| johninCR |
Chipco issued the challenge so here it is:
A 2 meter tall array of 16 NSB drivers as a front loaded horn. Bill said it couldn't be done but I believe the combined Sd of 800cm is what does the trick. HornResp is showing me 102-105db/w/m from 40hz to 200hz with 8 ohm wiring. That should match well with the array since I don't think the horn will get array projection. If the room makes it too bass heavy just add the optional final flare and move it out of the corner.
Using a corner to form the final horn segment, it's only 16.5cm (6.5") wide at the front, 23cm (9") in the back and only 50cm (20") deep....... but 2 meters (6.5ft) tall. It does need to go in a corner, but using side wall placement will only cost you about 3db between 40 & 80hz. I stayed middle of the road with the rules of thumb for horns and trying different driver and construction variances got the same result. It should work with just about any 4" driver and be forgiving to minor construction variances. Changing the angle of the cab to the side wall has little effect so placement can be used to fine tune the horn's upper cutoff and to suit your listening position.
3 sheets of 3/4" or 18mm ply plus $16 worth of NSB's and whatever you are going to do about the tweets will get you from 40hz on up. It still has over 92db/w/m at 30hz, so with room gain it should have nice extension.
Construction will be pretty simple. Just 7 rectangular pieces each 2 meters long, plus top and bottom caps gives you the basic construction. 27 narrow rib divider/braces would be ideal and might be tedious. I believe rounded corners are unnecessary due to the low 200hz upper cutoff, but if you wanted to, simple 3/4" corner moulding would be sufficient for most of the corners due to the narrow widths of the passages.
I'll attach the Hornresp parameters and response graph if someone tells me how.
If I've lost my mind and completely missed the boat somehow, will someone please throw me a rope.
Below is a drawing done in paint. Sorry I don't have any kind of construction drawing program, so it's not to scale. |
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| bzdang |
| Hi, I don't know much about horn design but I can advise on ribs. It looks as though each rib could be made up of two or more parts so make a set of templates out of thin hard stuff like masonite and cut out copies on the router table with a template bit. |
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| johninCR |
Actually I have the dimensions all calculated for the ribs to be cut from the same plywood. Their all just long skinny triangles easy to rip with the table saw. Those for the 1st and 2nd segments would probably be best cut as one piece, so I don't have open space at the turn. Again easy with the table saw. I need so many because the throat starts as only 324sq cm which with 11 dividers is 12 channels with an 18mm width (easy spacing with scraps) by 150mm high. That's an 8/1 ratio and I think much larger than that is problematic. The termination of segment 2 is a total cross sectional area of 980 cm sq comprised of 12 150mm high channels a hair over 54mm wide, a good ratio. Pairs of channels coming out of segment 2 will flow into single channels using the 5 ribs in section 3. They'll start at 318mm by 55mm, ratio of less than 6/1 and terminate at less than 4/1. The final segment should probably be channeled too, but we'll see how it sounds without it.
With all those effective braces, it should be as solid as a rock. There's no dead spaces to waste volume and with the back not closed, some fudging at the end will be easy. The 2 side panels will go on last, so if something is off a little at the end it will make zero difference unlike a typical box.
I really got lucky how everything just flowed together perfectly from what I had pictured in my head. Once I got the horn length range down to a single wavelength at 200hz, the response gelled in the models almost instantly. At first I was thinking that I needed a much longer horn, but it was unnecessary due to the height of the cab.
Just have to set aside a day to build them. I already have a pair of 12 driver 7" wide hardwood baffles I can use, so no holes to cut, which makes 1 long day realistic. 12 of the B/K 4"ers I have have a +/- 2db from 42-200hz with the same dimension cab, so it's worth a shot. |
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| chipco3434 |
| quote: | | Chipco issued the challenge |
I'm not smart enough at this stuff to challenge anything! Looks like a GREAT project! |
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| chipco3434 |
| ...and this could be the final project before she throws me out. At least I can live in these cabinets. |
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| johninCR |
Chipco,
Can't live in these. It's only about 14" wide and 20" deep with the optional final segment built on, so it takes up less real estate than a typical floor plant. Using the corner and wall as the final segment, the cab is only 6.5" wide (9" at the back) by 20" deep.
