Hi All
A few questions have come up as well as a need to explain how two small drivers can make a lot of low bass.
The mouth bubble is an unofficial term and an explanation of what a horn does is needed first.
One thing a horn does is provide what is like a transformer in the acoustic realm, it can (in a perfect world) load the driver with an acoustic load much larger than it would feel based on its radiating area. It is the increase in loading which allows a horn system to have a greater electro acoustic efficiency or as seen by the user, more sound per Watt of input.
This transformer effect also has a high pass filter effect set by the horn’s rate of expansion. For example, if one wants a horn to work to 30Hz, the rate of expansion if its cross section must be at a rate that doubles about every 24 inches. If you make a horn that doubles its area every 12 inches, you have a 60Hz horn instead or if you want 15Hz, it must expand more slowly, doubling its area every 48 inches.
The Horn effect actually extends outside the enclosure to a varying degree.
For example, if one places the spud enclosure in a room corner with the outlet at the floor/wall/wall junction, it is easy to see that the familiar “corner loading” where floor and walls act like a horn in that the expansion is confined to one eighth space.
From where ever the outlet is, keeping the rate of expansion in mind, one can plot out what the confined area is immediately outside the horn is and what ever that rate of expansion is, that is the frequency above which the physical surroundings continue the horns actual acoustic path and length.
To the degree you are within this zone, you are experiencing the low end more like the pressure zone experience in an automobile as opposed to the radiated pressure in free space. Obviously, this is an analogy, the thing is you have a greater pressure here for free which like Bill Murray said “Which is nice”.
This allows the sound where you are to be louder for a given amount of LFE in the bedroom or kitchen above.
Drivers.
We don’t use exotic drivers in any of our products, with a few exceptions, what is perceived as “exotic” is a marketing concept and not an engineering concept, we do use some made to spec drivers and variants of stock drivers but the way I see it, for the most part, you can’t get anything that performs better than the drivers that are designed to perform in a market where many folks can and do measure things.
Anyway, I suspect many of you would be surprised how great a horn system can sound when you get rid of a lot of the problems, our SH-50 will reproduce a square wave and can do it over a wide frequency range (over a decade), spanning both crossover points, possible because the design has none of the phase shift or driver to driver interference normal in multi-way speakers.
What I do in the design process is look at what is available and try a few of the best bets in the computer model. The sort of nearly last resort is sending out driver spec’s for a purpose built driver but sometimes that’s the only solution.
The best designs resolve one acoustical problem or another and that is what given them there strength as it were. Like the Synergy horns are a way to combine the outputs of multiple drivers over multiple ranges and have then “knit” together seamlessly acoustically such that what is radiated is as if it were one source in time and space.
In the case of the Spud driver, these are simply a very beefy 8 inch woofer with the right parameters and the new driver is a beefier one which we found / tried just recently and it is around a hundred bucks each in OEM quantity and no I don’t think we want to sell drivers.
You can’t just plunk any driver in a Tapped horn however as explained below.
A pretty common reaction to our Tapped horns subwoofer is surprise in how much sound what ever driver and box size can produce.
The Tapped horn allows the horn to be made much smaller than a conventionally driven horn.
In a normal bass horn, if one makes it too small (like a more convenient size) one finds the response droops off and exhibits a series of peaks and dips which are caused by the acoustic loading changing as a function of frequency (from being too small physically relative to the wavelength produced).
The thing I would say is the breakthrough in the Tapped horn, is that if you can get all the various driver and horn parameters just right (something which one couldn’t do by accident, these are much harder than any other type of enclosure and have to be designed by computer) then one has a nice flat response to a much lower frequency than the horn size would normally have allowed the normal way.
By driving the horn at two different points in exactly the right way with just the right driver parameters, one can arrange the geometry so that where the lowest peak would have been, only one face of the drivers feels the load which where the dip had been, now both sides of the driver feel the acoustic load. The result is a nice broad response where the internal reactance’s have annulled leaving a nice broad band device.
It is more complicated to design than a normal horn but when the horn must be as small as possible, the effort can be well worth it.
In effect, it has a variable driver of sorts, who’s properties change with frequency compensate for the opposite changes the horn passage presents.
If look at the size of the spud and look at Ivan’s half space TEF measure of one box (on edge, the minimum external horn bubble), you can imagine this is impossibly small for a “20Hz bass horn” doing it the old way.
It is not uncommon to find the effect of the Tapped horn adds 10dB in sensitivity to the driver relative to it as a direct radiator.
In this case, a convenient example, an increase in sensitivity of 10 dB is brought about by a significant increase in acoustic load and as one then expects, one finds that the driver excursion is reduced by about a factor of 3.3 for a given radiated SPL. Also the excursion limited SPL is raised by about 10 dB (again, relative to that driver as a direct radiator).
RMK, I am glad you bought a second unit, in the design of it I had assumed people would use two under or behind a couch.
Actually Ivan suggested the under couch part first (a location I hadn’t thought of) and that lead to the flat shape. One of the hard parts is scaling what we do commercially down in size for the home. How much is enough, how much overkill is just enough?
The TH-50’s and SH-96’s look tiny on my computer screen or in a big room like the photo below but much less so in a living room.
Anyway, I think you will be pleased with a pair, there is hardly anything more gratifying than making a first time guest uncontrollably jump to there feet in genuine wide eyed terror at what to them was certainly a car crashing into the house.
Best,
Tom Danley
