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Old 11th February 2010, 09:29 PM   #1
max axe is offline max axe  United States
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Default small horn + waveguide PA?

I am looking for suggestions on how to design a unique loudspeaker.

I need a unique portable sound reinforcement speaker based on a horn of around 8 inches in diameter. The device would naturally be limited in frequency range. The question is how to extend the low end below the horn cutoff. But theres more; This thing must look like some kind of novel musical instrument.

I am a musician, not a speaker buff. I was hoping that I could mine the depths to which you folks have gone into this field and come up with a few gems to get me headed in the right direction. I am new to this forum. Assume I dont know anything about speaker design.

Recently I have been playing a midi wind instrument and it is very disconcerting to not have local sound! A midi wind instrument only sends data to a synth module of some kind and using typical floor monitors on stage to hear yourself is not good! (I am using sampled sounds of trumpet, trombone, clarinet, oboe and some strings, triggered by a Yamaha wx5 midi wind instrument.)

The goal is to create a portable monitor attached to the midi wind instrument in the form of a horn. A low mid-range horn. This serves an important second function of looking as though horn like sounds should be coming from this otherwise boring looking instrument.

An audience responds to visual ques. A part of it can pass over and rest on my shoulder. Think trombone

Accurate, or should I say flat response is not critical. Coloration might even improve the sound! The audio source is not so pure and great that
it couldnt even benefit from some real world resonance, especially for horns; the speaker is now a part of the sound of the instrument, not an audiophile reproduction monitor. Since It will be carried it cant weigh too much or be too bulky. The desired frequency range is 250 Hz to say 2500 Hz preferably higher. Of course a broader range would be better.

The problem as I see it is that the low cutoff of a horn for 250 hz is about 17 inches diam. Too big!

A trombone mouth diameter of 8 inches cuts off at 540 Hz if I understand it. (How does a trombone play down to below 100 Hz?) I would like to extend the lower octave of an 8 inch horn by some tricky means that doesnt look too strange on my shoulder. A tube or two of appropriate length coming from the rear of the driver may function like the bose cannon concept, reinforcing the lower octave in waveguide fashion.

So thats my first concept; a TL or wave guide rear coming off a trombone sized horn. Another option is to use a larger driver in the mouth of the horn, say a 5 inch full range, and let the now sealed horn act as a labyrinth. I have considered even using a defunct trombone for the horn as well as some other parts. Note how small the diameter would be if I used the
curved section of a trombone tube to bring the TL back around. If anyone wants to eyeball the flare curvature of a trombone to guesstimate its sonic applicability, I can post a graphic.

Yet another option is to handle whatever frequency range the held monitor cant produce with a nearby free standing unit. I am thinking of a trombone case sized tube that would double as a case for the instrument. It would be desirable to have a range say from 80 hz to the low end of the horn, which would be around 600 Hz for the 8 inch dia I guess. It would be preferable to get the formants locally though. The ideal would be to get enough range from a single driver so that I didnt have to mess with crossovers.

So I need geometry suggestions and driver suggestions. On drivers, I was hoping to get away with less than a 4 inch driver. This thing will be slung over my shoulder...it has to look pretty slick. In fact I already ordered two Peerless 830970 2" Full Range 4 Ohm. That brings up another design option which is just to front fire both without any horn loading and try to get the tubular structure to act well enough as a labyrinth or whatever. I could imagine a cavity but no larger than an ostrich egg...

Questions that occur to me are;
Has anyone had success tying a TL to the rear of a horn?
How much can wave guides be narrowed? What benefit is flaring the end of a transmission line if its mouth is smaller than the horn cut-off? I mean to say is a larger diameter port much more efficient than a small one, given that both have a circumference much lower that the wavelength in question. Will I need a high pass filter to save the little speaker? There are so many sub forums, how can I reach interested
enthusiasts with this topic?

I think that the sound pressure levels need to be pretty high. I stand next to a full drum kit and blaring monitors. It would be ideal to achieve a spl the same as a real trumpet.

