JBL Synthesis 1400 Array BG loudspeaker Measurements

Sidebar 3: Measurements

Other than the in-room and nearfield measurements, for which I used an Earthworks QTC-40, the quasi-anechoic measurements of JBL's Synthesis 1400 Array BG were all performed using DRA Labs' MLSSA system and a calibrated DPA 4006 microphone. The JBL's specified sensitivity of 89dB/2.83V/m was confirmed by my measurement. While this is not as high as with some horn-loaded designs, it is still usefully above average. In addition, the 1400 Array is an easy load for the partnering amplifier to drive, with an impedance magnitude that remains between 6 and 8 ohms over almost all the range above 200Hz, with a low electrical phase angle (fig.1). However, a minimum value of 4.8 ohms at 103Hz and a combination of 6 ohms and –52° phase angle at 71Hz will mean that the amplifier needs to be comfortable driving a 4 ohm load.

Fig.1 JBL 1400 Array, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The traces in fig.1 are mainly free from the wrinkles that would suggest the presence of cabinet resonance modes. However, the woofer enclosure sounded lively when rapped with a knuckle, and performing cumulative spectral-decay tests on the output of a simple plastic-tape accelerometer fastened to the enclosure's panels revealed that the small impedance wrinkle at 180Hz was associated with a resonant mode (fig.2). However, high-level modes at 250, 313, and 400Hz were also present on all surfaces, and I would have thought these would add some congestion to sounds rich in lower-midrange energy. However, while LG was aware of these resonances—he could feel them when he rested his fingertips on the cabinet while playing the half-step–spaced toneburst track from Editor's Choice (CD, Stereophile STPH016-2)—he couldn't hear any coloration that might be laid at their feet.

Fig.2 JBL 1400 Array, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of woofer-enclosure side panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

Listening to pink noise from behind the cabinet with the horn section disconnected, I could hear a slight hollowness coloring the sound. However, this came not from the lively cabinet, but from some low-level peaks in the port's output at 400 and 600Hz (fig.3, red trace). The fact that the port faces to the speaker's rear will reduce their audibility with music. The saddle centered on 29Hz in the impedance-magnitude trace indicates that the flared, 4"-diameter port is tuned to this low frequency; its output does indeed peak between 20 and 50Hz, and the minimum-motion notch in the woofer's output (fig.3, blue trace) does lie at 29Hz. (This is the frequency where the back pressure from the port resonance holds the cone stationary.) The woofer rolls off smoothly before its crossover to the horn array at the specified 750Hz. Above the crossover frequency, it rolls off steeply.

Fig.3 JBL 1400 Array, acoustic crossover on tweeter axis at 50", corrected for microphone response, with nearfield responses of woofer (blue) and port (red), plotted below 350Hz and 800Hz, respectively.

The horn array (fig.3, green trace) has an equally steep rollout below 1kHz, and its response on the tweeter axis is basically flat (with subjectively inconsequential ripples) in the region covered by the midrange horn. A sharp but very narrow suckout is evident at the upper crossover frequency of 8kHz, and above that frequency the tweeter appears to be balanced a couple of dB too high in level. Above 300Hz, fig.4 shows the 1400 Array's response on the tweeter axis at 50"; below 300Hz, it shows the complex sum (taking into account both acoustic phase and the different distances of the woofer and port from a nominal farfield microphone position) of the woofer's and port's nearfield responses. Around half of the apparent boost in the bass region is an artifact of the nearfield measurement technique, but it does appear that the 1400 Array's low frequencies are generously balanced and well extended. As LG noted, the JBL's bass shook the air in his room "and rattled loose radiator panels." A slight depression in the midrange is followed by a flat low treble and, again, a touch too much energy in the region covered by the tweeter.

Fig.4 JBL 1400 Array, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with complex sum of nearfield responses plotted below 300Hz.

