TIP #22:
Although in tip # 8 we talked about subwoofers, new technological advancements in bass extension and impact enhancement cry out for a second look at this fast advancing area of audiophile exotica. In other words, let’s yak about new subwoofers. For an in depth look at conventional subwoofer technology, refer back to tip # 8. Right now, we will touch on the basics of a subwoofer before we go on to the esoteric.
A subwoofer reproduces only low frequency information. It is an enclosure containing one or more bass drivers. In order for it to sound as though it belongs with the main speakers, it is necessary to divide and distribute the signals. This ensures that only the highs go to the main speakers and only the bass goes to the subwoofer. The electronics used to do is the crossover.
The lower the frequency, the more power required to reproduce it, and is the reason why your radio (which has little bass) seems to play louder than your CD player (which uses more power, at the same volume level, to reproduce the deep bass). Thus most every high quality subwoofer comes with an amplifier.
The sonic gains achieved by adding a powered sub/crossover combination are lower bass response extension and increased midrange detail. Midrange and bass are usually handled by one driver in each of the main speakers. Removing the bass requirements creates a dedicated driver to more accurately reproduce the midrange. Adding a powered sub/crossover combination also increases clarity, by freeing up all the power the existing amp used for bass reproduction and distributing it with more authority for mid and high frequency reproduction. Plus, by virtue of more power, there is a higher sound pressure capability.
The latest technology in subwoofers is toward lower, more accurate bass reproduction. As well as more refined control over the critical crossover point, where the speakers give way to the sub, and the sub gives way to the speakers. Now with the popularity of integrating Pro-Logic multi channel processors, the sub with 5 or even 7 channels rather than just two, balancing and controlling the bass response has taken on new meaning.
Recent philosophy, which demands that sound reproduction be as accurate as possible, has led speaker designers back to the fundamentals of the original recording. If one can precisely recreate the original recording process in reverse, then one has perfect reproduction. All live recordings use omni-directional microphones. These mics pick up in a 360 degree pattern, and all the sounds in the venue end up at this one point of pickup. They record not only the music, but how the environment interacted with the music (People cheering and coughing, wall, floor and ceiling reflections, etc.) In order to reproduce this process in reverse and create from our original point of pick up a “point source” (a much heralded audiophile term) a 360 degree radiating speaker is the logical choice. While most of the world’s most esoteric loudspeakers have arrived at this conclusion, in subwoofers the point was missed -until now. 360 degree subwoofers have several benefits over conventional subwoofers. The main one is lower frequency extension with less distortion.
For every action there is an equal and opposite reaction, so with a moving woofer on one side of a box, the box moves in the opposite direction, vibrating and changing the drivers resonance (distortion). By building a very ridged enclosure to minimize box resonance, mounting a driver opposite the first, and driving them in phase, one pushes against the other with equal pressure, canceling this effect. Bass is air motion, and with twice as much driver area, bass extension increases.
Use of active sensors also helped in the quest for lower more accurate bass. Sensors mounted in the voice coil of the woofers (dead center) feed the information gained to a processor, which compares this signal to the incoming signal f or accuracy, then modifies the signal, if the signal is different. The woofer instantly corrects any potential anomaly – aren’t computers great?
Since the original microphone had only one pickup point, and no loudspeaker can accurately reproduce the full range of music, we must use multiple drivers to do the job, with or without a sub. Making the transition between each individual driver with as few sonic burps as possible is critical to accuracy. The transition from the main speakers to the sub must be done seamlessly. To this end, a crossover with a wide adjustment range is needed.
Quality crossovers have controls for
- Phase (which way the woofer moves)
- Volume (so they match the output of the main loudspeakers)
- Low Pass Filter controlling the frequency at which the sub starts to work and setting the point where the main speakers will “roll off”.
Exotic models also have a high pass filter allowing you to set where the main speakers “roll off” and, most importantly, a “Shape” or “Kneel control. This is a control dedicated to the precise point where the two driver’s jobs intersect. These controls allow perfectly seamless blending of the main and sub speakers. These new refinements, coupled with center channel mixing for tonally seamless pans from one main speaker through the center channel, to the other main speaker, is revolutionizing the exotic subwoofer world for home theatre applications, and will filter their technology down to the more affordable realm, over time. In short, if you don’t have a subwoofer in your system, you don’t know what you are missing!
