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This half space loudspeaker set-up will show a level enhancement of up to 6dB at very
low frequencies where the loudspeaker response is almost omnidirectional. Results
correlate nicely with the real-world listening experience.
ii) The second set-up is where the loudspeaker is placed on or near the ground and its
acoustical crossover axis is tilted towards a “boundary” microphone. The microphone is a
small, but accurate, omnidirectional measurement microphone with its capsule facing
the direction of the loudspeaker – but placed snugly against the ground. This half space
microphone set-up will show a level enhancement of up to 6dB over the full frequency
range of the loudspeaker being tested, as long as the surface is hard and smooth.
The method can be problematic, though, depending on the loudspeaker’s off-axis polar
response and ground irregularities at high frequencies.
The boundary method also tends to enhance the mids and highs when compared with
real-world listening conditions - where most listeners' ears tend to be several
wavelengths above the floor at mid and high frequencies. The pit method (i) is preferred.
6. There are still some manufacturers who state “open space” or “4∏” (spherical) conditions. It’s a
difficult condition to meet in practice as you need a measurement chamber that is anechoic
down to the lowest frequency to be measured. Unless, of course, you hoist the loudspeaker and
measurement microphone high above the ground...
To summarise measured maximum spl problems:
There is usually no way of knowing what test bandwidth a manufacturer has used as this is rarely
stated. Maximum spl measured at a response peak is useful for alarm sounders but meaningless
if you’re interested in broadband performance.
Different manufacturers use different test signals and these can have significant effects on, for
instance, peak measurements, as accompanying rms levels will be different. Remember that pre-
recorded contemporary music can be more compressed than classical recordings – so peak-to-
rms factors may be relevant to your installation.
Some manufacturers don’t allow adequate warm-up time or measurement duration so
maximum spl figures don’t include long-term compression. Maximum peak or burst spl figures
are useful – but they should only be used to supplement the normal AES power rating and not to
replace it.
Manufacturers rarely define the test conditions adequately. Not all “half space” set-ups are
equal. Half space loudspeaker measurements can make perfect sense but half space microphone
measurements may yield mid-high figures that are up to 6dB higher than is achievable at ear-
height.
Again, distortion figures are rarely mentioned. Efficient designs tend to generate less distortion
than inefficient ones for the same spl. Beware products with impressive maximum spl and AES
power rating figures but no mention of distortion. Power hungry and loudspeakers doesn’t
necessarily equate with musicality and projection.
www.funktion-one.com © Funktion One Research Limited 2011
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