LX521 - Reference LoudspeakerFor 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
The Listening Room
The specifics of the phantom acoustic scene that is rendered by a pair of loudspeakers and perceived by a listener's brain depend upon the radiation characteristics of the loudspeakers, their location in the room, the reflective, diffusive and absorptive properties of the room and the listener's location. The first requirement is for lateral symmetry of the loudspeaker and listener setup with respect to large reflecting surfaces. Secondly, the loudspeakers must be placed at some minimum distance from those large surfaces in order to delay specular reflections by more than 6 ms. This allows the brain to give primary attention to the earlier arriving direct sound from the loudspeakers, if the reflected sound streams have similar timbre and spectral content as the direct sound. This in turn requires loudspeakers with constant, frequency independent radiation characteristics in order to illuminate the room spectrally neutral. The LX521 represents such loudspeaker to a high degree. The typical box loudspeaker is omni-directional at low frequencies and becomes increasingly directional as its baffle and driver dimensions become larger than 1/4th of the radiated wavelength. The power response typically changes by more than 10 dB between low and high frequencies. The power response of the LX521 is 4.8 dB lower at low frequencies and nearly constant up to 7 kHz before it decreases.
Richard Taylor has analyzed the required room dimensions and speaker setups to obtain a >6 ms delay for the first order reflections. He also determines the toe-in angle for equal strength of side wall and front wall reflections in a small room with a dipole source . I consider it more important to minimize the side wall reflection and to diffuse, not to absorb, the front wall reflection for optimal imaging.
James Heddle contributed a spreadsheet, which calculates the strength and delay of first order room reflections. Enter your speaker's distances from the walls and adjust the toe-in angle to minimize the reflected energy from the closest side wall. Suppression of this reflection widens the sweet spot and this top baffle orientation places the aural scene between the loudspeakers even for far off-center listeners. Compare the calculated dipole reflections to those of a monopole.
A room that is not open or acoustically dead behind the listener is likely to cause problems due to rear wall reflections. If the distance dr between listener and wall is l/4, i.e. when the reflected wave has traveled l/2 relative to the incident wave, then incident and reflected waves will cancel. The frequency response at the listener's location therefore has a notch at frequency f = 340/4*dr . Ideally this notch is at a low 10 Hz, which means that dr = 8.5 m (28 feet), a very long room. A more common listener-to-rear wall distance of 1.5 m (5 feet) would put the notch around 57 Hz, right into the bass range and be very problematic. Thus every attempt should be made to attenuate the rear wall reflection. Use a variable low frequency sinewave signal source to hear the notch frequency range at your listening place. Pink noise is not a good test signal for this.
The ideal listening room acts like a waveguide with the loudspeakers at some distance from the diffuse (live) end of the room and sound traveling past the listener to the open (dead, absorptive) end of the room (see the drawing below). Sound reflections from the wall behind the listener should be attenuated as much as possible, particularly below 200 Hz., the frequency range where discrete room resonances tend to dominate the bass distribution in the room. The LX521 being a dipole has the advantage of minimally exciting lateral room modes due to the null in its radiation pattern.
A sound processing room is usually designed to be quiet and acoustically dead. It is a work environment and not at all representative of the typical living space where people listen to a stereo recording for enjoyment but also pursue other activities. The processing room has to be dead to ensure a direct to reverberant sound ratio of no less than -6 dB at the work place and to minimize the influence of reflections and reverberation due to the colored illumination of the room by the typical box-type monitor loudspeakers. Close-field monitors at short listening distance relax the reverberation time requirements. They approach headphone listening. Headphones are completely unsuited for judging the spatial rendering of a stereo recording that is intended for loudspeaker playback. Headphones are optimally suited for analyzing tonal artifacts in a recording but completely distort distance perception. Recording engineers often claim that they "can hear through the flaws of their monitors" to the real sound. Then why are there so many technically poor recordings? They would be well advised to listen to their work/mix via LX521 Reference Loudspeakers.
An acoustically small dipole radiator in a room with T60 = 750 ms will have the same D/R ratio as an omni-directional radiator in a room with T60 = 250 ms. One room is dead, the other very live, but at the same distance from loudspeakers the room contributions to the sound at the listener's ears are equally subdued compared to the direct sound coming from the loudspeakers. The dipole loudspeaker reaches by a factor 1.73 = sqrt(3) deeper into the room. The two graphs below and the Listening_distance.xls spreadsheet show what this means in actual numbers. Note in particular how steeply the required amount of wall absorption must increase to obtain a 250 ms reverberation time. My preference is for a reverberation time of around 450 ms, which also provides a pleasant environment for talking and reading in addition to critical listening. The wall behind the loudspeakers should be diffusive in order not to lose the rear radiated sound from the LX521. Specular reflection from the side walls must not be attenuated as this reduces high frequency energy in the room. The wall behind the listener should be lossy to attenuate room modes. In addition, cloth wall hangings, rugs, pictures, upholstered chairs, open cabinets, plants and other decorative elements are all that is needed to interface a dipole loudspeaker with the room regardless of whether it is intended for work or pleasure.
See also: Sound Field Control for Rendering Stereo
LX521 Characteristics & Specifications
From F3 to LX521
I changed the project name to LX21 as this experiment might now become my 21st completed loudspeaker design. After several iterations I found on 5/21/12 a baffle shape that worked well with SEAS CA22RNX and FU10RB drivers. They were combined with 1st order quasi-Butterworth filters, which are 6 dB down at the crossover frequency. Both drivers then add in-phase over a limited frequency range. Some frequency response irregularities still had to be cleaned up, particularly around 2 kHz.. During my F3 experiments I had investigated the SEAS U18RNX/P. I liked its smooth response due to a new Curv cone. SEAS provided me with a 8" Curv cone prototype using a short voice coil. I want low voice coil inductance to have low Le(x) and Le(i) non-linear distortion in the midrange. SEAS furthermore changed the FU10's surround damping to optimize the driver's upper midrange response for my application. With two well behaved and capable midrange drivers of different size, but covering nearly the same frequency range, it became feasible to build a very wide bandwidth, 120 Hz to 7 kHz, dipole midrange bandpass filter and keep group delay variation low in its passband by using a 1st order crossover filter around 1 kHz. Group delay variation relates to envelope distortion, which in a 4-way design can be kept lower in the critical midrange than for a 3-way.
The two SEAS 10" woofers ended up in a V-frame baffle after I had first tried a W-frame for force cancellation. I did not like the complexity of baffle construction, given my DIY skills for square joints. I also wanted the baffles to use as few wood parts as possible, to eventually provide a low cost flat-pack of parts. The V-baffle also has a less pronounced resonance above the working range of the woofer than a W-frame. Even with the W-frame much mechanical vibration was coupled to the midrange/tweeter baffle when it was placed directly upon the woofer baffle. Therefore a bridge is placed over the V-frame woofer, which detaches the woofer from the midrange/tweeter baffle.
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 loudspeakers 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 Reference Loudspeaker.
After listening to a great variety of good, bad and so-so stereo recordings I am tempted to call it MAGIC521. The music comes through!