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|6th March 2005, 01:09 PM||#1|
Join Date: Jul 2004
Moved: Driver choice (Belgium) and LF design considerations for 50€ Line Array
I decided to move my post from " thoughts on hi-vi b3s line array /w single tweeter" to a separate thread:
I'm trying to do the same for a friend of mine who is still studying and lives in a Student Dorm.
I was planning to start a separate thread on this, but I think this thread is the best place to do it, since I think our goals are very similar. I hope this is not considered thread jacking, more something like thread 'expanding'
Here are the design considerations and specifications I have typed the last few days:
An as sensitive as possible speaker who is composed of minimum component *variëty* (for ease of construction and low price), and who will give as full range as possible at 10 watts RMS, without reaching the excursion limits of any of its drivers at this
specification. Total price of 2 finished speakers should be less than 50€.
Since this will almost certainly require active equalization, a high sensitivity is my main goal, as to be able to EQ (on the PC) a flat response. For a high sensitivity there are 2 options: horn-loaded and a line-array. Horns are very difficult to construct and multiple horns would be nessesairly for a high bandwith. Looking at price and ease of construction, a line-array with multiple cheap extended range drivers would be the best choice for my design.
If it is to be an array, then it can be used as a small PA for presenting the annual quiz of the Counsel of the Student Dormatory. The design of the array itself will be optimized for this purpose. 12 Drivers in one speaker should give a 10dB increase in efficiëncy which is adequate and economical from #drivers/gain point of view. Also 24 drivers in total is a good number to reduce the shipping costs to an acceptable portion of the total cost. Power handling for a single driver should then be 1watt RMS so one speaker can handle 10 watts.
I have found out bij Googleing that for speech intelligibility the array should cover 250hz-4000hz. If the soundspeed in air is 340m/s, then for 4000hz the wavelength is 8.5 cm. (full wavelength) If for the design to work as an array the maximum distance between the point sources aka driver centers can be no more than 1/2 WL, then even 7cm diameter drivers should go up till (almost, a 7cm driver is not a true point source) 10000hz, ensuring the speechrange will be radiated according to line array theory. The lower freq limit of the LA is defined bij the length of the array. Earhight while sitting on a chair is 1.25m, and thus the lower limit is 136Hz, also outside the defined speech range.
Now the LA-specifications are defined, lets look at the normal (horizontal) radiation behavior. For good sound quality (and my friend who I'm designing this speaker for also expressed his concern for this), the high freqency range should extend high enough. I'd therefore like it to extend till 15000hz. Obviously this requires an as small as possible driver. This again compromizes low frequency responce, where I'm aiming for an extension till 40-50hz if possible.
The ideal driver should therefore be oval in shape. The smaller cross-section can be coupled together for the LA effect, and the taller cross-sections will give a larger surface area to get as much displacement as possible. Given the 8.5 cm LA limit from above, and assuming a 3/4 ratio for the oval shape, the largest possible driver is 8.5cm/11,5cm. Again, smaller drivers for better HF are preferred.
Now I have come to a point where there are 2 possibilities to cater for the low frequency responce, and this is where I would like some brainstorming from you guys: bass reflex and hornloading the back of the array.
I know some things about horns, since I'm still busy creating a spreadsheet according to the Leach Model, and I have modeled a few horns in HornResp. But for bassreflex I don't have a feeling what is possible or what to expect.
Summing up 12 7cm circular drivers, gives one equivalent 24cm driver, but still with 1mm Xmax! (assumed 1mm xmax for the drivers). Finding parameters to model 12 extended rangers in Hornresp is difficult, but with some educated guesses about BL factor etc ... I found out that Hornloading is a good way to reduce excursion at lower frequency.
The lower the tuning of the horn, the lower the excursion, however that backhorn should be fitted in the max 1.25m height, so there will obviously be some lower limit. I haven't looked in the lowest freq one could expect at max excursion at 10watt RMS yet, but I expect it should be under the 250hz-limit of the line array. At 1 watt I hope to get as low as 40-50hz.
For Basreflex I have no idea how much the excursion will be, and how low it can be tuned, so hopefully someone can give me some hints. I also worry about how much SPL is possible at 1 watt. If there is lets say 10dB less from 300-50hz, then a horn would
obviously be better for downfill.
Some toughts about Fs Vs F3: i think it was Thiele who stated that drivers should only be operated one octave under their Fs and still have acceptable quality. However, since with a backhorn the loading and excursion should be a lot more civilized than in BR, I think Fs is not an issue for that. In BR I could lower the Fs a bit (and smooth out the High end - remember I'll use an an Eq anyway to extend HF responce, so some sensitivity loss up there is not a real problem) by spraying some sort of waterproofing spray in tiny layers on it, but i doubt that would be nessesairly.
So to summarize the driver I'm looking for:
- 1 watt RMS
- Oval shaped
- Max size 8.5cm/11.5 cm
- As much Xmax as possible
- Around 1€/piece.
- Low Fs
- Preferably available/sold in Belgium.
