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Old 11th April 2013, 09:02 AM   #1
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Default String mounted dipole

I've purchased a pair of BG Neo8s and I'm considering my options on what type of system to build with them. I want to pair each Neo with one or two 6-10" woofers probably in an OB configuration. I'll use active XO and digital EQ and run the Neo8s's without additional tweeters. I hope to use the woofers from about 60-70 Hz (I use subs below that) up to about 300-400 Hz where I'll XO with the Neos.

I'm new to speaker building so I need some tips. One option I'm considering is to mount all the drivers only using some kind of vibration dampened string in a metal or wooden frame. I figured this will minimize vibrations spreading from the drivers. I can't find much information about this sort of mounting on the web (I remember seeing some pictures of it though). What's the downsides to this type of driver mounting and where can I find more information about it? Additionally, I'm considering covering all the free space between the frame and drivers with Basotect or similar acoustic damping material. I figured that would make the baffle area larger and thus extending the lower range of the drivers (at least the Neo's). Is it a good idea?

I'm also looking for tips on what 6-10" woofer drivers would be suitable to use with the Neo8s in an OB configuration. Price up to about $100.

Thanks.
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Old 11th April 2013, 02:25 PM   #2
Rudolf is offline Rudolf  Germany
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Quote:
Originally Posted by phazer99 View Post
One option I'm considering is to mount all the drivers only using some kind of vibration dampened string in a metal or wooden frame. I figured this will minimize vibrations spreading from the drivers. I can't find much information about this sort of mounting on the web.
It doesn't matter whether the string is vibration damped. You don't need to damp vibrations along the string, but perpendicular to it. What you use, is the basic pendulum effect. Don't string the speaker between elastic cords. This will result in a spring-mass-system going along with resonances. Just hang it from two simple cords.
Quote:
What's the downsides to this type of driver mounting and where can I find more information about it?
You will loose a very small part of the driver excursion. You will win the missing force of the driver transmitted to a baffle or a box. But for most people who mount their driver this way, it is just a convenient way to avoid a baffle altogether.
Quote:
Additionally, I'm considering covering all the free space between the frame and drivers with Basotect or similar acoustic damping material. I figured that would make the baffle area larger and thus extending the lower range of the drivers (at least the Neo's). Is it a good idea?
If you want a baffle, why don't you want to mount the driver on it? Make sure you understand how a baffle changes the response of a driver like the Neo8.

Rudolf
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Old 11th April 2013, 03:40 PM   #3
zmyrna is offline zmyrna  United Kingdom
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My neo8s are on the post and I am planning a 3 or 4 way OB with them as well.
I think it would be difficult to make neo8s work down to 300-400Hz without a rather large baffle. I am planning to cross them around 800-1000Hz (nude or with a small waveguide).

Placing drivers on separate baffles solves vibration issue best. A vertical brace in the middle of the baffle is also very useful.

Absorptive baffles are difficult: any absorptive material would leak under certain freq, absorb certain freq, and reflect the rest. Do you have the time, patience, skills and equipment to experiment?
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Old 11th April 2013, 04:31 PM   #4
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Quote:
Originally Posted by Rudolf View Post
It doesn't matter whether the string is vibration damped. You don't need to damp vibrations along the string, but perpendicular to it. What you use, is the basic pendulum effect.
And this will prevent the string from resonating?
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Originally Posted by Rudolf View Post
Don't string the speaker between elastic cords. This will result in a spring-mass-system going along with resonances.
I thought that using elastic strings of non-uniform mass and of different lengths (sort of spider web like) would dampen the resonances occurring in the strings and driver.
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Originally Posted by Rudolf View Post
If you want a baffle, why don't you want to mount the driver on it? Make sure you understand how a baffle changes the response of a driver like the Neo8.
My idea was that a string mounting would introduce less audible distortion than a normal baffle in wood, but if that's not the case I'll probably go a normal wood OB design.
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Old 11th April 2013, 04:36 PM   #5
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Quote:
Originally Posted by zmyrna View Post
My neo8s are on the post and I am planning a 3 or 4 way OB with them as well.
I think it would be difficult to make neo8s work down to 300-400Hz without a rather large baffle. I am planning to cross them around 800-1000Hz (nude or with a small waveguide).
Looking at the specs you're probably right, but it's not a big issue to raise the XO to about 800 Hz.
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Originally Posted by zmyrna View Post
Placing drivers on separate baffles solves vibration issue best.
Do you mean completely separate baffles standing on the floor? How do you align the drivers vertically in such a setup?
Quote:
Originally Posted by zmyrna View Post
Absorptive baffles are difficult: any absorptive material would leak under certain freq, absorb certain freq, and reflect the rest. Do you have the time, patience, skills and equipment to experiment?
I think any baffle material would leak under some freq and reflect as well. Well, I have time and patience to experiment, but I'm lacking in skill and tools for wood working so I'm looking for a relatively simple design
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Old 11th April 2013, 10:39 PM   #6
Rudolf is offline Rudolf  Germany
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Quote:
Originally Posted by phazer99 View Post
And this will prevent the string from resonating?
Sorry, no. What I should have written is: "You don't need to damp vibrations along the string, you only have to cope with vibrations perpendicular to it."
And "to cope" means effectively: Forget about them.
Let me explain:
Typically the moving mass (cone/dome, coil and spider) of a driver weighs 50-100 times less than the fixed mass (basket and magnet). And the surface of the suspending strings will be 10-100 times smaller than the surface of the cone/dome. This leads to the string excursion being 50-100 times less than the cone/dome excursion and the air volume moved by the "resonating" strings being 1000-10000 times less than the volume moved by the cone/dome.

