From altecpro.com (I doubt they'll mind overmuch...)

Altec Lansing's 604 loudspeaker-based systems are probably the most
famous broadcast studio and recording studio monitors of all time. For
over a quarter of a century, the 604 was employed by more studios than
all other monitor systems combined. The 604 approaches transparent
sound reproduction more than any other transducer. As one well-known
engineer stated, "When my eyes were closed, the original performance
was experienced as though it was not an electromechanical
reproduction." The 604 is known worldwide as the original point
source
standard by which other monitors are referenced. Two of the audio
industry's highest compliments were bestowed to this loudspeaker:
=B7 Doug and Sherwood Sax developed a special dividing network for 604-
based monitors used in over 800 of the world's most prestigious
studios.
=B7 A major loudspeaker manufacturer used the Altec Lansing 604 as
original equipment for one of this competitor's most famous studio
monitors.
Today's marketplace boasts many "digital-ready" loudspeaker
systems.
Most manufacturers suggest that only their newest models can manage
the dynamics presented through the digital medium. Nothing could be
further from the truth with Altec Lansing's 604. Whether a vintage
model, or our newest model, the 604-8L, the digital medium only
improves the performance of the 604. In fact, the 604 makes
"digital"
sound better.
The Altec Lansing 604-8L includes a two-way loudspeaker and a dividing
network. This Duplex=AE loudspeaker features a 15 in. (381 mm), low
frequency cone and a 1 in. (25 mm), high frequency compression driver.
Both are mounted to a single 16 in. (406 mm) diameter frame. The dual
magnet construction allows each driver to be structurally,
magnetically,
electrically, and mechanically independent of the other.
The 604-8L utilizes a dual-section dividing network with dual
equalization (mid and high frequency). The division is centered at
1,500Hz and provides 12dB/octave of attenuation for the low
frequencies and 18dB/octave of attenuation for the high frequencies.
The low frequency cone driver features a 3.0 in. (76 mm) diameter,
edgewound,
copper-ribbon voice-coil and a 5.6 lbs. (2.54 kg) ferrite magnet -
producing a gap flux density of 1.3 T. The high frequency compression
driver features a 1.75 in. (45 mm) diameter, edge-wound, aluminum
ribbon voice-coil and a 2.5 lbs. (1.13 kg) ferrite magnet - producing
a gap flux density of 1.6 T. The high frequency driver is channeled
through the low frequency magnetic structure and coupled to a 60=BA
x 40=BA MantarayTM constant-directivity horn that provides tight control
of frequencies above 1,500 Hz.
The 604-8L provides high acoustic output level capability while
maintaining smooth response, wide bandwidth and constantdirectivity
control of the critical mid and high frequencies.