You can sell the wife on the fact that it's smaller than you have now.
I've been thinking that with all that bracing, 1/2" ply will be fine which will cut an inch off of those dimensions. You'll barely be able to fit the drivers.
Probably a strip of thick vinyl or something similar would be a good way to attach it to the wall for stability and to seal the final bend, while leaving the angle somewhat adjustable. |
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| chipco3434 |
John, what would the dimensions be with a double row of 4" drivers? I'm hankering to build upon the McIntosh XRT series.
What is your solution for the HF on these?
As for the wife, she's really quite amenable to all this wackiness. |
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| johninCR |
To get down to 40hz, you need the 16 drivers. Whatever the layout, you have to retain the horn length and cross sectional areas through that length. Essentially the volume has to remain the same and if you cut it in half, the cab width would need to double and the distance from the wall would need to double. It would be 40" tall and require approximately 2ft by 2ft of floor space in the corner, since the mouth would need to be 13-15" wide. Still not bad considering $16 of drivers and sensitivity of about 100 from 40hz on up. Each would take less than a sheet of 3/4" ply. You'd give up line array dispersion except in the higher frequencies, so that might affect tonal balance. Without the array affect the 16 drivers won't keep up with the horn, so you'd have to also have to EQ the bass down some.
A lot less divider/braces would be required. Just 3 in the 1st horn segment and 1 in the 2nd and 3rd segments should be sufficient for keeping the channel width/hieght ratio low enough, but I'd go with 5 and 2 to minimize panel resonances. |
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| pblossom |
| Here are the Hornresp input parameters and SPL chart from the data john supplied. |
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| pblossom |
| Here's the input parameters |
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| chipco3434 |
| quote: | | To get down to 40hz, you need the 16 drivers. |
No, no, no... I am currently running a WAF credit acount.
I'm talking about 32 drivers per side. Two rows of 16.
What's the tweeter selection? Pyle's? |
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| johninCR |
| quote: | Originally posted by chipco3434
No, no, no... I am currently running a WAF credit acount.
I'm talking about 32 drivers per side. Two rows of 16.
What's the tweeter selection? Pyle's? |
Yes, I plan to use the pyle tweet because I know it works and I already have a pair not in use.
Double the drivers means double the throat area needed and that doubling extends through the entire horn. Essentially 50%-100% increase in width gets you no more extension, but a very flat response between 40 and 200, plus a slight efficiency gain.
You could redesign the whole thing and get down to 35hz, maybe even 30hz, but you need a much much bigger cab. It was like me trying to get below 40hz with 16 drivers and it just wasn't going to happen and still get a smooth response.
I tried it and 35hz looks ok. It takes about double width and probably 6" added depth. The horn is longer and gives you a 160hz cutoff and length which might work well with the driver's response. But 32 more holes to cut and drivers to wire for an extra 5hz of extension doesn't make sense to me. Plus a 120% increase in cab size and detriment to horizontal dispersion of having 2 line arrays side by side are just 2 more strikes against 32 per side. You'll have to figure out the folds yourself. |
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| johninCR |
Don't cut any wood yet. The experts were supposed to catch a major faux pas. A rear horn is supposed to have a horn length equal to an ODD NUMBER OF 1/2 WAVELENGTHS OF THE UPPER HORN CUTOFF FREQUENCY. As it is now, the horn output would be directly out of phase with the front of the drivers at the crossover point. This would cause cancellation and a big dip in response at 200hz.
