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Appendix C – Maximum spl
A note about “maximum spl” specifications
Funktion One publishes a calculated “maximum spl figure” for guidance only. Potential purchasers
should be aware, however, that neither calculated nor measured maximum spl figures are a reliable
figure of merit or comparison as there are no hard and fast rules when it comes to assessing maximum
sound pressure level.
Manufacturers tend to pick ’n’ mix the parameters that make their product look best. Some calculate
maximum spl, but ignore factors that might detract from that all-important magic number. Others
measure responses but choose to ignore vital musical parameters such as power-bandwidth and
distortion. Some only quote peak spl using ridiculously short test durations that bear little relationship to
musical performance criteria.
And don’t be fooled into thinking that manufacturers with impressive “educational” programmes publish
more relevant figures. Sadly, the small print would suggest otherwise.
In short, it’s a marketing jungle out there and the buyer should beware.
Introduction
Maximum spl figures are often thought of as a key parameter when assessing the suitability of a
loudspeaker system. They are the starting point for sound designers’ spl versus distance predictions
when working out which loudspeaker system will provide the required performance for a particular
installation or event.
But are manufacturers’ maximum spl figures reliable – or, indeed, comparable?
Manufacturers’ small print suggests that a wide variety of calculation and measurement methods are in
use. This makes it impossible to compare different manufacturers’ data directly.
Most manufacturers quote calculated maximum spl figures based on their product’s sensitivity and their
driver supplier’s power ratings. Calculations are easier to do than actual measurements and most
manufacturers are honest enough to admit this, stating a lack of suitably isolated locations or facilities.
One manufacturer, however, has been known to imply that mics aren’t available to measure beyond
140dB. They’ve obviously never heard of Brüel & Kjær.
Manufacturers who calculate maximum spl figures don’t usually indicate the applicable bandwidth or the
likely distortion levels. See the Calculation notes below.
Manufacturers who measure maximum spl figures quote a variety of test signals, endurance time and
boundary conditions – or none at all. As each measurement criterion can affect the resultant maximum
spl figure by between 3 and 6dB, it is virtually impossible to compare different manufacturers’ spl
specifications once all possible criteria are taken into account. See the Measurement notes further
down.
Calculation
You’d be forgiven for thinking that maximum spl ought to be fairly easy to calculate. After all, we know
how to measure loudspeaker sensitivity. We simply apply one watt of signal and see what spl we
measure at one metre on axis. And loudspeakers’ maximum power ratings have been standardised since
the Audio Engineering Society (AES) published the original recommendations, AES2-1984, several
decades ago. This was revised in 2003 and is still commonly used.
www.funktion-one.com © Funktion One Research Limited 2011
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