Soundlab (Chinese) 204mm (8") drivers in open baffle array

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That is what I am hoping for. The total area should be approaching the equivalent of a 16 inch driver.

Thanks for that. I don't understand why though. Is this specific to open baffles?

Here is the little Lepai amp powered up from the batteries. There is a reassuring click of a relay around half a second after switching on. I will mount the amp & battery pack on the rear of one of the baffles to make the speakers 'active'. The source will be an old iPod mini connected with a 1/8" lead.

The last time I made speakers was before CDs let alone iPods, class-D amps and line arrays.

I have just bought 100m 79-strand 2.5 sq mm speaker wire for £13 (from Wilkinson Plus). I am sure this was almost £1 a metre 30 years ago. With a nominal rating into 8 ohms of 3.2KW (!) it should not interfere with the transients.



Many thanks!

You're quite welcome.

Okay, the trick with the series resistor is actually to lower the damping effect of the amplifier at low frequencies.
By allowing the cone to "ring" slightly at low frequencies (nothing anyone could take offence at), the output of the driver around its fundamental resonance increases.
So, the amplifier lets go of the cone a little, giving more output in the low frequencies.
A more complex answer is that the Qes (electrical damping) of the speaker increases, thus raising Qts towards Qms (mechanical damping).

Fine-tuning can take a while, but I would try to use less than 4ohm in order to keep power losses reasonably low.

Chris
 
Hi,

No. It simply won't work at all and the application of "theory" is simply wrong.

Contrary to what you say, I posted how to use the drivers for the application.

Somebody "complaining" about their ideas being criticised isn't even listening.

I'll repeat, it simply won't work at all as it stands. The drivers are very likely
too low Q and lack excursion capability. By all means build it if you want to
pretend it will be allright, it wont, it will be terrible for the intended purpose.

rgds, sreten.

With a 10" wide baffle effective cone area will plummet in the bass. a 15"
driver in a 24" baffle is about equivalent to a single 8" in a box, here you
will end up in the bass with the equivalent of 4 x 2" drivers, it will not work.

Cheap drivers tend to have a high Q as a result of cost saving on the magnet. These drivers look to me as the sort of thing found installed in ceilings in shops, or other such systems that will need a high Qts for reasonable bass output.
I'm also curious about where you got the equivalent sizes for drivers on different baffles. I can't see how one can make a direct comparison between two low frequency systems that are so different.


Tim, how loud will these need to play?
I would be worried about driver over-excursion at low frequencies, if you're going to be asking PA levels of them. A low-cut filter
With a narrow baffle, depending on listening distance, you might find the need to eq up the lower midrange. This would eat up some amplifier power.
What are the AA batteries you're planning on using?

Chris
 
Cheap drivers tend to have a high Q as a result of cost saving on the magnet. These drivers look to me as the sort of thing found installed in ceilings in shops, or other such systems that will need a high Qts for reasonable bass output.
I'm also curious about where you got the equivalent sizes for drivers on different baffles. I can't see how one can make a direct comparison between two low frequency systems that are so different.

Chris


Hi,

No.
A cheap 8" driver with 96dB sensitivity is most likely low Q, and no excursion.
You can increase Q with a series resistor, but your losing senstivity, and
for a portable battery powered system that is not a good approach.

Like I said before, for a 10" baffle width any discussion of bass is pointless.

The equivalence is only approximate, but e.g. :
http://www.quarter-wave.com/OBs/OB_Design.pdf
You cannot expect more overall bass output than a decent 8" in a box.
Bass output capability of the above is nothing like the driver boxed.

The lower you go the worse it gets due to the baffle loss of OB's.
(Why Quad ELS57's go no louder in the bass than LS3/5A's.)
The comparison is based on baffle loss, the lower you go and
the narrower the baffle the worse it gets. People simply do
not realize its a real loss, not a frequency response shaping.

For this application : a 4 x 1 line array of 8" drivers that are likely low
Q with limited excursion, powered by a low powered amplifier, so any
real bass boost is out of the question the optimum solution is simple :

A vented box, fairly wide to drop the baffle step frequency. Drivers
mounted offset on a 0.6 width of the baffle. A reasonable guess is
50 to 70 litres tuned to 40-45 Hz. (Driver Fs is apparently 40Hz.)
(Tuning low is always best when guessing, 44Hz = bass bottom E.)
The efficient way of increasing driver Q is using box volumes.
Vented bass efficiency is far higher than sealed, best for low power.

The above will have oodles of bass capability compared to a 10" OB.
Outdoors it will still massively struggle, walls and corners will help.

rgds, sreten.
 
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Tim, how loud will these need to play?
I would be worried about driver over-excursion at low frequencies, if you're going to be asking PA levels of them. A low-cut filter
With a narrow baffle, depending on listening distance, you might find the need to eq up the lower midrange. This would eat up some amplifier power.
What are the AA batteries you're planning on using?

Chris,

Not loud at all they are for background music at a garden party. I may crank up the volume when I get them inside for myself.