Back to the drawing board. I should have know it was too good to be true. |
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| mike.e |
I replied to you on decware :D
Seal the back chamber to prevent 1/2wave cancellations.I dont know how vocals would sound coming beside the walls:xeye: |
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| johninCR |
| quote: | Originally posted by mike.e
I replied to you on decware :D
Seal the back chamber to prevent 1/2wave cancellations.I dont know how vocals would sound coming beside the walls:xeye: |
Mike,
First, that's with the 49 cent pioneers, so it's $16 of drivers plus shipping total for 2 speakers. Add 2 $20 super tweets, some kind of xover due to the 7k peak on the cheap pioneers, an Lpad for the tweet, and 3 sheets of 12mm plywood. At a listening distance of 3-4 meters, for about $100 a pair plus work, you end up with a pair of speakers that's equal to a point source speaker with sensitivity of about 103db/1w/1m from 40-20khz , impedance of whatever you choose, and uses only a 24"x24" corner, but is 6.5ft tall.
I can't seal the back chamber because it's a rear horn. I got the upper cutoff cancellation just about licked, but I need to choose a cutoff frequency to finalize the layout. 120hz matches the -3db of the driver in a sealed box, but gives me a 3db dip in response centered at 50hz. Should I even worry about that dip?
Since it's a corner horn everything comes from the corners, but that should work great with the big tall image presented by an array.
The 2nd harmonic distortion part I don't understand. Is there a way estimate that? Maybe that's the hole in the whole idea of squeezing 40hz out of 16 drivers that have an Fs of 105hz. |
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| mike.e |
You wont hear -3db unless you listen,in an anechoic room without room modes
You really cant estimate distortion.You could measure it. I guess with the combined area of a 15" the drivers wont be moving far at all :D
The horn will drop the Fs down due to the mass of air in the horn.
operating drivers below Fs is not a good idea,i only do it on my 2226 because it has SFG and shorting ring so distortions still quite low. |
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| johninCR |
| quote: | Originally posted by mike.e
operating drivers below Fs is not a good idea..... |
I've read in more than one place that the old time experts said that rear horns need drivers with an Fs higher than the low end cutoff.
Also, this isn't for high power, high output. I doubt that each driver will ever see even close to 1 watt. At 1/2 watt for each driver, 8 for the array, it will be at about 107db, which at a 4 meter listening distance will be the same as 113db from a point source due to the line array effect. Hopefully the low power operation will keep distortion at bay even though it's operating so far below Fs. |
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| mike.e |
Yep thats true-because the horn drops the Fs
We wont know where Fs is on the horn :P You can calculate it
It will be great probably! |
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| johninCR |
| I don't know about great. I'll settle for pretty good for a first try at a horn. I just have my fingers crossed at this point. |
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| GM |
| quote: | Originally posted by johninCR
The experts were supposed to catch a major faux pas. |
'Supposed to'?! I was in the process of explaining why Bill is right, answering your other Qs, and giving a bit of insight into the finer points of BLH design until I read this......
GM |
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| johninCR |
GM,
I sincerely apologize for the wording. Nobody is "supposed" to do anything here to help others, and I greatly appreciate any and all help, guidance, and opinions from everyone, especially those who are highly experienced. For everyone's benefit, not just mine, please do take the time to post your insights regarding this project.
At the time I was happy that I caught the mistake and sad at the same time because I needed to redo the design. I wanted to post something quickly, so I wouldn't be one to spread incorrect info online. BTW if anyone has contact info for the designer of the Big Fun Horn, please thank him for posting all that great info about the design and that he should correct the error in horn length vs the upper frequency cutoff. |
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| chipco3434 |
Come on guys... you have to read between the lines.
Keep up the calculations John! |
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| rcavictim |
| quote: | Originally posted by GM
'Supposed to'?! I was in the process of explaining why Bill is right, answering your other Qs, and giving a bit of insight into the finer points of BLH design until I read this......
GM |
GM,
When I had read that comment by JohninCR it came across clearly to me that John was making it in playful jest.
Taking the time as you do to make calculations and figure things out to help other members here is I am sure appreciated very much by all and is an example of the core value of these forums in how they will inspire participation such as yous. Please do not be upset. |
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| johninCR |
Paul is going to be kind enough to post my 2 newer versions.
