The Decibel

On Thu, 10 Jul 2003 13:47:05 GMT, Nathan Higgins -spam
wrote:

I understand what a decibel is and how it is calculated, what I don't
understand is how sound level meters reference the sound to give a
reading
in dB. Surely every power meter would give a different reading depending
on
what its reference is for 0dB. I have the figure 0.02 mPa in my head as a
reference but should all meters be calibrated to this pressure ? Is there
a
gentleman's agreement to what 0db should be referenced to with sound
meters
(i.e.. to measure audio in a concert).

Any help appreciated.


I don't know the numbers, but I remember reading that the agreed point for
0dB is the quietest sound audiable to a person with normal hearing.

Maybe that'll help a bit.

--
Jim H

In article ,
Jim H wrote:
I don't know the numbers, but I remember reading that the agreed point
for 0dB is the quietest sound audiable to a person with normal hearing.

Yes, but it's a different measurement from the 0 dB commonly associated
with amplifiers, etc.

0 dB commonly used with electrical rather than acoustic sound levels
originates from the early days of telephones, and is 0.775v across 600
ohms. Which is also 1 millwatt.

--
*Suicidal twin kills sister by mistake.

Dave Plowman London SW 12
RIP Acorn

I don't know the numbers, but I remember reading that the agreed point for
0dB is the quietest sound audiable to a person with normal hearing.

Maybe that'll help a bit.

Oh, so it varies according to the person's age, then?

0dB depends on age! Now I've heard it all!

SPL = 20.log_10 (p_rms / p_0) ; p_0 = 2*10^-5 Nm^-2 RMS

Therefore 0dB = 2*10^-5 Nm^-2 RMS

Hearing Level (HL) is a different thing, and tends to increase
with age (AFAIR). However, it is also expressed in dB. You should
not compare or confuse it with SPL expressed in dB.

Then there is A-weighted dBA, which is SPL weighted dependent
on frequency, to better reflect how noisy it sounds, given the
ear's frequency response. You often see figures quoted for how noisy
equipment (a.g. cars) are, expressed in dBA.

Having said that, the ear's frequency response becomes flatter
at higher SPLs! Its bass response is particularly poor at
low sound levels. This is the reason old amps used to have
"loudness" buttons, which boosted the bass response (and treble
to a lesser extent) at low settings of the volume control, to
compensate. It's also why without such controls, the output
sounds "tinny", lacking in bass.


Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.fleetie.demon.co.uk

Martin said "levels. This is the reason old amps used to have
"loudness" buttons, which boosted the bass response (and treble
to a lesser extent) at low settings of the volume control, to
compensate. It's also why without such controls, the output
sounds "tinny", lacking in bass."


I have a few new amps without such controls......they do not sound tinny or
lacking in bass

Unless you have a highly anomalous hearing system, with a flat
response even at low sound levels, you should be able to
notice less bass at very low listening levels. Unless your amps
have such a "loudness" control built in and concealed, which is
so unlikely as to be negligible as a possibility.

I am not wrong on this about the ear's response flattening
as sound levels increase, especially at low frequencies. Do the
research yourself if you doubt me. By the way, my degree is in
electroacoustics. We were taught all this extensively.

I am not entirely sure what you are trying to say wih your post.

It seems to be that either:

a) You have golden ears;
b) You have an amazing amp or speakers that defy what I posted;
c) You just don't believe me;
d) You just want to contradict me for its own sake.

Perhaps you could clarify.


Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.fleetie.demon.co.uk

In article ,
Dave xxxxxx wrote:
I have a few new amps without such controls......they do not sound tinny
or lacking in bass

Well, subjectively they do. But if you're used to this effect you'll be
happy with it, and wouldn't like a loudness circuit. Some amps years ago
had a second 'volume' control that varied the frequency response with
level - said to mimic the ear. This didn't work either. ;-)

Sound mixers in radio working on sessions for late night shows in the old
days where there was specially recorded music were recommended to monitor
at very low levels - this forced them to produce a slightly bass heavy
balance, and a smaller dynamic range than usual.

