Home » Sound and Oscillation » Unit of measuring Intensity of sound

Unit of measuring Intensity of sound

What is intensity of sound?

“Sound energy passing per second through a unit area held perpendicular to the direction of propagation of sound waves is called intensity of sound.”The sound waves transfer energy from the sounding body to the listener.The intensity of sound depends on the amplitude of sound waves.Intensity is a physical quantity and can be measured accurately.The unit of intensity of sound is watt per square meter (Wm-2).

Sound intensity level:

The human ear responds to the intensities ranging from 10-12 Wm-2 to more than 1Wm-2 (which is loud enough to be painful).Because the range is so wide,intensities are scaled by factors of ten.The barely audible and the faintest intensity of sound is 10-12 Wm-2 is taken as reference intensity,called zero bel (a unit named after Alexander Graham bell).

The loudness of a sound depends not only on the intensity of sound but also on the physical conditions of the ear.The human ear is more sensitive to some frequencies rather than the others.

The loudness (L) of a sound is directly proportional to the  logarithm of intensity i.e.,

L ∝ log I

L =K log I  ……(1)

Where k is a constant of proportionality.Let Lbe the loudness of the faintest audible sound of  intensity Iand L be the loudness of an unknown sound of intensity I,then by equation (1),we can write

L0 =K log I0……..(2)

Subtracting equation (2) from (1),we get:

L – L0=K(log I -log I0)

=K log I/I0

This difference ,(L-L0),between the loudness L of an unknown sound and the loudness L0 is called the intensity level of the unknown sound.Therefore,the intensity level of an unknown sound is given by

Intensity level = k log I/I0    ……..(3)

The value of k depends not only on the units of I and I0 but also on the unit of intensity level.If intensity I of any unknown sound is 10 times greater than the intensity Iof the faintest audible sound i.e., I=10 I0 and the intensity level of such a sound is taken as a unit,called bel,the value of k becomes 1.Therefore ,using k=1 in equation (3) becomes:

Intensity level = log I/I0 (bel)   …..(4)

Bel is a very large unit of intensity level of a sound.Generally,a smaller unit called decibel is used. Decibel is abbreviated as (dB).It must be remembered that 1 bel is equal to 10 dB.If the intensity level is measured in decibels,then equation (4) becomes:

Intensity level = log I/I(dB) ……….(5)

Using equation (5) ,we can construct a scale for measuring the intensity level of sound.Such scale is known as “decibel scale”.The intensity level of different sounds in decibel is given in the table:

Sources of sound Intensity (Wm-2) Intensity level (dB)
Nearby jet airplane   103 150
 Fast train   101  130
Siren 100 120
Lawn mover 10-2 100
Vacuum cleaner 10-5 70
Mosquito buzzling 10-8 40
whisper 10-9 30
Rustling of leaves 10-11 10
Faintest audibel sound 10-12 0

The decibel scale is a logarithmic measure of the amplitude of sound waves.In a logarithmic scale ,equal intervals corresponds to multiplying by 10 instead of adding equal amounts.

Logarithmic scale Decibels (dB) Linear scale Amplitude (m)
0     1
20       10
  40        100
60         1,000
   80          10,000
100           1000,000
120           1,000,000

Watch also video about intensity of sound:

About admin


  1. I am not positive the place you are getting
    your information, however great topic. I must spend some time studying much
    more or understanding more. Thank you for great information I sed to be in search of this
    info for my mission.

  2. Hello my loved one! I wish to say that this post is amazing, great written and come with almost all significant infos.
    I would like to see extra posts like this .

  3. I like reading through an article that can make people think.
    Also, many thanks for permitting me to comment!

  4. At this time I am just disappearing to perform my breakfast, afterward having my breakfast coming again to
    learn additional news.

  5. Incredible points. Outstanding arguments. Continue the good

  6. Heya i am the very first time here. I stumbled on this board and that i believe it is truly useful & it helped me
    to out much. I hope to present something back and aid others like you
    aided me.

Leave a Reply

Your email address will not be published. Required fields are marked *



Check Also

How do you measure sound

What is speed of sound? Sound waves can be transmitted by any medium containing particles ...