by Andy Lewis, Founder, Acme Sound

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 = (10 x log Pm) +S1, where

Om = maximum output
Pm = maximum input power
S1 = sensitivity measured at one Watt/one meter

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 is 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:

• He has something to hide, and
• He thinks you don’t know the above equation.

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.

Recently, I received a call from a gentleman who had obviously reading 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?
Another method that can be used to deceive you is that of expressing sensitivity at 2.83 volts. Not so, you might insist, because 2.83 volts applied to an 8 ohm speaker is equal to 1 watt. Therefore, expressing sensitivity at 2.83 volts is the same as expressing it at 1 watt. Right?

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.