LOUDSPEAKER SYSTEM COMPONENT COMPLEMENT
(4) 2242H subwoofers (2.1% conversion efficiency each or
8.4% in tandem)
(4) 2227H high-efficiency 15" woofers (one pair each side, about 17%
efficient each pair)
(2) 2123H high-efficiency 10" midrange (3.5% efficient each)
(2) 2382A horn with 2450H compression driver (30% efficient each)
Efficiency or "conversion efficiency" is
defined here as the ratio-expressed as a percent-of the acoustical output
obtained for a given electrical input. Thus a transducer that delivers 5
acoustical watts to the air when fed a 100-watt electrical input signal is
said to be "5% efficient."
In case you're wondering if the poor 10"
mid with its mere 3.5% efficiency, can keep up with the horn, rest assured I
needed 10 dB of attenuation on the mid to get flat frequency response.
For amplification, I used two BGW SPA-3
triamplifiers. BGW was happy to set up the triamplifiers to provide
high-pass filtering for the two 15's at 80 Hz, band pass for the 10"
midrange from 300 Hz to 1200 Hz, and band-pass and horn EQ filtering with
the lower end of the high frequency band at 1200 Hz. The crossover slopes
are Linkwitz-Riley type 24 dB/octave.¹ The amplifier's input section also
includes switched attenuation and built-in signal delay to adjust the
acoustic time of arrival for acoustically aligning the cones and compression
driver. Although the amplifiers are each only 5.25 inches of rack space,
they each produce up to a total output of 1000 watts, providing 600 watts
for each pair of fifteen's, 200 watts for the mid and 200 watts for the
horn. This represents an average of around 30 dB of headroom above normal
living room listening levels, which generally range in
milliwatts
for these speakers. Even though this much headroom and power seem to be
overkill, I assure you that is not the case. I include here a table from my
Audio Engineering Society workshop on basic audio-it includes the results of
about 130 hours of measurements I did of live and recorded material using a
$7500 Brüel & Kjær true-RMS voltmeter and a $40,000 Brüel & Kjær audio
analyzer to record the peak-to-average power ratio of various signals.
(1997 note: these are no longer
available except on special order. I recommend using four solid-state amps
of your choice-my choice is Adcom-and building a 48 dB/octave (8th order)
Linkwitz-Riley crossover).
In order for untrained
listeners to perceive no obvious squashing of dynamics, audio
reproduction systems should be capable of the following peak-to-average
ratios for these stimuli:
SOURCE MATERIAL
|
CREST FACTOR
|
ROCK MUSIC
|
10 dB
|
HORNS (legato notes)
|
10 dB
|
REEDS (legato notes)
|
12 dB
|
STRINGS (bowed)
|
15 dB
|
SPEECH
|
20 dB
|
PIANO
|
30 dB
|
POP MUSIC
|
40 dB
|
STRINGS (plucked)
|
40 dB
|
DRUMS
|
40 dB
|
ORCHESTRAL MUSIC
|
50 dB
|
INDIVIDUAL PERCUSSION INSTRUMENTS
|
60 dB
|
GENERAL HIGH-FIDELITY REPRODUCTION
|
60 dB
|
¹ Linkwitz-Riley 24 dB per
octave (fourth-order) or 48 dB per octave (eighth-order) crossover
filter slopes produce flat acoustic energy summation through the crossover
frequency region.
The BGW SPA-3 turned out to be the elegant
and simple alternative to a large rack of gear. Having done similar projects
many times in the past, I can honestly say I would not go back to the racks
and cabling and connectors and ground-loop chasing always necessary as long
as the SPA-3 is available to eliminate all the little gremlins that tend to
pop up when one builds up complex systems.
Note to builders: I don't recommend trying
to build passive crossovers for these units. Acoustic time-of-arrival delay
of sufficient time is not practical with passive devices and system
performance really suffers without it. Additionally, to obtain anywhere near
the performance of the tri-amped system using coils, caps, power resistors
and such, the crossover would be far more expensive than a pair of SPA-3's
and would probably weigh somewhere in the neighborhood of 100 pounds-because
of all the huge coils.
There have been many questions in the
letters I've received about doing this project some other way, with
modifications, with different components etc., perhaps to accommodate
someone's favorite component or fit some particular space or budget. I
cannot address these questions individually without addressing the
particular goal of the individual asking. I can do this-it is in fact, my
livelihood-I will be happy to design a custom system for you, but be aware
that this must be done on a consulting time basis. This project however, is
what it is, and as such, it pleased all the audio golden ears at Disney
enough that they insisted on using my lab pair for the yearly Halloween show
with an audience of 2500 people spread over an outdoor area covering 235
degrees, as well as company meetings with Eisner and Wells in huge tents
with thousands of people in the audience. This with a sizable company
inventory of all manner of THX-rated large theater systems, rock-concert
boxes form various manufacturers, etc.
© 1997 Drew
Daniels
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