Hello all, I have a friend (I swear!) who had a question for me I can't answer.
He wants to build a massive horn for an outdoor art exhibit. I did a bit of crunching, and for a full-wave horn, wavelength of 10 Hz came out to something crazy like 100ft. Design goal is 120 th at 10 Hz.
Anyway, building this isn't supposed to BE the project, it's just an add-on.
Then I got thinking about making it out of fabric. I could hang it from a streetlight at the halfway point. Hmmm.
How stiff would the fabric have to be? I could treat the fabric with some kind of rigidising compound, but the more flexible the better, for storage/transportation purposes.
Other ideas I considered:
-cardboard
-make in sections, each one larger than the last, slip over each other, like old-fashioned spyglass
So I guess the overall question is:
What is the minimum rigidity required to get a horn effect for bass frequencies?
Katou
He wants to build a massive horn for an outdoor art exhibit. I did a bit of crunching, and for a full-wave horn, wavelength of 10 Hz came out to something crazy like 100ft. Design goal is 120 th at 10 Hz.
Anyway, building this isn't supposed to BE the project, it's just an add-on.
Then I got thinking about making it out of fabric. I could hang it from a streetlight at the halfway point. Hmmm.
How stiff would the fabric have to be? I could treat the fabric with some kind of rigidising compound, but the more flexible the better, for storage/transportation purposes.
Other ideas I considered:
-cardboard
-make in sections, each one larger than the last, slip over each other, like old-fashioned spyglass
So I guess the overall question is:
What is the minimum rigidity required to get a horn effect for bass frequencies?
Katou
Katou,
1) 120 th? Level is measured in dB SPL (sound pressure level) at a specified distance. For large horns the measurement distance must be approximately the diameter of the horn to be valid.
2) Any movement of the fabric is a reduction in acoustical output. Ideally, one would construct the horn from several inches of reinforced concrete. Fabric, unless fiberglass with polyester or epoxy impregnated, will be far too "floppy" to guide low-frequency waves.
3) Cardboard works for low SPL levels, but is too absorbtive at high levels.
4) That concept uses far more material than a folded horn, as folded horns share walls, and can be easily braced between walls with less material needs than external bracing of "Dixie cups".
5) For a horn of the size you contemplate, well braced 1" thick Baltic Birch would be a good choice. Tapped horns work well with a path length of 1/4 wavelength, so around 28.5" of folding is all that is needed for a 10 Hz horn.
Look at DSL's "Matterhorn" for an even more ambitious version of what your friend plans.
https://www.youtube.com/watch?v=o36Kp6veJ6c
The Matterhorn is a 40-driver, 40 kW self-powered Tom Danley designed tapped horn subwoofer which can produce 105 dB of output at 250 meters, with a response -3 dB at 12 Hz.
That is the equivalent of about 153 dB at one meter, 123 dB at 31 meters.
120 dB at one meter at 10 Hz probably could be achieved with a large TH using 6 drivers powered with 6000 watts.
Art
Last edited:
If the output is only needed for a few seconds at a time over a narrow bandwidth, a series-tuned 6th order BP with both chambers tuned to the same frequency would do it.
You'd need something with a very strong cone and a lot of thermal mass to avoid melting. Pick a car audio driver with a big voicecoil. Use big ports and big chambers.
That said, if you wanted 120dB at 10Hz and some other frequencies, it'll be very difficult to do. One-note wonders are easy.
Chris
You'd need something with a very strong cone and a lot of thermal mass to avoid melting. Pick a car audio driver with a big voicecoil. Use big ports and big chambers.
That said, if you wanted 120dB at 10Hz and some other frequencies, it'll be very difficult to do. One-note wonders are easy.
Chris
I wish there was an SPL measurement plot. Maybe at 100m away instead of the usual 1m? 
So at equiv 1 watt input for reference, 2.83v drive each driver is getting 25mV ? That's like what 10 micrometers of displacement ? Must be very low distortion assuming the shipping container is well damped. Or is baseline measurement 40 watts of input with each cone getting usual 2.83v?
So at equiv 1 watt input for reference, 2.83v drive each driver is getting 25mV ? That's like what 10 micrometers of displacement ? Must be very low distortion assuming the shipping container is well damped. Or is baseline measurement 40 watts of input with each cone getting usual 2.83v?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.