Richard, everyone above makes good points.
There seem to be three advantages to seeking high sensitivity during the design of any speaker system:
--to use lower-powered amplifiers (= a simpler amp, which is often more linear/more transparent)
--to reduce voice coil temperature swings (sensitivity decreases as the voice coil's impedance climbs with temp, a form of "power compression")
--to be able to play it more loudly (assuming it can handle the power AND has the excursion).
Personally, I think #2 is the main "hi-fi" reason a speaker designer should seek the highest sensitivity raw driver for a given task.
There are several disadvantages/obstacles to increasing a driver's sensitivity:
--no matter how large/powerful the magnet, we can only get a finite amount of magnetization into the pole piece, located inside the voice coil.
--about half the moving mass of a cone or dome driver is in the voice coil. All the high-sensitivity woofers and mids have very short voice coils, to reduce their moving mass. For a woofer, that shorter coil reduces the maximum stroke available, which limits loudness and low bass excursion (think Lowther). A mid driver that has a short stroke forces the use of a higher-order crossover to keep it from running out of stroke, and that crossover always screws up the time coherence.
--the lower the moving mass, the more compliant that driver's suspension must be, to keep the driver's frequency response flat before a crossover is even applied. And to let the driver go just as low as its lower-sensitivity competition. Except there's a limit as to how compliant the suspension can be manufactured, for consistency from unit to unit and to keep the voice coil centered.
Most high-sensitivity speakers do not go below 45Hz, flat, because of their suspensions' intentional lack of compliance- done to limit the stroke of their short voice coils. They do not have to be "boomy" however- that's the result of poor engineering somewhere in the design process, not sensitivity.
If multiple drivers are used to increase sensitivity, there are many problems:
-little chance of time coherence.
-room positioning becomes critical, as off-axis, the tone balance and time-coherence (if any) are unpredictable.
-more reflections off the larger cabinet face.
The highest sensitivity, high power-handling drivers must be carefully assembled. One of the biggest decreases in sensitivity comes from widening the voice-coil gap for sloppier, faster assembly and out-of-round voice coils. This is why there are a lot of 83-87dB designs out there.
The best woofers, made by larger manufacturers, with high sensitivity and low distortion are from PHL Audio and Volt Loudspeakers. However none of those are "ideal"- some are not flat in tone balance, and others do not respond to small signals.
There is the 100dB-sensitive Stage Accompany 6" ribbon tweeter, if one could afford it (>$600 each). Regardless, finding a mid driver to mate to it seems impossible, without loading the SA down with resistors to turn it down (reducing its sensitivity).
In a complete speaker system that uses only a single driver per frequency range, the upper limit to sensitivity is now 90-91dB, if you are to have any useful bass response, a cabinet that's not too large, and wide dispersion.
Sealed-box woofer boxes are smaller than ported woofer boxes, for the same low-bass extension. To go that low, they have longer voice coils- they need the stroke since there's no port. Which means more moving mass, thus less sensitivity.
Sealed-box woofers have softer suspensions than ported woofers, so the sealed box itself can become the primary "spring"- a more perfect suspension than any man-made one, for lower distortion and better response to very soft signals.
Sealed box designs were necessitated by the onset of stereo reproduction, as no one really wanted TWO big Altec or E-V speakers in the living room back in the '50's, or could afford two. When those sealed boxes started selling in big numbers, that was one impetus for amplifier companies to increase power outputs. The other was "marketing".
Yes Richard, you did open a Pandora's box. Shame on you.
Best regards,
Roy
Green Mountain Audio
There seem to be three advantages to seeking high sensitivity during the design of any speaker system:
--to use lower-powered amplifiers (= a simpler amp, which is often more linear/more transparent)
--to reduce voice coil temperature swings (sensitivity decreases as the voice coil's impedance climbs with temp, a form of "power compression")
--to be able to play it more loudly (assuming it can handle the power AND has the excursion).
Personally, I think #2 is the main "hi-fi" reason a speaker designer should seek the highest sensitivity raw driver for a given task.
There are several disadvantages/obstacles to increasing a driver's sensitivity:
--no matter how large/powerful the magnet, we can only get a finite amount of magnetization into the pole piece, located inside the voice coil.
--about half the moving mass of a cone or dome driver is in the voice coil. All the high-sensitivity woofers and mids have very short voice coils, to reduce their moving mass. For a woofer, that shorter coil reduces the maximum stroke available, which limits loudness and low bass excursion (think Lowther). A mid driver that has a short stroke forces the use of a higher-order crossover to keep it from running out of stroke, and that crossover always screws up the time coherence.
--the lower the moving mass, the more compliant that driver's suspension must be, to keep the driver's frequency response flat before a crossover is even applied. And to let the driver go just as low as its lower-sensitivity competition. Except there's a limit as to how compliant the suspension can be manufactured, for consistency from unit to unit and to keep the voice coil centered.
Most high-sensitivity speakers do not go below 45Hz, flat, because of their suspensions' intentional lack of compliance- done to limit the stroke of their short voice coils. They do not have to be "boomy" however- that's the result of poor engineering somewhere in the design process, not sensitivity.
If multiple drivers are used to increase sensitivity, there are many problems:
-little chance of time coherence.
-room positioning becomes critical, as off-axis, the tone balance and time-coherence (if any) are unpredictable.
-more reflections off the larger cabinet face.
The highest sensitivity, high power-handling drivers must be carefully assembled. One of the biggest decreases in sensitivity comes from widening the voice-coil gap for sloppier, faster assembly and out-of-round voice coils. This is why there are a lot of 83-87dB designs out there.
The best woofers, made by larger manufacturers, with high sensitivity and low distortion are from PHL Audio and Volt Loudspeakers. However none of those are "ideal"- some are not flat in tone balance, and others do not respond to small signals.
There is the 100dB-sensitive Stage Accompany 6" ribbon tweeter, if one could afford it (>$600 each). Regardless, finding a mid driver to mate to it seems impossible, without loading the SA down with resistors to turn it down (reducing its sensitivity).
In a complete speaker system that uses only a single driver per frequency range, the upper limit to sensitivity is now 90-91dB, if you are to have any useful bass response, a cabinet that's not too large, and wide dispersion.
Sealed-box woofer boxes are smaller than ported woofer boxes, for the same low-bass extension. To go that low, they have longer voice coils- they need the stroke since there's no port. Which means more moving mass, thus less sensitivity.
Sealed-box woofers have softer suspensions than ported woofers, so the sealed box itself can become the primary "spring"- a more perfect suspension than any man-made one, for lower distortion and better response to very soft signals.
Sealed box designs were necessitated by the onset of stereo reproduction, as no one really wanted TWO big Altec or E-V speakers in the living room back in the '50's, or could afford two. When those sealed boxes started selling in big numbers, that was one impetus for amplifier companies to increase power outputs. The other was "marketing".
Yes Richard, you did open a Pandora's box. Shame on you.
Best regards,
Roy
Green Mountain Audio