Today's Active Topics · FORUM INDEX · Search Forum · Who's Online? · FAQs/Help · Calendar

Main Forums >> Production - Mixing, Mastering, Gear & Techniques

Pages: 1
Fruitfly

Joined: 19/04/10
Posts: 21
Audio Engineering Question - Decibels
#988353 - 19/05/12 12:27 PM
Morning SoundonSound,

I am currently studying sound engineering and I've got a question if you could help me solve.
It's a topic under Decibels. The question is:

How many Decibels are in between 4mV and 800mV

I can't seem to figure out the formula for it but I have the answer. The answer is +46dB

Can anyone help me?

Hugh Robjohns
SOS Technical Editor

Joined: 25/07/03
Posts: 18383
Loc: Worcestershire
Re: Audio Engineering Question - Decibels [Re: Fruitfly]
#988354 - 19/05/12 12:33 PM
You must have been given the formula!

Decibels is a ratio of two quantities, in this case signal voltages.

Decibels = 20 x log(v1/v2)

So 800/4 is 200

Log 200 is 2.301

20x2.301 is 46

I claim my test marks!

Hugh

--------------------
Technical Editor, Sound On Sound

Fruitfly

Joined: 19/04/10
Posts: 21
Re: Audio Engineering Question - Decibels [Re: Hugh Robjohns]
#988362 - 19/05/12 01:20 PM
Hi Hugh,

First of all, thank you so much. Does this work for V, pascals and the others as well?

dmills

Joined: 25/08/06
Posts: 2130
Re: Audio Engineering Question - Decibels [Re: Fruitfly]
#988382 - 19/05/12 03:13 PM
Sort of:
The dB is actually a ratio of power, not voltage, and is defined as dB = 10 * log (p1/p2), but power is proportional to the square of voltage, so dB = 10 * log ((v1*v1)/(v2*v2)) = 20 log (v1/v2).

Thus for other things you need to decide what relation they bear to power, (linear or square law) in order to decide what the multiplier needs to be used.

Regards, Dan.

--------------------
Audiophiles use phono leads because they are unbalanced people!

C.LYDE
member

Joined: 22/10/02
Posts: 209
Loc: South Africa
Re: Audio Engineering Question - Decibels [Re: Fruitfly]
#988394 - 19/05/12 04:55 PM
Quote Fruitfly:

Does this work for V, pascals and the others as well?

Pascals? Basically convert the ratio in question to a logarithmic value : 10x log(desired value/reference)

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

--------------------
C.LYDE
http://soundcloud.com/c-lyde

Jack Ruston

Joined: 21/12/05
Posts: 4066
Re: Audio Engineering Question - Decibels [Re: Fruitfly]
#988407 - 19/05/12 05:45 PM
Fruit Pascals?

--------------------
www.jackruston.com

robi

Joined: 13/07/07
Posts: 25
Re: Audio Engineering Question - Decibels [Re: Fruitfly]
#988425 - 19/05/12 08:25 PM
Quote Fruitfly:

Does this work for V, pascals and the others as well?

For Pascals you have to use the same formula as for V. So, dB=20log(Pa1/Pa2).

As for "others", I don't know what you mean

Persuazion

Joined: 29/10/05
Posts: 1559
Loc: Scotland
Re: Audio Engineering Question - Decibels [Re: Jack Ruston]
#988427 - 19/05/12 08:41 PM
Quote Jack Ruston:

Fruit Pascals?

--------------------
http://www.loverslanestudios.co.uk

Fruitfly

Joined: 19/04/10
Posts: 21
Re: Audio Engineering Question - Decibels [Re: dmills]
#988452 - 20/05/12 05:09 AM
Thanks! you guys have been great

But I've actually tried both the formulas to calculate the dBs of Voltage and others Which is:

10 * log ((v1*v1)/(v2*v2)) = 20 log (v1/v2)

I cant seem to get the right answer for it - is all the answer suppose to be negative?
I'm doing an online quiz and it seems to be telling that it's not the right answer.

the questions are:

1. How many dBs between 50 MicroPascals and 200 MicroPascals
I got 16 for this.

2. How many dBs between 2V and 200V
I got -10.

3. How many dBs between 5mW and 500mW

They all seemed to be wrong. The Bels are killing me

narcoman
active member

Joined: 14/08/01
Posts: 8469
Re: Audio Engineering Question - Decibels [Re: Fruitfly]
#988461 - 20/05/12 08:01 AM
Quote Fruitfly:

They all seemed to be wrong. The Bels are killing me

get your bracketing right!!! Also - what Log are you using on your calculator?

