# How do I use a db meter ?

Discussion in 'Electronic Basics' started by Abbie, Nov 22, 2003.

1. ### AbbieGuest

I have a multimeter, an analog/mechnical one, and one of
the scales specifies db's. What do I use this for and in
what type of measurement ? Can I use this scale to build
a loudness meter / decibel meter ?

2. ### bgGuest

The DB scale has as many uses as a voltage/ohms/amps/watts scale ---
Basiclly it is a ratio, which means that what you measure is being compared
to some other value.
DB's are very handy for making comparisons, for example frequency response,
distortion and noise levels, gains and losses, as opposed to making an
absolute measurement of the voltage or power.
is and how to use it.
bg
Abbie wrote in message ...

3. ### BaphometGuest

--
Baphomet - reply to: fanda at catskill dot net

In audio circuitry, db is a measurement of relative signal level. The
reference signal is usually stated as
0 dbm which signifies 0.707 volts into 600 ohms. For example, a power
amplifier might state that for a 0dbm input (even though the imput impedance
is usually 10K or above, the source impedance supplying the signal must be
600 ohms), the output power is so and so watts. VU meters on audio equipment
are calibrated in db. http://www.sfu.ca/sonic-studio/handbook/VU_Meter.html

A loudness meter is a different animal. You would usually be interested in
ambient noise and more often than not, whether it will contribute to hearing
loss to due excessive levels (explosions, aircraft, heavy metal rock music,
etc.). You would need an input transducer (microphone) to capture the noise
and convert it to an electrical signal. The signal would then have to be
amplified and conditioned (weighted) depending upon the type of audio
response required for the measurement. Then the signal would be fed to a VU
meter for display.
http://www.nt-instruments.com/X0-ASP-pLngCateId_126-pIntLevel_4-pLngPageId_453-X1-default.htm

4. ### AbbieGuest

The question remains, what do I do with a decibel scale on a voltmeter ?

5. ### JeffMGuest

what do I do with a decibel scale on a voltmeter ?1)In the manual for the meter, find the impedence to which it corresponds.
2)Make an AC measurment read on the dB scale. This is dBm (relative to 1mW).
OR
This is the relative strength of the 2 signals in dB.

6. ### AbbieGuest

As I explained, I have a db scale on my multimeter. I must say
I am still puzzled as to what to do with it. Let me ask it this way:
My multimeter has no DB mode on the rotary switch, only volts,
amps, and ohms (ac/dc). Could it be that it is simply an unused
feature, in other words, the designers of this multi meter didn't
need a DB scale, but this is what they had so they used it. Or is
this normal. Next: Suppose I wanted to measure sound level
from a circuit with an electret and an amplifier. Do I just connect
the output of the amplifier to a voltmeter and that's it, or do I need
to calibrate, if so, how.

7. ### Michael BlackGuest

Well since you'd be measuring AC volts, then you need to set it to
an AC volt range. And then you use it as an alternate scale to
the AC volt scale.

Keep in mind that dB is relative. "0" is defined as a certain voltage
into a certain impedance, and then the scales up and down from there
are relative to that reference point.

In those cheap meters, "0" may not be to any particular standard.
If you put the standard voltage into that meter (and the circuit is
the standard impedance), it may not read "0". Not just an inaccuracy,
but because the meter isn't set up for the standard.

But that doesn't matter, since there are plenty of cases where you'd
want a dB meter where the reference is not the standard. You'd have
some voltage, absolute or arbitrary, and that would be "0". Then as
you adjust things, you could look at the dB scale in reference to
that absolute point. Double the voltage being measured, and you'll see
a 3dB increase; half the input voltage, and it decreases by 3dB.

Think of a tape deck with a dB meter. When it reads "0" the voltage
measured may not be the standard voltage. But something is adjusted
so when the meter reads "0", it is a certain level that doesn't overload
the circuit, but which also gives a good signal to noise level. If
the signal level goes below "0", you know it is getting weak and maybe
down into the noise. If it reads too high, and such meters usually are
marked to indicate "too much", then you risk distortion. The absolute
voltages do not matter to the end user, only that the input signal keeps
the meter within a certain range of levels.

Michael

8. ### dBGuest

Abbie, do you have an email address to which I can reply?

9. ### bgGuest

Now I see your problem. DB meters are voltmeters. IF the voltage is being
measured across a known resistance, then it is also a wattmeter (sort of ).
Assuming that this meter is intended for audio, a very common standard in
audio is 0db = 1 milliwatt across a 600 ohm resistor. This is called 0 dbm.
If the meter was set up for 1 milliwatt into 600 ohms, then you can use this
info to determine how many dbm zero is equal to for each setting of the
range switch.
The voltage needed to produce 1 milliwatt into 600 ohms is .774 volts. Look
on your voltage scale. Above or below the DB scale should be a voltage scale
and the 0dbm should line up with .774 volts. Wherever your range selector
switch needs to be set for this voltage scale, would also be your 0 dbm
range. Range selector switches usually step by 10db increments. As the range
switch steps up or down, the voltage scale will alternate between a scale
that ends with a one and a scale that ends with a three.
Another possibility is that the 0db mark on your db scale is not referenced
to any standard at all. I have an old Boonton RF meter that does this.
Because DB is a ratio, the ratio is still valid anywhere
on the scale. For example going down 6db , cuts the voltage in half.
Therefore if you were measuring a voltage and the db scale was on a -10db,
dropping the signal level to -16db, just cut your voltage in half. We don't
need to know what the voltage is or what the resistance of the load is. All
we would know is that we cut the level in half or 6db. If we increase the
level by 6db, we doubled the signal voltage. You can find a chart of these
ratios or use these formulas to figure it out for yourself -----
For volts or amps db = 20 log E1/E2 (or I1.I2)
For watts db = 10 log P1/P2

Here are a few key levels to remember
a 3db increase multiplies power by 2 and voltage by 1.414
a 3db decrease multiplies power by 1/2 and multiplies voltage by .707
For voltage -
every 6db change = 1/2 or times 2
every 10db change = 1/3 or times 3
every 20 db change - 1/10 or times 10

Abbie wrote in message ...

10. ### John JardineGuest

The "Db" scale on most mechanical multimeters is pretty near useless.
The scale is meant to be read when the meter is switched to an 'AC Volts'
range and some kind of low audio range frequency is required to be measured.
It's a hangover from the "olden days" when audio rated voltmeters were
somewhat rare and engineers measured everything (mostly telephone stuff)
in "Db's" wrt a 600ohm based, 0Dbm reference (.707 Volts RMS=0Dbm).
The frequency response of the meter will most probably be crap and well near
useless for the kind of thing you are thinking about.
Even the simplest, electronic amplifier/rectifier circuits will give you a
much superior frequency range and flatness for audio loadness/Db use.
Use the mechanical meter only to provide a nicely printed Db scale when used
with some external electronics.
regards
john  