If at all, I would be more concerned about sound diffraction at the bulky rubber strings as shown here:
http://i.imgur.com/6kUhD5z.jpg

Quote:
I thought that using elastic strings of non-uniform mass and of different lengths (sort of spider web like) would dampen the resonances occurring in the strings and driver.
See my last sentence above.
Quote:
My idea was that a string mounting would introduce less audible distortion than a normal baffle in wood, but if that's not the case I'll probably go a normal wood OB design.
This could be true, but it would very much depend on the baffle construction and the frequency range - woofer, mids or treble.

Rudolf
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Old 12th April 2013, 06:15 AM   #7
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Yes, to be honest, I like this idea very much - to try Neo 8 marry with Alpha 15A which I already have on OB paired with Ciarre CH250, just 1st order crossed.
Then probably I will make different baffle for Neo 8, just have not idea how width is necessary...
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Old 12th April 2013, 07:50 AM   #8
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Quote:
Originally Posted by Rudolf View Post
If at all, I would be more concerned about sound diffraction at the bulky rubber strings as shown here:
http://i.imgur.com/6kUhD5z.jpg
Yes, that's why I plan to use damping material as a baffle if I chose this type of mounting.

I looked at your website, lots of useful info there. I'm still unsure what woofer driver to use with the BG Neo8s. I'm considering the HiVi M8N 8" driver because of the good reviews and performance, reasonable price and nice looks . Would a pair of those be a suitable choice, and what baffle dimensions would you recommend to get good performance down to about 60-70 Hz?
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Old 12th April 2013, 03:30 PM   #9
Rudolf is offline Rudolf  Germany
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Quote:
Originally Posted by phazer99 View Post
I'm considering the HiVi M8N 8" driver because of the good reviews and performance, reasonable price and nice looks . Would a pair of those be a suitable choice,
I believe that nobody but me would give a pair of those drivers (per side) any chance in a dipole, but you can risk it if you
- don't sit much farther than 2 m away from them
- are content with modest volumes (nothing to wake up the neighbours)
- are prepared to roll off anything below 70/80 Hz
- have subwoofers below.
Quote:
... and what baffle dimensions would you recommend to get good performance down to about 60-70 Hz?
You should have a small H-frame around them - shallow enough to allow an upper Xover at ~500 Hz. The H would be about 25 cm wide. I could give you precise dimensions when the project takes off.

Rudolf
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Old 12th April 2013, 03:46 PM   #10
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Remember that the gravity force is your friend...
Or, it's the field that we're immerged in, so....
Just hang the drivers on rubber bands- they'll need to be stretched to the lenght that permits the load to oscillate BUT only out of audio band, i.e. at subsonic frequencies.
So each driver would find its static position and it would be able to move only out of audible band.
Another thing is the kind of sound from dipole radiation, or the sound of a quasi-cylindrical source such as a Neo 8, but that's another theme ...
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