This is very close to Parts Express' famous 4" Extended Ragers at 0.69$, but I think it should be possible to find similar ones that are more to my specs.
A driver I found (available in numerous shops that sell electronic parts here in Belgium) is the SP-170. I googled an even found a price quote for it here:
The price is a few times higher than I'm looking for, but it's also the largest possible driver I would use, so I think I can find a smaller model at lower price.
Someone mentioned that 1 watt/driver wouldn't be enough for the LA to handle 10 watts, so I posted this reply (which also belongs now to the design-text above):
I think the remark about power handling was directed at my design (which i will move to a separate thread - it doesn't really belong here, sorry for that ), so I will answer it.
12 drivers of 8 ohm can be connected in 3 clusters of 4 drivers in parallel. One cluster has an impendance of (8/4=) 2 ohm. These 3 clusters in series makes a total impendance of 6 Ohm.
If we want to put 10 watts in the array, then the voltage is
the Square root of (6 Ohm * 10 Watts) = 7.75 Volt over the array.
The voltage over one cluster is calculated by the Voltage Divider formula, so we divide the impendance of one cluster (2 ohm) by the sum of all the impendances of the other clusters (2 Ohm + 2 Ohm + 2 Ohm) and multiply this with the total voltage drop across the array (7.75 Volt). Hence 7.75 * (2/6) = 2.58 Volt across each cluster. (The 2 other clusters are identical to this one.)
The parallel wiring of the 4 drivers in a cluster makes that the drivers in that cluster (and thus each of the 12 drivers in the array) has 2.58 Volt across it.
The power dissipated in the driver is (Volt*Volt)/Resistance.
So (2.58V*2.58V)/8 Ohm and thus each drivers sees 0.83 Watts of power. I didn't calculate it till now, but my gut feeling of 1 watt/driver wasn't that far off
One last remark: if one of the clusters has more drivers than the other clusters (= different impendance), then the Voltage Divider will give different voltage drops for each cluster, and it is obvious there will be differences in power between the clusters (and thus drivers) in the array. This is called "power tapering", but I'll not elaborate on that further.
Sorry for the long post guys (and girls)
-- If in doubt: overdesign!
|9th March 2005, 03:15 PM||#2|
Join Date: Jul 2004
During the last few days, I've been looking around for suitable drivers.
The electronics parts-shop I was talking about sells Monacor and Visaton speakers in the size range I'm looking for, but unfortunately I can't find prices or extensive driver specs on their website, so I spent half a week digging trough the sites of Monacor and Visaton to find suitable drivers. (assuming they can get other drivers from these companies if I would order them.)
Monacor's site was absolutely NOT helpfull, but the one from Visaton is a gem. I found this driver, which seems to have all I need, and even more xmax and power handling than I hoped. They come with round and square baskets, and in 4 & 8ohm versions, so if you find more than one link on their site, that's it.
It's called "FRWS 5":
But ... These links come from the "industry" part of their site, and so they have no price indication. If you look for the same drivers in the "single drivers/accesoiries" part, then you'll see the're about 8€ for ONE driver, making the total cost over 200€ for 2 speakers.
I just sent an email to the E.P.-shop to ask them for a price quote. Maybe the 'industrial' drivers (large amount of production)are a lot cheaper than the 'single' drivers ... or maybe they can get them a lot cheaper.
Just in case they really are that much overpriced, I've already started looking for larger (4-5") full rangers with whizzer cone etc, like the "BG 17" 8 ohm for 16.5€/piece, making my project even cheaper than expected. ... but still ...
Does anyone have some experience with this kind of Visaton drivers? (both the BG 17 or the FRWS 5 ?)
oh, I've given up on using oval drivers. The high frequency range seems very ragged and/or the upper octave is almost absent.
P.S. If a moderator reads this, could you please remove the "Moved:" from the topic title? I couldn't find out how to do this myself.
-- If in doubt: overdesign!
|24th March 2005, 04:20 PM||#3|
Join Date: Jul 2004
In the past few weeks I've been modeling in Hornresp a backhorn for the Visaton BG 13.
I used Goldwave's highpass filter and various songs to determine that I wanted (well ... hoped) downfill till 50Hz, (I have a Sennheiser HD 25-1 (70 Ohm) headphone that I consider a full-range source for reference,) so I made the horn countour expand at 40Hz.
After spending days and days to determine the horn parameters, the last nights I finally came around to actually folding the horn and making a sketch.
For the folding skeme I was inspired by Mr. Zhang, and I will post the URL to his design that sparked my interest.
(If you read this, thank you very much, Mr. Zhang)
You can find the scanned sketch and design calcs in this zip file:
Feel free to add any coments. (aldough due to the lack of replies to this thread I'm not hoping for it. )
Within a few days I will write more extensive documentation on the design and folding process, as a reference for any interessted designers.
Now I'll send the sketch to the friend I'm designing these speakers for, and he will put it in a Solidworks model and determine how many square meters of wood are needed. Then I can give him a real price quote.