Whether this top-octave boost will be heard as such will depend on the speaker's radiation pattern in the same region. The 1400 Array's horizontal dispersion, normalized to the response on the tweeter axis, is shown in fig.5. Other than some discontinuities at the top of the passband of the vertically oriented midrange horn, it is remarkably even and well controlled. It appears as if the large-diameter woofer's fairly narrow dispersion in the midrange is beautifully maintained through the lower treble by the midrange horn's own pattern. And while the speaker becomes more directional in the top octaves, its dispersion is very even with frequency, and doesn't show the usual narrowing above 20kHz. In the vertical plane (fig.6), the 1400 Array's response, again normalized to the tweeter-axis response, doesn't change over quite a wide listening window, other than the discontinuities at the top of the midrange unit's bandpass. This is just as well, given that the tweeter is a high 46" from the floor.

Fig.5 JBL 1400 Array, 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.6 JBL 1400 Array, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15–5° above axis, reference response, differences in response 5–15° below axis.

Whether or not the JBL will sound as if it has too much high-treble energy will depend on the size of the listener's room and the absorptivity of its furnishings. LG did comment on "terrific treble extension." I performed my usual spatially averaged response in LG's room; the result is shown in fig.7. (To generate this graph, I took and averaged twenty 1/6-octave smoothed responses for each speaker individually, in a rectangular grid measuring 36" by 18" and centered on the positions of Larry's ears in his listening chair.) The broad boost in the mid- and upper bass is due both to the residual room effects and to the 1400 Array's own generous output in this region. Note also that the JBLs are generating full output down to 25Hz. But, good grief! Look at the in-room response above 200Hz: It is extraordinarily flat and even, falling within superb ±1dB limits other than a very slight boost at 2kHz. The fact that the in-room balance remains flat above 5kHz, despite the increasing absorptivity of the room furnishings, does mean that the JBL's highs will sound "airy," if not exactly "hot." Compare this graph, for example, with fig.8 in the measurements, taken in the same room, accompanying LG's review of the Revel Ultima Salon2. The Revel is close to being perfectly neutrally balanced above 5kHz, and sounds that way. But the JBL's overall achievement is still remarkable.

Fig.7 JBL 1400 Array, spatially averaged, 1/6-octave response in LG's listening room.

Fig.8 JBL 1400 Array, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

In the time domain, the 1400 Array's step response on the tweeter axis (fig.8) indicates that all three drive-units are connected with positive acoustic polarity, and that the tweeter output arrives at the microphone half a millisecond before that of the midrange, which in turn arrives half a millisecond before that of the woofer. This is definitely not a time-coincident design, though the fact that the ear/brain does integrate arrivals over a longer period than 1ms should mean that this won't matter much. LG was impressed by the stability and accuracy of the JBLs' imaging, which you'd think might be adversely affected by the lack of time coincidence. But as far as the lower-frequency units are concerned, the 1400 Array's step response is at least time-coherent, in that the overshoot of the midrange unit's step smoothly leads into the woofer's step. This suggests an optimal crossover implementation.

Finally, the JBL 1400 Array's cumulative spectral-decay plot (fig.9) is not as clean as I would have liked to see, presumably due to reflections of the driver outputs from the edges of the horns. Nevertheless, LG found the JBL's treble to sound smooth and grain-free.

Fig.9 JBL 1400 Array, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).

I have been a fan of the 1400 Array's designer, Greg Timbers, ever since I met him and listened to prototypes of his JBL 250 on a visit to the company's Northridge, CA, factory in 1981. (Sadly, as I wrote these words, it was announced that while research and design will remain in Southern California, the manufacture of JBL and other Harman Group loudspeakers will move to the company's maquiladora factory in Mexico.) So while I'm suspicious of horn-loaded designs, I'm not surprised that the Synthesis 1400 Array BG offers both superb speaker engineering and superb measured performance. I keep returning to that remarkably flat and even in-room response: Good grief!—John Atkinson

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COMMENTS
Jimmy_G's picture

It would be interesting to compare how much of Project Array's magic that Mr. Timbers was able to distil into his curious Studio 5 series as both designs feature large bi-radial horns and trapezoidal cabinets.  

Full disclosure, my own curiosity got the better of me last January and I purchased the 530s for my smaller 2 channel system and I haven't found reason to take them out yet. I figured that anyone implementing a compression driver mated to a horn, and sensitivity isn't the goal, has a design that certainly warrants a listen.  

I haven't had the opportunity to listen to any of the Synthesis Project systems yet so I've always been left wondering.  

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