KEF LS50 Anniversary Model loudspeaker Measurements

Sidebar 3: Measurements

I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the KEF LS50's frequency response in the farfield, and an Earthworks QTC-40 for the nearfield and spatially averaged room responses. My estimate of the KEF's voltage sensitivity was 84.5dB(B)/2.83V/m, which is within experimental error of the specified 85dB. This is a little lower than average but 2dB higher than the LS3/5A's.

Somewhat optimistically specified at 8 ohms, the LS50's impedance (fig.1, solid trace) drops to 4 ohms at 200Hz and to 5.4 ohms at the top of the audioband. The electrical phase angle is generally mild, but the combination of 5.3 ohms and –41° at 135Hz, a frequency where music often has high energy, will make the speaker work at its best with a good, 4 ohm–rated amplifier.

The small blip at 39kHz in the magnitude trace in fig.1 indicates that the tweeter's fundamental dome resonance lies at this very high frequency, but the traces in this graph are otherwise free from the small lower-frequency discontinuities that would suggest the presence of cabinet resonances of some kind. Cumulative spectral-decay plots of the cabinet walls' vibrational behavior, calculated from the output of a plastic-tape accelerometer, didn't uncover any midrange resonances on any of the surfaces, though the sidewalls flexed a little at the frequency of the port tuning frequency (fig.2).

Fig.1 KEF LS50, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

Fig.2 KEF LS50, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of side panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The port is tuned to 52Hz, confirmed by the minimum-motion notch at that frequency in the woofer's nearfield output (fig.3, blue trace). The port's nearfield response (red trace) peaks sharply between 40 and 70Hz, and though some upper-frequency output is visible, this is well down in level. The LS50's overall response, averaged across a 30° horizontal window centered on the tweeter axis, is commendably even, with some small dips balanced by small peaks. The tweeter's output remains at full level out to the 30kHz limit of this graph, and unlike in some earlier generations of the Uni-Q concept, no discontinuities are visible in the mid-treble that might be due to destructive interference between the direct radiation from the tweeter and reflections of that radiation from the circumference of the woofer cone.

Fig.3 KEF LS50, anechoic response on HF axis at 50", averaged across 30° horizontal window and corrected for microphone response, with nearfield responses of woofer (blue) and port (red) and their complex sum (black), respectively plotted below 312Hz, 1kHz, 312Hz.

The LS50's horizontal and vertical dispersion, referenced to the tweeter-axis response, are shown in figs. 4 and 5, respectively. The radiation pattern in both planes is very uniform, with the usual but well-controlled narrowing of the pattern in the top octaves, though the vertical dispersion is wider than I had expected from my auditioning.

Fig.4 KEF LS50, lateral response family at 50", normalized to response on HF axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.

Fig.5 KEF LS50, vertical response family at 50", normalized to response on HF axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.

What matters most is the result of this quasi-anechoic behavior in the listening room. I performed my usual spatially averaged response measurements of the LS50s in my listening room. Using SMUGSoftware's FuzzMeasure 3.0 program and a 96kHz sample rate, I average 20 1/6-octave–smoothed spectra, taken for the left and right speakers individually, in a vertical rectangular grid 36" wide by 18" high and centered on the positions of my ears. This eliminates the room acoustics' effects, and integrates the direct sound of the speakers with the in-room energy to give a curve that I have found correlates reasonably well with a speaker's perceived tonal balance.

The red trace in fig.6 shows the KEF LS50's spatially averaged response. It is commendably even from the lower midrange through the mid-treble, though with a slight emphasis in the upper midrange and low treble. The low frequencies are boosted a little by the mode in my room at around 63Hz, but the output has dropped sufficiently below that frequency to get no significant reinforcement from the 31Hz mode. The top two audio octaves slope down a little, due to the increased absorption of the room furnishings in this region, and the in-room spectrum drops sharply above the audible range, presumably due to the tweeter's limited dispersion at ultrasonic frequencies.

The blue trace in fig.6 shows the spatially averaged response of my 1978 pair of Rogers LS3/5a's, taken under identical circumstances, but with a 3dB-higher level at 1kHz to equalize the two pairs of speakers' midrange outputs. The peak between 1 and 2kHz that lends this classic speaker its characteristic slightly nasal coloration can be seen, coinciding with a small dip in the LS50's response. The LS3/5a has noticeably more energy in-room in the top three octaves, this audible as extra "air" in direct comparisons with the KEF. At the low end, the LS3/5a has a bigger response "bump" in the upper bass, and the slower rolloff offered by its sealed-box alignment allows it to excite the lowest-frequency mode of my room enough to usefully extend the bass.

Fig.6 KEF LS50, spatially averaged, 1/6-octave response in JA's listening room (red); and of BBC/Rogers LS3/5a (blue).

The red trace in fig.7 again shows the spatially averaged in-room response of the KEF LS50s, this time compared with the spatially averaged response of the Bowers & Wilkins CM5 speakers (green trace). The two speakers' in-room behavior is very similar in the lower midrange and bass, but the CM5 excites the lowest-frequency room mode more than does the LS50, resulting in better low-frequency extension. Though the KEF has less energy apparent in the upper midrange, the B&W is more laid-back in the low treble. However, the extra energy above 5kHz produced in-room by the CM5 lent it a lighter balance overall. The B&W's high-amplitude tweeter resonance just below 30kHz makes its presence known in this graph, but this is well above what anyone can hear.

Fig.7 KEF LS50, spatially averaged, 1/6-octave response in JA's listening room (red); and of B&W CM5 (green).

Turning to the time domain, the LS50's step response on its tweeter axis (fig.8) indicates that the tweeter is connected in positive acoustic polarity, the woofer in negative polarity. However, the smooth integration of the decay of the tweeter step into the start of the woofer step implies optimal crossover design and correlates with the excellent integration of their outputs in the frequency domain seen in fig.3. The cumulative spectral-decay plot on the tweeter axis (fig.9) shows an astonishingly clean decay at all frequencies. (Ignore the black ridge of delayed energy just below 16kHz in this graph, which is due to interference from the computer's video-display circuitry.)

Fig.8 KEF LS50, step response on HF axis at 50" (5ms time window, 30kHz bandwidth).

Fig.9 KEF LS50, cumulative spectral-decay plot on HF axis at 50" (0.15ms risetime).

The KEF LS50 may be relatively affordable, but it offers superb measured performance.—John Atkinson

GP Acoustics
US distributor: GP Acoustics
10 Timber Lane
Marlboro, NJ 07746
(732) 683-2356
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