Anatomy of a subwoofer's design and response
The 15 EXP is a potent self powered (active) subwoofer system that is designed for use in high-end stereo and home theater systems. It uses a long-throw 10 inch treated paper cone woofer in a bandpass cabinet and a potent 200 watt full-featured amplifier to produce quick, accurate, powerful bass response. The back of the woofer is mounted in an acoustic suspension sub enclosure that produces quick, accurate, transient response. The front of the woofer fires into a vented sub-enclosure that shapes the woofer’s response, producing flat, extended, bass.
The hallmark of the 15EXP is its ability to integrate well with large and small speakers, even with tiny speakers such as our Role Audio Skiffs. The 15EXP's unusual bandpass design produces flatter bass response over a wider frequency range and faster transient response than most subwoofers.
While the frequency response of most active subwoofers is primarily dependent on elaborate artificial shaping of the response by electronic means, the 15EXP subwoofer achieves exemplary, linear frequency response primarily through the natural and harmonious mating of its high quality driver and its sophisticated bandpass cabinet design. As a result, bass produced by the 15 EXP is natural sounding, musical, extremely neutral, highly articulate, and simply "right."
The 15EXP's amplifier includes both high and low level inputs, continuous phase adjustment, auto on/off switch (activated by input signal), adjustable volume level, and an electronic crossover that is continuously variable from 50 to 120 Hz. The amplifier sums the right and left stereo inputs to a mono output, so that one subwoofer may be used per system. The amplifier has thermal, overload and fuse protection for years of reliable use.
BASS REINFORCEMENT VS BASS REPLACEMENT
The 15 EXP is designed for bass reinforcement as opposed to bass replacement. In the old days of passive subwoofers, integrating the responses of a subwoofer and satellites speakers had to be precise with little or no overlap in frequency response because passive subwoofers did not have active phase control to bring the frequency response of the subwoofer and satellites into phase alignment. With the advent of the active subwoofer it is best to think of a good subwoofer as providing bass reinforcement. The response of the subwoofer and satellites can overlap provided both responses are in phase and neither response is "peaky." Because the 15EXP has a very flat frequency response over an extended frequency range, you can set the crossover high and allow considerable overlap between the subwoofer and satellites speakers' outputs, provided the 15 EXP and satellite speakers are brought into phase alignment.
As a combination of acoustic suspension or sealed box and vented box design, the 15EXP bandpass cabinet produces the excellent transient response characteristics of sealed box designs and the high bass efficiency of vented designs.
Without the electronic contouring of the frequency response most active subwoofers would be un-listenable. In contrast, the response of the 15EXP without any crossover, active or passive, is "text book" neutral +/- 3.5 dB from 32 Hz to 180 Hz and an even more impressive +/- 2.5 dB from 35 Hz to 175 Hz which meets the THX standard for subwoofers. Therefore, even when driven directly with any amplifier without a crossover the natural frequency response of the 15EXP would mate well with just about any loudspeaker because of its neutrality and the natural 12 dB per octave frequency response pattern above 100 Hz produced by its bandpass design.
The active 200 watt amplifier provides convenience, flexibility, and an additional 12 dB attenuation to tame irregularities in the pass band of the frequency response of the 15EXP. As shown in Figure 2, below, with the crossover set to its minimum level, the frequency response of the 15EXP, driven by a 76.8 millivolt test signal into its 200 watt amplifier, shows a "text book" 24 dB per octave roll-off pattern from 100 Hz to 200 Hz. Notice that in all 5 graphs, the the low-end frequency roll-off pattern is always a less steep 12 dB which, with natural room reinforcement of bass frequencies, allows the 15EXP to deliver exceptional low bass performance.
Figure 2: 15EXP with 200 watt amplifier small signal frequency response with the crossover set to minimum level
The setting of a subwoofer's crossover rarely has a direct 1 to 1 result on its frequency response. Setting the crossover to 100 cycles does not mean the subwoofer response stops at exactly 100 cycles, because the subwoofer's bass response is a combination of the responses of the subwoofer amplifier, the woofer, the volume of the input signal and the enclosure. Moreover, in a subwoofer like the 15EXP, where the shape of the subwoofer's frequency response is primarily determined by the bandpass cabinet design, the the effect of changing the crossover setting from minimum to maximum shown in Figures 2 and 3 mostly attenuates its frequency response and its slope above 100 Hz.
Figure 3: 15EXP with 200 watt amplifier small signal frequency response with the crossover set to maximum level
The crossover mostly attenuates the pass band of the 15EXP and combines with the natural roll of of the 15EXP to form a 24 dB roll off pattern. Because of this, the volume and phase can be equally critical as the crossover seating for integrating the 15EXP with speakers.
Figure 4: 15EXP with 200 watt amplifier large signal frequency response with the crossover set to minimum level
The interplay between driver, amplifier, and subwoofer cabinet that produces the frequency response of a subwoofer is not static but dynamic and changes with the input signal to the amplifier. Therefore the following two figures are mostly illustrative. Most subwoofers when driven very loudly depart significantly from neutral frequency response and tend to produce one-note, homogeneous bass. The 15 EXP's performance is different. As the 15 EXP is driven louder, its frequency response tends to flatten and to extend in both directions, as can be seen by comparing Figure 4 and in Figure 5.
Figure 5: 15EXP with 200 watt amplifier with large signal frequency response with the crossover set to maximum level
Notice this graph of the 15EXP's frequency vs amplitude response, shows that when the 15EXP is driven at very high volume with the crossover wide open there some squiggles in the pass band of the 15EXP above 500 Hz but it is more than 40 dBs down from the midband response which renders it inaudible.