Lumen White Whiteflame loudspeaker Measurements

Sidebar 3: Measurements

Although my 89dB(B)/2.83V/m estimate of the Lumen White Whiteflame's sensitivity was a couple of dB below the specified figure, this is still higher than average. Its impedance magnitude (fig.1) drops below 4 ohms only in the upper bass and the mid-treble, though the combination of 5 ohms impedance and -45 degrees phase angle at 78Hz might give inadequate amplifiers conniptions. The saddle at around 40Hz in the impedance-magnitude trace gives a clue to the tuning frequency of the vertical slot-shaped port on the speaker's rear, implying only moderate bass extension.

Of more concern in the fig.1 plots are the peaks and discontinuities at 120Hz and 845Hz, which indicate the presence of major resonances at these frequencies. Fig.2 is a cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the center of the top panel. A strong mode can be seen at 120Hz, with a second a little lower in frequency. Higher in frequency, however, the decay is very clean, which was also the case on the large side panels. Despite their large radiating area, these were virtually free from delayed energy at 120Hz and 845Hz. The plywood enclosure seems to be much better at transmitting broadband acoustic energy than in storing it.

Fig.1 Lumen White Whiteflame, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Fig.2 Lumen White Whiteflame, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet's top panel. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

The complicated-looking traces in fig.3 show the nearfield responses of the midrange unit (blue), one of the woofers (red), and the port (green). The notch at 42Hz—the lowest note of both the four-string double bass and the bass guitar—in the woofer's output confirms the tuning frequency of the port, the latter's output broadly peaking at the same frequency. But note the mess in the woofer and port traces between 90Hz and 150Hz. There is a major enclosure resonance in this region that affects the behavior of both the woofers and the port. I certainly found this resonance audible on pink noise from behind the speaker. What was surprising was how inaudible it was with music when I auditioned the speakers in Michael Fremer's system.

Fig.3 Lumen White Whiteflame, nearfield responses of the midrange unit (blue trace), woofer (red), and port (green).

With the levels in this graph scaled according to the square root of the radiating areas, the crossover between the woofers and the midrange unit appears to lie at around 200Hz. The latter's nearfield response appears to gently roll out below 200Hz, but is marred by a very narrow peak at around 850Hz, suspiciously close to the frequency of one of the discontinuities in the impedance-magnitude trace. The port is still radiating considerable energy in the midrange; this is because, being a mere slot, it is wide open to the woofers' backwave.

Fig.4 shows the Whiteflame's farfield response on the tweeter axis, spliced at 300Hz to the complex sum of the nearfield responses shown in fig.3, and taking into account both acoustic phase response and the different distances of the port and the woofers from the nominal farfield measuring point. In the low frequencies, the severe enclosure resonance at 120Hz makes its presence known, while the speaker's bass is 6dB down from the nominal 1kHz level at around 35Hz. The peak at 850Hz in the midrange unit's nearfield output seems in this graph to be more associated with a distinct step in the farfield response, the middle of the midrange being shelved down by a few dB. Other than a couple of low-treble ripples, the high frequencies appear flat, though with a gentle rolloff apparent in the top octave in this spatially averaged graph. (I average seven responses taken at 5 degrees intervals across a 30 degrees horizontal window centered on the tweeter axis.)

Fig.4 Lumen White Whiteflame, anechoic response on listening axis at 50" with grille on, averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the nearfield midrange, woofer, and port responses plotted below 300Hz.

Lumen White
Acoustic Dreams
RR 5, Box 429
Fairfield, IL 62837
(618) 847-7813