surfaces (walls, floor, ceiling) of the room then begin to reinforce the acoustic energy in the room, making it stronger. This is an acoustic phenomenon that happens in the room and the room air, after the acoustic energy has left the radiating woofer and/or subwoofer. So this boundary reinforcement phenomenon happens to all woofers, regardless of type, including the TRW.
       The TRW design works hand in glove with this boundary reinforcement phenomenon, in order to benefit the TRW's performance, by giving it flat response down to DC. The TRW, being a DC device, inherently and effortlessly extends all the way down to DC. But that extension to DC does not automatically mean that its frequency response is actually flat at these very low bass frequencies. The manufacturer's measured frequency response curve for the TRW-17, measured in a real listening room, is indeed flat, and flat down to DC. But the TRW relies on the boundary reinforcement phenomenon, present in all real rooms, to achieve this flatness of response. In other words, the TRW is designed to have flat frequency response in real rooms, which of course is how you will be listening to it.
       If, on the other hand, you were to listen to the TRW radiating hypothetically (say) into the open air outdoors instead of into a real room, then the TRW would still extend effortlessly down to DC, but the curve of its response extending down to DC would not be a flat curve, and would instead have a very gradual, shallow declining slope, probably at 6 dB per octave. Why? As discussed briefly above, the TRW's radiation resistance declines far more gently, at lower frequencies, than conventional subwoofers, being a mere linear decline rather than the square law decline faced by conventional subwoofer drivers (the TRW achieves this shallower decline because of the fact that its effective diaphragm area increases, linearly, with lower frequency). But a decline is still not flat, so this gentle decline in radiation resistance, with lower frequency, would mean that the TRW's frequency response should decline gently, with lower frequency, if played into the open air outdoors. However, when playing into a real room with boundaries, the boundary reinforcement phenomenon boosts the acoustic energy in the room at very low frequencies, so the TRW's frequency response curve becomes essentially flat.
       Each listening room has different dimensions, and thus a slightly different characteristic for this boundary reinforcement phenomenon. The TRW system includes a parameter adjustment that can customize the TRW's performance, so that it matches your room dimensions, in order to achieve the flattest possible frequency response, when working with your particular room's boundary reinforcement parameters. Interestingly, this TRW adjustment parameter actually tailors the high frequency end of the TRW's spectral range, and does not do any equalization of the low frequency portion of the TRW's spectral range. We mention this because, with conventional woofers and subwoofers, when equalization is employed it is applied at the low frequency end, which causes all kinds of further problems (with excess excursion, further degrading of transient response, etc.).
       It's also worth noting in passing that, if the TRW were used outdoors instead of into a real room, and if its frequency response curve did thereby decline, then the TRW would still have excellent time domain transient response, hence excellent bass quality. That's because its rolloff slope would be very shallow, a mere 6 dB per octave, and bass systems with this very shallow rolloff slope still have essentially perfect time domain transient response, with no negative overshoot and no ringing. In contrast, all conventional subwoofers roll off at much steeper rates, with slopes twice as steep or worse (12 to 36 dB per octave), which, as we discussed above, is why they evince such bad spurious misbehavior in their time domain transient response, i.e. in their actual performance in real time.