The 120hz upper cutoff version is substantially smaller than the original. If you painted it the same color as the wall, you almost wouldn't notice it in the corner. The 240hz version is bigger than the original but has a tiny chamber, only 7.5L for 16 drivers.
120hz seems like the right cutoff for NSB's in a small sealed box but 240hz seems to match the open air rolloff. So which is better?
Another concern I have is that both chambers are quite small. What kind of affect will that have on the sound coming from the front of the drivers? HornRESP only tells me what the horn is supposed to do. I have no clue how the front radiated sound is affected. |
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| pblossom |
| Here is the input parameter screen and the SPL response for the 120 hz (upper cutoff, I think) horn. |
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| pblossom |
| Here is the input parameter screen and the SPL response for the 240 hz (upper cutoff, I think) horn. |
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| mike.e |
| oh,when theres a halfwave difference between driver radiation and horn radiation,youl have a huge null somewhere above 100hz |
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| johninCR |
Mike,
Your point is why I haven't built 1 yet. I'm still hoping that someone with rear horn design experience steps in to offer some advice.
While the much shorter horn isn't as smooth on the low end, it's much more preferrable in terms of space and construction materials. I'm still uncertain which should be better. Both show really strong output according to HornRESP.
The 1/2 L per driver chamber size of the long horn worries me in terms of front radiation sound quality, but the horn output and front output are only directly out of phase at 160hz, where the horn output should be much greater.
The 1 L + chamber size for the short array seems better to me but the 2 waves are directly out of phase at 240hz which HornRESP show there is still 100db of output from the horn. Also,
the larger chamber give me more flexibility to reduce the chamber volume for fine tuning purposes.
With either, maybe the answer is to EQ down the horn output below it's upper cutoff frequency, then these out of phase points won't be significant because one output would be far greater than the other at these critical points. |
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| Tim Moorman |
Hey John,
Getting in on this thread late...I think I see what you are trying to do but I'm not sure I'd do it that way. I doubt you will be satisfied with the performance, running the driver that low below Fs. My immediate reaction is I doubt that you will be able to effectively load the horn much below 80 Hz or so(guessing). You would need to keep impedance peak issues controlled as the driver is no longer loaded properly by the horn at some point. This can be measured but you then need to alter the design as you go to compensate.
If you want to build a trial horn, use foam core laminated to sheet goods, and some duct tape. This is sold as 1.5" thick insulation (pink) panels in 4 x 8 sheets around here. Not terribly expensive either, and will provide the anti-resonant behavior you need, plus it's easy to cut and use for the interior dividers. A small jig saw works great on the hard board-backed type, or electric carving knife on just the rigid foam sheets. The driver mounting baffle, of course, would need to be solid.
I'm not wild about rear horns with a long pathway developing higher frequencies(above 150 Hz) either. They usually sound bad as they resonate the enclosure more than low bass frequencies(yeah, hard to believe), and they tend to roll off HF due to the folding. Rear horns can sound colored as well unless you damp out some of the reflective splash directly behind the driver.
I'd just build a straight front horn (along with a rear chamber) to the longest dimension you can live with, and go from there. Say, start with modeling at 100 - 150Hz cut off to keep the design small as possible, yet still give useable output down low enough to tie in with a sub plate amp. My guess is ~30" deep. Conical won't load as well low, but prove to be very good sounding in the mids do to the fast flare rate. Try an exponential flare if loading lower is an important design consideration.
Maybe use a couple of 15"s/18"s each side in a w baffle or the equivalent for dipole bass duties.
Sorry, probably not what you wanted to hear, but I just have my doubts... Can't hurt to try though as a trial unit so long as materials are kept cheap, and by building just one side.
Tim |
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| johninCR |
Tim,
Thanks for stopping in. You've pushed me over the edge to try the short version (120hz upper cutoff). Regarding the impedance
issue, HornRESP is showing a 30ohm spike at 40hz and a few ohm rise at 120hz.
