This thread is to document the design and build of my 2nd open baffle speakers.
The first were a proof of concept using all cheap drivers - http://www.diyaudio.com/forums/showthread.php?s=&threadid=128015&perpage=25&pagenumber=1
I then started a thread to discuss good, affordable drivers for a 3 way open baffle, dipole design - http://www.diyaudio.com/forums/showthread.php?s=&threadid=133147&perpage=25&pagenumber=1
These threads and my experience, reading and simulation so far has led me to the following driver line-up, which may of course be altered if it doesn't work out:
* Eminence APT80 compression tweeter (comes with small horn)
* Eminence Beta 8 midrange driver
* Skytronic 902.222 poly cone woofers
So we've got PA style tweeter and midrange, supplemented with twin 12" woofers of the cheap variety. The woofers were chosen because of their extreme low price (£30/pair new on Ebay) and suitability for open baffle loading (Fs=29hz, Qes=0.71, 3mm xmax - enough with 2 drivers per side).
Bass will be enhanced with Graham Maynard's T-bass (transformer bass) circuit, which passively provides up to 6dB of boost at LF. It also provides some cut of upper bass, which is essential for a passive open baffle design. Driver energy storage is reduced and reproduction of low frequencies is greatly enhanced.
The goal is to produce a speaker with great timing, dynamics and tone. They also have to fit into a tiny room, which has limited me to the use of 12" woofers.
I've attached an initial sketch of how they might look.
Simon
The first were a proof of concept using all cheap drivers - http://www.diyaudio.com/forums/showthread.php?s=&threadid=128015&perpage=25&pagenumber=1
I then started a thread to discuss good, affordable drivers for a 3 way open baffle, dipole design - http://www.diyaudio.com/forums/showthread.php?s=&threadid=133147&perpage=25&pagenumber=1
These threads and my experience, reading and simulation so far has led me to the following driver line-up, which may of course be altered if it doesn't work out:
* Eminence APT80 compression tweeter (comes with small horn)
* Eminence Beta 8 midrange driver
* Skytronic 902.222 poly cone woofers
So we've got PA style tweeter and midrange, supplemented with twin 12" woofers of the cheap variety. The woofers were chosen because of their extreme low price (£30/pair new on Ebay) and suitability for open baffle loading (Fs=29hz, Qes=0.71, 3mm xmax - enough with 2 drivers per side).
Bass will be enhanced with Graham Maynard's T-bass (transformer bass) circuit, which passively provides up to 6dB of boost at LF. It also provides some cut of upper bass, which is essential for a passive open baffle design. Driver energy storage is reduced and reproduction of low frequencies is greatly enhanced.
The goal is to produce a speaker with great timing, dynamics and tone. They also have to fit into a tiny room, which has limited me to the use of 12" woofers.
I've attached an initial sketch of how they might look.
Simon
Attachments
The midrange driver
Let's start by looking at the mid driver. I've chosen a driver based on a few factors:
* small mids tend to sound wrong to me
* high sensitivity is a good thing
* PA drivers always sound interesting and detailed
* Eminence parts are affordable
* I've heard rave reviews of the basic Eminence ranges used as mids
* In the few 8" mid or mid/bass speakers I've heard the midrange comes across well compared to 6.5", 5" or 4" cones
* an 8" cone should barely move when producing midrange, so distortion will be low
It has been noted that the Eminence Beta 8 has a rising response, but I've noticed this is almost exactly the opposite of the roll-off after the dipole peak in my speaker shape.
I think this response may very well be to my advantage. I've taken a screen shot from the Beta 8 official response curve and overlaid it on the same frequency scale as my Edge plot for that shape of baffle and driver.
See attached pic.
Let's start by looking at the mid driver. I've chosen a driver based on a few factors:
* small mids tend to sound wrong to me
* high sensitivity is a good thing
* PA drivers always sound interesting and detailed
* Eminence parts are affordable
* I've heard rave reviews of the basic Eminence ranges used as mids
* In the few 8" mid or mid/bass speakers I've heard the midrange comes across well compared to 6.5", 5" or 4" cones
* an 8" cone should barely move when producing midrange, so distortion will be low
It has been noted that the Eminence Beta 8 has a rising response, but I've noticed this is almost exactly the opposite of the roll-off after the dipole peak in my speaker shape.
I think this response may very well be to my advantage. I've taken a screen shot from the Beta 8 official response curve and overlaid it on the same frequency scale as my Edge plot for that shape of baffle and driver.
See attached pic.
Attachments
Here are specs of the Beta 8:
http://www.eminence.com/proaudio_sp...tail_link=BETA-8A&speaker_size=8&SUB_CAT_ID=2
Here's some info on the APT80:
http://www.eminence.com/tweeters.asp
and here (without horn):
http://www.eminence.com/proaudio_drivers_detail.asp?web_detail_link=APT50
The tweeter doesn't seem overly fussy about where it's crossed over but to avoid a harsh sound I wish to avoid crossing over too low. I'll start at about 4Khz 1st order, and also try 2nd order.
