Floor Loudspeaker Reviews

My relatively small room in Santa Fe measures around 19' by 16.5', with a 9' 3" ceiling broken up by 9" vigas—raw pine logs. The room is carpeted, and there are patches of Sonex foam on the ceiling to damp primary reflections of the sound. The other wall has RPG Abffusors behind the listening seat to absorb and diffuse what would otherwise be early rear-wall reflections of the sound that might blur the stereo imaging precision. Tube Traps are used in the room corners to even out the room's bass resonances, the result being a relatively uniform reverberation time of around 200ms from the upper bass to the middle treble, falling to 150ms above 10kHz.

The loudspeakers were positioned for the best sound (with only one pair of loudspeakers in the listening room at a time), generally some 3' from the rear wall (which is faced with books and LPs) and approximately 5' from the side walls (which also have bookshelves covering some of their surfaces).

Amplifiers used were a solid-state, dual-mono Mark Levinson No.333, a tubed Mesa Boogie Baron, and a tubed Cary Audio Design CAD-300SEI integrated. The preamplifier was the remote-controlled Mark Levinson No.38S, with either a Mod Squad Phono Drive EPS or an Audio Alchemy VAC-in-the-Box used to amplify LP signals from a Linn Sondek/Cirkus/Trampolin/Lingo/Ekos/Arkiv setup on an ArchiDee table. Digital sources were Mark Levinson No.30.5 HDCD and Parts Connection Assemblage D/A processors driven by a Mark Levinson No.31 transport via Madrigal and Illuminati AES/EBU cables and Meridian 518 or Genesis Digital Lens jitter-reduction units (No.31) or Sonic Frontiers UltraJitterbug (Assemblage). I also used the impressive Sonic Frontiers SFCD-1 CD player. Laserdiscs were played on a Panasonic Prism LX-1000, the digital signal taken from its TosLink output.

Interconnects used were AudioQuest's AudioTruth Lapis X3 alternating with XLO 1.1 Signature; speaker cable was a bi-wired set of Cardas Cross. All source components and preamps used in my listening room were plugged-in to a Power Wedge 116 Mk.II, itself plugged-in to a dedicated AC circuit and fitted with the Power Enhancer option. The amplifiers were plugged-in to a Power Wedge 110, again fitted with the Power Enhancer.

Each pair of speakers was broken-in before audition by being placed face-to-face and driven with out-of-phase, high-level pink noise for 36 hours, followed by the Burn-in Noise track on Stereophile's Test CD 3 for another 12 hours.

Other than impedance, for which I use an Audio Precision System One, all acoustic measurements were made with the DRA Labs MLSSA system and a calibrated B&K/DPA 4006 microphone. I place the speaker under test on a turntable/stand so as to place its tweeter about halfway between the listening-room floor and ceiling and midway between the sidewalls. Then, on the floor between the speaker and microphone, I construct an acoustic "black hole" out of graded layers of acoustic foam and fiberglass damping material, which kills the forward floor reflections of the sound emitted by the speaker. The primary reflection in the MLSSA's calculated impulse response is, therefore, that of the tweeter from the ceiling, which arrives about 4ms after the direct sound. This 4ms reflection-free time window results in a measurement valid down to 300Hz or so. To minimize reflections from the test setup, the measuring microphone was flush-mounted inside the end of a long tube. Reflections of the speaker's sound from the mike stand and its hardware will be sufficiently delayed not to affect the measurement.

The frequency response is calculated from this impulse response using the Fast (Discrete) Fourier Transform and is corrected for the measuring microphone's on-axis departure from a flat response. I then take the impulse responses of the low-frequency driver(s) and port(s)/passive radiator(s) in the nearfield, with the tip of the microphone capsule almost touching the centers of their cones/apertures, and append these responses to the upper-frequency ones. To calculate the overall low-frequency rollout, DRA Labs' software allows you to add the complex responses (amplitude and phase) of the woofer and port or sub-bass unit in the ratio of their diameters.

Because Santa Fe's 7000' altitude reduces the sensitivity of all loudspeakers, I calculate sensitivity by comparing the measured, B-weighted level at 50" for a given voltage input, using a noise signal, with that obtained for a sample Rogers LS3/5A that I've measured both in Santa Fe and at sea level. For my in-room spectral analyses I average six measurements at each of 10 separate microphone positions for left and right speakers individually, giving a total of 120 original spectra. These are then averaged to give a curve that, in my room, has proved to give a good correlation with a loudspeaker's perceived balance. I use an Audio Control Industrial SA-3050A spectrum analyzer with its own microphone, which acts as a check on the MLSSA measurements made with the B&K mike. I also used the Goldline DSP-30 automated spectrum analyzer (currently under review).

For my cabinet resonance tests, the speaker is placed on upturned cones (which allow all the various modes to develop to their fullest) and excited with a 2kHz bandwidth MLS signal at a level of 7.55V RMS. (See Stereophile, June 1992, p.205; and September 1992, p.162, both reprinted here.)—John Atkinson