I realize that one or even a few of these little things won't load a horn properly. In fact HornRESP show only a spike at 80hz with just a few. Essentially, I'm talking about stacking 16 horn mouths together in a 2 meter stack. 16 tiny voice coils with a combined BL of 18 working together to move a cone area the same size as a 15", with 98db sensitivity combined. An added advantage of being tall is the tiny amount of corner space needed.
I think I can make one with 1 sheet of 1/2" ply and some scraps all of which I already have. I already have a 12 driver baffle and the 4"ers I have model a little better than the NSB's due to their slightly lower Fs of 95. The hard part is done, so I might be able to do it all in a day, just have to get off my butt and go for it. I just hope HornRESP is right. |
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| Tim Moorman |
John,
There will be a delay between the direct radiator vs horn output due to the length of the pathway, but since the horn is working(now) at frequencies below the direct radiator output, you should not have the strong interaction and comb filtering problems that arise when producing the same frequencies.
In your experiments, I wouldn't hesitate to flip the baffle around, create a tall sealed rear chamber with the help of horn response, and call it a front horn. Of course you would lose the high freq output above cutoff from the direct radiator loss, but that's what numerous cheap drivers are for - just add another row in front with their own front horn flare - Ha!.
What ever it ends up being, it will be real damn loud, which is kinda fun by itself, but it may also give you a glimpse into the possibilities of the great bass available through horn loading.
Tim |
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| johninCR |
| quote: | Originally posted by Tim Moorman
John,
There will be a delay between the direct radiator vs horn output due to the length of the pathway, but since the horn is working(now) at frequencies below the direct radiator output, you should not have the strong interaction and comb filtering problems that arise when producing the same frequencies.
In your experiments, I wouldn't hesitate to flip the baffle around, create a tall sealed rear chamber with the help of horn response, and call it a front horn. Of course you would lose the high freq output above cutoff from the direct radiator loss, but that's what numerous cheap drivers are for - just add another row in front with their own front horn flare - Ha!.
What ever it ends up being, it will be real damn loud, which is kinda fun by itself, but it may also give you a glimpse into the possibilities of the great bass available through horn loading.
Tim |
I've built arrays, so I know it will be loud. Extension is what I'm looking for. Also, I got an incorrect formula for rear horns online and the first design put the horn length exactly at 1 wavelength of the acoustical crossover frequency, so it was back to the drawing board. If I do end up needing to go front horn plus another array for the front radiation, I'll go back to the original dimensions and wire the horn out of phase.
Isobarik loading may help too, with double the motor strength to move and half the Vas to move the same cone area. I need to model that. |
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| johninCR |
IT WORKS....BUT.....!
I made a test horn today using an OB array I have. I used styrofoam and duct tape to close in the chamber and plywood to make a straight horn out of the back.
Although HornRESP told me I'd get 100db, I only got 87db at 40hz on 1 watt. The difference is due primarily to the makeshift construction. The styrofoam really vibrated and alot of sound was coming from there. Also, a straight conical horn is less efficient. In addition I wasn't using a corner or side wall and the 4ft tall narrow shape probably resulted in less benefit from the floor boundary.
Although the test was successful and I believe that 1 watt and 16 NSB's will get you 101db at 40hz in a compact but tall array horn, the limitation is that you can only put 1 watt into it. The cone excursion increases drastically below 50hz up to .7mm at 40hz, and with the xmax of 1mm even .7 is higher than you want to be.
I may go ahead and build a pair to go with my low powered tube amp, but my enthusiasm is shot because I really wanted to get to 110db on 10 watts from 40hz on up using cheap 4" drivers, but that just isn't going to happen and 50hz is too high to put a high pass filter. |
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| pblossom |
| Which design did you use? |
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| johninCR |
It was really neither version because the plywood I had was in between the 2 horn lengths. I used the same throat adjusted for the 12 drivers instead of 16 and a mouth size the modelled similarly to my layouts.
It doesn't really make any difference. With that amount of Sd you need more excursion down low. You can double the drivers and enlarge the throat and chamber and get a similar response and cut excursion in half, but then you're in the same boat at 2 watts. If you drastically increase the horn length, you can get a more extended flat response and extend the controlled cone movement into lower frequencies.