Everything will be passive.
http://www.eminence.com/proaudio_sp...tail_link=BETA-8A&speaker_size=8&SUB_CAT_ID=2
Here's some info on the APT80:
http://www.eminence.com/tweeters.asp
and here (without horn):
http://www.eminence.com/proaudio_drivers_detail.asp?web_detail_link=APT50
The tweeter doesn't seem overly fussy about where it's crossed over but to avoid a harsh sound I wish to avoid crossing over too low. I'll start at about 4Khz 1st order, and also try 2nd order.
Everything will be passive.
And here it is with some boost, to show (very, very roughly) what T-bass will be doing.
I think a 2nd order low-pass filter at about 100hz or so should be a good starting point to match up to the midrange.
I might invest in some speaker software to make filter design easier, but I'll be making measurements using ARTA at the very least.
Simon
I think a 2nd order low-pass filter at about 100hz or so should be a good starting point to match up to the midrange.
I might invest in some speaker software to make filter design easier, but I'll be making measurements using ARTA at the very least.
Simon
Attachments
I've posted this twice before in other threads but it's very interesting - the X-Baffle simulation for some woofers, including the Skytronic ones I'll be using here.
See how I'll get a flatter response to work with from the Skytronic woofers than my previous ones (7" sub driver x4).
Credit to Thorsten Loesch for writing the spreadsheet and Panomaniac, as well as others, for putting me onto it.
Baffle Construction
This is an area I'm open to suggestion on, but with the following constraints:
* cost must be relatively low
* complexity must be low
* materials must be readily available (e.g. from Arnold Laver timber merchants)
I know full well that 18mm MDF doesn't really cut it in open baffle speakers, which are massively excited by multiple woofers rocking the whole assembly back and forth.
I have heard of MDF & chipboard being an effective combination. I could have a cheap, pretty and hopefully solid baffle made from, say, 15mm veneered MDF stuck to 12 or 15mm chipboard...
Simon
See how I'll get a flatter response to work with from the Skytronic woofers than my previous ones (7" sub driver x4).
Credit to Thorsten Loesch for writing the spreadsheet and Panomaniac, as well as others, for putting me onto it.
Baffle Construction
This is an area I'm open to suggestion on, but with the following constraints:
* cost must be relatively low
* complexity must be low
* materials must be readily available (e.g. from Arnold Laver timber merchants)
I know full well that 18mm MDF doesn't really cut it in open baffle speakers, which are massively excited by multiple woofers rocking the whole assembly back and forth.
I have heard of MDF & chipboard being an effective combination. I could have a cheap, pretty and hopefully solid baffle made from, say, 15mm veneered MDF stuck to 12 or 15mm chipboard...
Simon
Attachments
Simon I wonder if there is any way to raise the total "Q" factor of those Skytronic woofers without robbing them of too much SPL??
Adding mass will lower Fs and raise "Q" but at the expense of SPL/efficiency.
From the little work I've been doing a woofer Q of around 1 seems to be the best compromise; above that the peak at resonance uses up all of the woofers excursion and below that there is less and less bass as the "Q" decreases.
What we need is a dedicated 12 or 15inch OB woofer that we all can afford. Yes I know I've said it before, I need to learn to accept compromise.
Adding mass will lower Fs and raise "Q" but at the expense of SPL/efficiency.
From the little work I've been doing a woofer Q of around 1 seems to be the best compromise; above that the peak at resonance uses up all of the woofers excursion and below that there is less and less bass as the "Q" decreases.
What we need is a dedicated 12 or 15inch OB woofer that we all can afford. Yes I know I've said it before, I need to learn to accept compromise.
I don't think the Qts of 0.49 is too bad - it's 0.09 higher than what I'm listening to now - and this works. At least it's a little closer to the ideal, and with bags more displacement (2 x 12" vs 4 x 7") so if EQ ever comes into it, they won't die on their bottoms completely. I'm going to get the speakers designed and built before deciding if any driver modifications or alternative drivers are required.
Remember - T-bass makes things possible that most OB builders haven't experienced with quite ordinary drivers.
In the UK at least, the Alpha 15 is the woofer you speak of, but I can't accommodate them in this room... just give me a few years to buy a house of my own
Simon
Remember - T-bass makes things possible that most OB builders haven't experienced with quite ordinary drivers.
In the UK at least, the Alpha 15 is the woofer you speak of, but I can't accommodate them in this room... just give me a few years to buy a house of my own
Simon
SimontY said:
I did have positive experience with these shelf-mounting profiles
An externally hosted image should be here but it was not working when we last tested it.
screwed to some flimsy chipboard.And speaking "massively": If you put a small (40x40 cm) concrete paving slab on top of the U frame that will stop any rocking efficiently.
Rudolph that is simply a great idea, cheap readily available and very stiff for the mass involved.
Also I happen to have some spare. I'll try those on my next unsupported panel. Would you use just screws or a bead of epoxy as well??
Simon you may need to look at putting the "FEET" on the base a little forward of the baffle for stability. I did on mine and it has improved thing markedly.
Regards
Ted
Also I happen to have some spare. I'll try those on my next unsupported panel. Would you use just screws or a bead of epoxy as well??
