The Decibel (dB)
The decibel (dB) is one tenth of a Bel. The Bel is named after Alexander Graham Bell and is equivalent to twice (or half) the loudness (level or signal amplitude) of a given sound. This scale of measurement is relative (instead of absolute measures of signal in voltage) and logarithmic.
* Doubling or halving loudness represents a 10dB change.
* Doubling or halving power or wattage represents a 3dB change.
* Doubling or halving voltage represents a 6dB change.
Frequency Response
Frequency response is measured in Hertz. Ideal measures for the human ears range from 20Hz to 20kHz. Real-world measures range from 80Hz to about 16kHz (measures lower or higher is likely to be a low rumble or a hiss respectively). The frequency response is the allowed change in decibels within a specified range.
A high-quality audio device has a frequency response of 80Hz to 16kHz +/-3dB. This means that the device loudness at one frequency does not differ by more than 6dB at another frequency. Such a device responds to the allowed frequencies with relatively little change in loudness. A mid-quality audio device may have the same range, 80Hz to 16hHz but a "loudness delta" of +/-10dB. This is a much larger change in loudness (with the possible exception of microphones such a large change indicates a low-quality audio device).
Dynamic Range
Frequency response does not take into account noise and headroom (it is confined to loudness). Dynamic range begins to qualify sound levels by adding together the signal to noise ratio (S/N) and headroom (levels above 0dB before distortion occurs). Good S/N is 70dB or greater; so a device with a S/N of 80dB and 10dB of headroom has a dynamic range of 90dB.
Sensitivity
Sensitivity can describe microphone output against a given level of input with respect to a 0dB reference level. So a microphone with -40dB sensitivity is superior to one at -60dB.
It also describes speaker output against a given level of power with respect to the 1 watt/ 1 meter standard. Efficient speakers rate around 100dB when a microphone is placed 1 meter away and given 1 watt of power. Inefficient speakers have ratings below 90dB. Recall from our discussion of the decibel, that just a 3dB change means double or half the power is needed to power a speaker at a decent level of sensitivity.