58
the AES power rating. This is perfectly reasonable for highly efficient loudspeakers, such as
Funktion One systems, as most musical levels will be sitting well below the loudspeaker’s AES
rating. But inefficient, power hungry systems are likely to be running much closer to their AES
ratings producing unacceptable levels of distortion and mush.
Current measurements of peak displacement limit maximum driver excursion (Xmax) to 10%
deviation from linear displacement but manufacturers can get around this by choosing their test
bandwidths carefully. The recommendations allow manufacturers to choose between quoting
input current distortion or percentage deviation of displacement. The two types of distortion are
not the same (and don’t necessarily increase linearly with excursion) so manufacturers could
simply choose whichever result looks better in the calculations. Unfortunately, distortion is
rarely quoted so all this work could be wasted anyway.
There is now a proposal to measure Xmax at 10% total harmonic pressure distortion or at 10%
2nd or 3rd order modulation distortion using a two-tone signal where the upper frequency
component is 8.5 x the frequency and at a 12dB lower level than the lower frequency
component.
3. AES2-1984 (r2003) recommends that the test signal is band-limited pink noise and states that
“The manufacturer shall state the upper and lower cut-off frequencies (-3dB) of the noise
signal”. Most loudspeaker system manufacturers simply say “band-limited pink noise” without
quoting upper and lower cut-off frequencies so it’s impossible to know if low frequency
excursion is going to be a limiting factor in practice.
4. AES2-1984 (r2003) also recommends that the test noise has a 2:1 peak-to-rms voltage ratio – i.e.
a 4:1 peak-to-average power ratio. This leads some manufacturers to quote peak power ratings
of four times the long-term AES rating. This is quite permissible as long as the peak power figure
is only used to supplement the normal AES power rating – and, of course, the peak power rating
doesn’t cause over-excursion at very low frequencies. Again, some indication of upper and lower
frequency cut-offs would be helpful.
5. Some manufacturers also add a nominal 6dB spl to their calculated maximum spl figure to allow
for “half space” or “2∏” (hemispherical) floor or wall loading. This can make sense at low
frequencies, where small loudspeakers exhibit omnidirectional pressure characteristics. It also
makes sense for subwoofers whose response extends low enough for the listener to be regarded
as existing in half-space. It doesn’t make sense, however, for directional horn sections whose
coverage doesn’t wrap round to the boundary. Again, an indication of bandwidth would help
here.
To summarise calculated maximum spl problems:
The quoted sensitivity – which is, after all, the basis of most calculated maximum spl figures -
may be optimistic by up to 10dB at the upper and lower ends of the frequency range.
Likely distortion figures are rarely mentioned.
Most manufacturers’ calculations ignore long-term voice coil heating and the resultant
compression.
Manufacturers who state peak spl may have added 6dB to figures calculated using their driver
suppliers AES power ratings.
Manufacturers who state “half space” or “2∏” conditions may have added a further 6dB to their
figures.
www.funktion-one.com © Funktion One Research Limited 2011
To Next Page
Loading...