--
*TEAMWORK...means never having to take all the blame yourself *

Dave Plowman London SW 12
RIP Acorn

"harrogate" wrote

"Dave Plowman" wrote in message
...
In article ,
Jim H wrote:
I don't know the numbers, but I remember reading that the agreed point
for 0dB is the quietest sound audiable to a person with normal hearing.

Yes, but it's a different measurement from the 0 dB commonly associated
with amplifiers, etc.

0 dB commonly used with electrical rather than acoustic sound levels
originates from the early days of telephones, and is 0.775v across 600
ohms. Which is also 1 millwatt.

Herewith an invitation to be flamed.

The 'dB question' was a favourite catch-out when interviewing technicians
for promotion, and I'm afraid Dave you have fallen into the trap.

A decibel is a ratio - nothing more, nothing less - so it can technically be
used to measure anything.

I think he _probably_ knew that. He just didn't say it. The original
question (if I remember rightly) was couched in terms of "What is
0dB [when referring to sound level]?", so maybe he didn't feel the
need to point out that it's 10* or 20* log_10(X / X_ref).

Say the alcohol in 1 pint is 0dBAlc; I'm drinking one now. By 7AM tomorrow
morning, I expect to have consumed about 12dBAlc. Hic!


Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.fleetie.demon.co.uk

In article , Glenn Booth
wrote:
Hi,

In message ,
harrogate writes


However in electronics the dB can also be used as a ratio but the
reference must ALWAYS be given, otherwise it is meaningless. Your
quotation of 0dB being 775mV across 600R, coincidently being a
milliwatt, is incorrect - that is the spec for 0dBm into 600R - notice
the 'm' which gives the reference as 1 milliwatt. Likewise 0dBm can
also mean 1mW into 50R in RF terms - in which case the context must
again be quoted. The one that, like SPL, is a generally accepted
reference point is 0dBW or 1 watt, which is, unless otherwise stated,
always into 50R.

I don't disagree that the reference must always be given, but for
measurements of power, such as those that reference dBm (dB referenced
to 1mW) surely the impedance is totally redundant? (Unless what one is
really trying to describe is voltage, but using a power ratio to do
so). The 50R says nothing that I can see about the power, it only
allows one to relate the voltage that will be dropped across that
particular impedance/resistance with that dB worth of power being
dissipated.

Fair point. However, bear in mind two points:

1) That in most cases (in RF at least) the quoted systems will be based
upon assuming the system is impedance matched and then give the power that
will be delivered to the source. Hence quoting the impedance tells the user
that this is the required matched impedance for optimum power transfer.

2) That in most cases the receiver will tend to be designed to work over a
given voltage range due to finite voltage rails, etc. Hence the impedance
is useful for establishing the voltage levels that must be expected.

It is therefore useful to confirm the assumed impedances. In RF/microwave
we have the annoyance that 50 Ohm is common for system and lab work, but
other impedances like 75 Ohm, etc, crop up for specific purposes/areas.

In principle, though: yes, once you've quoted the signal power in dBm
you've established the power available. You could then use a transformer to
alter the impedance (and hence signal voltage) if so desired.


The other common use of the dB is dBuV or decibels relative to 1
microvolt - a very common and popular reference used by the BBC when
quoting field strengths.

... And dBu, dBv (if you're American) and dBV, which are all in common
use in audio, but not definitively accepted (to the point that the AES
has several papers on the inadequacies of their definitions). As you
say, better to define the terms each time.

I also recall that for a while dBf (dB w.r.t. a femtowatt) was popular with
VHF/FM tuner makers and reviewers. IIRC this was the result of a decision
to make it the standard as it made relating 75Ohm and 300Ohm input
sensitivites easier. However it seemed to vanish (in the UK at least) after
a year or two.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

In message ,
harrogate writes

"Dave Plowman" wrote in message
...
In article ,
Jim H wrote:
I don't know the numbers, but I remember reading that the agreed point
for 0dB is the quietest sound audiable to a person with normal hearing.

