Ehh...baffle step compensation...I have an idea that its not needed or even wanted in the surround channels...the front speakers should take care of that ?
I would not think this important since there is no baffle. Its at the end of a tube. Almost a perfect dipole.
Dr. Geddes,
I have a question about the high frequency behavior of the Oblate Spheroid. Please correct me if I’m wrong. As I understand it, other horn profiles like the tractrix can have a flat on axis high frequency response because of their narrowing directivity. Since the Oblate Spheroid does not have a narrowing directivity with increasing frequency, it has a drooping high frequency response. This drooping frequency response is not a problem when matching the 108dB compression driver with a direct radiator that is 95dB because the response can be made flat again by attenuating the mid band of the compression driver.
Now for the real question. If one was trying to achieve the highest sensitivity and flattest high frequency response with the Oblate Spheroid; wouldn’t it be beneficial to use a driver that has a rising high frequency response? Hopefully this could result in a flatter response that would require less attenuation. What are your thoughts on this? Thank you.
Rgs, JLH
I have a question about the high frequency behavior of the Oblate Spheroid. Please correct me if I’m wrong. As I understand it, other horn profiles like the tractrix can have a flat on axis high frequency response because of their narrowing directivity. Since the Oblate Spheroid does not have a narrowing directivity with increasing frequency, it has a drooping high frequency response. This drooping frequency response is not a problem when matching the 108dB compression driver with a direct radiator that is 95dB because the response can be made flat again by attenuating the mid band of the compression driver.
Now for the real question. If one was trying to achieve the highest sensitivity and flattest high frequency response with the Oblate Spheroid; wouldn’t it be beneficial to use a driver that has a rising high frequency response? Hopefully this could result in a flatter response that would require less attenuation. What are your thoughts on this? Thank you.
Rgs, JLH
Your absolutely correct on all accounts. You seem to have a grasp on the situation. Actually the DE250 is JUST senitive enough at 10 kHz to match the output of the 15TBX100. Hard to believe right. At 10 kHz the crossover is basically at 0 dB attenuation - to be flat on axis.
But many have noted that due to the OS having such a great deal of HF energy out to the sides and into the room, they always seem to sound bright if made flat on axis. A few dB down seems to correct this. In terms of power the response it is still flat, but the axial response is falling a bit. Thats how much more energy is being distributed to the room because of the true CD at the upper end. The OS really do distribute the HF energy very wide even at the very highest frequencies.
Many people say that the directivity is limited by the throat radius, but thats not true, either theoretically or in practice.
But to your specific question, the response for any compression driver has to fall above its mass break point, it can't rise. Its just not possible because the mass of the diaphragm. But anything that brings up the top end, like copper caps etc. is a plus.
But many have noted that due to the OS having such a great deal of HF energy out to the sides and into the room, they always seem to sound bright if made flat on axis. A few dB down seems to correct this. In terms of power the response it is still flat, but the axial response is falling a bit. Thats how much more energy is being distributed to the room because of the true CD at the upper end. The OS really do distribute the HF energy very wide even at the very highest frequencies.
Many people say that the directivity is limited by the throat radius, but thats not true, either theoretically or in practice.
But to your specific question, the response for any compression driver has to fall above its mass break point, it can't rise. Its just not possible because the mass of the diaphragm. But anything that brings up the top end, like copper caps etc. is a plus.
JLH said:
Yes, just like we've done a lot in cone drivers with front horns.
But, does such compression driver exist? I don't think so.
Cone drivers can't rise in response either, at least not as a piston (they can have resonance break-ups which extend their response, but one should not be using the driver in this range.) The total power radiated by a piston has to fall at high frequencies just like a compression driver - for two reasons - its mass and its inductance.
The only reason that piston appear to rise is because their directivity is decreasing - the falling power is radiated into an equally falling area.
The only reason that piston appear to rise is because their directivity is decreasing - the falling power is radiated into an equally falling area.
Dr. Geddes, I actually will not agree with you on the no bass issue for surrounds. A lot of newer action movies are being recorded with full range surround channels. A few movies, such as the Transformers movie, have pans front to rear or vice versa that require bass or the pan sounds as bad as a left to center pan with mismatched speakers. I experimented with surrounds with limited bass and found that some of these newer movies require speakers capable of relatively full sound and overall similar tonality to the main speakers. While these sorts of movies are still rare, I foresee them becoming more and more common. I feel that HD DVD and Blueray have really transformed this issue as well, both formats had recordings with much better sound in many cases, especially the high res uncompressed tracks.
As for BSC in the surrounds, it wouldn't be necessary if the surrounds are being used on the wall, but if they were to be hung free space, it might be necessary. With Dr. Geddes view that no bass is needed, then of course no BSC is needed, but with my view, then you would maybe need to spend a little more time on the crossovers.
Dr. Geddes how much power do you have going to your rear surrounds? What are they as of now? While I believe you when you say they can't keep up and you are blowing tweeters, I can't help but wonder if they are less efficient and you are clipping your rear amp constantly, causing the blow tweeters.
As for BSC in the surrounds, it wouldn't be necessary if the surrounds are being used on the wall, but if they were to be hung free space, it might be necessary. With Dr. Geddes view that no bass is needed, then of course no BSC is needed, but with my view, then you would maybe need to spend a little more time on the crossovers.
