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'[OT]: How to read dB?'
2001\05\24@231957 by

Hi!

What is dB? what is the relativeship of value in dB with voltage
(caculation formula)?

If for volume, 20dB, 0dB,-10dB and -50dB which is bigger? Coz I am
totally blind in
reading value in dB....

T.Q.

regards,
William

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>
>If for volume, 20dB, 0dB,-10dB and -50dB which is bigger? Coz I am
>totally blind in
>reading value in dB....

Decibels (literally, 10ths of a Bell) are relative power units.
You can only correlate them to voltage if you already know the impedance.
A gain or loss of 3dB is half or twice power
When you express power in dB, you must give it as dB relative to something.
3dB is literally saying "twice as much power" If you want to say an
absolute amount of power in dB, you relate it like dBm (dB milliwatt) or
dBW (dB watt)

-50dB is MUCH smaller than +10dB.  In fact, it's 60dB down from 10dB, or
1/1000000th the power.

30dB is a factor of 1000.

My audiologist once told me that at a specific freq, I had a "minor"
hearing loss of 60dB.
I asked her how she'd like a "minor" paycut of 60dB. She'd be making about
\$0.10 that year.

I got a new audiologist now.
:)

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dB has no units.
dB is a relationship between two values.

Speaking of voltage:   dB=10*log(V1/V2)=10*(logV1-logV2)

for example, if you want to know the gain in dB for an amplifier you take
Vin and Vout and apply the formula:

Vin=0.5v
Vout=10v

10*log(Vout/Vin)=10*log(10/0.5)=13dB

0dB is when the relationship between the two numbers is 1, for example:

10*log(1/1)=0dB

Another way of seeing this relationship is as 'gain' or 'loss'. When you
have a negative value in dB it means that the relationship between the
numbers is less than 1, for example:

10*log(1/5)= -7dB (aprox)

One common reference used when dealing with power is 1 mW, so if you have
1mW of power out of something compared to the reference you have 0dB. But if
you have, for example, an amplifier that gives out 100W of output for each
mW of input, you then have an amplifier with a gain of:

10*log(100/0.001)=50dB

Bye

Calvin

{Original Message removed}
Hi,

----- Original Message -----
From: David VanHorn <dvanhornCEDAR.NET>

| >
| >If for volume, 20dB, 0dB,-10dB and -50dB which is bigger? Coz I am
| >totally blind in
| >reading value in dB....
|
| Decibels (literally, 10ths of a Bell) are relative power units.
| You can only correlate them to voltage if you already know the impedance.

It's a little bit of math.

Relative power, in dB: 10*log(Po/Pi).

OK, but since P=(V^2)/Z, the voltage *gain* in  dB is directly:
20*log(Vo/Vi), taking the same reference impedances.

For voltage expressed directly in dB, ussually (if no otherwise specified)
it's implicitly considered a 600 Ohms impedance (from telephone lines...).

| 30dB is a factor of 1000.

That's for power relations. For voltage relations, 30 dB is a factor of
aprox 31.6 (between Vo & Vi: Vo/Vi) .

Best Regards,

S.-

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>
>That's for power relations. For voltage relations, 30 dB is a factor of
>aprox 31.6 (between Vo & Vi: Vo/Vi) .

Right, but as I said, you can't get an absolute voltage without a
reference, and an impedance.
You can get the ratio of Vo/Vi, but not the absolute.

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You do not require resistance to define an "absolute"voltage.

ie.  dbV = 20  log (V2/Vref). You can define the standard you are referring
to.
eg. dBuV = 20  log (V2(uV)/ 1uV)

You may need resistance for POWER measurements if you want to state it in
terms of Voltage or current ratio.

ie. dbW = 10 log (P2/P1)

= 10 log ((V2^2/R2) / ((Vref^2/Rref))

Regards

David Huisman

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'[OT]: How to read dB?'
2001\06\02@203834 by
I know this is an old post but I had some time to think about it.

----- Original Message -----
From: David VanHorn <dvanhornCEDAR.NET>

{Quote hidden}

Did she tell you how to do your work?  Why should you tell her how to do her
work?  That was not something worth switching professionals.  Auditory
perception works on a different scale, AND she's saying relatively speaking,
compared to people that have it much worse and still manage to live life
hearing.

Generally 10x real power means only about 2x percieved loudness difference.
So your 60dB difference would be 2^6 ~= 1/64th the percieved power.  It's
still a big drop but it's actually quite workable.

Jeff

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>
> > I got a new audiologist now.
>
>Did she tell you how to do your work?  Why should you tell her how to do her
>work?

Because she's working for me.
My new audiologist agrees too.

>  That was not something worth switching professionals.  Auditory
>perception works on a different scale, AND she's saying relatively speaking,
>compared to people that have it much worse and still manage to live life
>hearing.

I thought she wasn't paying adequate attention to my condition.
I went elsewhere. You have a problem with that?

>Generally 10x real power means only about 2x percieved loudness difference.
>So your 60dB difference would be 2^6 ~= 1/64th the percieved power.  It's
>still a big drop but it's actually quite workable.

You try it.
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At 08:32 PM 6/2/01 -0400, Jeff DeMaagd wrote:

>Generally 10x real power means only about 2x percieved loudness difference.
>So your 60dB difference would be 2^6 ~= 1/64th the percieved power.  It's
>still a big drop but it's actually quite workable.

Sorry - not right!  60 dB is a voltage ratio of 1000 to 1.  Lets put it
this way: the hiss level on your car cassette deck is about 45 dB below
peak level.  60 dB is 15 dB quieter than that.  A good quality studio grade
open reel tape deck (Ampex, Studer) is about 60 dB dynamic range.  The best
I've ever seen in a cassette deck is about 50 dB.  Note: all of these tape
noise figures are with NO noise reduction (which is how you align tape decks).

So, yes, a hearing loss of 60 dB is QUITE something to be worried about!

dwayne

Dwayne Reid   <dwaynerplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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----- Original Message -----
From: David VanHorn <dvanhornCEDAR.NET>

> > > I got a new audiologist now.
> >
> >Did she tell you how to do your work?  Why should you tell her how to do
her
> >work?
>
> Because she's working for me.
> My new audiologist agrees too.

OK then.

> I thought she wasn't paying adequate attention to my condition.
> I went elsewhere. You have a problem with that?

So the audiologist putting things in relative perspective made the situation
such that you weren't recieving proper care?  I guess I don't know enough
specifics.

> >Generally 10x real power means only about 2x percieved loudness
difference.
> >So your 60dB difference would be 2^6 ~= 1/64th the percieved power.  It's
> >still a big drop but it's actually quite workable.
>
> You try it.

Actually I have all my life, I have hearing aids.  Human hearing simply does
not scale directly to electrical engineering and they are both analyzed from
different perspectives.

Jeff

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