The human ear is an incredibly flexible hearing device. It has a smart built-in mechanism that reduces its own sensitivity as the sound level goes up and it also has the extraordinary ability to handle a vast range of sound power levels.

It can hear the sound of a coin dropping close-by as well as the booming of a jet engine miles away.

What is a decibel?

The decibel (symbol: dB) is a relative unit of measurement equivalent to one-tenth of a bel (B). It is used to exhibit the ratio of one value of a power or root-power quantity to another on a logarithmic scale. A logarithmic quantity in decibels is called a level.

Two signals whose levels vary by one decibel have a power ratio of 101/10. Zero decibels (0 dB) is the lowest sound audible to a healthy human ear. From there, every increment of 3 dB means doubling of sound power or acoustic intensity.

Decibel’s definition originates from the readings of power in telephony of the beginning of the 20th century in the Bell System in the United States of America. One decibel is one-tenth (deci-) of one bel, named as a tribute to Alexander Graham Bell; however, the bel is rarely used.

Today, the decibel is used for a wide spectrum of measurements in science and engineering, particularly in acoustics, electronics, and control theory. In electronics, the gains of amplifiers, attenuation of signals, and signal-to-noise ratios are usually measured in decibels.

Sound power and loudness

The relative loudness that we perceive isn’t something that can be objectively measured; it depends on the individual. Most of us identify a particular sound to be two times as loud as another one when they are about 10 dB apart; for example, you will find a 60-dB air conditioner twice as loud as a 50-dB refrigerator.

Yet that 10-dB variation is actually a ten times increase in intensity. A 70-dB dishwasher will sound approximately four times as loud as the 50-dB refrigerator, when it comes to acoustic intensity, the sound it makes is 100 times as strong.

Here's another example: If the sound from one typewriter registers 60 dB, then ten typewriters clicking would register 70 dB (not 600 dB!), and they would sound only two times as loud as one machine. You would need 100 typewriters to attain a noise level of 80 dB, and together they would sound only four times as loud as a single typewriter.

The possibility for a sound to damage hearing is proportional to its intensity, not its loudness. That's why it's delusive to rely on our subjective perception of loudness as a sign of the risk to our hearing.

Here are some linear and non-linear sound levels in our circumambience :

Phons and Sones

The phon is a non-regular noise unit that is intended to reflect perceived loudness, and is based on psychoacoustic tests in which volunteers were asked to adjust the decibel level of a baseline tone of 1 kHz until it was the equal loudness as the signal being measured.

So for instance, if a sound is 70 phons, that means it sounds as loud as a 70-dB, 1-kHz tone. The dBA scale is now largely used rather than phons. The sone is a more intuitive measure of loudness, because a doubling in the number of sones equals a doubling in loudness that you perceive(unlike the logarithmic phon scale). Noise levels of household fans are usually measured in sones.

A-weighting dB(A) and C-weighting dB(C)

Measuring noise levels relating to loud noise at work are usually given in dB(A) or dB(C) - those frequency weightings are applied to the decibel measurements (A and C frequency weightings), actually they are decibel scale readings that try to replicate the sensitivity of the human ear to various sound frequencies.

• A-weighting (A-frequency-weighting): ‘A’ Weighting is predominantly used and covers the full frequency range of 20Hz all the way up to 20 kHz. The human ear is most sensitive to sound frequencies between 500 Hz and 6 kHz while at lower and higher frequencies the human ear is not that sensitive. The 'A' weighting adjusts the sound pressure readings to display the sensitivity of the human ear and is therefore mandatory all over the world for hearing damage risk readings.
  
  
• C-weighting (C-frequency-weighting): The C-weighting is mainly focused at the effect of low-frequency sounds on the human ear.m Compared with the A-weighting it is essentially flat or linear between 31.5Hz and 8kHz, the two – 3dB or ‘half power’ points. Peak Sound Pressure Measurements are made using the C- frequency weighting. Measurements are primarily displayed as dB(C) or dBC. Or for example as LCeq, LCPeak, LCE – where the C shows the C-weighting.
  
  

Sound Level Measuring

A sound level meter is the device usually used to measure noise levels on the decibel scale. Several factors affect the noise level reading:

•  The gap between the meter and the source of the sound
•  The direction the noise source is facing, relative to the meter
•  Noise dissipation and reverberation

In order to have the most use of the reported sound value, you need to take all of the mentioned factors in account, and state the conditions in which the reading was taken.

Conclusion

This article goes into the basics of decibels and their use in measuring sound. Decibels (dB) are a logarithmic unit of measure which express the ratio between two quantities of power, such as sound pressure or electrical power. The decibel scale is not linear but rather follows the well-known “bell curve” distribution.

A person’s hearing threshold changes with age, being most sensitive around age 20, and gradually becomes less sensitive. Once the threshold is reached, it takes a much greater intensity of sound to perceive or hear it.

About half of people in the U.S between ages 30 and 70 have problems hearing high frequency sounds due to age-related hearing loss. Although not everyone suffers from such hearing loss, it is still a good idea for most people to wear ear protection when exposed to loud sounds, as this type of noise does pose a risk in terms of hearing damage.