Thank you
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Old 12th February 2010, 01:17 PM   #2
bjorno is offline bjorno  Sweden
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Quote:
Originally Posted by max axe;2082214...I am looking for suggestions on how to design a unique loudspeaker...[/quote

...I need a unique portable sound reinforcement speaker based on a horn of around 8 inches in diameter. The device would naturally be limited in frequency range. The question is how to extend the low end below the horn cutoff...But theres more; This thing must look like some kind of novel musical instrument...
Hi max axe,

Here is a quick HR simulation that might be a solution for the lower end of the FR:

b
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Old 12th February 2010, 09:08 PM   #3
max axe is offline max axe  United States
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bjorno

Thank you. Thats very cool. Its interesting that a trombone with its very narrow pipes can hit 80 Hz at 130 DB. Maybe flapping lips are the new driver paradigm.

This geometry would be great for a floor standing bass augmentation. If I could get a good seal I could open it up like an instrument case and carry the wind instrument inside. What is your thought to specify rubber on the long section?

My original query, naive as it may be, is what would the frequency response be if you replaced the closed section with a horn of mouth diameter 20 cm driven by a rather small full range, like Peerless 830970 2", (that would project to the left if we are modifying your diagram) and then used a scaled down cross section of your TL to the right. Aiming for perhaps 200 Hz. This would be the Bose concept of rear phase addition, except the shorter of the TL legs is substituted with a horn. The idea is that I want to attach it to the instrument and carry it whilst playing....

Im curious if you compared the response of your bass design using a straight 8 inch tube of the same length....

thank you
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Old 13th February 2010, 04:58 AM   #4
max axe is offline max axe  United States
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I have recently tried using the pyle 40watt megaphone. I was too curious, since it had the approximate look and size of this concept. It wasnt what one would hope, although it did give an interesting immediate presence to the horn sounds. Im trying to figure out if the irritating sound is simply from the band width or irregularities in freq response, or something else. Its re-entrant. Brash is an understatement
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Old 13th February 2010, 07:58 PM   #5
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a straight horn divided into three pieces then each piece cut in half length wise would be very portable and can be attached to you speaker cab. the longer the horn the lower the frequency.
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Old 13th February 2010, 09:04 PM   #6
max axe is offline max axe  United States
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Yes, breaking it down into pieces.....

You have to have intimate contact with the instrument youre playing on stage. Its a stressful situation, loud noises coming from everywhere. Only in concert halls do you have anything like good acoustics to play in, even then, you rely on stage monitors.

The core idea here is to have something that is actually a part of the instrument, which by itself makes no sound at all, its a synth. I could use a local monitor, but better would be something that also increases the visual impact. The instrument in question is a Yamaha wx5 and the audience is confused that big brass sounds can come from it.

I think to myself, maybe I can get an old trombone bell and fit it with a driver and now I can hear myself and it looks more interesting. But then I look at the mouth size, and mysteriously, even though a tombone plays loudly down to 80 Hz, unfortunately, a speaker of same dimensions cant. The cut-off is something like 550 hz. So how can you get an extra octave at the bottom, ragged or not? But maybe bjornos design, or something just a bit larger, that I could build as a case for the instrument and its horn...
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File Type: jpg visualization1.jpg (23.2 KB, 477 views)
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Old 13th February 2010, 09:39 PM   #7
max axe is offline max axe  United States
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So I wondered, in happy ignorance, if bringing the rear wave around, in the manner of the (apparently reviled) bose acoustimass could extend the low cut-off, maybe a ragged octave, say 250 Hz. Since this is close to the lower notes of the trumpet and clarinet, it would be good enough perhaps for the purpose, in as much as the full range sound is also going into the main PA system from the synth engine.

But now Im inclined to mount a 5 or 6 inch in the mouth of the bell, and use the rear as a labyrinth. Sad to let go of the horn sensitivity and quality though.

Its amazing that you cant achieve the range and SPL with a 2 lb magnet and 50 watts as you can a pair of lips and a light tube of brass.
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Old 13th February 2010, 10:40 PM   #8
bjorno is offline bjorno  Sweden
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Quote:
Originally Posted by max axe View Post
bjorno

Thank you. Thats very cool. Its interesting that a trombone with its very narrow pipes can hit 80 Hz at 130 DB. Maybe flapping lips are the new driver paradigm.

This geometry would be great for a floor standing bass augmentation. If I could get a good seal I could open it up like an instrument case and carry the wind instrument inside. What is your thought to specify rubber on the long section?

You can use any airtight material for the pipe but the rubber pipe idea i.e. using a soft bendable, expanded closed cell rubber insulation pipe, originates from a sound leaking tube test I made years ago with a 4” driver I mounted on one end of a 2m closed lambda/2 tube where the driver also faced a closed box of fc= 85 Hz at the basket side.

