DaleShirk Posted November 13, 2011 Share Posted November 13, 2011 These measurements document the differences between a rear mounted tweeter and a front mounted tweeter. The tweeter is a K77 factory rear mounted in a 1978 La Scala, vs. front mounted in the same. To front mount it I trimmed (beveled) the opening with a pocket knife so that the horn flare fit from the front, then attached the horn driver from behind. I did not recess the flange flush. I countersunk the holes in the horn flange slightly and used small flat-head screws typically used for kitchen cabinet hinges. Measurements were made using a TEF20 analyzer driving one channel of a Stewart PA-100 amplifier directly connected to the tweeter with no crossover components. Drive level was 1 Volt RMS sine sweep. Measurements were taken at 1 meter distance using an Earthworks TC30K. Assuming a nominal 8 Ohm impedence, one can get the 1Watt/1 Meter response by adding 9 dB. Quote Link to comment Share on other sites More sharing options...
DaleShirk Posted November 13, 2011 Author Share Posted November 13, 2011 This measurement overlays the on-axis response of front mount vs rear mount. Front mount is somewhat smoother. The rear mount does have an edge in output in the 5K-6K region, maximum about 3 dB at 5800 Hz. I believe this is the result of the extra air loading in front of the horn when it's rear mounted. Functionally this lowers the flare rate slightly, making it more efficient at the lower end of its response. This might be important when using a passive crossover since the actual crossover frequency and slope is the combination of electrical components and the acoustic passband of the driver. Quote Link to comment Share on other sites More sharing options...
DaleShirk Posted November 13, 2011 Author Share Posted November 13, 2011 This measurement really shows where the bones are buried in the rear mount scenario. It is the rear mounted tweeter with on-axis overlayed with 15, 30, 45, and 60 degrees off axis horizontally. Quote Link to comment Share on other sites More sharing options...
DaleShirk Posted November 13, 2011 Author Share Posted November 13, 2011 This measurement shows why I believe it's worth the minimal effort required to change the tweeter to front mount. It is the front mount horizontal off axis set. Note how much smoother it is. Also of note is the lower frequency ripple in the 45 and 60 degree sweeps. The ripple spacing of about 1000Hz suggests a path length difference around 1 foot. I suspect this is diffraction from the side edge of the cabinet, which is getting much closer to the direct path at these shallow angles. Conclusion: For my set of La Scalas, which will be actively crossed over and EQ'ed, I'm definitely going with the front mount. 1 Quote Link to comment Share on other sites More sharing options...
moray james Posted November 13, 2011 Share Posted November 13, 2011 great measurements. have you done any polar comparrisons on mounting your tweeters vertical Vs horizontal? Best regards Moray James. Quote Link to comment Share on other sites More sharing options...
DaleShirk Posted November 13, 2011 Author Share Posted November 13, 2011 I did take +/- 15 and 30 vertical front mount and rear mount in the La Scala, and the front mount was slightly smoother. However they both exhibited much more diffraction effect from the nearby top edge of the cabinet and the squawker opening. Mounting the tweeter vertically on a larger motorboard would be smoother. Generally, based on simple observation the horn would show wider dispersion across it's small dimension at lower frequencies and narrower at higher frequencies, while across the wider dimension it would ne more uniform low to high. Quote Link to comment Share on other sites More sharing options...
mustang guy Posted November 13, 2011 Share Posted November 13, 2011 I did take +/- 15 and 30 vertical front mount and rear mount in the La Scala, and the front mount was slightly smoother. However they both exhibited much more diffraction effect from the nearby top edge of the cabinet and the squawker opening. Mounting the tweeter vertically on a larger motorboard would be smoother. Generally, based on simple observation the horn would show wider dispersion across it's small dimension at lower frequencies and narrower at higher frequencies, while across the wider dimension it would ne more uniform low to high. Great experiment with some positive results. Are you going to test the squaker too? Also, the woofer slot is a straight edge, and perhaps it can be bevelled from inside to increase the efficiency of the horn. I am afraid of this one a little. Permanent damage for an experiment. This would be a nice idea for a LaScala III. edit: Are LaScala II's front mount tweeter horns? I have regular LaScala's Quote Link to comment Share on other sites More sharing options...
DizRotus Posted November 13, 2011 Share Posted November 13, 2011 Very interesting. PWK said that front mounting the horns. "doesn't make a dime's worth of difference." He was right in terms of the economics of manufacturing, which is not to say that doing it isn't worthwhile, as supported by your tests. It would've cost Klipsch, i.e., the consumer considerably more than a dime to have the horns front mounted. Quote Link to comment Share on other sites More sharing options...
