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"Maximum Output" Specifications
Traditionally in the speaker industry manufacturers have supplied a figure indicating
sensitivity as measured at an input power of 1 Watt, 1 meter away from the
speaker, on axis with it. This figure is often notated as SPL 1W/1M. The frequency at which
the measurement was taken is also sometimes supplied.
The sensitivity, power capacity and maximum output of a loudspeaker are related
by a precise mathematical relationship:
Om = maximum output
As an example, let us consider the JBL E120, a highly efficient 12” guitar
speaker. The JBL E120 is rated by the manufacturer to have a sensitivity
of 103dB 1W/1M. This would be considered a very high sensitivity for a moving
coil loudspeaker. Additionally, with a maximum input power of 300 Watts,
it is a very loud 12” speaker indeed. In practice, the JBL E120 is loud enough
to kill a small child.
The above equation, based on the manufacturers’ specifications, indicates
a maximum output of 127.8 dB SPL.
Why is this important? I’ll tell you why. Because when a manufacturer uses
a non-standard specification, such as “Maximum Output”, it’s for two reasons:
If, for example, a little red ad in a Bass Guitar magazine stated that a
little bass guitar speaker had a power capacity of 250 watts and a “maximum
output” of 123 dB SPL, this would automatically imply a sensitivity of 99.0
dB. This speaker would be celebrated world-wide as a marvel of efficiency.
I’m not aware of any compact bass guitar speakers which are regarded as highly
efficient. Are you?
If, on the other hand, this speaker had a much lower sensitivity, say in
the 85 to 90 dB range, and the same power capacity, the “maximum output”
would automatically be reduced to 109 to 114 db SPL. This is physics, folks,
and there’s no room to wiggle.
This is why our Acme units don’t have an advertised “Maximum Output” figure.
We publish standard sensitivity and maximum power figures, as does 99% of
the speaker industry worldwide. We have nothing to obfuscate or hide, and
will not use non-standard specifications in order to deceive you or cover
up the fact that our speakers are fairly inefficient. If you’re curious about
their maximum output, feel free to use the above equation to calculate it.
But remember, this is continuous output, as opposed to peak
output, which is vastly higher in number, and subject to change with respect
to an amplifier’s peak, rather than continuous, power
output.
Yesterday, I received a call from a gentleman who had obvoiusly reading
just just materials. He asked,"How loud is your Low B-4 4x10" speaker with
six hundred watts RMS?" (He specifically mentioned "RMS.") I did a quick
calculation in my head, estimated 93+20+6+1.5, and replied,"120.5 dB." (In
fact, I was wrong. A more accurate figure would be 120.78, according to the
above equation.) This innocent gentleman concluded that the 1x10" speaker
he had been reading about was louder, with less power than our 4x10." I tried
to explain the concept to him, but I’m not sure he got it. He’ll probably
figure it out when he tries to play rock and roll through that 1x10.")
2.83 volts?
Not necessarily! Is it an 8 ohm or a 4 ohm speaker? What they don’t tell
you is that 2.83 volts into a 4 ohm speaker isn’t 1 watt, it’s 2 watts! Twice
the power. So, a 4 ohm speaker with a sensitivity of 90 dB 1w/1M will have
an output of 93 dB when driven by 2.83 volts.
When you see a speaker’s sensitivity measured at 2.83 volts input, instead
of 1 watt, check the impedance. If it’s a four ohm speaker, the publisher
of the specification is trying to deceive you.
Just to be precise, and fair, the gentleman who does the technical work
for a competing company, after reading this page, pointed out to me, correctly,
that there are inherent difficulties with using the “one watt” standard.
Namely, that because a speaker presents a reactive, not a resistive load
to an amplifier, power itself has an varying relationship to voltage, and
voltage therefore is a more precise way to express sensitivity. He is right.
I would only counter that this is the standard that the industry uses, even
if slightly flawed. I believe it’s better to compare apples with apples,
and this is the standard that is used to give shoppers a fighting chance,
but I can see his point.
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