in a unit of time.
The SI unit of frequency is called hertz (Hz, after its inventor Heinrich Hertz). One hertz corresponds to one cycle (or full oscillation) per second.
Example: Audible sound waves have a frequency ranging between approximately 20 Hz to 20 000 Hz. This means that at any point along the path of the sound wave, the pressure will oscilate from high to low 20 to 20000 times every second.
The frequency of a wave refers to how many times per second the wave transitions from its highest point to its lowest point and back again. This is typically measured in hertz (Hz), or number of cycles per second. The frequency of a wave determines its pitch. High frequency waves have a high pitch, and lower frequency waves have a lower pitch. The average human can hear frequencies from 15 or 20Hz to roughly 20,000Hz (20kHz).
The amplitude of a wave refers to half the distance between a wave's highest point and its lowest. The larger the amplitude of a wave, the louder its volume, which is typically measured in decibels (dB). The decibel range for human hearing is complicated and depends on the frequency of the sound in question, age, and listening conditions, but roughly ranges from 0 to 120db, with each change in 10db corresponding to a doubling of the perceived volume.
The absolute threshold - ATH - is the volume level at which one can detect a particular sound 50% of the time.