High Efficiency Speaker Asylum: Re: We disagree on a couple of points by tomservo Need speakers that can rock with just one watt? You found da place.

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Posted by tomservo (M ) on February 19, 2006 at 20:11:02

In Reply to: We disagree on a couple of points posted by EGeddes on February 19, 2006 at 16:49:34:

Hi Earl

I would disagree also, THD or the lack of, is an indicator of linearity or is that no longer desirable?
The fact is, even with the same driver in each, a proper bass horn can have a fraction of the distortion at the same level as the direct radiator.
While THD is not a direct comparison, when one hears a system with say 1/10 the distortion, there is a difference you hear.

For a cone driver in a horn, non linearity is primarily motion related (see BT-7 example below). The level, same driver, when the motion is loaded by the horn to say one sixth, will also produce less distortion.
A compression driver like you mention (worked with) is a very different animal, much more loading on the moving system, much wider acoustic bandwidth.

Don’s paper (s) were the case at the time (he worked at EV then I think).
He had a number of horn design papers actually and many have used and still use his design approach.
On this forum, some years back there were debates on Marshal Leach’s math vs that traditional one. If you want maximum acoustic power and a proper (designed to) bandwidth, then Marshal’s math gives much better results.
By better, for one thing I mean it includes a proper acoustic roll off in the response.
As with a Bandpass box, that low pass filter is a good thing for the crud all drivers produce above the actual input signals.
I have been designing low frequency horns this way since the first Servodrive in the 80’s.
I have never worked for a big company, the products sold primarily as a result of side by side comparison and measurement in the industrial areas.
Over at the Serovdrive (A company I used to design for) web site, look up the BT-7.
There are measurement’s there for the distortion at 40 Volts RMS.
Granted using a commuatated motor means no motor Xmax, the low distortion levels could still only result if the air path were not a source of distortion here. The driver is the main source of non linearity under normal conditions.
So model up a direct radiator /vented box or BP system, figure out how big a system which can produce that 142 dB continuously with low 2nd and 3rd distortion like that has to be.
Include power compression, be conservative as this was actually measured with a slow swept sine at 40V RMS.
Figure out how much it weighs, figure out how many cubic feet of truck space it takes up, how much power it takes, how much it weighs and you will see.
You will see why say Clair Bros audio used them for many tours like U-2, Garth Brooks, MJ and so on, when they had an endless supply of “free” recones from the previous (big) company from California.
They replaced 32 double 18”s in the same sized boxes with (usually) 12 BT-7s, 6 per side. They went from breaking 12 to 20 drivers per show to going 51 shows before a BT-7 needed service. They got rid of half the amp racks and bass distro too.
This was the first time they ever used a speaker product in there rig they didn’t make themselves. Trust me, I didn’t bribe those guys to use them.

Keep in mind I designed those many years ago now, in fact they were part of two of those AES presentations from long ago. A REEEALY powerful, rotary motor driven speaker with forced air cooling that eliminated power compression, very while well attended and the Q&A went on for another 45 min, it apparently wasn’t interesting enough subject to consider printing in the journal.

I recently found a way to make a “smaller horn” work better too, four of our TH-115’s (using the Tapped horn) are Equal in sensitivity to the 4 BT-7, but are half the cabinet volume, half the weight ( miss the BT-7 low cut off by a few Hz) and about half the cost.

Bottom line, I’m not saying the bass horn is the answer to most low frequency needs, just that the grim level of performance you describe is relatively easy to surpass, at least in the sizes I deal with.
Don’t take the lack of lf horn products and AES papers from any of the large companies as an indicator of what works. It is perhaps more due to the big driver companies not making suitable drivers for high power LF horn loading. Like back when the Lab sub was starting, Eminance ended up making some to my target, a massive strong driver and totally unlike what Don’s math calls for..

We will have to disagree on this horn thing, I’ve simply built, listened to and measured far too many of both kinds to agree.
Do a study? Ha that’s rich, unless it becomes an advantage for the company for me to rejoin AES, I don’t feel the need to go through all that effort pouring the details of my work out only to the other manufacturers, although it was kind of fun.
Best,

Tom

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