The interesting thing about the decibel is that it is a “dimensionless quantity.”
For more detail, see here http://en.wikipedia.org/wiki/Dimensionless_unit
It is simply a unit of comparison between two different powers, pressures or voltages. So if two sounds are the same volume, we can say there is 0 dBs difference between them. It so happens that a human can just about detect a change of 3 dBs between two different sounds & a difference of 10 dBs between two sounds is perceived as a doubling of volume.
Usually we will be dealing with decibels of sound pressure level (dB spl) but there are other dB measurements you may encounter.
dBm is where the m stands for milliwatt
dBvu is where vu stands for volume unit. A meter with a needle for indicating level
dBu (a.k.a. dBv) where the u stands for volts using 0.775v as a reference
dBV where V stands for volts using 1v as a reference.
Is the rate at which an acoustic generator, vibrating body or electrical signal repeats a cycle of positive to negative amplitude. The number of cycles that occurs over the period of one second is known as that sound’s frequency and is measured in Hertz (Hz.)
So, a 50 Hz sound will complete 50 cycles, from positive to negative, over the course of a second. A 500 Hz sound will complete 500 cycles, from positive to negative, over the course of a second. Or, an object vibrating back & forth 5,000 times a second will produce a sound with a frequency of 5 kHz. Have a look at a picture of 3 pure sine waveforms at 50 Hz, 500 Hz & 5000 Hz.
Musicians usually refer to frequency as pitch. So for instance, in an orchestra a piccolo can produce very high pitched, or high frequency sounds where as a double bass can produce very low pitched, or low frequency sounds.
Listen to the eg
Humans with good hearing can hear sounds between about 20 Hz & 20,000 Hz (or 20 kHz.) There are plenty of sounds that have both higher & lower frequencies than this in the world it is just that ordinarily we can not hear them. The silent dog whistle is a good example. Dogs can “hear” higher frequencies than humans. Silent dog whistles produce frequencies well above 20 kHz. This means humans can not hear them, but dogs can & in reality, “silent” dog whistles are far from silent, at least for dogs!