Originally Posted by Zen Traveler
Originally Posted by arnyk
...To expand on your example, building speakers with horns increases both their sensitivity and efficiency.
BTW horns or waveguides obtain higher efficiency through the intersection of a number of different technologies. The driver portion of horn loaded speakers can be far more efficient than the comparable element of a direct radiator. And additional measure of efficiency or sensitivity comes from the horn itself - it works like an acoustic transformer that matches the impedance of the driver to the air better than we see with direct radiators.
What about in the case of the Klipsch RF-7 that has been referred to? They have a horn tweeter but is mated to dual 10" LF drivers... Bill Fitzmaurice in the speaker forum is always mentioning they can't be as Sensitive as the Klipsch specs show and going back to the article that Wayne linked to earlier which states they also may not be as efficient as those number indicate...For the record, the RF-7s also have a power handling rating of 250 watt RMS 1000 watt Peak...In the old days there was a formula using those numbers to get how much of an amp was needed, but if I remember correctly the discrepancy between that and the Sensitivity numbers is pretty drastic on the RF-7s.
Cone drivers can be built that are around 100 dB/W sensitivity, but only with some compromises. The usual compromise relates to bass extension versus the size of the enclosure.
The first compromise is shifting from an overhung voice coil to an underhung voice coil to boost efficiency.
• Coil height is greater than the gap's height.
• This method keeps the number of windings within the magnetic field (or flux) constant over the coil's normal excursion range.
• Higher coil mass, sensitivity medium to high.‡
• Soft non-linearity as the coil exceeds limits.
• Gap's height is greater than the coil's height.
• This method keeps the magnetic flux that the coil experiences, constant over the coil's normal excursion range.
• Low coil mass, sensitivity low to medium.‡
• Hard non-linearity as the coil exceeds limits.
The second compromise is shifting from a high mass cone to a low mass cone. This has a side effect of increasing the enclosure size for a given bass extension.
High efficiency and high Xmax mean a stronger, wider magnetic field in the magnet assembly, which means more metal, most of it being that expensive magnet stuff.
If you check into the world of live sound you can find systems in the 100 dB/W range that probably meet spec, but the boxes are large for their bass extension. For example the EV ZX-5 is a nice robust speaker in a 27 x 16 x 17.5 enclosure which is an easy 2 cubic feet. Its -3 dB point is about 60 Hz, which is very high if this were a speaker for home audio of this size. http://www.electrovoice.com/product.php?id=252