|Home | Audio Magazine | Stereo Review magazine | Good Sound | Troubleshooting|
JBL says that its new J Series of loudspeakers has been designed to provide exceptionally accurate, uncolored, and balanced sound at affordable prices. The line consists of five “bookshelf” speakers and two larger “tower” models, plus a center-channel speaker specifically for home theater applications (the others are equally usable in conventional music systems or home theater).
The J820M falls in the middle of the line in size and price. It is a two-way system suitable for shelf or stand installation. Its 8-inch woofer, which has a polymer-fiber cone, operates in a vented enclosure. The crossover, at 3.5 kHz, is to a 14-millimeter (approximately ½-inch) titanium-dome tweeter. The rated system response, at the —6-dB points, is 50 Hz to 20 kHz. Sensitivity of the nominally 8-ohm system is specified as 90 dB, and it is recommended for use with amplifiers rated between 10 and 125 watts output.
The speaker’s port opening is on its front panel, simplifying its installation against a wall if desired. For listening and most measurements, however, we placed the speakers on 25-inch stands several feet from the room walls. The terminals, on the rear panel, are spring clips that accept stripped wire ends. Unlike most such connectors, they are 3/4 inch apart and would also accept dual banana-plug connectors (though not as easily as connectors designed for that purpose).
Beginning with this speaker, we have slightly modified the room-response measurement technique we have used for many years. Previously, a frequency-sweeping signal generator and a synchronized chart recorder plotted the speaker’s frequency response on graph paper. The purpose of this measurement was to establish the effective frequency response of a pair of speakers in a fixed “real-room” environment, under physical and acoustic conditions that are maintained as nearly identical as possible for all speakers tested. That process remains fundamentally unchanged, except that we now use our Audio Precision System One to generate the test signals and digitally process the output re turned by the Bruel & Kjaer 4133 measurement microphone. By measuring the speakers’ room response under identical conditions in the same environment with both types of instrumentation, we have established that the results from the new procedure are very similar to those obtained with the old, with the advantage that the new procedure is considerably faster and more convenient to perform.
The room-response curve for the JBL J820M speakers (the average of the left and right speakers’ outputs, smoothed and corrected for the known high-frequency absorption characteristics of the room) was basically very similar for the two measurement methods. It was very flat, with relatively little response variation over most of the audio range (our averaging and smoothing process minimizes the effects of the unavoidable room standing-wave patterns). Since the smoothing function provided by the Audio Precision system is not identical to that of our former instrumentation, there were numerous minor differences between the results of the two tests, but their graphical characteristics were strikingly similar.
= == =
DIMENSIONS: 10 inches wide, 19¼ inches high, 10 inches deep
WEIGHT: 18 pounds
FINISH: Black woodgrain
PRICE: $300 a pair
MANUFACTURER: JBL, Dept. SR, 80 Crossways Park W., Woodbury, NY 11797
= == =
Essentially, the response curve had two relatively flat plateaus, each covering about half the spectrum. Be tween approximately 100 Hz and 1 kHz the smoothed output varied over a ±1 -dB range. There was a distinct 4- dB drop between 1 and 1.5 kHz and a strikingly flat response (less than 1 dB overall variation) from 1.5 to 10 kHz. Applying the high-frequency room correction (which we have been using on all speaker measurements for about twenty-five years) extended the flat high-frequency response to 20 kHz. It was interesting (and gratifying) to find that our former measurement method produced the same basic curve shape as our new procedure, though with slightly more variation and departure from uniformity.
A close-miked woofer-response measurement indicated maximum out put at 135 Hz, falling by 6 dB at 80 Hz. The port radiation dominated at frequencies below about 90 Hz. Such a measurement is inherently less ambiguous than the room measurements, since it is essentially free of room interaction. Unfortunately, it is not al ways easy to splice such a bass curve to a room curve in a meaningful manner. Nevertheless, there was no doubt that the J820M easily met its response specification of 50 Hz to 20 kHz at the —6-dB points. The actual lower limit is hard to determine precisely because of room-boundary effects, but listening tests indicated that it was somewhere between 40 and 50 Hz. Overall, the J820M’s composite measured response was admirably smooth and extended for a speaker of its size.
Quasi-anechoic MLS response measurements confirmed the impressive flatness of the speaker’s output. The axial 2-meter response was flat within ±3 dB from 300 Hz to 13 kHz, with a tweeter-resonance peak of 8 dB at 15 kHz. This measurement also showed a pronounced notch at 3.8 kHz, apparently due to the crossover (the notch did not appear in room measurements and was not audible).
The J820M’s measured sensitivity with a 2.83-volt input was 93 dB sound-pressure level (SPL), somewhat higher than rated. At a constant input of 2 volts, equivalent to a 90-dB SPL output, distortion was between 0.7 and 1 percent from 100 Hz to 2 kHz. It rose at lower frequencies, reaching 3.7 percent at the rated low-frequency limit of 50 Hz. Below that point the output dropped off fairly steeply, al though the distortion did not exceed 10 percent down to 20 Hz.
Those results suggested that at reasonable average levels, driving the J820M moderately hard at low-bass frequencies would not cause the sound to muddy up with distortion, and that was confirmed in listening tests. The audible bass output decreases as the frequency drops below 50 or 60 Hz but remains reasonably clean in the process.
Taken as a whole, the JBL T820M is a very good small speaker. We listened to it at length before making any measurements and were immediately impressed by its smoothness and balance across the audible spectrum. As it happened, the only other speakers available for comparison were considerably larger and more expensive than the JBL’s, and correspondingly good in their performance. A comparison between them and the J820M’s was inevitable, and it produced surprising results.
In its balanced, uncolored sound, the J820M easily held its own in the comparison. The larger speakers had a clear advantage in the low bass, but the J820M had a more extended high- frequency response and greater perceived airiness. Teamed with a sub- woofer, it could be a worthy competitor to the larger system.
Bottom line, the JBL J820M is an extraordinarily fine-sounding speaker at a bargain-basement price. It de serves a careful audition if you are in the market for a compact, inexpensive, and thoroughly satisfying speaker.
Source: Stereo Review (09-1995) JULIAN HIRSCH • HIRSCH-HOUCK LABORATORIES