Because sound pressure levels are measured on a logarithmic scale, (power ratios of 10) an increase in decibels beyond a certain point means that the energy impacted on the ear can quickly pass a threshold where damage can occur. As shown in the chart below, even an increase of 5 dB can greatly reduce the duration before hearing loss can occur.
Note that sound levels above 70 dB begin to interfere with voice communication, while 75 dB and above are generally characterized as annoying. (but dependent on frequency as shown below)
The general effects can be studied in the response table below:
Fortunately, sound pressure levels also decrease with the square of the distance, so moving twice as far away means that the power level decrease by a factor of 4. Bear in mind that 3 decibels (dB) reflects a change in power level by a factor of 2, with +3 dB being twice as loud and -3 dB being half as loud.
Also, the human ear does not have a “flat” frequency response – so frequencies at the extreme edges of hearing perception (from 20 Hz to 20,000 Hz) are not perceived to be as loud as those centered around 1 KHz – 3 KHz which is in the range of normal human speech.
Therefore, different measuring scales on sound meters are used to distinguish between “human” hearing (dBA) and “machine” hearing (dBC) which has a flatter frequency response.