Dr. Geddes how much power do you have going to your rear surrounds? What are they as of now? While I believe you when you say they can't keep up and you are blowing tweeters, I can't help but wonder if they are less efficient and you are clipping your rear amp constantly, causing the blow tweeters.
The technology of having the right amount of flexibility in a cone is a totally different field. If we look at the frequency and phase response of drivers like the Jordans, we can see that they do not rely on breakup to extend the response, but rather have good control of how it flexes. Although it may not be the ultimate solution, but still performs quite well.
Directivity of cone driver rely on how the cap and cone work with each other. One of my drivers use a different cap design that resulted in droop of SPL above 4KHz, but the sweet spot was widened after the right amount of EQ, pretty much the same as OS wave guides require some EQ.
Directivity of cone driver rely on how the cap and cone work with each other. One of my drivers use a different cap design that resulted in droop of SPL above 4KHz, but the sweet spot was widened after the right amount of EQ, pretty much the same as OS wave guides require some EQ.
pjpoes said:
Even in commercial theaters not much effort is spent on the LF capability of the surrounds and in a large space like that it would be far more important than a small HT. If you want bass from your surrounds then go for it, I just don't see the need based on the multitudes of movies that I have auditioned and the fact that I have subs all over the place which handle the lows for the surrounds. There is nothing below 150 Hz going to my surrounds.
The rears are far less efficient and maybe the amps clipped at some point, that would certainly take out the tweeters. But I keep my surrounds fairly low in level, never at anything like the mains.
You are probably right to say that anything below 150hz isn't necessary, well I might argue 100hz, but I think they need to be strong down to that point. Again, I agree that below 100hz sub woofers will take care of things. I was more thinking that my own experiments with small surrounds, which were output limited below 250hz made for some discontinuity of front rear pans. 3.5" drivers will do that to you (I know why so small, I was experimenting so shoot me). When I switched to a setup using 6.5" woofers things improved greatly, but still aren't perfect. While the 10" coaxial suggested would probably be much stronger, I was a little concerned that the enclosure you mentioned would cancel too much of the low midrange/upper bass.
I was proposing 8" and I guess that I wasn't thinking coax, but wizzer cones. On a tube some two feet long. the dipole moment will be pretty large and the low end won't sag so much. But at least there isn't a 1" tweeter to blow out every month or so, and the 8" B&C's in series/parrallel (haven't decided) would handle a lot of power without any problems. I need to actually try this, of course, but I'm not going to do small speakers again.
Curious as to your preference for dipoles; IIRC they were originally developed when all we had was mono surrounds and low separation.
I also prefer monopole or bipole pointed away from the listeners as I hear plenty of output down to the 100 Hz XO I use.
I guess it's just another tradeoff - bipoles for better diffusion of ambient effects (which admittedly seem to predominate) but less specificity for localized and less bass; monopole/bipole for better localization of pinpoint effects and more bass.
I also prefer monopole or bipole pointed away from the listeners as I hear plenty of output down to the 100 Hz XO I use.
I guess it's just another tradeoff - bipoles for better diffusion of ambient effects (which admittedly seem to predominate) but less specificity for localized and less bass; monopole/bipole for better localization of pinpoint effects and more bass.
I think surrounds in an HT system should have the same extension as the mains. But it really depends on recording, processing, methods.
Idealy if the content requires to creat an image of somthing comming from behind the listener or directly from the side, it would be rather awkward to hear the inballance in sound, especially when comming from the side.
The kind of surrounds used also depends on the kind of room the system is going to be used in. If one has to keep the speakers against the wall, then too much spread in directivity does not make sense.
Theoretically, it takes 4 speakers to project 3D sound image movements, just like the GPS system requires 4 signal sources to determine 3D position. In actual practice, probably 6 would make a system compatible with almost all formats on the market not considering the subs, and probably will handle future formats as well.
Idealy if the content requires to creat an image of somthing comming from behind the listener or directly from the side, it would be rather awkward to hear the inballance in sound, especially when comming from the side.
The kind of surrounds used also depends on the kind of room the system is going to be used in. If one has to keep the speakers against the wall, then too much spread in directivity does not make sense.
Theoretically, it takes 4 speakers to project 3D sound image movements, just like the GPS system requires 4 signal sources to determine 3D position. In actual practice, probably 6 would make a system compatible with almost all formats on the market not considering the subs, and probably will handle future formats as well.
noah katz said:
Its not so much my preference as Thomason Holman - formerly of Lucas Sound. He recomends dipoles with the listener on the null axis. I can see why this is though. One does not get any direct sound, only ambient sound from the room and this is what the surrounds are supposed to do.
Then there is Floyd Toole who swears the opposite. Says that the surrounds need to be direct to add the proper spatial effect.
From my knowledge of these two indivisuals I would say that the preference difference comes from usuage differences. Holman is a film guy and Floyd is a orchestra guy. Not surprising that they might differ on expectation.
I only use the surrounds for film, hence I follow Holmans recommendations.
soongsc said:
Except that no theater in the world does that. Only a few misguided home theaters that I've seen have this. (Must have been designed by the speaker salesman.)
soongsc said:
Except that the analogy is completely wrong because the technology is completely different.
gedlee said:
That seems true if you only look at your exposed part of the whole message.gedlee said:
😉
If you just consider HT systems as a thriller gadget, then the standards are of course lower.😀
I was talking about the GPS reference, and it only takes 3 satelite signals to determine position. Its called "triangulation".