Yes, but it's a different measurement from the 0 dB commonly associated
with amplifiers, etc.

0 dB commonly used with electrical rather than acoustic sound levels
originates from the early days of telephones, and is 0.775v across 600
ohms. Which is also 1 millwatt.


Herewith an invitation to be flamed.

The 'dB question' was a favourite catch-out when interviewing technicians
for promotion, and I'm afraid Dave you have fallen into the trap.

A decibel is a ratio - nothing more, nothing less - so it can technically be
used to measure anything. However a ratio can only be measured against a
reference. Hence it is quite correct to measure sound - or even more
correctly sound pressure levels or SPL - as a ratio against a given
reference, in the case of sound this being generally accepted as the
quietest sound that a human with 'normal' hearing (whatever that is) can
hear. Obviously this is not accurate enough so the absolute PL defined by
other contributors was developed. Unfortunately it has become common
practice to refer to sound levels just as 'dB' without quoting the
reference - although in SPL terms 0dB NEVER changes.

However in electronics the dB can also be used as a ratio but the reference
must ALWAYS be given, otherwise it is meaningless. Your quotation of 0dB
being 775mV across 600R, coincidently being a milliwatt, is incorrect - that
is the spec for 0dBm into 600R - notice the 'm' which gives the reference as
1 milliwatt. Likewise 0dBm can also mean 1mW into 50R in RF terms - in which
case the context must again be quoted. The one that, like SPL, is a
generally accepted reference point is 0dBW or 1 watt, which is, unless
otherwise stated, always into 50R.

The other common use of the dB is dBuV or decibels relative to 1 microvolt -
a very common and popular reference used by the BBC when quoting field
strengths.

Flame invitation withdrawn.



It's common in digital telecomms to use dBV, ie, dB referred to 1V.
--
Chris Morriss

In message , Glenn Booth
writes
Hi,

In message ,
harrogate writes

Herewith an invitation to be flamed.

Invitation accepted :-)

However in electronics the dB can also be used as a ratio but the reference
must ALWAYS be given, otherwise it is meaningless. Your quotation of 0dB
being 775mV across 600R, coincidently being a milliwatt, is incorrect - that
is the spec for 0dBm into 600R - notice the 'm' which gives the reference as
1 milliwatt. Likewise 0dBm can also mean 1mW into 50R in RF terms - in which
case the context must again be quoted. The one that, like SPL, is a
generally accepted reference point is 0dBW or 1 watt, which is, unless
otherwise stated, always into 50R.

I don't disagree that the reference must always be given, but for
measurements of power, such as those that reference dBm (dB referenced
to 1mW) surely the impedance is totally redundant?
(Unless what one is really trying to describe is voltage, but using a
power ratio to do so). The 50R says nothing that I can see about the
power, it only allows one to relate the voltage that will be dropped
across that particular impedance/resistance with that dB worth of power
being dissipated.


The other common use of the dB is dBuV or decibels relative to 1 microvolt -
a very common and popular reference used by the BBC when quoting field
strengths.

... And dBu, dBv (if you're American) and dBV, which are all in common
use in audio, but not definitively accepted (to the point that the AES
has several papers on the inadequacies of their definitions). As you
say, better to define the terms each time.


OdBm ref 600 Ohm is 775mV rms, but 0dBm ref 50 Ohm is something like
120mV. That's why it's often easier to use dBV.
--
Chris Morriss

X dB* reduction

I also recall that for a while dBf (dB w.r.t. a femtowatt) was popular with
VHF/FM tuner makers and reviewers. IIRC this was the result of a decision
to make it the standard as it made relating 75Ohm and 300Ohm input
sensitivites easier. However it seemed to vanish (in the UK at least) after
a year or two.


Vanish!!?.

It is vanish itself. About somewhere around the acoustic power of a
farting mosquito!.

Or the square root of f*** all....
--
Tony Sayer