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All we have to do is work out the AES power rating in dB, with reference to one watt, and then add that
figure to the sensitivity figure, surely?
Working out the AES power rating in dB, with reference to one watt, and then adding that figure to the
sensitivity figure is, indeed, a common way of calculating maximum spl so most manufacturers have little
choice but to follow suit. This industry tradition appears to allow potential purchasers to compare and
contrast different products from different manufacturers. But, unfortunately, the figures can cause
unfair and misleading comparisons making the whole exercise pointless.
Here are some points to consider:
Sensitivity
1. Loudspeakers rarely have ruler flat spl versus frequency characteristics. Some manufacturers
take advantage of this and quote the loudspeaker’s sensitivity for a single octave centred on the
highest peak, rather than quote an average sensitivity for the loudspeakers’ full frequency range.
This means that a loudspeaker with a really nasty mid frequency resonance in its response could
look better in terms of both sensitivity and maximum spl.
2. Quoted frequency ranges are not always the ± 3dB you might expect. They’re often defined as
the upper and lower frequencies where the loudspeaker’s spl drops 10dB lower than the average
level of the most sensitive octave (see below). So, instead of ±3dB, they’re +0dB, -10dB with
respect to an octave averaged response peak. This means that a system’s sensitivity may be
10dB lower at the upper and lower ends of its frequency range.
3. Loudspeakers don’t usually have flat impedance versus frequency characteristics either. Typical
direct radiator loudspeaker impedances tend to peak at the main system resonance, drop back a
little, and then rise at high frequencies due to the voice coil inductance. They are also designed
to be voltage driven rather than power driven.
And most power amplifiers are designed to deliver a flat voltage versus frequency characteristic.
So it would be more realistic to quote a loudspeaker’s sensitivity in terms of dB spl (on axis at
one metre) versus drive voltage.
Some manufacturers already do this, working out the nominal voltage required to deliver one
watt at their quoted nominal impedance. But sensitivity is still rarely quoted versus frequency so
points 1 and 2 still prevail.
AES power rating
1. A loudspeaker driver’s AES power rating is its long-term (typically, two hours) free air power
rating. Many loudspeaker manufacturers simply reiterate their driver supplier’s figures which
may not allow for the effects of voice coil heating (compression) under real-world loading
conditions. This is particularly relevant to inefficient, heavily processed loudspeakers whose
manufacturers like to get into the race for ever more unrealistic power handling claims.
Funktion One tries to avoid this kind of power war, by concentrating on turning the electrical
signal into sound through highly efficient loudspeaker system designs.
2. AES2-1984 (r2003) states “The rated power of the device shall be that power the device can
withstand for two hours without permanent change in acoustical, mechanical, or electrical
characteristics, greater than 10%”. This means that a loudspeaker’s AES power rating only refers
to its resistance to permanent change (or failure), not to its linear operating range. It is quite
permissible for a loudspeaker to generate excruciating levels of distortion or to suffer from
several dB of output compression as long as a permanent change doesn’t take place.
AES2-1984 (r2003) recommendations mention distortion measurements being made at 10% of
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