What a weird mess!
I too was confused by the awkward use of decibels until I went to
Rowlett's page and found that there are many different decibels. I have
highlighted the definition of the two decibel units seen on the package in
red.
I would interpret the "106 dB spl/mW +/-4 dB" to mean that for each
milliwatt of power applied to the speaker, there is 4 Pa (2.5 - 6.3
Pa) of pressure applied to the ear drum or produced
by the speaker diaphragm. I found this by setting 106 = 100 +
20�log10(P/2) and solving for P. The
decibels are positive because the pressure is greater then the 20 μPa reference.
I'm not sure how I would interpret the "-39 dBV/Pa +/-5 dB. The
value of V calculated is 11.22 mV (6.31 - 20 mV). The negative value of
the decibel means the "voltage" is less then the reference value of 1 V.
I'm guessing this might mean that for every pascal of pressure produced by the
diaphragm of the speaker, 11.22 mV needs to be applied to the coil. Is
this suppose to be what is meant?
Well, anyway, this whole decibel thing is a mess. Everything could
just as well be left in SI units. Pascals per watt and volts per pascal
(better yet, pascals per volt) would be just as sufficient.
John
dB -
a symbol indicating that a measurement is made using a logarithmic scale
similar to that of the decibel (see below) in that a difference of 10 dB-
corresponds to a factor of 10. In each case, the actual measurement a is
compared to a fixed reference level r and the "decibel" value is defined
to be 10 log10(a/r). Many units of this kind have been
used and only a few of the more common ones are mentioned in the next entries.
In each case the dB symbol is followed by a second symbol identifying the
specific measurement. Often the two symbols are not separated (as in "dBA"), but
the Audio Engineering Society recommends that a space be used (as in "dB A").
dB A, dB C
units of sound intensity, exactly like the decibel except that before the
measurement is made sounds of high and low frequencies, heard poorly or not at
all by the human ear, have been filtered out. The letters A and C refer to two
filtering methods.
dB c
a unit of signal strength used in electronics, especially in measuring noise
levels. The signals are measured relative to the strength of the carrier signal,
which is the desired signal. A typical statement might be that a certain noise
level is -50 dB c, meaning that the noise is 50 "decibels below carrier" or
10-5 times the carrier signal strength.
dB FS
abbreviation for "decibels full scale," a unit of power as measured by a
digital device. A digital measurement has a maximum value M depending on the
number of bits used. If the actual power measurement is p, the dB FS
value displayed is 20�log10(p/M) dB FS. Since p cannot
exceed M, this reading is always negative.
dB i
a unit measuring the gain of an antenna. The reference level is the strength
of the signal that would be transmitted by an isotropic antenna: one radiating
equally in all directions. For example, an antenna rated 20 dB i transmits a
signal in the desired direction 102 = 100 times stronger than an
isotropic antenna.
dB m, dB W
logarithmic units of power used in electronics. These units measure power in
decibels above the reference level of 1 milliwatt in the case of dB m and 1 watt in the case of
dB W. A power of n watts equals 10 log n dB W; conversely, a power
of p dB W equals 10(p/10) watts.
The same formulas link dB m to milliwatts. An increase of 10 dB m or 10 dB W
represents a 10-fold increase in power. Since 1 watt = 1000 milliwatts, 0 dB W =
30 dB m.
dB rn
a symbol for "decibels above reference noise," a unit measuring noise levels
in telecommunications. The usual reference level is -90 dB m, which is
equivalent to a power of 1 picowatt (1 pW). For example, 50 dB rn equals -40 dB
m.
dB spl
a logarithmic unit of sound intensity as computed from
the sound pressure level. The reference level is a pressure of 20 micropascals. If sound waves exert a
pressure of P pascals, the sound intensity is 100 + 20�log10(P/2) dB
spl.
dB u
a logarithmic unit of power, similar to dB m but computed from voltage
measurements. The reference level is 0.775 volts, the voltage
which generates a power of 1 milliwatt across a circuit having an impedance of
600 ohms. A
voltage of V volts corresponds to a power of 20�log10(V/0.775) dB u.
dB V
a logarithmic unit of power, similar to dB m but
computed from voltage measurements. The reference level is 1 volt. A voltage of V volts
corresponds to a power of 20�log10(V) dB V.
dB W
see dB m above.
dB Z
a unit of radar reflectivity used in meteorology. The unit measures the
amount of energy returned to a weather radar site as a function of the amount
transmitted. The scale is logarithmic, a difference of 10 dB Z indicating a
10-fold increase in energy returned. For display purposes, dB Z values are
grouped as follows:
(Level 1, 18-30 dBZ) - Light precipitation
(Level 2, 30-38 dBZ) - Light to moderate rain
(Level 3, 38-44 dBZ) -
Moderate to heavy rain
(Level 4, 44-50 dBZ) - Heavy rain
(Level 5, 50-57
dBZ) - Very heavy rain; hail possible
(Level 6, >57 dBZ) - Very heavy rain
and hail; large hail possible
The colorful "radar images" shown on television are actually plots of these
levels.
----- Original Message -----
Sent: Sunday, 2003-02-09 22:30
Subject: [USMA:24798] Plantronics Stereo PC
Headset
> I bought a
Plantronics Stereo PC Headset at Best Buy today. Noted the
>
following....
> K used as symbol for kilo instead of k
> no spaces
between numbers and unit symbols (I prefer that they be used,
> although
that is not a stated requirement of the SI.)
> awkward statements using
decibels:
> 106dBSPL/mW +/-4dB
> -39dBV/Pa +/-5dB
>
>
But what I *really* liked about the package label was that there were
>
absolutely no non-SI units! That includes such statements as:
> Cable
Length & Connection: 2.9 meters (3.5mm plugs)
> Speaker Driver Size:
28mm diameter
>
> Not one single, solitary FFU, WOMBAT, American
Hodgepodge unit!!! None
> anywhere in or on the package! And the bar code
display includes the words
> "Audio60 - US" so they were intended for
marketing in the US. Hot diggitty!
> By the way, they sound great;
haven't tried the microphone yet, though.
>
> I think I'll write
them a nice note and include some incidental suggestions
> for improving
their SI formatting.
>
> Jim
>
