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**davidtrinh****Member**- Registered: 2016-12-03
- Posts: 8

I am going to write down a problem below.

(ex) What is the approximate combined loudness in decibels of a garbage disposal truck and a washing machine, each with a sound intensity of 80 decibels?

It looks like a simple problem. I thought the answer was 160 decibels. I got that by multiplying 80 by 2 since the two sounds have the same intensity. But, the answer key says 83 decibels.

Is the answer wrong? Please give me your comments. Thanks.

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**bobbym****bumpkin**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 109,606

Hi;

I have two very loud air conditioners from the two zombies who live next door to me and require air conditioning when the outside temperature is 51 F. I guess they have to keep their skin from rotting off their already ugly faces...

I needed to know the intensity of the noise they were producing. I got it from these two sites

http://www.engineeringtoolbox.com/addin … -d_63.html

https://noisemeters.com/apps/db-calculator.asp

If you use their calculator you will see the book answer is correct.

They appear to be using this formula:

with

**In mathematics, you don't understand things. You just get used to them.****If it ain't broke, fix it until it is.**** Always satisfy the Prime Directive of getting the right answer above all else.**

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**bob bundy****Administrator**- Registered: 2010-06-20
- Posts: 8,012

hi davidtrinh

The decibel scale is logarithmic** so to add two sounds you need to 'anti-log' the amounts then re-log the result.

https://en.wikipedia.org/wiki/Decibel

decibel is one tenth of a bel so you need to convert first:

80 db = 8 b

so compute 10*log(10^8 + 10^8)

This comes out as 83 db

** so if the db goes up by +10 the sound is 10 times as loud.

Bob

Children are not defined by school ...........The Fonz

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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**davidtrinh****Member**- Registered: 2016-12-03
- Posts: 8

Thank you very much, bobbym and bob bundy.

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