Racetrack shaped drivers?

Shawn_K

Racetrack shaped drivers?

I have noticed that many drivers have a useable frequency beyond where you would usually cross them over at due to beaming. For example, a 4 inch driver may have a smooth frequency response up to 5 kHz or higher if the cone breakup is damped effectively. I usually would cross a 4 inch driver at or above 3 kHz in for a high performance loudspeaker design due to beaming – I like wide dispersion for stereo imaging.

I’m thinking about an MTM layout with a Hiquphon OW2 tweeter and a Racetrack shaped driver with an outside frame dimension of 126 x 150mm (W x H). A driver with these dimension would have around 77cm^2 effective cone area with a 28mm Voice coil and phase plug; which is 35-54% more cone area then a typical 4 inch driver. I would like to have a 2.6-3 kHz X-over point which the racetrack driver would have the horizontal dispersion to accomplish but would sacrifice some vertical dispersion and the vertical driver spacing would be more which would effect lobbing. The ideal driver would have a Qts low enough to achieve a closed box Qtc of 0.5 (critically damped). High pass filter 2nd order filtered at about 180 Hz Via electronic or digital means. A larger 8.5-10 inch woofer would be mounted lower on the baffle in a MLTL alignment and powered with a DSP enabled subwoofer plate amplifier. The racetrack drivers would only need about 3.5mm of X-max with 154cm^2 cone area to produce low distortion down to 180 Hz. I would probably mount the tweeter behind the baffle and have a 3.25 inch diameter waveguide to align the dispersion profile between the driver for more even power response and better driver offset time alignment. Except for the custom driver needed for this system to work what do you guys think of this idea?

Shawn   

dcibel

https://www.madisoundspeakerstore.com/approx-5-woofers/sb-acoustics-sb15sfcr39-8-5x8-paper-cone-woofer/

Shawn_K

I might also consider using SB Acoustics MW13P or MR13P drivers for the MTM. Dispersion is excebtable up to 2.5 kHz but that dip around 1.5 kHz is ugly - I'm guessing its a resonance in the surround? 

Wolf

Nope- it's cone edge resonance, not necessarily the surround.

dcibel

That low in frequency a dip is almost certainly a result of the half roll surround. What happens at this frequency is that the surround starts moving opposite to the cone, essentially acting as a braking force. The cone resonance is what's going on higher up at 8.5kHz.

Wolf

Cone resonance meaning material, yes- at 8.5k. I said cone edge resonance. I did not say cone resonance.

Shawn_K

Have you guys seen the surround edge coating solutions that Troels Gravesen has experimented with. He will edge coat your MR13P or W12CY003 drivers for 100 euro's plus shipping, He is keeping his fomula proprietary.
http://www.troelsgravesen.dk/SBA-741.htm
http://www.troelsgravesen.dk/W12CY003.htm

PWRRYD

JMHO but no... 100 euros plus shipping?!?!  I would rather keep things in the awesome spirit of DIY and buy 4 drivers (for the same overall cost) and perfect my own edge coating technique on the first two sacrificial drivers then modify the remaining two.  Sending money to someone for that?  No way Jose.  Just sayin 

Shawn_K

I wasn't going to send him drivers to do it - Just pointing out that this guy has enough confidence that his modification works that he is willing to do it as a service. I might start trying this modification on lower priced 4-5.5 inch divers first. Does anyone have any suggestions for lower cost drives that have this kind of cone edge surround interaction phenomenon?

ani_101

start with adding different sticky substances. The thread for modifying PM180 by jburner over at PETT or Diyaudio has some suggestions. But i haven't tried it - not looking at the measurements, it does fine playing music.

dcibel

Wolf said:

Cone resonance meaning material, yes- at 8.5k. I said cone edge resonance. I did not say cone resonance.

The cone edge is which part??

dcibel

Shawn_K said:

Have you guys seen the surround edge coating solutions that Troels Gravesen has experimented with. He will edge coat your MR13P or W12CY003 drivers for 100 euro's plus shipping, He is keeping his fomula proprietary.
http://www.troelsgravesen.dk/SBA-741.htm
http://www.troelsgravesen.dk/W12CY003.htm

It's just added mass at the surround lip, doesn't need to be any special proprietary secret formula, just some flexible material that will stick to rubber. Try RTV silicone.

It's funny, you find this stuff on many cheap small TB full range drivers, but not in the flagship SB drivers. Some high end drivers actually have a varying thickness to the surround from one end to the other, so the edge that meets the cone is thicker than the edge that meets the frame.

ani_101

dcibel said

It's just added mass at the surround lip

On the inside lip?, where the cone meets the surround and not where the surround meets the frame?

ani_101

Image from TG

dcibel

Yes, where the surround meets the cone. The point is to dampen the transition of differing materials. Feel free to read the words on the page you pulled that image from, and look at the other images too.

Wolf

You normally see the sticky rim on TB smaller wideband midbasses in the 'crevice' of the cone/surround junction, not on the underside of the surround.
HiVi M8 has a sticky 0.5" wide application on the underside of the cone rim.
The TB underhung W8 has the entire underside of the surround coated.

It really depends on where it needs to be the most effective.

dcibel- haven't you ever heard the surround called the 'cone edge' before? You can't say: cone breakup/resonance = cone edge resonance. They are usually in different frequency ranges. Breakup/material-resonance is not the edge-resonance. What is so difficult about this?

dcibel

Yes, it is the surround...now go read your reply to Shawn again. It's not the surround, it's the surround! 

Semantics, but the term "cone edge resonance" is very misleading. It's not the cone, it's the surround, and it's not really a resonance either, but more of an breakup antinode.

Shawn_K

I think I understand what is going on with the cone edge resonance - if that's the official name for it. If anyone reading this discussion has a bonified PHD in loudspeaker transducer engineering and design please chime if there is an official name for what we are calling a "Cone edge resonance"
Thanks,
Shawn 

Shawn_K

I want to apologize for my last post - I came off snarky and I don't want to offend anyone. Everyone's opinion's should be heard and appreciated. I have been lurking on these DIY boards for many years and learned probably 90% of the things I know about loudspeaker design by the people who have been willing to share. I thank you all.  

dcibel

I thought you were trying to be funny

6thplanet

I always assumed cone edge resonance was the junction of cone to surround. I call a cone, a cone and a surround, a surround. Figured the half of the surround that's part of Sd measurement being a different material caused this issue. Therefore, it could be called "Beginning of surround resonance"

ani_101

Does the material of the surround have any influence or the stiffness? Say cloth vs foam vs rubber - would a material have a more characteristic impact?