I tried changes in driver specs and it made very little difference.
To get more output you need more excursion or much more total volume of your horn.
I still think that adding a bass horn is a good option for an array. I have some cheap 5" drivers that have more xmax that I'm going to try. |
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| Tim Moorman |
John,
Well, at least you tried to experiment, which is more than I can say for a lot of the people. Nothing ventured...right? I think you just lost control of the cone from not being able to load the horn properly that low with that driver. Too far below resonance(Fs). You should see very little movement, if properly loaded.
If still interested in horn bass, I would try the Adire Tempest horn shown in the diy part of their site. Didn't you have a couple of subwoofers lying about? Big box, though.
Maybe just a single driver in a test horn with a suitable 90 - 100 Hz cut-off?
Tim |
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| johninCR |
Thanks Tim,
Using HornResp, I've plugged a bunch of different drivers in the same horn with pretty similar results, even single 15's. Without the benefit of a much bigger horn to tune lower, it takes .5-.7mm of excursion to get to 100db at 40hz, so you need more excursion than a 1mm xmax to get more higher output and still sound good.
My goal was to try to do it just with NSB's, but a 100db limit isn't enough. I have a bunch of $2.50 5"ers that sound almost as good as my 4's. It looks like they have about 3 times the xmax and the modelling is showing I can get them up to 105db at 40hz and less than .5mm excursion with a 16 driver array in a total footprint of about 18"x18". Half of that is the final flare, so we're still talking about small, but tall.
There's no way to get into subwoofer territory without a big horn. I can't wait to have an attic or basement to hide one in. |
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| Tim Moorman |
The old rules of thumb for full sized horns: axail length = wavelength (HZ) at cut off. Mouth circumference equals wavelength at cut-off. 1140 ft/sec speed of sound/40 Hz=28.5
40 Hz = 28.5 Ft wavelength divided by 4 for 1/4 length= 7.125 ft. both in pathway and mouth circumference. 14.25 ft for 20 Hz.
These have been replaced by Marshall Leach's math, Horn Resp, and some other guidelines, but it will give you an approximation of the scale of things, as you probably now know.
They do grow rapidly with lower frequency. Foreshortening just adds irregularities to response.
Nice try, anyway.
Tim |
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| Tim Moorman |
I cheated down on the mouth size for the corner loading (1/8 space). This would be for each driver, and you would gain some close coupling numerous drivers with the added total mouth size increased proportionately.
Fact is, a horn starts loading at 1/4 wavelength, but does an increasingly better job as you approach 1/2 wavelength, with mouth size about 1/3 wavelength or greater.
Most of the bass designs of recent release use the room boundaries to effectively extend the mouth, as you had originally planned with corner loading.
Tim |
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| johninCR |
Tim,
By pathway do you mean horn length. I'm getting a good graph with a 143cm (4.7ft) horn length and a 6,368 sq cm (6.85 sq ft) mouth. Do you think that's compromising too much to sound good with corner loading?
I do think the actual results may be somewhere between 1/8 and 1/4 space due to the tall narrow shape, but the 1/4 space graph looks pretty good too. |
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| Tim Moorman |
John,
Those were just approximations based on the 40 Hz. I didn't actually model the horn. It was merely to illustrate the very large mouth one would need for the numerous driver array. Additionally, too, the large flare needed to support 40 Hz for just a single driver. Check your CIR to see if it is 1 or greater.
I have a foreshortened midbass horn for a 15" driver that has a 9" x9" throat, and exponential flare out to an ~17" x 26" mouth at 23" long. Rear chamber is probably 30 liters or so. That is a very short horn for 80 Hz cut-off, but good to perhaps ~95 Hz with one side on the floor. You can do that sort of thing with a 15" driver in an exponential horn with compression.
If you model it, it will show a pronounced peak from 100 to 200 Hz, with not much but roll off either side. It is more extended above 250 Hz than shown, so HornResp won't include this.