As in all exams, showing your working is everything. Just giving an answer means nothing. So, show your working for the first one and it'll be easy to show where you're going wrong.

ef37a

Joined: 29/05/06
Posts: 5626
Loc: northampton uk
Re: Audio Engineering Question - Decibels [Re: narcoman]
#988473 - 20/05/12 09:08 AM
Quote narcoman:

Quote Fruitfly:

They all seemed to be wrong. The Bels are killing me

get your bracketing right!!! Also - what Log are you using on your calculator?

As in all exams, showing your working is everything. Just giving an answer means nothing. So, show your working for the first one and it'll be easy to show where you're going wrong.

Indeed!^
And who uses microPascals? I make 50 of 'em ~ equal to 8dBSPL? But then I am not well you know!

Dave.

dmills

Joined: 25/08/06
Posts: 2130
Re: Audio Engineering Question - Decibels [Re: ef37a]
#988502 - 20/05/12 01:13 PM
Underwater acoustics (My day job) uses 1uPa as the default reference level.....

Regards, Dan.

--------------------
Audiophiles use phono leads because they are unbalanced people!

narcoman
active member

Joined: 14/08/01
Posts: 8469
Re: Audio Engineering Question - Decibels [Re: Fruitfly]
#988505 - 20/05/12 01:34 PM
eyes going.

though that said underwear acoustics

Hugh Robjohns
SOS Technical Editor

Joined: 25/07/03
Posts: 18383
Loc: Worcestershire
Re: Audio Engineering Question - Decibels [Re: Fruitfly]
#988524 - 20/05/12 03:37 PM
Quote Fruitfly:

I cant seem to get the right answer for it - is all the answer suppose to be negative?

You'll get the same answer with a + or - symbol depending on which value to put as v1 and v2.

If the question asks for the difference then only the value is relevant, not the sign.

Quote:

1. How many dBs between 50 MicroPascals and 200 MicroPascals
I got 16 for this.

I get 12dB. Pascals is a pressure, so 20xlog ratio

Quote:

2. How many dBs between 2V and 200V
I got -10.

I get 40dB

Quote:

3. How many dBs between 5mW and 500mW

I get 20dB (this one is a power, so 10xlog ratio.)

It really is very simple: for powers it is 10x the log of the ratio of the two values. For other quantities like voltage, sound pressures etc, it is 20 x the log of the two values.

Hugh

--------------------
Technical Editor, Sound On Sound

Martin Walker
Watcher Of The Skies

Joined: 28/02/01
Posts: 16387
Loc: Cornwall, UK
Re: Audio Engineering Question - Decibels [Re: narcoman]
#988680 - 21/05/12 11:28 AM
Quote narcoman:

eyes going.

though that said underwear acoustics

And with the 'subtly enhanced and padded' versions available nowadays for those who want to simulate butt extensions, such acoustics could actually be measurable!

Martin

--------------------
YewTreeMagic

Pages: 1

 Rate this thread 1 star 2 star 3 star 4 star 5 star Jump to --FORUM INDEX-- *Main Forums* -----   Recording Techniques   Production - Mixing, Mastering, Gear & Techniques   Product & Industry News   Music Business   Mac Music   PC Music   Apps and Other Computers/OS   Guitar Technology   Keyboards & Synthesis   DIY, Electronics, Studio Design & Acoustics   Music Theory, Songwriting & Composition   Live Sound & Performance   User Reviews   Musicians' Lounge*SOS Forums* -----   Magazine, Website & Forum Feedback   SOS Support Forum

 Extra Information 1 registered and 18 anonymous users are browsing this forum. Moderator:  David Etheridge, James Perrett, zenguitar, Martin Walker, Hugh Robjohns, Zukan, Frank Eleveld, Will Betts Forum Permissions       You cannot start new topics       You cannot reply to topics       HTML is enabled       UBBCode is enabled Rating: Thread views: 1746

May 2013
On sale now at main newsagents and bookstores (or buy direct from the
SOS Web Shop)
Click image for May 2013
Other recent issues:
DAW Tips from SOS

Email: Contact SOS

Telephone: +44 (0)1954 789888

Fax: +44 (0)1954 789895

Registered Office: Media House, Trafalgar Way, Bar Hill, Cambridge, CB23 8SQ, United Kingdom.

Sound On Sound Ltd is registered in England and Wales.

Company number: 3015516 VAT number: GB 638 5307 26