KEF Home THX Loudspeaker System Measurements
Sidebar 2: Measurements
I only measured the AV3 Left, Right, and Center loudspeaker, as there are no agreed-upon standards for surround-speaker performance. The AV3's impedance magnitude and electrical phase angle (fig.1) were measured with an industry-standard Audio Precision System One. As usual with a THX design intended to be used with a separate subwoofer, the sealed box is tuned to a highish frequency, this revealed by the magnitude peak at 93Hz. Overall, the AV3 appears to be quite a demanding 4 ohm load. Its sensitivity was lower than specification at a calculated 86dB/2.83V/1m (B-weighted). This may be due to differences in measurement technique, or the fact that I use a B-weighted figure.
For the acoustic measurements, I used an MLSSA v.8.5 system from DRA Labs, in combination with an Outline computer-controlled loudspeaker turntable and a B&K 4006 microphone calibrated to be flat on-axis at the typical measuring distance of 50". To minimize reflections from the test setup, the measuring microphone is flush-mounted inside the end of a long tube. Reflections of the speaker's sound from the mike stand and its hardware will thus be delayed so as not to affect the measurement.
Looking at the AV3's frequency response averaged across a 30° horizontal angle on its central tweeter axis at a microphone distance of 50" (fig.2), though the AV3's balance is flat through the midrange and smooth overall, it is gently tilted-down in the treble. The bass also appears to peak up a little before rolling out below 100Hz.
By itself, I don't think this sufficient to be the reason why JGH found the system somewhat uninvolving to listen to. But the plot of the speaker's horizontal dispersion (fig.3) suggests an additional reason. (Only the changes in response are shown in this graph, which means the on-axis response appears to be a straight line.) The AV3's output at the top of the woofer passband falls off more rapidly than at the bottom of the tweeter region. In all but very lively rooms, this will tend to make the speaker sound rather polite. Coupled with the AV3's on-axis response, there just isn't enough happening in the treble.
To give the KEF designers their due, getting the balance just right is not trivial. The mid-treble flare in the speaker's horizontal dispersion seen in fig.3 means that if the speaker had a more evenly balanced on-axis treble response, the speaker might well sound too bright in acoustically live rooms.
In the vertical plane (fig.4), the use of spaced, multiple drive-units results in comb-filtering much above or below the central listening axis. It looks as though the listener's ears should be within ±5° of the AV3's central tweeter axis if the speaker's not to sound rather sucked-out in the upper midrange.
There are no surprises in the time domain, the step response (fig.5) revealing the triple tweeters to be connected in inverted acoustic polarity, with their output arriving at the microphone a little in front of the positive-going output of the twin woofers. The cumulative spectral-decay or waterfall plot (fig.6) reveals a clean initial decay, but with some low-level hash developing in the lower treble.John Atkinson