String mounted dipole

[phazer99]

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.

 

[Rudolf]

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.

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.

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

 

[zmyrna]

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?

 

[phazer99]

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?

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.

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.

 

[phazer99]

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.

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?

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

 

[Rudolf]

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

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.

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

 

[bonzite51]

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...

 

[phazer99]

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?

 

[Rudolf]

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.

... 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

 

[picowallspeaker]

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 ...

 

[picowallspeaker]

Moreover, the art of good sound reproduction must pass through isolation of the transducers from the environment.
Also the absorption of the back wave, it can only be good with the use of natural wool or felt, as it has the characteristics for the duty.
Just make a tall frame and hang the drivers with rubberbands, using also the magnet for pivoting it on axis . Use lots of wool or felt on the back to absorb the backwave.
That's it !

 

[phazer99]

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

What's the main problem with the driver, too low Qts value?

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.

Thanks. Do you have some link to info on how to calculate the dimensions for an H frame?

 

[Rudolf]

What's the main problem with the driver, too low Qts value?

There is no problem with the driver itself. But most people would consider two Peerless SLS 10 per side as the minimum. They have >4 times the air moving capability compared to two HiVis.

Thanks. Do you have some link to info on how to calculate the dimensions for an H frame?

I do it with the H frame worksheet of Martin J King:
Quarter Wavelength Loudspeaker Design
Look at http://www.quarter-wave.com/OBs/U_and_H_Frames.pdf too. Page 3 gives a basic calculation for the frame depth.

 

[phazer99]

Did a first test with the Neo8s yesterday. I mounted it on a 20 cm wide open baffle and active XO with a 10" woofer in a sealed box. First impression was that this driver is loud (had to turn it down about 8 dB compared to the woofer)! The measurement showed the expected peak at about 11 kHz. After Audyssey XT32 calibration the frequency response was pretty flat and integration with the woofer was good. In this setup with digital EQ there's no problem in running the Neo8s from 400-500 Hz to 20 kHz. Below that it dips quite steeply.

The driver sounds good. Comparing the sound with an Acoustat Spectra 11 ESL panel I would say it's a little bit brighter (maybe because my Acoustat panels are old), but with less detail and not as clean separation of sounds. I would say I prefer the sound of the ESL but the Neo has much wider dispersion, is easier on the amp and of course doesn't require separate power, so it's to work with for sure.

Is there a break in period for the Neo8s and can I expect the sound to change much over time?

 

[phazer99]

There is no problem with the driver itself. But most people would consider two Peerless SLS 10 per side as the minimum. They have >4 times the air moving capability compared to two HiVis.

I don't understand how you came to that result. The SLS 10 has an effective area of 346 cm2 and xmax of 8.3 mm giving a volume displacement of 287 cm3. The M8N has an effective area of 214 cm2 and xmax of 5.8 mm giving a volume displacement of 124 cm3. So the SLS moves about 2.3 times more air than the HiVi. Or did I calculate incorrectly?

I've looked at many woofers and currently the SLS 10" and the ScanSpeak Discovery looks like reasonable priced alternatives. The main differences are that the SLS has larger xmax and thus moves more air, but the ScanSpeak has higher Qms, lower Qts and is more sensitive. I figured lower Qts means more control of the cone and might be an advantage when using the driver up to 300-400 Hz. Is this a correct assumption? Are there any other important differences between the woofers when using them in an open baffle? Are there any other good 8" or 10" woofer alternatives?

 

[Rudolf]

I don't understand how you came to that result. The SLS 10 has an effective area of 346 cm2 and xmax of 8.3 mm giving a volume displacement of 287 cm3. The M8N has an effective area of 214 cm2 and xmax of 5.8 mm giving a volume displacement of 124 cm3. So the SLS moves about 2.3 times more air than the HiVi. Or did I calculate incorrectly?

From http://www.diyaudio.com/forums/planars-exotics/233909-string-mounted-dipole.html#post3450979 I got the impression that you were thinking about 2 HiVis total - not pairs. Your calculation is correct of course.

... but the ScanSpeak has higher Qms, lower Qts and is more sensitive. I figured lower Qts means more control of the cone and might be an advantage when using the driver up to 300-400 Hz. Is this a correct assumption?

I find the Discovery tempting because of the cast frame and the lower Fs. But I don't buy into this "cone control" thing at all. And Qts has absolutely lost any meaning four octaves above Fs.

Rudolf