Lansche Audio 5.1 loudspeaker Measurements
Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Lansche 5.1's frequency response in the farfield, and an Earthworks QTC-40 for the nearfield and spatially averaged room responses. The Lansche is specified as having a voltage sensitivity of 91dB/2.83V/m; my estimate on the tweeter axis was 90.5dB(B), which is both within experimental error of the specification and usefully higher than average.
Fig.1 shows the Lansche 5.1's electrical impedance phase angle, following the modification to the crossover. The impedance is considerably higher in the region covered by the ionic tweeter. This will not be a problem with a typical solid-state amplifier, with its low source impedance, but it will bring up the level of the treble with tube amplifiers. At lower frequencies, however, the speaker is very difficult to drive. Not only does the impedance drop below 5 ohms below 200Hz, reaching a minimum magnitude of 2.65 ohms at 80Hz, there is also a combination of 3.4 ohms and 48° phase angle at 62Hz, a frequency where music can have significant levels of energy. An amplifier capable of driving 2 ohm loads with aplomb is a must with this speaker.
One peculiarity I noted while measuring the 5.1's impedance was that there was noticeable RF noise in the oscilloscope trace. (I always use an oscilloscope to monitor what I am measuring with the Audio Precision, as a visual check on what is happening.) I suspect that this RF noise is due to the fact that the Lansche tweeter is transformer coupled; some of the arc generator's noise is reflecting back through the transformer to the partnering amplifier. This will not be an issue with tube amplifiers, which tend to have a low level of overall negative feedback and a curtailed ultrasonic response. But with high-feedback, solid-state amplifiers, the speaker terminals are also an input port via the feedback circuit, and not all amplifiers will deal gracefully with RF noise input in this manner. It's possible that this might have been, at least in part, why I felt the tubed Audio Research amplifier proved a more synergistic match with the Lansche than the wide-bandwidth, solid-state MBL amplifiers.
The traces in fig.1 are free from the small discontinuities that would imply the existence of cabinet resonances. Checking the panels' vibrational behavior with a plastic-tape accelerometer, I found a couple of low-level modes between 200 and 300Hz on all surfaces (fig.2). Overall, the Lansche's enclosure is well damped. There was a higher-level mode present at 350Hz on the top panel, but this was eliminated by the VPI bricks I used for my listening.
The saddle centered on 25Hz in the impedance-magnitude trace suggests that this is the tuning frequency of the large port near the bottom of the speaker's rear panel. The nearfield output of the woofers (fig.3, blue trace; the woofers behaved identically) does indeed have the expected minimum-motion notch at that frequency, which is when the back pressure from the port resonance holds the woofer cones still. The port's output (fig.3, red trace) peaks between 15 and 60Hz, with a sharp rolloff above that region. Three resonant peaks are visible in the port's midrange output, however, though these are well down in level and their audibility will be reduced by the fact that the port faces away from the listener. The woofers roll off above 80Hz, but the midrange drive-unit (fig.3, green trace) comes in a little too high in frequency for optimal integration with the woofers.
These nearfield responses are all boosted by a few dB, due to the implicit assumption in the measurement methodology that the radiators are mounted in a true infinite baffle; ie, one that extends to infinity in all directions. But they do suggest that the Lansche 5.1's low-frequency alignment is somewhat overdamped. These measurements were taken with the rear-panel low-frequency jumper set to "0"; setting this to "3" reduced by 3.6dB the output level at 75Hz, but the woofer response below 50 and above 200Hz wasn't changed.
Higher in frequency in fig.3, the overall balance is flat, though with a slight excess of energy apparent between 1.5 and 3kHz. This is actually due to the tweeter having too high an output at the bottom of its passbanda horn artifact?though the midrange unit rolls off smoothly in this region (not shown). Before the modification to the crossover, this excess of energy extended more than an octave higher in frequency. The high-frequency jumpers on the terminal are marked "+1," "0," "1," and "2." Before the modification, these numbers roughly corresponded to the change in the tweeter level; after the modification, the difference in this level at 10kHz between "+1" and "2" was just 1dB.
While the Lansche's horizontal dispersion in the bass and midrange is wide and superbly even, the tweeter's horn loading makes the speaker increasingly directional as the frequency rises (fig.4). This might well make the 5.1 sound too mellow in large rooms. In the vertical plane (fig.5), a large suckout develops at the crossover frequency more than 5° above the tweeter axis, but the speaker maintains its balance below that axis. This is appropriate, given the height of the tweeter, which is 37" from the floor.
The red trace in fig.6 shows how its quasi-anechoic response measurements translate to the Lansche speaker's in-room behavior. This spatially averaged graph is produced by using SMUG Software's Fuzzmeasure program to take 10 response measurements for each speaker, in a vertical grid centered on the positions of my ears. (The speakers were driven by the Lamm amplifiers for this measurement, with the tweeter jumpers set to "+1.") Other than a slight excess of energy at the bottom of the tweeter's passband, the treble is smooth, with a slight downward trend due to the increasing absorptivity of the room's furnishings in the top octaves. However, the lower midrange is shelved down, and while the low frequencies are aided by the lowest-frequency resonances in my room, they are still a little light compared with the level in the treble. The blue trace in fig.6 was taken for the speakers before the crossover modification. Even though the tweeter jumpers were set to "1," there is just too much low-treble energy present in the room, which is why I found the original speakers to sound so forward. This excess also threw into contrast the shelving-down of the 5.1's lower midrange, emphasizing the leanness. The production change to the crossover brought the speakers' in-room response into better balance (red trace).
In the time domain, the Lansche 5.1's step response on the tweeter axis (fig.7) reveals that its tweeter and woofers are connected in positive acoustic polarity, the midrange unit in inverted polarity. The setback of the horn tweeter means that the start of the midrange unit's negative-going step can be just seen before the positive-going spike of the tweeter's step at the 4ms mark. The cumulative spectral-decay plot on the tweeter axis is superbly clean in the upper midrange and treble (fig.8).
The Lansche 5.1's measured performance indicates that it will never be a warmly balanced speaker, but the revision to its crossover brings that extraordinary tweeter into better balance with the speaker's lower ranges.John Atkinson