Totem Acoustic Forest loudspeaker Measurements

Sidebar 3: Measurements

The Totem Forest's voltage sensitivity came in a little under specification at an estimated 86dB(B)/2.83V/m, but the 1dB difference is within the margin of error in this measurement. It may be of only average sensitivity, but the Forest's plot of impedance magnitude and electrical phase angle against frequency (fig.1) revealed it to be a very easy load for the partnering amplifier to drive. Even what is probably the worst-case condition—a combination of 7 ohms magnitude and around 25 degrees capacitive phase angle in the lower midrange—will hardly stress even the most current-challenged tube amplifier. The saddle in the magnitude trace at 50Hz indicates the tuning of the rear-firing port, while the small blip at 26.5kHz is due to the metal-cone tweeter's "oil-can" resonance.

Fig.1 Totem Forest, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

Of more concern are the wrinkles in the impedance traces at 270Hz, 520Hz, and 690Hz, which imply the presence of some major cabinet resonances. Investigating the panels' vibrational behavior with a plastic-tape accelerometer (a strip of piezoelectric plastic similar to an acoustic guitar pickup), I found evidence of resonances at these frequencies on all the cabinet surfaces. Fig.2, for example, is a cumulative spectral-decay plot calculated from the accelerometer's output when it was fastened to one of the sidewalls 6" from the top. However, while the modes are visible, they are still relatively low in level; I suspect that the ridges of delayed energy in this plot are actually due to the behavior of the air enclosed in the cabinet.

Fig.2 Totem Forest, cumulative spectral-decay plot calculated from the output of an accelerometer fastened to the cabinet sidewall 6" from the top. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

Additional evidence for this can be seen in fig.3, which shows both the individual responses of the tweeter and woofer at a distance of 50" on the tweeter axis, and the nearfield responses of the woofer and port. The last is the bandpass response centered on the woofer's minimum-motion frequency of 50Hz—you can see some resonance peaks in its output at the same frequencies as the wrinkles in the impedance plot. That these modes are of very high Q (Quality Factor) and that the port faces to the speaker's rear will both mitigate against their audibility. However, they were still strong enough to cause glitches in the woofer's nearfield response. Note that I neither filled the speaker's lower cavity with sand for this measurement, nor did I place the Beaks on the cabinet top, as I wanted to examine the worst-case behavior.

Fig.3 Totem Forest, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield woofer and port responses plotted below 300Hz and 1kHz, respectively.

Slight peaks and dips in the woofer's output can be seen, but the response trend is basically flat. The acoustic crossover to the HF unit appears to be set around 3kHz, with the tweeter then a little lively in the mid-treble. Its output drops back a bit in the top audio octave, before rising again to a peak 20dB high at the diaphragm's primary resonance frequency of 26.5kHz. This resonance is too high to be heard, and in any case will not be excited by restricted-bandwidth CDs (as they all are, of course). But it will ring with the new wide-bandwidth digital media, as well as with LPs played back with an MC cartridge, in which cases it might have an unpredictable effect on sound quality.

Fig.4 shows how these individual responses sum on the 33"-high tweeter axis, averaged across a 30 degrees lateral window. The bass rolls off with the usual steep reflex slope below 50Hz, much as Larry Greenhill found in his auditioning. The slight measured boost in the upper and midbass will be mostly due to the nearfield measurement technique, which assumes a 2-pi acoustic environment. But the upper-frequency tilt is real and, all things being equal, will probably add to the sense of perceived detail rather than make the speaker sound top-heavy.

Fig.4 Totem Forest, anechoic response on-axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the nearfield woofer and port responses plotted below 300Hz.

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