Learn physics with iLectureOnline.com: what are decibels (dB) and why we use them to measure sound intensity?


Decibels, written as dB, are a more convenient way to measure the intensity of sound. The actual measurements for the intensity of sound are in Watt/m^2 and, after you will see the upper and lower limits in w/m^2, you will understand why we use dB as units.

First off, I want to praise Michel van Biezen from iLectureOnline.com (YouTube channel) for the astrounding work he’s put into the hundreds of short videos about algebra, calculus, physics, chemistry, astronomy and so forth. Finally I was able to understand better what sound is, how it forms, how the molecules wiggle a tiny bit in there and how to calculate sound in different situations.

There are about 47 short videos in the sound section and I’ve watched them all at 2xspeed and still could understand what he explained in there.

Back to the sound intensity: why db instead of watt/m^2? Because the lower threshold of sound we hear is about I0 = 1 * 10^(-12) Watt/m^2 and the upper limit is about 1 watt/m^2.

As you see saying that the human speech has the intensity of about 10^(-6) w/m^2 is not easily digestible comercially speaking. Here come the decibels in play:
I in dB = 10 log I/I0.

I is the intensity we just measured in W/m^2, I0 is 1 * 10^(-12) Watt/m^2 and log is the logarithmic function in base 10.

In any case, using this transformation we get :
0 dB for I0 = 1 * 10^(-12) Watt/m^2
10 dB = 1 * 10^(-11) Watt/m^2
20 dB = 1 * 10^(-10) Watt/m^2
30 dB = 1 * 10^(-9) Watt/m^2
40 dB = 1 * 10^(-8) Watt/m^2
50 dB = 1 * 10^(-7) Watt/m^2
60 dB = 1 * 10^(-6) Watt/m^2
70 dB = 1 * 10^(-5) Watt/m^2
80 dB = 1 * 10^(-4) Watt/m^2
90 dB = 1 * 10^(-3) Watt/m^2
100 dB = 1 * 10^(-2) Watt/m^2
110 dB = 1 * 10^(-1) Watt/m^2
120 dB = 1 * 10^(0) Watt/m^2 = 1 Watt/M^2

A whisper is at 0dB, normal human voice speaks with about 60dB, a baby crying is at 80dB and the threshold of pain is at 120dB. Also, in order to generate a sound in our ear drum the molecules need to be displaced by only about 7-8 nm, just a tiny-tiny bit. The human ear is an incredible thing.

Also, if you have a speaker and add another speaker, then the dB will have +3 and if you use 10x as more of the said speaker then the dB rise up +10. So that you know.

I any case, make sure to subscribe to iLectureOnline right now and start being smarter.

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