Here db(logarithmic unit) stands for Decibel. In humans the
audible range of frequencies is usually said to be 20 Hz
(cycles per second) to 20 kHz (20,000 Hz). It may be harmful
for human ear if it exceeds that range.Hence the value of dB
is always -(minus).
The decibel ( dB) is used to measure sound level, but it is also widely used in electronics, signals and communication. The dB is a logarithmic unit used to describe a ratio or it is the unit of sound.
its value is showing in minus(-) because mathematically we take antilog of the value of sound/noise