Do you really like the way HDTV looks?

"Randy Yates" wrote:

"Albert Manfredi" writes:
[...]
So it's not really correct to differentiate between "absolute" and
"relative." It's always relative. The only question is relative to
what?

There is a semantic problem here.

The term absolute means, in this context, known. For example, even
though dBm is "relative" in the sense that it is relative to a
1reference power (1 milliwatt), it is a known quantity since the
reference power is defined. This means (e.g.) dBm is an absolute power
level.

Since all the disagreements in this infinte thread related to
understanding SPECIFICALLY what decibels are, I think semantics are very
important. There seemed to be evidence of fundamental misconceptions on
this matter.

To me, when you say, "absolute," you are talking about Watts, for
example. The minute you stick the dB in there, the person using that
number has to appreciate that he's looking at some ratio, even if the
reference used in computing the ratio might not be immediately obvious
to the reader. (Always good to look it up, in that case.)

In my opinion, the term "absolute," in this context, should not be used.
It leads to just the sort of confusion that we saw in the thread. There
are no two definitions of dB. There is only one definition, and the only
thing that changes is the reference used in computing a ratio.

Bert

"Richard Harison" wrote in message
...

A previous posted point of mine was that SPL is also measured in db.
After that it gets a little vague as to source. What =0db?

Normally, by convention, SPL is referenced to 20 micropascals. So 0 dB
would work out to that SPL. Why choose that reference? Because that is
by convention the "softest sound" a human can hear.

It applies to "normal" hearing of a young person who has not been
subjected to lots of loud music.

Bert

"Albert Manfredi" writes:
[...]
There seemed to be evidence of fundamental misconceptions on this
matter.

If you cannot admit that certain dB definitions are absolute power
while the basic dB definition is not, then you're not seeing the
light.

For example, dBm is absolute power, although in a unit different than
Watt. This should cause no less problem than, e.g., stating power in
horsepower. Both quantities are absolute power, but in units which are
different than Watts. In one case the relationship to Watts is linear;
in the other case the relationship is nonlinear. In both cases the
translation to an absolute power level in Watts can be made.

Conversely, dB is NOT power. You cannot translate a dB to an
absolute power level in Watts.

Note that this does not in any way diminish the fact that ANY dB
measurement is based on a ratio of a referred power to a reference
power. It is just that in cases of absolute units, the reference power
is known so that the referred power can be computed.
--
% Randy Yates % "And all that I can do
%% Fuquay-Varina, NC % is say I'm sorry,
%%% 919-577-9882 % that's the way it goes..."
%%%% % Getting To The Point', *Balance of Power*, ELO
http://home.earthlink.net/~yatescr

I stand corrected. 140 is about right for a jet. The 170 was for a Saturn 5
rocket

--
All the Best
Richard Harison
"Randy Yates" wrote in message ...
"Richard Harison" writes:

Thanks...
I was trying to make reference to human terms. (normal conversation= 70 db
SPL,
Lear Jet @ 20 feet(been there) 170 dbSPL

You also stated that SPL is measured in dB. That is incorrect. "dB"
and "dB SPL" are two different units. The first is relative
(unitless), the second is absolute.

Yes, normal conversation is about 70 dB SPL. I don't think a jet, even
at 20 feet, is 170 dB SPL. You would lose your hearing within a few
seconds at that level. The following lists at 140 dB SPL a jet engine
at 3m.

http://www-ccrma.stanford.edu/~jos/mdft/DB_SPL.html

Historically 0 dB SPL was a measurement of the quietest sound a human
could hear at their peak sensitivity of around 4 kHz.

--Randy




--
All the Best
Richard Harison
"Randy Yates" wrote in message
...
"Richard Harison" writes:

Agreed! Relative to a base of some source.
A previous posted point of mine was that SPL is also measured in db. After
that
it gets a little vague as to source. What =0db? An anechoic chamber 200
feet
underground? A whisper?

dB SPL is a well-defined unit and is the sound pressure level
referenced to 0.0004 dynes/cm^2 or, equivalently, 20 microPascals. If
the sound is traveling in a plane wave, this can also specify sound
intensity and is referenced to 10^{-12} W/m^2.

@BOOK{psychoacoustics,
title = "{Psychoacoustics: Facts and Models}",
author = "E.~Zwicker and H.~Fastl",
publisher = "Springer",
edition = "second",
year = "1999"}
--
% Randy Yates % "The dreamer, the unwoken fool -
%% Fuquay-Varina, NC % in dreams, no pain will kiss the brow..."
%%% 919-577-9882 %
%%%% % 'Eldorado Overture', *Eldorado*, ELO
http://home.earthlink.net/~yatescr



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"Randy Yates" wrote:

For example, dBm is absolute power, although in a unit different than
Watt. This should cause no less problem than, e.g., stating power in
horsepower.

Okay, let's say I grant you this. We understand what dBm is referenced
to, therefore we can it as a unit of power instead of Watts.

