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In Reply to: On apparent discrepancies between mouth size and reported response of bass horns posted by pk on January 11, 2003 at 03:04:56:
Hi Peter,Well, I don’t know about the respected bit, but I’ll try and explain where I’m coming from with my horn designs.
If you look at the 1850 horn on the speakerplans site you will see that the response does drop off like a rock below its cutoff point. The horn length is 1.59 meters or 62.59 inches. This equates to a full wavelength frequency of 216.77 Hz. So divide by 4 to get the quarter wavelength figure of 54.19 Hz. Again if you look at the plot you will see it fall sharply below 54 Hz, I also quote the cutoff frequency and state that the design is good down to 54 Hz. I also assume that at least 2 horns will be used in a stack and preferably 4. So I can’t see the problem or see why you could have a problem with this. If I quote that 2 horns have a cutoff of 30 Hz then I would be lying, but I don’t do this.
Concerning the HD 15 horn, I do quote that the design will operate down to 52 Hz which for one cabinet would be impossible, but it does state that it must be used in a stack of at least 6 cabinets. When used as such the combined horn mouth and extra horn length that can be achieved by combining horns will give a new cutoff of 52 Hz. Also note that I still say you should use a reflex or bandpass type cabinet below these horns to get a fully extended bass response below 50 Hz.
The 186 horn is meant to be used singly and with higher Qts drivers than a horn normally requires. This and an over sized rear chamber can give you a shallower rolloff below cutoff. This is depicted in the response, but you will also see that the cutoff has been raised in frequency from what the horn length dictates it should be. This is again due to the rear chamber being too large, a smaller rear chamber with a lower Qts driver will have a lower cutoff (or a cutoff that behaves predictably and is true to the 4/WL rule) and an over sized rear chamber will tend to shift the cutoff up in frequency and give a shallower rolloff, especially with drivers with a higher than recommended Qts and low BL. While not ideal I do prefer listening to horns with drivers with a higher than required Qts, good for the lounge but not good if you need a quick transient response and loads of SPL like at a concert.
So I hope that has explained the figures behind the designs. I always assume that my designs will be used in multiples. This is large scale PA for big venues and no one is going to use just one small 15” horn in there PA system, well I hope not. So my mouth areas are calculated assuming that a minimum of 2 1850 horns will be used together and 6 of the HD 15 horns will be used in one stack.
After liasing with David McBean, his wonderful Horn Response program (ver 5.60 and above) now lets you calculate the response for multiple horns. While not usable for everyone it’s really helpful in the PA world where multiples are the norm.
I don’t know what other manufacturers like LaScala and the others you mentioned are quoting for their horns, but all I can say is that I’ve always been honest with my figures. You can go to the speakerplans site and make the designs and it will cost you nothing, so I see no reason in fabricating or doctoring the results when I stand to gain nothing from the designs on that site. But if were talking about Big Mouth designs then the company has a 12” horn with a 1 foot horn path and a 8 inch square mouth that goes flat down to 16 Hz and will produce 16 Hz at 179 dB, honest. The good news is that they only cost $4,000,000 each.
Best wishes and happy resonating,
Rog Mogale.
Follow Ups:
Hi Rog,Thanks a lot for your reply, I am glad that you responded! Please, do not be offended by my questions. I have no reason to believe that you should be dishonest with your figues. My knowledge regarding horns is limited, and I really just want to learn about them!
From your post I learn that horn LENGTH is at least as important as mouth size when considering cutoff. I actually thought it was the other way around....
And now to a question:
In a previous post regarding the 1850 Folded Horn you clarified, that for home use you prefered the 186 Horn to the 1850 Folded Horn, since the 1850 Folded Horn sounded to 'hard' for hifi.However, in that previous post you also mentioned, that if the 1850 Folded Horn were extended, it would take away the hardness and make it much more suitable for home use.
I play around with the idea of making such an extended version of the 1850 Folded Horn (that is, with the PD1850 driver) for home use. I would like to go down to 40 Hz. Please correct me if I am wrong, but as far as I understand it, this would require a horn length of 2.15 meters or 546 inches [(344/40)/4].
Could you give me any clue about how much larger the horn mouth should be in order to obtain such a result when only using one cabinet per channel but placed on the floor against the back wall (not corner positioning)?
I thank you a lot in anticipation!
Regards
Peter K.
Hi Peter,I will contact you off air as it were, as I think I might have something for you.
Best wishes,
.
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