badman said:Even the old hands mix it upYou do indeed lose efficiency going isobaric, 3dB to be precise. Your sensitivity is what does not change (with 1 pair wired in parallel). With 2 pairs wired series-parallel, your efficiency and sensitivity will be the same as a single driver.
Why?
I plug the numbers for a pppp isobarik configuration vrs a real single driver i get 3 dB more efficiency.
dave
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badman said:Even the old hands mix it up
planet10 said:
I think badman's figures are right, except he may have missed that you now have twice the Sd. Therefore, the extra 3dB for the total system. For the same Sd, efficiency would be the same.
Re: Re: Re: Opposed drivers
I guess we do not share the same experiences. It was very audible to me. The most noticeable thing was it could go much louder before audible distortion set in. The tradeoff was when it distorted; it was right away and in your face. It was like a brick wall limit trip point on distortion, if that makes any sense.
How exactly is the sound from the back of the cone any different than the front of the cone? I agree you do have a pole vent to deal with, but that’s why you point it away from the listening position. However, inside of a horn the pole vent is a non-issue because it is enclosed by the horn.
I hear you on the WAF.
I have had several projects banned from the house. The most recent was a 20Hz tapped horn subwoofer that used a pair of Tang Band W8-740 woofers in push-pull. For some reason the wife got upset when one of her English tea pots rattled off the shelf and smashed into pieces. 
I just don’t get it. 
Dr. Geddes has the research and numbers to back it up, but I can still hear the difference. It must just apply to the mass-market or the audiophool stuff.
I personally like the sonic texture of removing most of the second harmonic. To my ears it removes the molasses from the leading edge and gives better dynamic resolution. It has a cleaner and tighter boogie factor.
I wouldn’t know anything about spider noise. I can’t ever recall having my attention drawn to spider noise before. Hmmm…
Rgs, JLH
planet10 said:
I guess we do not share the same experiences. It was very audible to me. The most noticeable thing was it could go much louder before audible distortion set in. The tradeoff was when it distorted; it was right away and in your face. It was like a brick wall limit trip point on distortion, if that makes any sense.
How exactly is the sound from the back of the cone any different than the front of the cone? I agree you do have a pole vent to deal with, but that’s why you point it away from the listening position. However, inside of a horn the pole vent is a non-issue because it is enclosed by the horn.
I hear you on the WAF.
I have had several projects banned from the house. The most recent was a 20Hz tapped horn subwoofer that used a pair of Tang Band W8-740 woofers in push-pull. For some reason the wife got upset when one of her English tea pots rattled off the shelf and smashed into pieces. I just don’t get it. planet10 said:
Dr. Geddes has the research and numbers to back it up, but I can still hear the difference. It must just apply to the mass-market or the audiophool stuff.
I personally like the sonic texture of removing most of the second harmonic. To my ears it removes the molasses from the leading edge and gives better dynamic resolution. It has a cleaner and tighter boogie factor.
I wouldn’t know anything about spider noise. I can’t ever recall having my attention drawn to spider noise before. Hmmm…
Rgs, JLH
Hmmm. 3 different ways of mounting 2 drivers in a 'torus'.
The top one in my illustration would have to have one wired out-of-phase anyway, as it's a single enclosure. If had them in phase, then the subby would be rocking along with everything around it (and probably pole on the first really big bass note). Easy to mount and dismount the drivers, but difficult to support the magnet of the driver on the right. Unfortunately, the wiring would be part internal, part external.
The middle set-up would mean that I could easily support the magnet and spider internally and that it would be easy to mount and dismount both drivers. It doesn't sound like much, what with a 7kg+ magnet and a cast frame, but you can sometimes get the frame to 'sing' with the right tone and this is my excuse for magnet support. I notice Dave uses lots of absorbent stuff on his modified drivers which probably does the same thing. The wiring up of the drivers would be internal, and as the EP2500 can pump out some reasonable current/voltage, that might be good too.
The bottom set-up would be easy to wire up, as it would be all exernal. But it might be a bit of a squeeze to get the drivers in and out. And it would be a hassle to fit any magnet bracing.