C. Fan Installation

       You'll want to hire a qualified carpenter or contractor, to do the planning and installation work for the TRW fan. The TRW's manufacturer offers design consulting services for hire, to help your contractor with the planning, especially useful if your house has an unusual configuration.
       If your listening room is immediately adjacent to your attic, basement, or garage, you might think that one could simply install the TRW fan in the listening room wall leading to that other space, thereby using the listening room wall as the mounting baffle for the fan. But it's better to actually install the TRW within that other space, on a baffle stretching across that space, near your listening room wall. This creates a chamber in front of the TRW fan, this chamber still being within your attic, basement, or garage.
       This chamber is then lined with sound absorbing material, which absorbs any unwanted high frequency noise from the whirring fan, before it reaches your listening room, while not absorbing any of the desired low frequency bass output from the fan. This chamber then in turn leads into your listening room, say via a large vent cut into the wall, ceiling, or floor (this vent can of course be covered with an attractive grille or cloth of your choice).
       The TRW fan, with its constant rotation, naturally makes a whirring sound, much like pink noise, just as every fan does, so its best to absorb and quiet this noise before it reaches your listening room (on the other hand, home theater devotees are already accustomed to living with projector fan noise, so the TRW's background fan noise might be tolerable for them if heard directly).
       Then, if you want to play the TRW really loud, its fan blades go to a steeper pitch, and this creates air turbulence around the fan blade tips, a turbulence which sounds just like the air turbulence created by a bird flapping its wings (note that it is not the fan blades themselves making the noise, nor are the bird's wings actually creaking and directly making the noise, but rather it is the air turbulence itself that makes this noise). Thus, if you want to play the TRW really loudly, there's a second reason to put a chamber with sound absorbing material between the fan and your listening room. On the other hand, if you are like me, and are more impressed by quality of bass rather than sheer quantity of bass, then you might wind up playing the TRW at lower volume levels that don't engender this fluttering air turbulence sound, so you might have less need for that front chamber with sound absorbing material.

D. High Frequency Limits

       Subwoofers are of course not designed to play high frequencies, and all subwoofers have high frequency limitations. With conventional subwoofers, these high frequency limits are reached when the cone starts breaking up (the larger the cone is for better bass, the sooner it reaches its upper frequency limit due to cone breakup), and when the enclosure has an internal response peak that must be avoided by crossing over the subwoofer to the main loudspeaker at a lower frequency.
       The TRW likewise has its upper frequency limits. The TRW realizes its superb, unique, fundamentally opposite bass performance because it is intrinsically a DC device. But this means it has to work a bit to be an AC device, and this work gets hard as the AC frequency gets higher. Because the TRW employs such radically different technology than conventional subwoofers, the mechanisms which impose the TRW's high frequency limits are naturally very different.
       For example, the motor in the TRW, which changes the fan blade pitch, obviously has to do its changing work faster and faster as the frequency goes higher. At some high frequency point, the mass of the fan blades (and connecting mechanics) cannot be changed any faster. Similarly, at some high frequency point of back and forth changing, the fan blades will start undesirably flexing and breaking up (much like cone diaphragm breakup), instead of staying rigid to push the air. There are striking engineering tradeoffs here, since making the fan blades heavier to become more rigid would make it harder to change their pitch fast.
       For another example, consider the fan's rotational velocity. As we discussed above, the fact that the TRW's fan keeps rotating at the same velocity, when the bass frequency being reproduced goes lower, gives the TRW a great advantage at low bass frequencies over conventional subwoofer drivers, which cycle ever more slowly at lower bass frequencies, since they are constrained by multitasking to track the signal.
              But this same constant fan velocity is a disadvantage for the TRW at high frequencies. The TRW fan keeps rotating at the same velocity for higher frequencies, whereas conventional cone drivers in contrast cycle ever faster as the frequency goes higher (which is one reason cone driver technology is still excellent for middle and high frequencies). Eventually, at some high frequency point, the input signal is cycling so fast that it catches up to the TRW fan's rotation, and the fan blades appear, to the input signal, to be standing still in rotation. In other words, the signal is cycling so fast that the fan has not rotated significantly during a signal cycle, and thus the fan blades have not taken a significant bite of air during the signal cycle.
       Thus, even if the TRW's blade pitch motor could change blade pitch fast enough to keep up with this fast cycling input signal, thereby performing its modulation function perfectly, there would not be enough airflow, from the fan's now too slow rotation, for the perfect modulation of that negligible airflow to have any effect. You can't modulate airflow in a fast cycle when there's no significant airflow within that cycle to modulate. The TRW could easily raise its high frequency limit, from this phenomenon, by simply raising the fan's rotational speed, but this would increase the fan's background noise level, and force some other design tradeoffs.
       As the design of the first TRW subwoofer evolved, these design tradeoffs, which come into play at higher frequencies, were carefully balanced against one another, to produce the best high frequency extension possible, consistent with various other considerations. This first TRW subwoofer, the TRW-17, has response that measures flat up to 40 Hz, and declines above that, probably showing some of the stress that the mechanics of the fan system are beginning to experience above 40 Hz. The manufacturer recommends setting the crossover point at 20 to 25 Hz, for the signal fed into the amplifier controlling the blade pitch modulation, so that the energy fed in, up at 40 Hz, is somewhat reduced and does not stress the fan system. The manufacturer makes and sells an optional crossover and power amplifier for the blade pitch modulation, which has a built-in rolloff corner at 25 Hz, with an 18 dB per octave downward slope above that.
       If you mate the TRW-17 subwoofer directly with main loudspeakers that extend down to 25 Hz, then you'll have full spectral coverage. But, as we established by our deliberate sonic experiment above, even if your main loudspeakers extend only down to 40 Hz, the TRW still sounds great, and integrates seamlessly with the main loudspeakers, and the combined system seems to cover the whole spectrum, since the human brain unconsciously, automatically synthesizes the missing 15 Hz gap in the spectrum, so you never hear anything missing.
       Thus, you can confidently use the TRW-17 with main loudspeakers that extend only down to 40 Hz, as well as with main loudspeakers that might extend lower than 40 Hz. In the future, Eminent Technology might produce a second TRW subwoofer model that could be crossed over at 40 Hz, but it would have some other design tradeoffs (perhaps higher fan noise and lower maximum loudness level). In any case, our sonic experiment with the TRW-17 proves that you don't need to wait for this second TRW model, even if your present main loudspeakers only extend down to 40 Hz.

