Designer's Notes

Bamberg Series 5 Bass Module
A unique approach overcomes traditional problems
    Single-black-cube subwoofer products seem to be a dime-a-dozen these days. Customer anticipation of a quick-fix to their bass performance issues only seems to quickly become disappointment.
    In spite of low pass filtering, all subwoofer drivers can show some harmonic distortion products. More often than not, this is the reason for the listener sensing where a single and off-to-the-side subwoofer is located in the room. To counter this, the traditional approach has been to move the crossover down ever lower in frequency. However, this makes blending the subwoofer and monitor quite problematic, as phase coherency suffers due to the inevitable subsonic roll off. Not only does the customer know where the bass is coming from, but it is strange sounding bass to boot. At BESL, we feel your pain.
    The Bamberg solution overcomes this by intentionally moving the cross point up to 125Hz. By using bass modules in stereo pairs, maximum loudness doubles (increases by 6dB), distortion is lowered, and there is absolutely no localization of the woofers. And placing the stereo woofer/monitor speakers out from the front wall a little provides quarter-space boundary reinforcement and increased amplifier headroom.

Design concept
    The Bamberg Series 5 bass module is a product we are especially pleased about. Consisting of the Peerless 10" XLS (Xtra Long Stroke) woofer and opposing 10" passive radiators, this dual drone system shows lower distortion than sealed or vented systems. If the parameters are just right (as they are with this design), very little EQ is required from the partnering amplifier in order to achieve flat response. This bass module measures beautifully flat down to 18Hz F3 as shown by the SPL graph. The roll off is approximately that of a 6th order system in the subsonic band.
    You may notice an absence of vents in all BESL speakers. Vents have a few negative side effects, such as midrange sound leakage, self-resonance, nonlinear operation, no cone control below tuned frequency, etc.  A drone system on the other hand prevents all such problems.
    One lesser known fact is that it is a good idea to couple the woofer with drones whose total cone area is about twice that of the woofer's. This reduces the drone excursion in the subsonic band, and thus increases maximum SPL output while reducing distortion. In a sense, the 10" woofer "migrates" to two 10" diaphragms at the lower frequencies. This is more total cone area than from a 12" woofer. (12" woofer designs generally build into twice the cabinet size.)
    By positioning the radiators low in the cabinet, the floor bounce dip is eliminated from the middle/upper bass range.

Construction
    The 10" woofer just fits the front panel of the cabinet, which tapers up to match the footprint of the Bamberg TM monitor. This provides an attractive narrow and stacking monitor/woofer cabinet set.
    Many competitors subwoofers are freestanding, and exhibit the "washing machine" effect. In other words, without being held rigidly still, the sound output is slightly compressed, making the bass sound muddy or soft. By contrast the Bamberg cabinet is intentionally heavy and very rigid. It can be spiked to the floor and is further weighted down by the matching Bamberg monitor. As a result, the bass sounds tighter, with more definition and "punch".

Performance
    We have performed some rather sophisticated measurements on the Bamberg woofer. Under free air conditions, it showed low distortion, met the advertised Xmax, and even exhibited composed performance to Xmax + 50%!
    Maximum half-space SPL is 102dB from 165 watts. We realize this figure is not as high as some competitors claims. Add about 5dB for baseboard loading (~107dB), or about 10dB (~112dB) for corner loading, and a few more dB still for the room gain effect. We are confident that you will be hard pressed to find true deep bass and high sound output from a package this compact. Furthermore, the power graph shows no compression up to the rated SPL limit.
    Finally, this woofer is even easy to drive. The impedance stays quite high all throughout the band of operation. Once again, it is a pleasant surprise that only marginal pass band EQ is required for this particular alignment.

Powering and filtering the XLS
    The next challenge then became one of finding appropriate power and low-pass filtering. In keeping with the high-end pedigree of the Series 5, outboard amplification and filtering is provided with the system. An external module houses both the bass power amp and monitor crossover. Proper crossover summation with the Bamberg monitor is absolutely guaranteed.
    Full-blown active tri-amp is also easily accomplised via the DCX2496 DSP crossover and power amps of your own choosing (and with BESL guidance). Conversion from bi-amp active/passive configuration to tri-amp all-active power - and back again - is a snap.

Summing up
    Honeslty, we always expect our customers to compliment the overall sound quality of their new Bamberg speakers. But invariably, they call us to rave about the punchiness and tonality of the bass!
    We fully expect the Series 5 bass module to be a new benchmark against which we invite any other similar-sized subwoofers to be compared.