Your experimental length of 4.7 ft axial length doesn't seem that far off if the combined Sd is close to a 15". But your Fs is still too high, right?
You probably need something in the ~50 Hz range for a 40 Hz horn.
How you model the throat will have a large bearing too on the other dimensions. That is, are you slot loading with some compression, just using the outside dimensions of the drivers stacked up, or actually using a "throat" into the horn for each device. Check if SD=S1, or is less. It will make a difference.
Tim |
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| johninCR |
Tim,
My thought process was as follows:
Since it's a rear horn, length is a big issue to get proper phasing where the front radiation takes over. I want to keep it at 1/2 wavelength without going too short. I settled on 120hz, 1/2 wavelength 143cm.
I wanted to keep the throat/sd ratio above .3 , but on the low side to keep the overall size of the horn small and extension is the goal not overall efficiency. Also, I want to keep the narrow width of the throat in combinations of 12mm &/or 18mm plywood thickness to easily make a spacer to ensure it is uniform throughout the length. I settled on a throat of 570 cm2 or 42% of the 1360 sd of 15 5" drivers I plan to use now and a width starting at 30mm.
For the flare rate, I just copied the hypex flare of the Jensen Imperial which also has 4 segments and a large compression chamber.
I have 2 areas of flexibility at the end. The mouth size and the compression chamber volume.
Bracing is needed anyway and I read that very narrow passages is bad. 15 braces through the 1st 2 segments gives me 16 passages that start at 3cmX12cm and end at 10x12 after the 2nd segment, where they will flow seamlessly into 8 passages and 4 for the final flare. That way I can cut 3 pieces of plywood that are dividers throughout the horn, 4 that run through the 3rd segment and 8 that cover only the 1st 2 segments. This won't be too difficult. Just cut the pieces, then cut out 12mm slots to slide the 3 vertical panels into making assembly quite easy. Once I get the first panel in, glued and sealed, the braces will form a skeleton for the rest of the horn.
One thing I don't understand is what to look for in the acoustical impedance graphs. The other is the effects of a large chamber. In playing around with chamber size my best response is with a large chamber, but I end up with an upper frequency cutoff based on throat and chamber that is lower than 40hz. ???? |
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| Tim Moorman |
John,
If I follow you, the upper cut-off was determined by the low end of the direct radiator output. Yes, a good plan, but as I described on the midbass horn, the program will not show the upper range very accurately and you can expect some output above cut-off. My guess is you will have some irregularities if you measure in-room response where they overlap but I don't think it will be that significant or annoying. You may have to experiment some with damping, for example, to absorb the HF. Look at the Jensen Imperial, as you mentioned, or the Altec A7 where both use a rear wave, one in a direct radiator and horn scoop, and one in a front horn with reflex enclosure, but both with bass output tuned well below the direct radiation. It can work with the right drivers and enclosure.
As far as compression and the throat chamber, I would keep the load on the drivers as low as possible to still yield the efficiency I was looking for. Generally, the higher the compression, the higher the driver load, and I'm not sure of the durability of your clearance units. Throat can also effect high frequency roll-off, which is not specifically the issue, but it is in an overall scope.
VRC should be set to zero in a rear horn, right, while the front compression chamber can be determined by the hypex calculator as a double check. It sounds as though some of the parameters have been mixed up. You may have to deviate some from the Imperial. Do it your way, then use the calculators, then use compare.
Acoustical impedance , near as I can tell in this display, is reactance/resistance in a mirror image and can overlap or diverge. There is a scaling factor to be used based on the design, but I don't know much about it. I look at the electrical impedance more closely. As far as horn design, the acoustic impedance is a reflection of the horn load. As it is a resonant device I expect that the irregular tracking of the graph is part and parcel of the resonant nature. I notice that the two should closely overlap and the peak amplitudes(?) should be kept as low as possible. Things really start to get erratic when you intentionally load bad numbers into the data base, showing that the driver is not being properly loaded by the horn.
Tim |
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