If their rated 96dB SPL at 1W/1m is correct, and given their combined 102dB, I may end up driving them with less than a tenth of a Watt! Either way their should be plenty of headroom in the amp, cabling and drivers.

The AAs are the low self-discharge rate NiMh cells with a 2100mAh capacity: 2.1Ah x 12V nominal = 25.2 Wh.

With an 8 ohm load, the Tripath 2020 amp has a mean THD+noise of around 0.02% (-74dB) until around 7W when it starts climbing a cliff face to 10% @ 13W. Its efficiency starts around 20% and rises to 90% at 13W. At 7W, ts efficiency is around 81%: 7W/ 0.81 = 8.64W input or approx 3 hrs battery life at max usable power with a corresponding SPL of almost 120dB at 1m. I understand the Lepai amp uses an opamp to drive the tone controls so the actual battery life should be a little less than Tripath figures alone would suggest.

Here is a link to the Tripath data sheet. I have lifted the relevant graphs and stuck them below.

http://www.e-ele.net/DataSheet/TA2020.pdf

Tim.
 

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Like I said before, for a 10" baffle width any discussion of bass is pointless.

The lower you go the worse it gets due to the baffle loss of OB's.
The comparison is based on baffle loss, the lower you go and
the narrower the baffle the worse it gets. People simply do
not realize its a real loss, not a frequency response shaping.

Sreten,

I disagree with your analysis.

As the wavelength gets large compared to the distance from the ground, the ground begins to act like a very large baffle. My design uses a minimal baffle purposely to remove the usual baffle lift from around 1Khz downwards - yes extremely high. The height of the drivers above the ground was chosen to reinforce only the low bass. The reinforcement begins gently around 150Hz and should reach a full 6dB at around 50Hz and below. The free-air resonant frequency of the drivers is quoted at 40Hz so the roll-off will be fairly fast after this.

The problem with large open baffle designs is that of trying to support the whole bass range. As I pointed out earlier on, this naive approach would require at least a 7m baffle. My design subtracts 6dB until the low bass in order to compensate for the usual weakness of an OB design.

I appreciate your challenging comments which have sent me much deeper into exploring OB design theory in order to check my approach than I would have otherwise gone.

Tim.
 
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Hi,

Sod it. Build it. Your not listening. Live and learn.
It won't work as you describe - you will find out.

rgds, sreten.

6dB ground lift @ 50Hz is bugger all compared to 10" baffle loss.
6dB more of no bass is still no bass, its all completely wrong.
You haven't "fixed" OB's. You simply do not understand them.
 
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Just realised if your using a baffle simulator e.g. :
Home of the Edge
Your arguments make some sense.

Sorry no. I only became aware of simulators due to this thread a few days ago and I haven't found one for Mac OS yet. The simulation is just on paper, in spreadsheets and my head :)

It is great to try out my ideas for a project before actually building it and I am genuinely grateful to all the contributors here.

Thanks!
 
Hi Tim,

I too started learning about OB theory only recently. I'd heartily recommend Martin King's excellent white paper detailing the design process for an OB system: http://www.quarter-wave.com/OBs/OB_Design.pdf

I've built a variation on Martin's design (using FF85K up top) and been very pleased with the results.

I have to agree with Sreten that I don't think you'll get any bass from your proposed design. Even ingoring baffle width and associated impact on bass, I suspect that those Soundlab drivers will have an Xmax of 2mm or less (common with full range 8" drivers), so you simply won't be able to shift enough air to get meaningful bass (unless you build a huge 20m horn and sit listeners right at the mouth!).

However, I would encourage you to build anything you want as you'll no doubt learn lots however the design turns out. I've built lots of crap over the years and I've always ended up wiser as a result ;-)
 
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For background music, the 4x8"s on open baffle might be enough, but I susppect you'd be pushing it if you asked for any more.

As an attempt at a compromise, why not build a sealed (or ported) enclosure around the bottom two drivers?

Another option would be a seperate subwoofer (in a more conventional cabinet), and cross over the 8"s. This would greatly reduce stress on the wideband drivers, allowing cleaner mid-high output.

Chris
 
Yes they do.

I remember seeing something a while ago where a small-ish system was being used outdoors, and some people found that the sound was much quieter off-axis with the speakers, enabling them to have a conversation without straining.

Unorthodox, sure, but never non-existent.
Here's another example.
Dipole PA system

Chris

(lets not forget the system being built here will be used for background music - nothing particularly demanding)
 
Sorry no. I only became aware of simulators due to this thread
a few days ago and I haven't found one for Mac OS yet. The
simulation is just on paper, in spreadsheets and my head :)

It is great to try out my ideas for a project before actually
building it and I am genuinely grateful to all the contributors here.

Thanks!

Hi,

Well all I can say is you've got something very very wrong somewhere.

300mm cuts off at around 400Hz and peaks somewhat higher higher up.

50Hz will be at least 18dB down, and due to baffle loss the effective
size of your 4x8" array will be less than 4x1" - completely useless.

