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[Tom Anderson]

I'm planning to build a large front-loaded horn subwoofer in the crawlspace under my media room.  The mouth of the horn will be approximately 8'x4' and emerge from the floor (with a metal grate covering), aiming for about 20 Hz on the low end.  However, room reinforcement should provide good response down to around 10 Hz.  So, should I size my horn's enclosed back chamber for 20 Hz or 10 Hz?  Same with the length... I was planning on trying to size it to a quarter wavelength at 20 Hz, but should it really be 10 Hz?  Also, do you think a low-pile carpet covering the mouth would cause too much damping or other distortion?  Finally, where would be a good source to buy a good compression driver for this horn?

I don't have any experience building such a horn, but I've been researching it for about a year or more and I think I understand all of the basic concepts.  However, any other tips would be much appreciated.  I've progressed to the point in my home construction where I need to start laying this out so that I can build the media room floor above it.

The idea is to make it a straight horn except for a bend up to the floor level.  The dimensions I have to work with are a little over 19'x12'x32", which should leave plenty of room to build a straight horn or three (I plan to include two fullrange horns too).

Here's how I arrived at my mouth opening:

Quote:

20Hz F(o)    [344 m/s]^2     1        1   Mouth Area = [-------]   * ------ * ----- = 2.942775 m^2           [ 20 /s ]     4 * pi     8                    = 31.6758 ft^2                      =~ 8' x 4'

However, my room dimensions actually just about meet a 10 Hz mouth area by extending the horn into the entire space:

Quote:

10Hz F(o)    [344 m/s]^2     1        1   Mouth Area = [-------]   * ------ * ----- = 11.7711 m^2           [ 10 /s ]     4 * pi     8                    = 126.7031 ft^2                      =~ 10' x 12'8"

Here's my length rationale:

Quote:

1/4 wavelength @ 20 Hz = 14.1076 ft   Acoustic length = physical len. + (mouth dia. * 0.6)   Physical length = 14.1076 - (6.38 * 0.6) = 10.28 ft

Therefore, I would derive the appropriate M for this length, so that A(10.28) = 32ft, where:

Quote:

A(t)   [      x/x(o)        -x/x(o) ]^2   A(x) =  ----  * [ (1+M) * e     + (1-M) * e        ]       4   [                 ]      A(x) = area of expansion at distance x      A(t) = area of the throat      x    = distance from the throat      x(o) = distance to the mouth      M    = degree of hyperbolic expansion

If anyone smarter than I am notices anything wrong with any of these calculations, please let me know!!!

Am I missing anything important?

[Scott Waldy]

Tom Anderson wrote on Tue, 29 May 2007 12:21

I'm planning to build a large front-loaded horn subwoofer

Ummmm.... what exactly is a "Front-Loaded Horn"?

S.

[Duane Massey]

Can't help with the math, but I'd nominate one of the TAD woofers. Built a couple of 20hz horns in the 70's and used the 1601a's in them (5 in each horn).

The beauty of a well-designed front-loaded horn is that you don't need a driver with long excursion. In the horns I've been involved with over the years the speaker cone movement is very small. The TAD 1601's were/are very accurate and have outstanding transient response. The only negative is their relatively low power handling, but this should not be an issue in a properly designed horn.

I'm not certain you will achieve anything useable @ 10hz, but good luck anyway.

[Sam Venning]

Why are you considering building a horn? Horns are ideal for P.A. where high sensitivity and long throw are required but for Home cinema    For use in a home Cinema i'd recommend an Infinite Baffle subwoofer. Providing that it is practical you can achieve realistic 10hz from a properly designed and EQ'ed IB. I could go into more detail but instead just try the following site...

http://ibsubwoofers.proboards51.com/

[Tom Anderson]

The main reason for a horn is efficiency.  I'm building an off-grid house which will have limited electrical generating capacity.  A horn amplifies sound naturally (without electricity) by providing directionality and by coupling the driver to the room volume.  And the enclosed back chamber provides damping.  An infinite baffle radiating into open space is much less efficient.  Going through the effort and expense to build a horn should save me on photovoltaic panels, batteries, and other components over the years.  Once built, the horn will need no maintenance or further expense.  Moreover, it should sound better too.  And I may even be able to drive it without a seperate amplifier, cutting out further expense, electrical load, and potential noise source.

As far as 10 Hz, there is no overriding need to attain that level, but the math would seem to suggest that room reinforcement would effectively increase the mouth size to a point where 10 Hz is possible.  Given that this is the case, my question is whether I should design the horn length and back chamber to that frequency, or the 20 Hz that the floor-level mouth is capable of?  I'm concerned that if I design it for 20 Hz, it will be over-shortened and compromise the sound, perhaps causing an uncomfortable distortion or booming, perhaps in the sub-audible region.  Does that make sense?  I'm not completely clear on what the implications would be, which is why I was hoping some pros might have some insight.

[Tom Anderson]

Scott Waldy wrote on Tue, 29 May 2007 20:52

Ummmm.... what exactly is a "Front-Loaded Horn"?

A front-loaded horn is one where the front output of the driver is horn-loaded while the back output is in a closed box.  Let's see how good my ASCII art is:

  ______________        }  |      |_|   |      ,/|  |     /  |   |  _.~`  |  | BC | D |FC  =`,_  H |  |     \ _|   |    ',  |  |      | |   |      `\|  --------------        }

BC = back chamber
D = driver
FC = front chamber
H = horn

[Tom Anderson]

Thanks for the suggestion.  However, I looked up the TAD 1601s on the Pioneer website, and the Fs is 28 Hz and the Qts is 0.31.  I was advised that Fs/Qts for front horn loading should be >120, but for the TAD 1601, it is 90.  Therefore, I am hesitant to take your suggestion and design for this driver.

[Sam Venning]

I still can see the reason for such a sub for home use... In pro audio the advantage of a horn's efficiency is considerable but in a home theater environment why bother? Due to the lack of loading (aka no Box) an IB will not require much power at all. With some Eq'ing the end result for an IB will ultimately be better than a horn. Is this system intended for movies or music?

[Sam Venning]

Also how large is the room? How do you intend to power such a sub? Do you plan on buying something such as the Feedback Destroyer Pro DSP1124P to allow you to EQ the subwoofer? Another important consideration is what main/surrounds are you using and how are they powered? Also another thing... Cabinet construction is an art no a science and despite what the 'Maths' might say designing and integration a Horn in a living room will take much trial and error, I really strongly advise you consider a large low tuned ported box or an IB (For movies) or build a horn from speakerplans.com   if you only listen to music.    Even them i don't think anything successfully will come from a horn...

Enough said

[Tom Anderson]

As I understand it, an IB has no damping and therefore has a high excursion which reduces the efficiency.  With a horn, I can use a low Qts driver with a small x-max.  The horn couples the driver to the room volume instead of having to push lots of air to make noise.  Horns can reach 1W/1m SPLs exceeding 120 dB.  It also produces sound with less distortion because of the tighter cone control.