Sidebar 3: Measurements
I used DRA Labs' MLSSA system with a calibrated DPA 4006 microphone to measure the KLH Model Three's frequency response in the farfield and an Earthworks QTC40 mike for the nearfield response of the woofer, both microphones used with an Earthworks preamplifier. I measured a different sample than those auditioned by Robert Schryer; its serial number was 20220401101.
The Model Three's excellent manual includes a lot of useful advice on optimizing the balance of the speaker. A diagram indicates that with the Model Three sitting on its stand, the listener's ears should be within a 20° vertical angle centered on the tweeter. I set the KLH on its angled stand and calculated that at an approximate distance of 96" the listener's ears, which on average are 36" from the floor, would be on the tweeter axis. I therefore performed a complete set of measurements with the microphone placed on that axis. Except when noted otherwise, all the measurements were taken without the grille and with the Acoustic Balance switch set to MID (–1.5dB above 400Hz, according to the manual).
KLH specifies the Model Three's free-field sensitivity as 85dB/2.83V/m. My B-weighted estimate was 85.6dB(B)/2.83V/m, within the margin of experimental error. The KLH's impedance is specified as 6 ohms, with a minimum value of 3.7 ohms at 320Hz. My impedance measurement, taken with Dayton Audio's DATS V2 system, indicated that the impedance magnitude remained above 6 ohms in the bass and treble and only dropped below 4 ohms with the Acoustic Balance switch set to HI (fig.1, bottom solid trace). The minimum value was 3.7 ohms at 304Hz—very close to the specification. The impedance is higher through the midrange and treble with the Acoustic Balance switch set to LO (fig.1, top solid trace). With the control set to MID, the magnitude lay between the two extremes in fig.1.
The woofer's response, measured in the nearfield (fig.3, black trace below 300Hz), peaks between 70Hz and 120Hz, rolling off below that region with the usual sealed-box slope of 12dB/octave. The apparent peak in the woofer's output compared with the midrange level will be due to the nearfield measurement technique, which assumes that the drive unit is mounted in a baffle that extends to infinity in both planes. Although fig.1 seems to show that the woofer is tuned to a fairly high 52Hz, the relatively slow rolloff below the tuning frequency means that the low frequencies will sound more extended in-room.
Fig.3 was taken without the grille and with the Acoustic Balance switch set to MID. Repeating the response measurement with the control set to HI increased the level between 500Hz and 20kHz by up to 1dB; setting it to LO reduced the output in the same region by 1dB. In neither case did the shape of the response change. The grille lowered the tweeter's output by 2–3dB between 3kHz and 6kHz and between 8kHz and 12kHz.
Fig.4 shows the Model Three's horizontal dispersion, normalized to the response on the tweeter axis, which thus appears as a straight line. The off-axis behavior is complicated, though it looks like the lack of energy between 900Hz and 1.2kHz in the tweeter-axis response deepens to the speaker's sides. The dispersion in the vertical plane is shown in fig.5. A suckout appears in the crossover region 10° above and 15° below the tweeter axis. It also appears that the KLH's output in the low treble is a little better balanced 5° below that axis, though this will be at a listening distance greater than 96".
Footnote 1: EPDR is the resistive load that gives rise to the same peak dissipation in an amplifier's output devices as the loudspeaker. See "Audio Power Amplifiers for Loudspeaker Loads," JAES, Vol.42 No.9, September 1994, and stereophile.com/reference/707heavy/index.html. Footnote 2: See fig.3 here.
Fig.1 KLH Model Three, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
The Model Three appears to be a relatively easy load for the partnering amplifier. However, the electrical phase angle (dotted trace) is high in the upper bass and midrange. Consequently, the effective resistance, or EPDR )footnote 1), lies below 3 ohms between 64Hz and 156Hz and from 161Hz to 939Hz. The minimum EPDR is 1.9 ohms at 85Hz and 1.98 ohms at 456Hz. The Model Three will therefore work well with amplifiers that don't have problems driving low impedances, and tube amplifiers will best be used from their 4 ohm output transformer taps.
The traces in fig.1 are free from the wrinkles that would imply the presence of enclosure resonances. When I investigated the cabinet's vibrational behavior with a plastic-tape accelerometer, I found a resonant mode at 242Hz on all the panels and another just above 1kHz on the side panel (fig.2). These modes are relatively low in level and have a high Q (Quality Factor), so they should not have audible consequences.
Fig.2 KLH Model Three, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of sidewall (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
Fig.3 KLH Model Three, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the nearfield woofer response plotted below 300Hz.
The trace above 300Hz in fig.3 shows the Model Three's quasi-anechoic farfield response, averaged across a 30° horizontal window centered on the tweeter axis. Other than a small peak at 700Hz, the midrange seems shelved down by 3–5dB compared with the level in the region covered by the tweeter. (The crossover frequency is specified as 1.6kHz.) The tweeter's output is respectably flat, though it rolls off in the top octave before the output rises due to the high-Q, high-amplitude "oil-can" aluminum-dome resonance at 24.2kHz. There is also a very high-Q resonance at 18kHz. I have no idea what this will be due to, but the tweeter of the KLH Model Five, which Ken Micallef reviewed in October 2021, behaved identically in this respect (footnote 2).
Fig.4 KLH Model Three, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.
Fig.5 KLH Model Three, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.
Turning to the time domain, the Model Three's step response (fig.6) indicates that the tweeter and woofer are both connected in positive acoustic polarity, with the tweeter's output arriving first at the microphone. The decay of its step doesn't uite coincide with the start of that of the woofer, which suggests that the optimal blend of the two driver outputs in the time domain will occur just below the tweeter axis. The KLH's cumulative spectral-decay plot (fig.7) is relatively clean, though ridges of decayed energy are present at the frequencies of the two tweeter resonances and just below 1kHz.
Fig.6 KLH Model Three, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.7 KLH Model Three, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
The KLH Model Three's measured performance suggests that care needs to be taken in setup and positioning in the room, and the Acoustic Balance switch utilized, to get the most neutral tonal balance.—John Atkinson
Footnote 1: EPDR is the resistive load that gives rise to the same peak dissipation in an amplifier's output devices as the loudspeaker. See "Audio Power Amplifiers for Loudspeaker Loads," JAES, Vol.42 No.9, September 1994, and stereophile.com/reference/707heavy/index.html. Footnote 2: See fig.3 here.
