As can be seen from the graph ( fig 2) on this technical page the room response is essentially flat even though the close mic response (fig 1) shows a roll-off. The close mic response indicates a reduction in response below the TL peak. If a subwoofer design is such that the frequency response is flat to subsonic frequencies in an anechoic or calculated room environment then it will have a extreme boost function when placed in a small to medium sized room. This boost function will be further enhanced by the driver receiving reflections (uncontrolled and unpredictable) that cause increases and decreases in the acoustic radiation resistance.
The advent of room treatments has grown more popular as the attempted use of the conventional subwoofers for music becomes more common place. This is due to the dual home theater-two channel system or for the audiophile to get as close to an invisible bass extension as possible. While room treatments can somewhat help reduce the problem of interactivity between drivers and room this plague cannot be eliminated by absorbing sound waves. It only reduces the intensity of the arrogant waves that have become the center of attention.
Room loading makes the use of conventional subwoofers even more unpredictable and is another reason why subwoofers remain out of audiophile systems. With no type of automatic filters available on a high-end system the effect of room loading makes it impossible to find a set it and leave it position on the subwoofer volume control. Some music sounds okay and other music overloads the room causing adjustments to be required.
ACOUSTICAL NOT ELECTRICAL PROBLEMS
95% of musical lovers live and play in ordinary size room spaces. The dimensions of the rooms are typically finite relative to long wavelength signals. This means that the more efficient upper bass frequencies are going to dominate in level even if the subwoofer driver is not involved in adding to the boost. These frequencies are generally associated with a dominating dimension in the room such as a ceiling height and in most homes this is 8 or 9 feet. A subwoofer driver cone exposed to this dominating dimension will further excite this frequency making for a masking (covering) effect on the rest of the sound. The loading of the room by extremely low frequencies as described above does not match this efficiency even if the subwoofer is capable of such low frequencies therefore the very low bass is masked as well. Equalizers, filters, time delays and other types of complicated signal processing has been available for years in professional audio and some of this processing is making its way into consumer audio. It is this type of signal manipulation being employed to correct for low frequency problems in small room conditions, when the processing was developed for large room events with low bass reflectivity. When there are little or no reflections from a room the acoustics situation is completely different and processing of the sound becomes a direct not reflected event. Large rooms support long wavelength signals but don't cause overload or boost.
WHAT CONSTITUTES A HIGH-END SUBWOOFER?
The majority of the current genre of active subwoofers and subwoofer amplifiers have the basic controls that allow adjustment of the volume, crossover frequency and polarity. If all is well acoustically this is all of the controls that you need. What does the typical high-end subwoofer have that the low-end subwoofers don't? Do the added features solve the problems that low end subwoofers have?
In any physical system it is important to prevent problems from occurring not correcting problems after they occur. If the wind attempts to force a car off the road the driver is much better off if inherent stability systems in the vehicle prevent him from noticing the wind. The alternative is to have the car moved by the wind and hoping that the driver can correct his course before he completely loses control. Any attempt to correct acoustical problems associated with long wavelength frequencies after they have occurred is analogous to the above situation. Low frequency room loading and upper bass mode stimulation of the driver wreak havoc on quality sound systems.
An audiophile sound system typically will have layers of problems to correct before it will achieve true high-end quality even with good components. An audiophile system is usually voiced without an external bass extension speaker of any kind. After everything sounds as musical as possible without the added bass extension perhaps we can experiment with a high-end subwoofer.
What makes it high-end the drivers, electronic components, amplifier quality/power, the rear loading scheme for the driver, thicker wood, etc? Most high-end subwoofers earn the title through the application of a myriad of electronic adjustments supposedly to correct for acoustical problems. The source signal itself will cause a need for correction even in real time while listening as the after the fact adjustments compensate for what goes into the subwoofer not what comes out. You can't tell the musician or recording engineer just how to play or apply the low frequency mix. The only solution is a true low frequency loading scheme, which is totally isolated from errant reflected signals opting to cause the driver to do something that the signal does not tell it to do.
TBI™ Bass Extension Modules have a driver that cannot be affected by room reflections therefore the radiation resistance remains unchanged with frequency. The driver is also relatively small and operating under a very high internal pressure making it even less receptive to external stimulation by low frequency reflections. Although the driver is relatively small, its motion remains linear even at very low frequencies but with decreasing output into the infrasonic range. Inherent acoustic isolation of the driver by an acoustic controlled enclosure (with acoustic high and low pass functions) operates in conjunction with the low frequency gain of the room to create a natural bass extension into the infrasonic frequency range. The bass is fast, deep and well integrated with your mains and the quality is consistent from one installation to another. You really don't have to worry about having to move to another location and losing your wonderful listening room. TBI™ Bass Extension Modules will smooth out the next room for you as a natural function.