A few dips were seen in the measurements I made but were not so easily heard when playing music.

Most of the midrange and the treble range were much attenuated but the mid bass and upper bass came through at a higher level and sounding quite smooth and of course attenuated.



Quote:
...My original query, naive as it may be, is what would the frequency response be if you replaced the closed section with a horn of mouth diameter 20 cm driven by a rather small full range, like Peerless 830970 2", (that would project to the left if we are modifying your diagram) and then used a scaled down cross section of your TL to the right. Aiming for perhaps 200 Hz. This would be the Bose concept of rear phase addition, except the shorter of the TL legs is substituted with a horn. The idea is that I want to attach it to the instrument and carry it whilst playing....
I don’t think a single Peerless 2” is the answer ‘SPL wise’ but 4 are, that is if a quadric or bell like aperture is placed in front of the drivers (See the picture of 4x Peerless 830970 in a HR simulation attempt targeting a Unity horn).

Each driver should be equipped with a small closed volume at the basket side.

An extra TL pipe or open back wouldn’t extend the FR much and as can be seen in the picture, but a small terminating volume (compliance) would reactance balance the horn to the better = flatter FR.


Quote:
...Im curious if you compared the response of your bass design using a straight 8 inch tube of the same length...
This would IMO result in an untamable and horrible FR throughout the whole BW and of course not that portable too.

b
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Old 13th February 2010, 10:42 PM   #9
bjorno is offline bjorno  Sweden
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Quote:
Originally Posted by max axe View Post
I have recently tried using the pyle 40watt megaphone. I was too curious, since it had the approximate look and size of this concept. It wasnt what one would hope, although it did give an interesting immediate presence to the horn sounds. Im trying to figure out if the irritating sound is simply from the band width or irregularities in freq response, or something else. Its re-entrant. Brash is an understatement
Hi again,

A megaphone always sounds bad with music though the sheer dimensioning involved looks about right.

I think that you for SQ reasons throughout should use furniture grade ply of 3-10 mm and build cross-braced framed stiff structures when forming the horns and the eventually needed enclosures.

I would cross brace every 10 cm using 3-5 mm ply.

You probably need at least 3 acoustic overlapping FR: s to cover the entire reinforcement BW range and you IMO, cannot avoid tapered shapes unless accepting an very overall low efficiency resulting in a low system SPL when a low voltage power supply also play a Joker.

b
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Old 13th February 2010, 10:56 PM   #10
GM is online now GM  United States
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Quote:
Originally Posted by max axe View Post
Its amazing that you cant achieve the range and SPL with a 2 lb magnet and 50 watts as you can a pair of lips and a light tube of brass.
Not really, just look at the compression ratio, i.e. your compressor (lungs) forcing air through your vocal chords, then further loaded by your low pass filter chamber (mouth) before squeezing it out through a tiny throat (lips) at high enough pressure to cause the wind instrument to create enough of a back pressure to keep your lungs from collapsing.

I have no idea how much volume displacement the average set of lungs has, but I imagine it's a good size radiator stroking an inch or more firing through a tiny slit with maybe a 1000:1 CR, so little wonder even a large format 10:1 CR horn driver (which is as good as it gets AFAIK for wide BW use) can't do it without a huge horn to give it enough back pressure to keep from bottoming it out long before its cut-off is reached.

Anyway, I don't have a clue how you're going to get a loud enough system in a small enough bulk, but to answer your Q, yes, you can TL or BH load the back of a horn. It's called a compound horn and can be simmed in HR and AkAbak. FWIW, the one compound horn I've experimented with used a large compression driver that I cut off at ~100 Hz and it was relatively huge and due to the small throats and the tiny chambers it required meant a rear path-length so long that the front/rear delay made it impossible to listen to for any length of time with my then youthful ears, so in my very limited experience this approach seems an exercise in futility. About the only positive thing it did was play painfully loud.

The driver-in-horn may have merit if you can make a large, long enough stand mounted horn with your instrument attached at the rear to mimic a 17th Century 'Voice of God' or French or......... horn. This would allow a large enough HE driver in a sealed cab to be 'hidden' in it. Being heard over a drum kit means it will have to easily play at 100+ dB with enough power handling for 115+ dB transients if your drummer is a loud one.

GM
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