Simon you may need to look at putting the "FEET" on the base a little forward of the baffle for stability. I did on mine and it has improved thing markedly.
Regards
Ted
Ted,Moondog55 said:
I found that screws worked well enough. If you feel the need you could add screws in the slits between the "official" mounting holes. I see these profiles as interim solutions until the baffle shape is optimised. After that I would recommend a massive baffle. So I try to avoid messing around with glue/epoxy on those profiles.
BTW: I have some massive but light weight aluminium profiles from a shop shelving system too. They are even more sturdy, but surely too expensive to buy. Maybe you stumble across those at a shop renovation ...
Rudolf
Just a couple of quick thoughts Simon.
2x Skytronic in parallel will have the same sensitivity as 4x in series-parallel. So 2x drivers on a baffle won't be inferior to 4x on the same sized baffle other than for being slightly down on maximum LF SPL.
Your two Skytronics will displace more air than a 15" anyway.
Also, do be aware that the dome of the APT (leading edge of the front magnet pole piece) needs to be level with the acoustic centre of the 8" driver at crossover, which might be the Beta's dust-cap dome too, or just a litle behind it.
Just mounting the horn of a horn driver on the same baffle as a mid-range is not acoustically correct !!!
(Look at Lynn Olson's sketches in 'Beyond the Ariel'.)
Cheers .......... Graham.
2x Skytronic in parallel will have the same sensitivity as 4x in series-parallel. So 2x drivers on a baffle won't be inferior to 4x on the same sized baffle other than for being slightly down on maximum LF SPL.
Your two Skytronics will displace more air than a 15" anyway.
Also, do be aware that the dome of the APT (leading edge of the front magnet pole piece) needs to be level with the acoustic centre of the 8" driver at crossover, which might be the Beta's dust-cap dome too, or just a litle behind it.
Just mounting the horn of a horn driver on the same baffle as a mid-range is not acoustically correct !!!
(Look at Lynn Olson's sketches in 'Beyond the Ariel'.)
Cheers .......... Graham.
Rudolf,
Thanks for the ideas, I will look at ways to get some mass on there. So your idea (a nice one) to put shelving brackets on is just to stiffen up a thin test baffle is it?
And thanks again for the advice you gave via e-mail.
Ted,
You're certainly on the money with the feet - I'll amend my diagram accordingly. The plinth on my mk1 baffles comes forward about 5cm and the speakers feel really stable.
Graham,
I've looked at the Lynn Olson thread but it's nearly 200 pages long, can you point me in the right direction? I've examined the drivers (they just arrived!!) and the front part of the metal assembly (where the magnet joins to the horn part) is just over 3cm from the front of the horn. On the Beta 8A, the front of the former/back of the dustcap is also 3cm from the very front of the driver (paper front-mounting gasket). So it looks like mounting them on the same baffle probably IS acoustically correct. I will take pictures later to confirm, as I'm not 100% sure.
About the drivers - other than one woofer frame being slightly bent at the edge from transit, they look great. The Beta 8A has a massive magnet, and it has the purple cone as hoped (to confirm it's the same as Panomaniac heard & loved). It has front and rear mounting gaskets and 8 fixing holes. The magnet has a nice rubber boot too. It's a helluva lot of driver for £33.
Simon
Thanks for the ideas, I will look at ways to get some mass on there. So your idea (a nice one) to put shelving brackets on is just to stiffen up a thin test baffle is it?
And thanks again for the advice you gave via e-mail.
Ted,
You're certainly on the money with the feet - I'll amend my diagram accordingly. The plinth on my mk1 baffles comes forward about 5cm and the speakers feel really stable.
Graham,
I've looked at the Lynn Olson thread but it's nearly 200 pages long, can you point me in the right direction? I've examined the drivers (they just arrived!!) and the front part of the metal assembly (where the magnet joins to the horn part) is just over 3cm from the front of the horn. On the Beta 8A, the front of the former/back of the dustcap is also 3cm from the very front of the driver (paper front-mounting gasket). So it looks like mounting them on the same baffle probably IS acoustically correct. I will take pictures later to confirm, as I'm not 100% sure.
About the drivers - other than one woofer frame being slightly bent at the edge from transit, they look great. The Beta 8A has a massive magnet, and it has the purple cone as hoped (to confirm it's the same as Panomaniac heard & loved). It has front and rear mounting gaskets and 8 fixing holes. The magnet has a nice rubber boot too. It's a helluva lot of driver for £33.
Simon
Here's a better sketch of how they might look.
I've now drawn the back up and made them 1cm wider to make sure the woofers fit on ok. I've reduced the side wings a fraction and altered the top driver layout a bit. I've also enlarged the plinth and made it extend out of the front.
I'd welcome practical, neat suggestions for adding mass and stability at this point.
Thanks,
Simon
I've now drawn the back up and made them 1cm wider to make sure the woofers fit on ok. I've reduced the side wings a fraction and altered the top driver layout a bit. I've also enlarged the plinth and made it extend out of the front.
I'd welcome practical, neat suggestions for adding mass and stability at this point.
Thanks,
Simon
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