Focus Audio Signature Model 88 loudspeaker Measurements

Sidebar 3: Measurements

With an estimated B-weighted sensitivity of 88dB/2.83V/m, the Signature 88 will play quite loud with only a few amplifier watts. Certainly the 30W Aleph 3 had no problem raising decent sound pressure levels in my room. The impedance (fig.1), however, drops to a low 3 ohms at 65Hz, with a punishing combination of 4 ohms and -50 degrees electrical phase angle just a little lower in frequency. Although the impedance remains above an amplifier-friendly 7 ohms above 120Hz, the Model 88 still needs to be driven by a beefy amplifier. Note that the shape of the impedance curve means that if the big Focus is used with an amplifier having a high source impedance, the response will be tilted-up. There are a couple of very small wrinkles in the impedance traces, at 350Hz and 650Hz, possibly indicating the presence of cabinet resonant modes of some kind.

Fig.1 Focus 88, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

The "saddle" at 26Hz in the fig.1 magnitude trace indicates the tuning of the port, this very low in frequency, implying good bass extension. The actual acoustic response of the port, shown to the left of fig.2, indeed does peak just below 30Hz, but actually extends an octave higher in frequency before beginning its steep descent. The woofer's output is the trace with a minimum-motion point at 29Hz. It actually peaks up significantly at 65Hz—the frequency of the impedance minimum in fig.1—before rolling off steeply and crossing over to the midrange unit at 180Hz. The latter actually starts to gently roll off below 300Hz. The upper crossover frequency is in the region of 2kHz; a sharp spike at 4kHz in the midrange unit's output is suppressed by 12dB or so by the crossover. There is a similar spike in the woofer's response at 800Hz. Again, this is suppressed by the crossover's low-pass filter.

Fig.2 Focus 88, acoustic crossover on tweeter axis at 50", corrected for microphone response, with nearfield woofer and port responses plotted below 1kHz.

Fig.3 shows how all the individual drive-unit outputs add up at a distance of 50" on the tweeter axis, averaged across a lateral window of 30 degrees. The very flat response meets excellent ±1.5dB limits between 200Hz and 7kHz, with slightly more energy apparent above that frequency, reaching +2.6dB at 14.3kHz. In the bass, the woofer region peaks up as expected, then rolls off to reach -6dB (relative to the level at 1kHz and 100Hz) at a low 25Hz. However, this is a little higher than suggested by the specification of -3dB at 22Hz.

Fig.3 Focus 88, anechoic response on tweeter axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with complex sum of nearfield woofer and port responses plotted below 300Hz.

The Signature 88 offers wide, even dispersion in the horizontal plane (not shown), the top octave progressively rolling off with frequency with off-axis angle once you get more than 20 degrees or so off-axis. (It rolls off a little earlier on the side away from the tweeter.) This behavior always correlates with well-defined, stable stereo imaging, something I did note in my auditioning. Vertically, the speaker also has good dispersion, the measured response not changing significantly over a ±10 degrees window centered on the tweeter axis.

In my listening, the spatially averaged response measurement (fig.4) reveals the woofer region to be elevated by 4dB or so with respect to the midband. This, of course, was not unexpected from my auditioning. The crossover region between the woofer and midrange is also a little depressed. But otherwise, the in-room response is very flat, with just a slight excess of energy apparent in the mid-treble. Again, this is something that doesn't surprise me.

Fig.4 Focus 88, spatially averaged, 1/3-octave response in JA's room.

In the time domain, the step response (fig.5) reveals that the tweeter and midrange are connected with the same positive acoustic polarity, the former leading the latter by a fraction of a millisecond. The woofer's step response can't be seen on this graph, but it too is connected with positive acoustic polarity. The Focus's cumulative spectral-decay or waterfall plot on the tweeter axis (fig.6) is commendably clean, which correlates with the speaker's exceptional treble clarity. A small amount of delayed energy, however, can be seen at 4kHz (the cursor position), which I believe is due to a cone resonance in the midrange unit. While this is well down in level, it is associated with a slight step in the on-axis response that might be associated with the degree of added presence I noted in my auditioning.

Fig.5 Focus 88, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.6 Focus 88, cumulative spectral-decay plot at 50" (0.15ms risetime).

Finally, despite the large area of the Signature 88's side panels—which, with a less well-constructed enclosure, might be thought to indicate the presence of high-level, low-frequency vibrational modes—they are well-braced and rigid. Fig.7 is a waterfall plot calculated from the output of a simple plastic-tape accelerometer fastened to the side panel above the woofer. A single midrange mode can be seen at 305Hz, but this is well-suppressed. There are some other modes present, but as these are also very low in level and higher in frequency, it is unlikely that they will have any subjective effect. The front baffle also had a mode present at 650Hz, but this too was well down in level.—John Atkinson

Fig.7 Focus 88, cumulative spectral-decay plot of accelerometer output fastened to side panel above woofer. (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz.)

Focus Audio
43 Riviera Dr., Unit #10
Markham, Ontario, Canada L3R 5J6
(905) 415-8773