High Sensitvity = good transient response ?

As as been alluded to, there is a tradeoff relationship between efficiency and bass extension (box size also factors in). It is possible to build a high efficiency system that truly goes deep, but the required box size is pretty big.

There also seems to be a correlation between efficiency and dynamic contrast.

Lately I've been playing around with vented bass systems in the range of 95-98 dB efficiency. By using an unusually large enclosure and tuning it very low, you can get transient response that rivals a fairly low Qtc sealed system. This approach is used with great success by Classic Audio Reproductions, and my own results haven't been too bad. Sean, you might want to try running a few simulations with, for example, a TAD 15" woofer in an 8 or 9 cubic foot vented box tuned to somewhere in the lower 20's. Compare that curve with a Qtc = .6 sealed box for the same driver, and you'll see the potential of this approach.

One of the reasons you typically see high-efficiency speakers using vented enclosures is that the driver parameters conducive to high efficiency (low Qts, medium to high Fs, and large Vas) really aren't suited for other enclosure types. Very low Qts woofers are suited for horn enclosures, but now we're talking refrigerator-size cabinets.

An example of the opposite extreme is the Carver Sunfire line of subwoofers. Carver has chosen to go with an extremely inefficient design (and equalization) in order to get deep bass in an incredibly small box. That's why the Sunfire has such an enormously powerful amplifier.

To the excellent responses above, I would just like to add this readdressing of the thread-head's main question: Yes, it is perfectly possible to achieve good transient response from a speaker design of average sensitivity. But I would also like to point out that the real-world transient response of a given speaker is going to depend a lot on the amplifier driving it, if your definition of "good" transient response means following the impulse accurately without excessive overshoot, undershoot, or overhang - and in the case of a lower-sensitivity speaker, this may require more from an amplifier. Some other things that enable a speaker to accurately track transient impulses include the drivers' ablility to resist compression through efficient heat dissipation, a non-resonant-in-its-operating-range (or well-damped) and rigid-yet-low-mass driver surface, and a wide linear (undistorted) travel range; a rigid and non-resonant mounting and cabinet (if there is a cabinet), as well as rigid coupling of the speaker to a non-resonant floor (or stand and floor); and a high-quality crossover construction that does not introduce its own delays, compressions, or resonances.

Duke, I've heard the CAR's ( or is it KAR's ?? ). I've commented here that i thought they sounded pretty good for a vented system. I do have to say that using very efficient drivers does give you a BIG advantage though when it comes to tuning vents. This has a lot to do with the gains achieved due to a more efficient motor design and the lower excursion rate that results from it.

If one uses a lower to average efficiency driver and uses vents, the driver will go from minimum to maximum excursion capabilities over the operating volume range. This is due to the increased drive levels that one must apply to achieve the amount of pistonic displacement necessary to achieve high volumes with a low efficiency driver. As such, this changes the amount of air and the associated turbulence that occurs in a vent over a pretty wide spl range. As such, most ports / boxes are a compromise that run most efficiently at one specific volume range. Going below or above that results in too little flow velocity or too much through the port. The sonic result of such a design is sluggish bass that lacks damping and / or increased port noise and "one note thud". Being able to find optimum performance on most ported designs typically means that you have a small window of listening levels. Anything below or above that results in slightly different tuning and the decline of performance.

Sealed systems never run into that specific problem as there is always a specific amount of "air spring" or "internal pressure" inside the box. As such, damping remains pretty consistent regardless of volume levels. Sealed boxes do have their problems though in that they do typically require far greater excursion out of the driver, which typically results in greater distortion. Obviously, the driver has to handle a lot of power to make long excursions too, so that is another drawback. Like anything else in audio, we pick our trade-offs and try to minimize the damage done.

Using a high efficiency driver with a port gives you the advantage of having a driver that will not have to "throw" or produce as much excursion to play as loudly. Since the driver will not have to produce as much excursion to perform over the range of average listening levels that most people use, one can more precisely tune the port for maximum flow velocity and damping. This results in a more linear performance over a wider range than what a typical port / low to mid efficiency driver combo would offer. There are times that the high efficiency / port combo may become "destabilized", but a good designer will strive to minimize that effect and / or place that range outside of what most users will run into.

The advantages of working with a low production / high quality manufacturer can come into play IF one really wants to fine tune their system. One can literally design a vented system to perform optimally if most of the variables ( drivers being used, listening room size, spl's required, type of music, etc.. ) are known. By factoring in how much power it will take to pressurize the room to achieve the average listening level and knowing the efficiency of the speakers, one can literally calculate how much air would be moving through a port and size it accordingly. This would maximize port velocity while minimizing such variables as port noise ( chuffing ), lack of damping, etc.. Obviously, this design would be even more specialized than the average "compromise" and may not work the greatest in other rooms or listening situations. Such are the perils of using a port and trying to be as "precise" as possible.

A newer port design on the market ( it's really been out for years but is just making it into commercial products ) is the "Aeroport" or similar products. This design is exactly what its' name indicates i.e. a port that is aerodynamically designed. Rather than using a straight tube, both ends are flared. The bend radius is reduced, which increases flow velocity through the port, minimizes port stalling, DRASTICALLY reduces "chuffing" or "port noise" at high volumes, etc... One can use a slightly smaller diameter port to maintain low spl flow velocity and damping and still not run into port compression ( stalling and chuffing ) until a very high spl was achieved. Since most people are not pushing their systems that hard, it should offer marked advantages and looks like a win / win situation if properly implimented. Sean
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Sean -

Thanks for your enlightening comments! I always enjoy your posts - and learn from them. Your explanation of the problems with low port velocity is new territory for me.

I haven't tried an Aeroport type port yet - but based on your recommendation I will soon. I had thought the only real advantage of flared ports was that you could get away with a smaller diameter port. Lately I've been experimenting with slotted ports running the full width and depth of the cabinet, but it's hard to adjust them once they're built.

...indeed thanks to Sean for such literate clarification. In just my listening experience I've only heard a few of medium efficiency i.e. 89...93 dB/W/m speakers that have a good bass.

I still do not understand people having fun from 100dB/W/m with SET amps. Bassically you can only listen to voice in this case.