About dB's - diyAudio
Go Back   Home > Forums > Design & Build > Parts

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 16th April 2003, 08:16 PM   #1
JDeV is offline JDeV  South Africa
diyAudio Member
 
Join Date: Oct 2002
Location: Cape Town
Question About dB's

I am bussy with a corespondence course in electronics and got the following question in an assignment. It sound a bit wrong to me (the uninformed one) and want to know if it make sense and what would the answer be? I don't grasp the dB thing yet, so please excuse silly questions (Maybe somebody can give me a "dB for Dummy's" explanation. )

Q:
Determine the dB level from a given sound power of 15dB, and a reference sound power of 20dB.

(Don't you measure sound power in Watts?)

Thanx for all help.
__________________
"Be carefull who's advice you buy, but be patient with those who supply it."
From Baz Luhrmann's - Everybody’s Free (To Wear Sunscreen)
  Reply With Quote
Old 16th April 2003, 08:32 PM   #2
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
I think the text writer needs the "dB For Dummies" course more than you do. Or at least a writing course. Sound power is given in watts OR in dB referenced to some standard power level.

Here's the entire course:

For power ratios, dB = 10 log (P1/P2)

Since power = V^2/R, and log (x^y) = y log x, then for a voltage ratio, dB = 20 log (V1/V2)


That's all there is, there ain't no more. If the dB quoted in the question are both referenced to the same standard power level, the answer is -5 dB.
__________________
And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
  Reply With Quote
Old 16th April 2003, 08:39 PM   #3
dhaen is offline dhaen  Europe
diyAudio Moderator Emeritus
 
dhaen's Avatar
 
Join Date: Jun 2002
Location: U.K.
dB's are a logarithmic scale. They are not absolute, and can be referenced to any point.
In this case they have told you the reference point (20dB).
The magic formulae I keep in my head are:
10*Log (P1/P2) (for power)
and
20*Log (V1/V2) for voltage (and other non squared values)
In other words, the dB figuer is describing the ratio between the 2 voltages, powers, or whatever...
You'll need to do some substitution...

Cheers,
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



New To Site? Need Help?

All times are GMT. The time now is 06:26 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2