Excerpted from Audio
magazine, April, 1976. pp. 44-56. By Richard S. Burwen.
This equipment has
taken about 12 man-years to design and build, starting as far back as
1962. My objective is to produce the maximum entertainment value from
prerecorded program sources and from live recordings. The room and
equipment are designed for reproduction over five speaker systems at
the original live sound levels and with more than 100 db dynamic range.
Speakers and Crossovers
Each of the five
speaker horns is 13 ft. deep and has about 64 sq. ft. of mouth area.
[i.e. 8 foot x 8 foot] The horns are conical, in preference to
exponential, in order to produce a gradual low frequency rolloff,
instead of a sharp cutoff. As it turned out, due to reinforcement
from the room, the average low frequency response on one third octave
bands is flat down to 16 Hz without equalization.
Each speaker horn
contains 30 Cerwin-Vega tweeters, a midrange horn with two JBL 2440
drivers, and two Empire 16 in. woofers. In addition, the left-front
and right-front horns have two 24 in. Cerwin-Vega woofers, mounted on
the doors. These woofers are equipped with feedback windings to
linearize their acoustic output over their range.
The rear speakers,
which are shown diagrammatically in the room plan, point towards the
rear of the room so as to provide reflected sound....the storerooms
between the front speakers constitute a 200-cu. ft. back enclosure.
The rear speakers vent into the room at the ceiling level in the
manner of a bass-reflex speaker....
The entire room is
solidly constructed of concrete, cinder clock and extra heavy
plaster. Although the horn walls are made of 4-in. filled cinder
block, I am amazed at how much these can vibrate. If I were to build
a room like this again, I would use 8-in. thick walls.
The ceiling is
wavy so as to diffuse the sound and the low points of the ceiling
conceal heavy steel beams which support the house above.
To drive these speakers there are a total of 17 Phase Linear 400
amplifiers [200 watts RMS/channel @ 8 ohms].... Each woofer, midrange
horn, and set of 9 or 12 tweeters is driven from one 200 watt
amplifier channel. With the electronic crossover at 50, 400 and 5,000
Hz ahead of the amplifier, the speakers can produce the same sound
level that would be produced by a single 20,000 watt amplifier....
The crossover filters utilize UM201 modules to produce 6-, 12-, 18-,
and 30-db/octave cutoffs for the various drivers. In addition, the
crossover provides equalization to lat acoustic response.
In designing and
adjusting the crossover network, I used one-third octave noise
measurements based on a six microphone average of omni-directional
microphones for low frequencies and an average of two cardiods
located in from of the left-front and right-front horns for high
frequencies. The microphones used for adjusting the crossover
frequency response are the same ones used for making recordings here,
so the entire system is flat. On one-third octave noise bands, it
measured flat within 1 1/2
db from 15 HZ to 20 KHz. Nevertheless, I find that when reproducing
much of the prerecorded program material availible I use from 3 to 30
db of low frequency boost
When you have a
high powered sound system like this, one of the difficult problems to
solve is to keep from blowing out 169 speakers at once. That is the
reason for having so many speakers in the first place. The tweeters
are wired three in series so that each can receive only 1/3 of the
amplifier output voltage. All the speakers can handle the clipping
levels of their respective amplifiers without mechanical damage.
Nevertheless, during sustained clipping the voice coils can get too
hot. To prevent overheating, there is an elaborate speaker protection
system in each of the rear horns and one for the three front horns.
The protection system has a total of 34 channels, each of which
measures the amplifier power output, computes the voice coil
temperture and disconnects its speaker with a relay before the voice
coil gets too hot. The same system eliminates turn-on and turn-off
thumps. In over a year of operation there has been no damage to any
of the speakers.