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Old 19th July 2011, 05:00 PM   #11
GM is online now GM  United States
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Quote:
Originally Posted by awol66 View Post
Thanks for the reply GM

ok , so i wasnt planning on actually building the full horn , but i thought that was my starting point , and then

presumably i would either scale the whole thing down to a useable size (i assume shrinking it would raise the BW (bandwidth?)

and reduce the efficiency ) or i use the upper section of the horn to construct a metronome design
You’re welcome!

Yes, shrinking it will shift its gain bandwidth [BW] higher, but trying to scale it isn’t the best way to do it. Instead, you cut a section out of it.

A Metronome appears to be a mass loaded [ported] conical pipe, so for a driver with a lower Fs, Vas, Qts than the FR6.5C it would theoretically require a much longer, slower expanding [narrower] horn tuned lower, so may need to be folded to get the driver ‘up’ to an acceptable listening height.

To get a quick approximation to build or just use as a starting point in a modeling program, I used the BIB pipe horn calculator [comes to a pointed closed end Vs the Metronome’s truncated one] tuned to 36 Hz [i.e. inputted 72 Hz to get an Fs tuning]:

L = ~93.5” [path-length]
Sm = ~200”^2 mouth [base] area
h = ~46.75” [folded]
w = ~11.89”
d = ~16.81”
a-b-c = ~8.41”
zdriver = ~6.92” [This would put the driver center ~40” off the floor]
net Vb = ~5.41 ft^3

All dims are inside [i.d.] and approximate. Yours would be inverted from the attached layout with a bottom plate to close off the terminus.

Based on the net Vb, the vent would be at least a 3.5” diameter hole 0.75” long [baffle thickness], but the pipe’s loading will dictate a larger hole and may need to ideally be a bit longer to ensure a low vent mach if the driver can handle much power down low, though I don’t recommend longer than ~3”. This can be calculated/modeled ‘close enough’ using MJK’s MathCad, HornResp, AkAbak or similar software or do as I did and find it empirically. The vent can be on one of the sides near/at the bottom or even on the bottom if spaced up off the floor like a Metronome, then model an alternate driver location if it’s too high off the floor.

Some baffle step compensation may be required to make it nominally tonally flat in-room.

GM
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File Type: gif BIB Layout.gif (29.5 KB, 76 views)
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Last edited by GM; 19th July 2011 at 05:03 PM.
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Old 19th July 2011, 07:02 PM   #12
awol66 is offline awol66  England
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thank you very much GM , i appreciate you going to the trouble of working out those numbers for me .

the next step for me is to go away and think about how near i get to those dimensions with the materials

i have to hand , got some blockboard offcuts about 48" x 11 1/2 " and some old shop shelving , also

blockboard , that will do the sides . a cunning plan is begining to form

thanks again GM , i will post up picks as i go along now ive found out wot the upload limit is .
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Old 29th July 2011, 03:31 AM   #13
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You're welcome! Hope you can come up with an inexpensive, yet great performing system.

GM
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