(That is max SPL at 50Hz will be < 4 x 1" boxed drivers with same excursion.)

Bass efficiency and max SPL will be appalling, it simply does not work.

rgds, sreten.

Head in the sand arguments are just so tedious, smell the coffee.
 
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Yes they do.

I remember seeing something a while ago where a small-ish system was being used outdoors, and some people found that the sound was much quieter off-axis with the speakers, enabling them to have a conversation without straining.

Unorthodox, sure, but never non-existent.
Here's another example.
Dipole PA system

Chris

(lets not forget the system being built here will be used for background music - nothing particularly demanding)

Hi,

Its still a stupid idea. It won't do what you say it will, at all, and
now it seems your making excuses to do something extremely badly.

rgds, sreten.
 
Yes they do.

I remember seeing something a while ago where a small-ish system was being used outdoors, and some people found that the sound was much quieter off-axis with the speakers, enabling them to have a conversation without straining.

Unorthodox, sure, but never non-existent.
Here's another example.
Dipole PA system

Chris

(lets not forget the system being built here will be used for background music - nothing particularly demanding)

Hi,

Its still a stupid idea. It won't do what you say it will, at all, and
now it seems your making excuses to do something extremely badly.

I'd suggest building with baffles that suit fairly wide boxes.
Then compare OB with vented boxes, no comparison.

rgds, sreten.

The bass end of the linked OB system uses 2x18" in a large folded baffle.
Its not a commercial system, max SPL is far too limited for the given
drivers, but with lots of power on tap at least it can work practically.
 
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Having done a rough experiment, I'm inclined to agree with sreten.

The set-up was as follows.

A single 8" (92dB@1w) driver on a 10" square baffle, sat on the floor, supported by a cushion leant on the sofa (so the cushion decouples the speaker from the sofa). Comparisons were to a single tapped horn (linked in my signature, ~84dB@1w), with the amplifier on the same volume setting.

Turned up to a reasonable level (actually, I was scared of pushing the 8" further), cone excursion was in excess of 8mm p/p. Bass output wasn't much at all. Some midrange was coming through (on a 24dB/oct low pass).
On the same volume setting, a single tapped horn gave physically tangible bass.

I had thought that the 4x8"s here would be roughly comparable to a 15" driver, but I had failed to account for the difference in baffle widths. This has been an eye-opener for me, as I've heard a pair of 15" drivers on open baffle produce plenty of bass (in a big room) with not a lot of excursion.

If you wanted to go ahead with this project, I'd suggest the following.
5 drivers per side, mounted with one roughly central in a large baffle, 4 around it. The central one does the whole frequency range, the other 4 crossed over so that they re-inforce the low frequencies.
The resulting wider baffle and eq provided by low frequency re-inforcement would mean much better bass output, and HF interference won't be a problem, as it'd be a single HF radiator per side.
I know you didn't want to use a crossover, but for an open baffle system, it really is the only way.

Chris
 
Having done a rough experiment, I'm inclined to agree with sreten.

If you wanted to go ahead with this project, I'd suggest the following.

Chris

Hi. I'd suggest at minimum using two doors, but don't think the drivers suit well, rgds, sreten.

P.S. your also right that series inductors on 3 of the 4 drivers is a good idea, or line level EQ.
Probably one inductor for two parallel drivers, and one in series of one of the other pair.
 
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OK, I have put my baffle-maker on hold. I accept that I made a mistake in not accounting for the extra 6dB baffle rolloff. Apologies for making you pull out your hair in trying to convince me! You succeeded. Thank you for all your suffering.

Also, there is now a new dimension in the equation. My brother had a clear out of his loft today and found a 15" woofer I bought almost 30 years ago. I thought it had long since been thrown away. Did I still want it ? Yes!!

When I bought it I knew the t/s parameters but not now. I have searched on the net and not even the make can be found let alone the model. It is probably not the best quality but does have a cast aluminium cage. The magnet looks like ferrite with dimensions 16cm x 20mm and it has a paper cone with rubberised edge.

So now I accept I need bass support, what is the best strategy with an unknown 15" woofer? It would be great to keep the main OB pair crossover-free, handling the 'critical range' low mids upwards and relying upon the natural bass roll-off instead. Then I could just put a corresponding low-pass on the woofer. Amps modules are cheap enough and I would like to avoid a huge box.

It has been a bit long-winded but basically I would like to have a pure OB, crossover-free pair handling all the critical range and use my unknown 15" woofer with filter and an extra amp in the smallest enclosure to handle what the OBs can't.

All ideas, pointers or links appreciated.

Thanks,

Tim.
 
Wahey, that's gonna help move some air.

With unknown T/S parameters, I think I'd try either a decent sized sealed cabinet, or a bigger ported one tuned ~40Hz.

I think it would be wise to put an active crossover in for the open baffles. They will be so much cleaner without the extra bass.

Any idea how much linear travel the woofer has?
 
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