Horn mouth dimension.. how low will it go?

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I guess it depends on what you are asking
is it a bass horn, or a fronthorn waveguide
with a bass horn, my guess is 35hz
for a fronthorn, maybe 100-200hz
and I would expect it to be slightly "less efficient", because of the dimensions and vertical design
but its guessing
sitting on the floor away from the walls is a 2PI loading.
Double the area to give the 4Pi equivalent.

Now find the circle with the same area as that 4Pi equivalent.
Now find the circumference of that equal area circle.
That is the limit for maximum wavelength that the horn loads the driver.
Any lower in frequency and ripples form, due to poor coupling.
The ripples in the frequency response get worse the lower you ask the speaker to reproduce.

The horn is considered truncated when it is shorter than a half wavelength of the lowest frequency you want to reproduce.
Truncations down to 60% are often adopted in commercial speakers. This seems to be an acceptable compromise.

The Flare rate of the horn also determines the lowest frequency that it can reproduce cleanly.

This is all based on the old fashioned and somewhat empirical rules for horn design.
Hornresp would do a better job of analysing your horn.

If you want to get a feel for what it might do, then using:
2PiArea = 1.8 * 0.6 = 1.08m^2
4PiArea = 2 * 1.08 = 2.16m^2
Diameter of circle for equal area = sqrt(4Piarea *4 / Pi) = 1.65m
Circumference of equal area circle = Sqrt(4*Pi*4Piarea) = 5.2m
Lowest frequency for full horn loading = speed of sound / circumference =340/sqrt(4*Pi*4piarea) = 340 / [4 * 3.14159 * 2.16] = 65Hz.
Minimum length for full horn loading = 5.2/2 = 2.6m
Thanks Andrew, thats an awesome reply!
The minimum length for the front horn loading is going to be hard to implement...! don't really want the speaker cabs to be more than a metre deep- how do the tannoy westminster's work?
I could reduce the mouth area by say, a quarter( 135 x 45 cm) - too tired to do the maths(1.30am) but if its around 100 hz that will be fine.
there are many pics of the Westminster showing the folding. I think one of Tannoy's brochures show the internals.
The Westminster is truncated.
From memory the horn is ~3.5m long. (if this is correct it could indicated a truncated 6m long horn)
The mouth area, as a result of truncation is quite small.
Presumably Tannoy experimented with many truncation ratios to come up with a commercially viable outcome. For £15,000 you would expect good bass performance.

the Westminster is a 3horn design.
The treble is front loaded compression driver. 1kHz to 20kHz
The mid is front loaded cone. 250Hz to 1kHz
The bass is back loaded cone folded. 18Hz to 250Hz
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I think I should explain the design- I want to build a floorstander from corian at around 170-200 cm high using a vertical rectangular front horn- its a 2 way with a small full range unit / front horn and a 15-18" pro speaker firing downwards(BR) at the base of the cab into an omni type flare. The BR volume utilises the spare space in the cabinet around the front horn and the cabinet shape will a sonus faber type shape.

I may be way over my head with this
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