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RE: Design parameters I hold dear... {Lynn Olsen} ...

Posted by Paul Eizik (A) on September 3, 2014 at 13:26:38

In Reply to: RE: Design parameters I hold dear... {Lynn Olsen} ... posted by Cleantimestream on September 3, 2014 at 09:51:36:

C

Horn mids (& tweeters) are frequently paired with reflex/vented woofers like your Klipsch Fortes, and large (mountainous) impedance swings can be expected with woofers with this kind of bass enclosure, and impedance compensation networks can be used to address this.

The German magazine Stereoplay (9/81)* published a comparison test of several horn and horn hybrid speakers with graphs showing the minimum and maximum impedance. Two are good representatives of the horn mid/tweeter with a vented woofer: Altec 19 (Max. 49.2 Ohms, min. 8.4 Ohms) and the EV Sentry lll Series 2 (max 68.7 Ohms, min. 6.1 Ohms). Compare these figures with a kit built K Horn (max. 13.4 Ohms, min. 4 Ohms), and the Klipschorn KDBR (max 32.1 Ohms, min. 4.8 Ohms). Also tested was a kit built Fostex mid horn along with what looks like an Altec A5/A7 replica for the woofer, which is horn loaded on the front of the woofer and vented on the back (max. 56.3 Ohms, min. 5.5 Ohms), and the EXA Studiohorn 3/60 B EXA which has a backloaded bass horn with direct radiator from the front of the woofer (max. 24.7 Ohms, min. 5.7 Ohms).

From this data it seems clear that horn loaded woofers can have more consistent impedance variances than comparable speakers with vented box woofers. Now it's unclear (with my college German) whether any of the speakers had impedance compensation networks, which would have skewed the results, but looking at the the 1981 time frame it's unlikely that they did, and no schematics of the crossover networks were provided.

The reason for the generally better impedance behavior of the horn loaded woofers is that the acoustic resistance of the horn dominates the performance of the woofer, and it is relatively consistent across the pass band of the horn. The direct radiator woofer is mass controlled in order to give the driver nominal flat response instead of an increasing high frequency response, along with the acoustic resistance of the box and the vent at the lower end, which are all variable according to frequency. At least this is a rather simplified take on what is a rather complex subject.

Paul
*A scan of the magazine article on a german website (Selbstbau, 11/14 2003) was linked to on the Hi-E forum here some time ago, however it's since disappeared. If anyone's interested, perhaps a deep search with the Wayback can turn it up (I made a paper copy for reference). To my knowledge Stereophile has never fully tested a speaker with a horn loaded woofer, and viewing Ed. John Atkinson admitted skepticism towards horns it's unlikely they will, unless Art Dudley shows up with a K Horn.

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