Conversely, dB is NOT power. You cannot translate a dB to an
absolute power level in Watts.

Not true. It had better not be true.

For the dB number to make any sense, it has to be based on a reference.
And if we can't translate it to an exact value, then all that means is
that we aren't using it properly and/or don't have enough information.

If I say that a 100W power amp is 3 dB down at 100 KHz, I may be saying
that it can provide max 50W at 100 KHz. I should be able to verify this
exactly.

Or maybe this refers to the drop compared to an output of 1W at 1 KHz.
So if a 1 KHz input creates a 1W output, that same level of input at 100
KHz will create 500 mW output.

But note, the second interpretation is different from the first one, and
the 3 dB points are very likely to be different in these two examples.
Only after knowing the reference can I verify these numbers.

No different from dBm, dB SPL, or dBA. If I don't know the reference,
the number is meaningless.

Note that this does not in any way diminish the fact that ANY dB
measurement is based on a ratio of a referred power to a reference
power. It is just that in cases of absolute units, the reference power
is known so that the referred power can be computed.

I think this is where we disagree. The reference power, or voltage, or
loudness, or brightness, *always* has to be known. The only difference
is that sometimes the reference level is some sort of pre-ordained
standard (1 mW, 20 uPascals), whereas other times it has to be specified
for that mesurement uniquely.

Bert

"Albert Manfredi" writes:
[...]
If I don't know the reference, the number is meaningless.

Absolutely untrue. Take, for instance, the response specification of a
speaker. We might specify the speaker response as "+/- 5 dB from 50 Hz
to 20 kHz." It's not even relevent what the absolute power is - the
useful information is in the speaker's relative response.

I think this is where we disagree. The reference power, or voltage, or
loudness, or brightness, *always* has to be known.

Bull****. No it does not.
--
% Randy Yates % "And all that I can do
%% Fuquay-Varina, NC % is say I'm sorry,
%%% 919-577-9882 % that's the way it goes..."
%%%% % Getting To The Point', *Balance of Power*, ELO
http://home.earthlink.net/~yatescr

"Randy Yates" wrote:

"Albert Manfredi" writes:
[...]
If I don't know the reference, the number is meaningless.

Absolutely untrue. Take, for instance, the response specification of a
speaker. We might specify the speaker response as "+/- 5 dB from 50 Hz
to 20 kHz." It's not even relevent what the absolute power is - the
useful information is in the speaker's relative response.

Excellent example. That sort of spec is qualified by a ton of different
variables, including but certainly not limited to the sound level being
generated. Just try to verify it and see what I mean. Think, for
example, on axis or off axis, at what level of distortion, and so on.
With speakers especially, these numbers are often little more than
marketing tools.

Like I said, without the reference information, the number is useless. I
couldn't have chosen a better example to make the point.

Bert

"Albert Manfredi" writes:

"Randy Yates" wrote:

"Albert Manfredi" writes:
[...]
If I don't know the reference, the number is meaningless.

Absolutely untrue. Take, for instance, the response specification of a
speaker. We might specify the speaker response as "+/- 5 dB from 50 Hz
to 20 kHz." It's not even relevent what the absolute power is - the
useful information is in the speaker's relative response.

Excellent example. That sort of spec is qualified by a ton of
different variables, including but certainly not limited to the sound
level being generated. Just try to verify it and see what I
mean. Think, for example, on axis or off axis, at what level of
distortion, and so on. With speakers especially, these numbers are
often little more than marketing tools.

Like I said, without the reference information, the number is
useless. I couldn't have chosen a better example to make the point.

Well, Albert, maybe I'm wrong. Please enlighten me and show me what
power level 5 dB translates to.
--
% Randy Yates % "Ticket to the moon, flight leaves here today
%% Fuquay-Varina, NC % from Satellite 2"
%%% 919-577-9882 % 'Ticket To The Moon'
%%%% % *Time*, Electric Light Orchestra
http://home.earthlink.net/~yatescr

On Wed, 27 Sep 2006 19:23:26 GMT, Albert Manfredi
wrote:

Conversely, dB is NOT power. You cannot translate a dB to an
absolute power level in Watts.

Not true. It had better not be true.

It damned well is true. Seems you are as ignorant as Gene-E-us.

For the dB number to make any sense, it has to be based on a reference.
And if we can't translate it to an exact value, then all that means is
that we aren't using it properly and/or don't have enough information.

Complete and utter ********.

On Wed, 27 Sep 2006 17:06:10 GMT, Albert Manfredi
wrote:

"Richard Harison" wrote in message
...

A previous posted point of mine was that SPL is also measured in db.
After that it gets a little vague as to source. What =0db?

Normally, by convention, SPL is referenced to 20 micropascals. So 0 dB
would work out to that SPL.

0 dB is 1.
0 dB SPL is 20 micropascals.

Can you not spot the difference?

Most of the problem with this is due to sloppiness - dB is used where
dB SPL is meant.