# Decibel Question

1. Oct 28, 2008

### jsg7377

Preface

This question proves my ignorance. You have been warned.

Background

- I am building a new "near silent" computer and I am looking at different types of fans.
- The case requires 3 fans.
- I am looking to purchase and install 3 Identical fans that have a rating of 10 dB.

Question

- One fan will obviously produce 10 dB of "noise"
- How many dB of "noise" will all three fans produce?

- The answer I assume is only 10 dB, however I could be wrong if dB can "accumulate" (meaning 3 fans would produce 30 dB). Furthermore, I may be completely off base and it is neither 10 dB or 30 dB.

I have been thinking about this too much and it is driving me crazy. Please enlighten me!

Thanks

2. Oct 28, 2008

### atyy

Let's suppose 10 dB corresponds to a pressure amplitude P, and you want to find Y dB corresponding to 3P.
Po is the reference pressure in both cases.

10 = 20log10(P/Po)

Y= 20log10(3P/Po)

Y-10
=20log10(3P/Po)-20log10(P/Po)
=20log10(3P/P)=20log10(3)=20(0.5)=10 dB

So if 1 fan produced 10 dB, 3 fans would produce (10+10)=20 dB, and 10 fans would produce 30 dB.

http://trace.wisc.edu/docs/2004-About-dB/ [Broken]

Last edited by a moderator: May 3, 2017
3. Oct 28, 2008

### jsg7377

Genius.

Thanks so much! It makes perfect sense.

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