All of them would be pretty much rock-steady, as the drivers would be moving in opposite directions - no difference there. A pity I can't mail John Kreskovsky to find out what he thinks of the arrangement and which would be best for removing harmonic distortion. Is it the physical direction in which the drivers are facing (in which case the bottom 2 would be better) or the direction of cone movement (in which case the top one would be better)?
I think I'll have to persuade my other half that we need a bigger stereo room so that I can build the thing
The top one in my illustration would have to have one wired out-of-phase anyway, as it's a single enclosure. If had them in phase, then the subby would be rocking along with everything around it (and probably pole on the first really big bass note). Easy to mount and dismount the drivers, but difficult to support the magnet of the driver on the right. Unfortunately, the wiring would be part internal, part external.
The middle set-up would mean that I could easily support the magnet and spider internally and that it would be easy to mount and dismount both drivers. It doesn't sound like much, what with a 7kg+ magnet and a cast frame, but you can sometimes get the frame to 'sing' with the right tone and this is my excuse for magnet support. I notice Dave uses lots of absorbent stuff on his modified drivers which probably does the same thing. The wiring up of the drivers would be internal, and as the EP2500 can pump out some reasonable current/voltage, that might be good too.
The bottom set-up would be easy to wire up, as it would be all exernal. But it might be a bit of a squeeze to get the drivers in and out. And it would be a hassle to fit any magnet bracing.
All of them would be pretty much rock-steady, as the drivers would be moving in opposite directions - no difference there. A pity I can't mail John Kreskovsky to find out what he thinks of the arrangement and which would be best for removing harmonic distortion. Is it the physical direction in which the drivers are facing (in which case the bottom 2 would be better) or the direction of cone movement (in which case the top one would be better)?
I think I'll have to persuade my other half that we need a bigger stereo room so that I can build the thing
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Just found your mistake.
You doubled the VAS in your excel spreadsheet.
If single driver got 50 liters VAS, isobaric get 25 liters VAS, then two isobaric pairs go back to 50 liters VAS.
You'll end up with the same efficiency.
I think if there was no efficiency loss, many people would use isobaric. Twice the powerhandling for a box which is half the size that can play 3 dB louder because of the new powerhandling. There need to be a disadvantage hehe !
You doubled the VAS in your excel spreadsheet.
If single driver got 50 liters VAS, isobaric get 25 liters VAS, then two isobaric pairs go back to 50 liters VAS.
You'll end up with the same efficiency.
I think if there was no efficiency loss, many people would use isobaric. Twice the powerhandling for a box which is half the size that can play 3 dB louder because of the new powerhandling. There need to be a disadvantage hehe !
planet10 said:
Don't forget that Re will change.
n0 = (p*BL^2*Sd^2) / (2*pi*c*mms^2*Re)
Re will be doubled if you connect the drivers in series. BL and mms will cancel themselves, so n0 will be half.
If you connect them in parallel, then BL will stay the same, Re will be half, but mms will double, so n0 will be half.
Since n0 will be half, you lose 3 dB efficiency with an isobaric pair.
I hope this is a better explanation !
"Dr. Geddes has the research and numbers to back it up, but I can still hear the difference. It must just apply to the mass-market or the audiophool stuff."
Ditto.
There is a HUGE difference between the sound of push-push vs push-pull.
I built 24 of this design for PA, -3dB at 31hz, 101dB/W/1M.
We used four of these with 1KW at a college homecoming dance about a month ago in a big ballroom with 1500 people dancing.
The bass was clean, and brutal.
Ditto.
There is a HUGE difference between the sound of push-push vs push-pull.
I built 24 of this design for PA, -3dB at 31hz, 101dB/W/1M.
We used four of these with 1KW at a college homecoming dance about a month ago in a big ballroom with 1500 people dancing.
The bass was clean, and brutal.
it's a very standard slot ported twin driver reflex loaded speaker.
But, it has a few nice features that make it particularly suited to portable PA duty.
The drivers are protected.
The sides are braced and rigid.
The drivers are arranged to push and pull to help cancel second harmonic when overdriven.
But, it has a few nice features that make it particularly suited to portable PA duty.
The drivers are protected.
The sides are braced and rigid.
The drivers are arranged to push and pull to help cancel second harmonic when overdriven.
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