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Measuring Sound

Introduction

Everyone is familiar with sound, we have ears to detect it after all!

But what is sound? Sound is a mechanical wave which is

created by a vibrating object. But how do we measure it?

Instrument

Sound Level Meter

Common Sound Intensity Levels

http://windchimes.co.za/photos/sound_intensity_levels.gif

Unit of measuring sound

Unit: Decibel (dB) Reference sound intensity level of 0

dB is an intensity of: I0=10-12 W/m2 (the quietest sound a human ear can detect)

All other sound intensities are measured in reference to this threshold hearing intensity.

Sound Intensity Level

When comparing a specific sound intensity to the reference intensity (Io), it’s known as sound intensity level.

Equation:

Where B= “bel” I= specific sound intensity I0 reference intensity (I0=10-12 W/m2 )

Question #1 :

A) You are at the Mariners soccer game where you are told the sound intensity is 10^-20

W/m2. What is the sound intensity level?

b) Is this level lower or higher than the threshold of human hearing?

*Hint*: What are you trying to find?*

Solution #1:

A) Since you know sound intensity level corresponds to “B”, you would use the equation below and simply plug in the values you know.

I= specific sound intensity= 10^-20 W/m2

I0= reference intensity= 10^-12 W/m2

B= what you are trying to find. Calculate! Answer: -80 dB b) Since the answer is negative it’s indicating that the sound is below the threshold level for human hearing (because the threhold for human hearing is 0 dB).

Question #2

You are at the Thunderbird Stadium watching a basketball game, where you find out the sound intensity level is 100 dB. What is the sound intensity in W/m2?

Solution #2

You will need to rearrange the sound intensity level equation, to find sound intensity (I) inside the Thunderbird stadium.

Solution continued...

Plug in known values:

Divide by 10:

Solution continued…

Since it’s log base 10, everything can be written in base 10:

Isolate for I0=

Final Answer: 10-22 W/m2

*To check*: Just plug back into original equation

Thank You!