Soundlab (Chinese) 204mm (8") drivers in open baffle array - Page 3 - diyAudio
Go Back   Home > Forums > Loudspeakers > Full Range

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 16th July 2011, 08:09 PM   #21
diyAudio Member
 
chris661's Avatar
 
Join Date: Feb 2008
Location: Sheffield
Blog Entries: 8
Quote:
Originally Posted by timmillea View Post
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
__________________
"Throwing parts at a failure is like throwing sponges at a rainstorm." - Enzo
My setup: http://www.diyaudio.com/forums/multi...tang-band.html
  Reply With Quote
Old 17th July 2011, 09:32 AM   #22
diyAudio Member
 
chris661's Avatar
 
Join Date: Feb 2008
Location: Sheffield
Blog Entries: 8
Quote:
Originally Posted by sreten View Post
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
__________________
"Throwing parts at a failure is like throwing sponges at a rainstorm." - Enzo
My setup: http://www.diyaudio.com/forums/multi...tang-band.html
  Reply With Quote
Old 17th July 2011, 11:13 AM   #23
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Quote:
Originally Posted by chris661 View Post
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.
__________________
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow

Last edited by sreten; 17th July 2011 at 11:43 AM.
  Reply With Quote
Old 17th July 2011, 11:16 AM   #24
diyAudio Member
 
Join Date: Jul 2011
Location: Huddersfield, UK
Quote:
Originally Posted by chris661 View Post
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.
Attached Images
File Type: jpg THD vs Power.jpg (55.9 KB, 168 views)
File Type: jpg Efficiency vs output power.jpg (39.1 KB, 165 views)
  Reply With Quote
Old 17th July 2011, 11:54 AM   #25
diyAudio Member
 
Join Date: Jul 2011
Location: Huddersfield, UK
Quote:
Originally Posted by sreten View Post
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.

Last edited by timmillea; 17th July 2011 at 12:02 PM.
  Reply With Quote
Old 17th July 2011, 11:56 AM   #26
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
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.
__________________
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow

Last edited by sreten; 17th July 2011 at 12:15 PM.
  Reply With Quote
Old 17th July 2011, 01:00 PM   #27
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Hi,

Just realised if your using a baffle simulator e.g. :
Home of the Edge
Your arguments make some sense.

But the above only applies to boxes not open baffles.

Baffle Step for boxes is not Baffle Loss for open baffles.

rgds, sreten.
__________________
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow

Last edited by sreten; 17th July 2011 at 01:06 PM.
  Reply With Quote
Old 17th July 2011, 01:12 PM   #28
diyAudio Member
 
Join Date: Jul 2011
Location: Huddersfield, UK
Quote:
Originally Posted by sreten View Post
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!
  Reply With Quote
Old 17th July 2011, 01:25 PM   #29
diyAudio Member
 
Join Date: Oct 2005
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 ;-)

Last edited by sharpi31; 17th July 2011 at 01:29 PM.
  Reply With Quote
Old 17th July 2011, 01:53 PM   #30
diyAudio Member
 
chris661's Avatar
 
Join Date: Feb 2008
Location: Sheffield
Blog Entries: 8
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
__________________
"Throwing parts at a failure is like throwing sponges at a rainstorm." - Enzo
My setup: http://www.diyaudio.com/forums/multi...tang-band.html
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
"Box" design and bracing for open baffle woofer array ryoshu Multi-Way 0 17th August 2008 06:07 PM
open baffle and/or line array lolojr1 Multi-Way 8 7th July 2006 04:47 AM
Open Baffle Line Array - using cheap drivers giambi77 Multi-Way 4 26th January 2004 09:29 PM
line array open baffle spkerguy Multi-Way 5 6th January 2004 08:42 AM
Open baffle line array ryoshu Multi-Way 15 15th January 2003 11:10 AM


New To Site? Need Help?

All times are GMT. The time now is 07:30 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2