E. Cost

       The TRW-17 is actually less expensive, than some of the huge conventional subwoofer systems that the TRW utterly dusts in performance. But it still is a substantial investment. Many of the parts in the TRW fan are custom machined, from expensive materials, in exotic shapes that were carefully optimized during the TRW's long design process. For example, the manufacturer had to build and try hundreds of different shapes for the fan blades, to optimize performance, since he was working in uncharted waters of radical new subwoofer technology.
       This makes each TRW-17 very expensive to build, and that is reflected in its price of $12,900 for the basic fan unit. Ancillary components are modestly priced, at $350 for the motor controller, and $700 for the modulator power amplifier with crossover (you could instead use your own audio power amplifier, rated at 200 watts or more, and your own crossover).
       You also should factor in the cost of hiring a qualified carpenter or contractor, to build the baffle board across your attic (or basement or garage) space, to cut in the vent to your listening room, and to actually mount and wire in the fan (Eminent Technology offers its own design services for hire, as well).
       Furthermore, if you agree with us that stereo bass is important, for believably recreating the spatial imaging of a large alternative venue, you might want to consider installing a stereo pair of TRW-17 subwoofers.
       The TRW subwoofer might be expensive, but it's worth every penny in the thrills and chills it will give you from your film soundtracks, far beyond what even the best conventional subwoofers can achieve.
       The TRW is also worth every penny for the far more accurate, much higher quality bass it will give you from all your music recordings, as well as the sonic benefits it brings to musical transients at all frequencies throughout the spectrum (better dynamics, focused coherence, tactile solidity, etc.).
       And, for me, the TRW is worth every penny, above all, for the amazing, unexpected magic wand it waves over your whole system, making your whole system sound cleaner, clearer, and more natural, making the whole system sound less like artificial hi-fi and more like real live music, apparently by curing distortions that we have had to live with and have accepted lo these many years, but from which the TRW frees us at last, forever more.
       How much would you pay for an audio component that totally transformed and upgraded your whole system in these ways, giving your whole system an extreme sonic makeover?

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