DaleShirk Posted November 14, 2011 Author Share Posted November 14, 2011 Are you going to test the squaker too? Of course. As far as the baffle edge, since the angles are less severe and the shortest wavelength much longer, I might simply bevel the plywood edge a little with a router bit, perhaps a roundover if the screws are far enough away. Also, the woofer slot is a straight edge, and perhaps it can be bevelled from inside to increase the efficiency of the horn. Considering the wavelengths involved, it would make absolutely no difference. BTW, for reasons beyond my understanding, I love your avatar. Is there an explanation or a larger image? Quote Link to comment Share on other sites More sharing options...
DaleShirk Posted November 14, 2011 Author Share Posted November 14, 2011 PWK said that front mounting the horns. "doesn't make a dime's worth of difference." With great respect for the legend and legacy of PWK, after taking these measurements I'd have to say "Bullshit!" [bs][] Quote Link to comment Share on other sites More sharing options...
DizRotus Posted November 14, 2011 Share Posted November 14, 2011 Don't be so hard on PWK. I agree that what you've done makes a positive difference and I suspect PWK would haved agreed. I interpret his comments to mean that it doesn't make enough of a difference to make it commercially viable. Klipsch took rather pedestrian components and got the maximum out of them. DIYers can "upgrade" to get improved performance, but those "upgrades" were not deemed by PWK to be justified by the marketplace. Quote Link to comment Share on other sites More sharing options...
Guest " " Posted November 14, 2011 Share Posted November 14, 2011 Also, the woofer slot is a straight edge, and perhaps it can be bevelled from inside to increase the efficiency of the horn. I am afraid of this one a little. Permanent damage for an experiment. This would be a nice idea for a LaScala III. edit: Are LaScala II's front mount tweeter horns? I have regular LaScala's you might find the attached of interest Quote Link to comment Share on other sites More sharing options...
mustang guy Posted November 14, 2011 Share Posted November 14, 2011 SF, That is a Khorn. Is the LaScala the same? I have only seen the slot side. Here is a link to the original image in super size. Quote Link to comment Share on other sites More sharing options...
consistent Posted November 19, 2011 Share Posted November 19, 2011 This is good and it would also be good to measure the 77 on the traditional horizontal plane v a vertical plane. I understand that in the 'vertical' it perfoms better (not as harsh). To my ears vertical is better Cheers Quote Link to comment Share on other sites More sharing options...
DaleShirk Posted November 19, 2011 Author Share Posted November 19, 2011 This is good and it would also be good to measure the 77 on the traditional horizontal plane v a vertical plane. I understand that in the 'vertical' it perfoms better (not as harsh). To my ears vertical is better It is an interesting question and since I'm currently on a crusade to learn more about diffraction it would be worthwhile. My preliminary guess is that the vertical might sound good when it's in the middle of a large baffle such as a Cornwall, not because it's vertical, but simply because it's well away from those edges. I did measure vertical on the LaScala, where it's on that narrow strip of wood with the top edge and the squawker edge each only an inch away and there is plenty of ripple. I also want to look into putting a foam covering on the face around the horns to reduce diffraction. Dale Quote Link to comment Share on other sites More sharing options...
djk Posted November 20, 2011 Share Posted November 20, 2011 "To my ears vertical is better" When rear mounted in a 3/4" baffle I would agree. Front mounting changes things. Quote Link to comment Share on other sites More sharing options...
consistent Posted November 21, 2011 Share Posted November 21, 2011 Good point. I would like to see results of the 77 measured horizontal, vertical, with and without baffle, front and rear mount. I think it may open up a whole (or is that hole) new level of discussion on its ability or lack of! Thanks Quote Link to comment Share on other sites More sharing options...
Chris A Posted November 21, 2011 Share Posted November 21, 2011 With great respect for the legend and legacy of PWK, after taking these measurements I'd have to say "Bullshit!" I'd have to agree with your assessment...I've never understood why the simple redesign of the Klipsch top hats to facilitate front-mounted tweeters was so difficult. In fact, it would have made the design even better since many folks would like to replace or refresh their Klipsch Heritage grills but find the task daunting due to rows and rows of staples...there is a better design that is available, and that faciliates grill maintenance. But that's another story...Dale, your particular speakers are Industrial La Scalas, and I assume that you are thinking about using these in commercial environment or at least in an evnironment where you might tend to move them from place-to-pace periodicially. However, if you were to install these in a home environment where you would tend to sit on-axis and fairly close to the speakers, I'd highly recommend placing the tweeters in a small L-shaped baffle and time-align them to the midrange drivers (i.e., aligning the front of their magnet structures with respect to the front of their cabinets. I believe what your will hear will astound you, and will also preserve the wide dispersion of the tweeter's polars horizontally and vertically. I also second your vote to tri-amp your La Scalas - and this will amend my suggestion to move your tweeters back, since you can time-delay the tweeters with respect to the midrange - this is the critical time-alignment parameter, and one the requires the most care in achieving alignment (i.e., to within 1/8th of a wavelength at the crossover frequency - about 20 microseconds to align well at 6000 Hz, which is +/-1/4 inch in physical time alignment). Regards, Chris Quote Link to comment Share on other sites More sharing options...
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