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For many years now I have refined, lived with and enjoyed the ORION (2002) and PLUTO (2005) loudspeakers. I have learned that the loudspeaker's radiation pattern and placement in the room are more important than the acoustics of the room.
Ever since I started building loudspeakers with cone-type drivers on open baffles several decades ago, I tried to find the optimum shape for those baffles given the drivers available. For ORION the drivers were selected primarily for their volume displacement and low distortion capabilities when used in a 3-way, active system. The LX521 evolved from experimentation with minimal width baffles, which can provide a more uniform dipolar radiation pattern at higher frequencies, if also suitable drivers are available. Inevitably this leads to a 4-way design, which I mostly tried to avoid in the past.
The shape of the midrange/tweeter baffle was arrived at empirically for the chosen SEAS drivers and after many acoustic free-field measurements. The two SEAS woofer drivers are housed in a V-frame baffle, which exhibits reduced resonance above the operating range of the woofer and some force cancellation.
A bridge over the woofer isolates the midrange/tweeter baffle from woofer cabinet vibrations. Woofer and midrange/tweeter baffles can be angled independently from each other. The bridge could also be built taller and the midrange/tweeter baffle tilted downwards to aim at the listener in front of a mixing desk. The woofer baffle must rest on the floor for ground plane reinforcement of its output.
The LX521 Analog Signal Processor splits the broadband line level input signal into woofer, mid and tweeter frequency bands. It equalizes driver and baffle response for each channel and filters it with LR4 response. The midrange signal is split after the power amplifier by a passive crossover filter into lower mid and upper mid driver inputs.
Frequency response on upper midrange axis and horizontal dipolar response in the frontal hemisphere are designed for neutral timbre of the the stereo phantom scene, when the loudspeakers are placed in the room as suggested. The aural scene is rendered with clarity and detail, both spatially and tonally.
LX521 Characteristics & Specifications
Outside dimensions: 49.25"H x 16"W at bottom x 15"D
3 parts, Woofer baffle, Midrange/Tweeter baffle and Bridge
Bridge can be built with increased height to raise Midrange/Tweeter baffle, if needed
Weight 67 lb est. (30 kg)
Open-baffle, dipolar radiator, < 20 Hz to 20 kHz,
Acoustically small in the horizontal plane through the upper midrange
Response -3 dB at 30 Hz (Q < 0.5) on ground plane, free-field
Dipolar response over +/-600 horizontal, < 120 Hz to 10 kHz
4-way loudspeaker system with woofer, lower midrange, upper midrange and tweeter
3-way ASP crossover/equalizer with LR4 filters at 120 Hz and 7 kHz,
assembled on two ORION ASP printed circuit boards
Passive 1st order crossover between lower midrange and upper midrange at 1 kHz,
located in base of midrange/tweeter baffle
Tweeters - SEAS 27TFFNC/G, H1396-04, coated textile dome,
front and rear of midrange/tweeter baffle
Upper Midrange - SEAS MU10RB-SL, H1658-04, Curv cone, (FU10RB)
42" above floor
Lower Midrange - SEAS U22REX/P-SL, H1659-08, Curv cone, (U18RNX/P)
Woofer - SEAS L26RO4Y, D1001-04, Aluminum cone
Push-pull mounted in V-frame baffle of 24"H x 13"W x 15"D
Minimum amplifier power: 8 x 60 W (e.g. ATI model AT6012)
60 W for each woofer, lower & upper midrange crossover, two tweeters in series
Room size: >240 ft2 (>22 m2) area, >8 ft ceiling
Speaker placement measured from tweeter:
>4 ft from wall behind it, >2 ft from side walls,
speaker separation >8 ft
Listening distance 8 ft to 18 ft depending upon loudspeaker application
Room acoustics: Fairly live with RT60 of 400 ms to 600 ms for a natural living space
LX521 as Mixing Monitor: - No need for RT60 = 250 ms to obtain a sufficient Direct/Reverberant sound ratio in the room.
- More articulated bass reproduction due to reduced coupling to room modes.
- Neutral timbre and fast time response
- Low non-linear distortion and high instantaneous dynamic range
- Optimum spatial rendering of the stereo phantom scene to judge the mix.
The frequency response on the upper midrange axis was designed to be flat. The free-field woofer/midrange response was shelved down by 4.2 dB to account for floor reinforcement. When listened to on program material in my room it became apparent that the high end had to be shelved down slightly. I use the same -3.3 dB shelf as for ORION. It merely requires a resistor and a capacitor on the circuit board. The resistor is easily removed to hear a flat response. A flat response is needed when the monitors are used for wave-field reconstruction purposes. For phantom acoustic scene creation, as in 2-channel stereo, a slightly rolled off high frequency response is indicated by the sphere derived HRTF for loudspeakers at +/-300.
The overall result is a speaker that provides a neutral sound, clarity, speed and spatial openness. Whether this is due to the improved polar response, due to inherent qualities of the midrange drivers, or the 1st order crossover, or all of these, I do not know. For its sonic qualities, wide range of applications and an important date in its development, I call it the LX521 Monitor.
http://www.linkwitzlab.com/LX521/Description.htm#F3
