Decibel Scale - Sound

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Have you ever been in a noisy environment that has left you uneasy and grimacing? Have you covered your ears while walking past a noisy factory? If you have you must have wondered what makes some sound louder than others. If noise is a problem for you then you must take steps to measure the noise levels so that you could reduce them. Measuring noise levels was not an easy affair earlier, but now with sophisticated and automated meters, one can measure sound intensity and volume easily and with accuracy. 

In this article, we will dive into understanding the unit of sound, which is the decibel, and get to know how a decibel scale (dB scale) works but before that let us understand how sound travels and what makes one sound louder than the other.

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Sound Waves and What Makes one Sound Louder than the Other

Sound is produced in a medium and caused by vibration. A vibrating guitar forces the air particles around it to compress and expand. This series of alternating compressions and rarefactions create a pressure difference and the disturbance travels through the particles of the medium transporting energy. The vibrating air particles then reach the listener and vibrate the small parts within the listener’s ears which produces sound.

The amount of energy being pumped by the source of the sound to the listener determines how loud the sound is. This energy is determined by the amplitude of the vibrating source (in this case the amplitude of the guitar). If you put more energy into plucking the guitar wires (i.e. the strings are displaced more), greater will be the amplitude of vibration. 

How is Sound Measured?

The sound intensity or sound power is measured in decibels (dB) with the use of a device called a decibel meter. The decibel meter gives out readout by sampling and measuring the pressure of sound waves emanating from a source of the noise. A decibel meter is also called a sound-level meter and one can even access the sound meter app on smartphones. The name decibels is in honor of Alexander Graham Bell who invented the audiometer and the telephone and also devised the decibel scale first.

How Does the Decibel Scale Work?

A human ear is more versatile than you can think. With rising sound levels, the built-in mechanism of our ears reduces its own sensitivity. Our ears also have an exceptional range of handling different levels of sound power. A human ear can discern the dropping of a pin in close range as well as the roaring of a jet engine far away. Although we can make out if there is a rise in the number of pins dropping, we can not really distinguish if there are 1000 or 1001 pins dropping since our ears are not a linear device.

A decibel scale is a logarithmic scale and works differently than a ruler (which is a linear scale). On a ruler 10 cm is twice as long as 5 cm or 30 cm is thrice as long as 10 cm but on a decibel, scale levels go up in powers of 10. This means that if there is an increase of 10 decibels on the dB scale, it translates to a 10-fold increase in sound intensity and it corresponds to doubling in sound intensity as well. 

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The lowest audible sound on a decibel scale is 0 DB. Here is a quick look at some of the everyday sounds we hear and how the noise level dB looks like for each of these sounds.

Decibel Level

Example From Everyday Life

0 dB

Close to complete silence

10 dB

Rustling of leaves

15 dB

A whisper

20 dB

Ticking of a watch

30 dB

Birds flying close by

40 dB

A quiet conversation like in a library

75 to 85 dB

Flushing of toilet

90 dB

A noisy restaurant

110 dB

Baby crying

120 dB

A jet engine

157 dB

Balloon popping

As per research, if one is exposed to noise level dB of 85 decibels for 8 continuous hours or 100 decibels for even 15 minutes, it could cause hearing damage.

Relationship of Decibel to Hertz

Hertz measures the amount or frequency of air pressure change by the vibration. Decibels measure the resulting change in air pressure which the vibrating object creates. This means that decibels measure the loudness or the power of sound while hertz measures sound’s frequency. We cannot convert from decibel to Hz and vice versa as they measure different components of sound waves.

Sound Level Meters

A sound level meter is a simple-looking device that has a pointy stick at the top which has a microphone that samples and measures sound. The body of the instrument is cut off from the microphone by the stick so that reflections do not perturb the readings and we can get more accurate results. The bottom of the meter has a square box that contains electronic circuits to measure the sound which the microphone detects. The detected sound is modified and filtered by this electronic circuit before the final readout is displayed in the LCD display.

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FAQ (Frequently Asked Questions)

1. What is Meant By “Threshold of Hearing” and What is the Range of Sound that Humans Can Hear?

Ans - The faintest sound that can be detected by humans is called TOH or threshold of hearing. The intensity of sound that a typical human ear can hear is  1*10-12 W/m2. This intensity corresponds to a sound that can displace air particles by a mere one-billionth of a centimeter. The most intense sound that can safely be detected by human ears (without causing any physical damage to ears) is more than 1 billion times the threshold of hearing. In decibels TOH is equal to 0 DB.

2. What are the Different Types of Sound Level Meters?

Ans - Sound meters come in many different qualities and standards. There are high-quality ones that meet international standards (IEC 60651, IEC 60804, and ANSI S1.4) and they are graded from type 0 to type 3.

  • Type 0 - These sound level meters are of the highest standards and are used in scientific laboratories or other situations where a high level of precision is required.

  • Type 1 - These are also high standard meters but slightly less accurate than type 0.

  • Type 2 - They do not suit lab-use work but can be employed for general use.

  • Type 3 - They are very cheap compared to other sound level meters and are used for rough survey work. They are also used where a preliminary reading is needed to figure out if more accurate readings need to be done by using type 0 or type 1 instruments or not.