-- If in doubt: overdesign!
|24th March 2005, 06:28 PM||#4|
Join Date: Dec 2001
can´t add much of my experience but if you wanna stick with visaton drivers you should have a look into the forums at http://www.visaton.de/vb/ .
Most threads are in german but no problem posting in english.
From what I´ve read people like the FRS8 8Ohm for TQWT (needles), little sealed speakers and it seems ideal for an open baffle line array.
Another favourite is the FR10 4Ohm.
one example : http://www.visaton.de/vb/showthread....ine+array+frs8
|1st April 2005, 03:52 PM||#5|
Join Date: Jul 2004
- I'll post in the Visaton forum and see if someone tried something similar. A Search in the Forum didn't give results regarding projects with the BG13 dough ...
- The FRS8 seems to have indeed a very flat response, but in the 'CT Effect' enclosure they sell from it, they have a High Freq rolloff so for some reason I don't believe the flat curve of the single driver. And the efficiëncy is WAY lower than the BG13. Too much discrepancy with the output from the backhorn.
- FR10 has less HF rollof but then again more xmax to boot. A Little less efficiency, but not that bad. Dunno ...
- Thanks for the tips dough
To the rest:
So, it took me some days more than expected to actually sit down and type out the design proces for my 'Take2"'s. (More like "Take 2.5"'s since I abandoned the idea of a Line Array )
The predicted SPL, impendance, xmax-curves and the drawing of the "Take-2" design are all found in this zipfile (2mB large)
Well, good tweeters are small and give good highs, but obviously you can't sweep much air for the bass. Good woofers are like the opposite of that, but their high freq are very ragged and dispersion is a real issue. Using a whizzer cone helps a lot, but still dispersion is a problem.
The conclusion is therefore that the choosen driver (visaton BG 13) must be as close to ear height as possible to have extention till 20.000hz. After sitting down and measuring, my earheight is about 125cm. So the height of the driver must be that high. (Also good for a natural height of the instruments and singers etc ...)
Now for the LF part. As I already explained above I choose a lower cutoff freq of 50Hz, which requires horn-loading to get this with only 3mm Xmax. Hornresp learned me that if you truncate a horn with a given expantion contour, you get dips en peaks AND the cutoff freq goes higher. A little trial and error made me choose a 40hz contour to get the desired 50hz cutoff. Horn length should be minimum 1/4 wavelength @ 40Hz, so 215cm. If we would fold in 2 parts, the lenght is 2*125cm = 250cm and we have our minimum 1/4 WL. However the mouth would be on top of the speaker if we choose minimum floorspace
Also, the mouth would only couple to the back wall, so we only radiate in half space. When I saw the design of Mr. Zhang on http://melhuish.org/audio/DIYRH4.html and I realized
that by putting the mouth sideways against the backwall AND the floor we are at least radiating in 1/4th space and if a corner is nearby, even in 1/8th. This leaves a lot more placing options in the room than simply designing a corner-backhorn!
Now for the dips and peaks ...
As you can see from the predicted SPL response, there is a considerable output from 100hz-250hz. Since all the bends are around 180 degrees, I hope the horn won't have significant output above 100hz. Even if so, the mouth radiates sideways so its HF output shouldn't interfere with the driver on the front. This still leaves us with 2 peaks: one at 80hz, and one at 50hz.
The 80hz peak should be less severe than predicted, due to the 180 degrees bends. If nessesairly, a notch filter (or equalizer) could be applied, keeping in mind that the front radiator still has significant output at that frequency. However, the THX-standard takes 80hz as the upper freq for subwoofers since directional information is lost at this freq.
Dual output from the hornmouth and the direct radiator shouldn't be a problem as long as its under 80hz.
The 50Hz peak could pose more of a problem. It is 10dB higher (thats 10 times more energy and about 2 times as loud) than the output from the direct radiator at the same wattage. The large dip between the 50hz and 80hz peaks, bottoms out at 105dB @ 10 watts and is on par with the output form the D.R. There are 2 options for the 50hz peak: a notch filter to reduce it 10dB, or choosing the highpass filter for the horn (to prevent overexcurtion, every horn needs it) a little higher than that, lets say around 54Hz. The steep rolloff curve of a 4th order filter and the 50Hz peak schould cancel each other out and give a flat responce till 50Hz.
So now we have a backloaded hornspeaker that gives flat responce from 20.000hz till 50hz, is 93dB @ 1watt/1m. It can take the desired 10watts over his full freq range without overexcursion. (highpass @ 54Hz) Has more placements options in a room than a *corner*-horn, due to its side-mouth and has perfect phase coherency above 80Hz due to it's single driver setup. There is a possibility for retrofitting it with a Fostex or even Lowther driver and cutting the "neck" to get a larger throat area at the expence of lower driver height and shorter horn length.
Drawbacks are the large witdth of 42cm (or size in general) and the neck can make it hard to transport by car.
Anyone has to add comments or point out flaws?
-- If in doubt: overdesign!
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