paulspencer:
>Seems to me it would make more sense to look at the amount of power that is needed to reach xmax in the box in question, even 1kw seems inflated to me.
Another question that seems relevant to which I don't have the answer, is at which point does a vent become too long? I have a 4" vent that is 1m long and it tends to ring - it would be better as a digeridoo than a vent!!!
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If you bother to check with a reasonably accurate program you'll see that it takes considerably more than 1kW to reach Xmax in 3ft^3/20Hz. This driver is very inefficient, potentially requiring mass quantities of power to reach HT reference levels depending on the app., so 1kW isn't necessarily 'inflated' (see below).
Sometime back, DW derived this somehow: 13560"/(20*Fb), which appears to work well enough. Some folks use considerably longer ones though and claim they sound fine, so as always, YMMV.
WRT 'ringing', vents are 1/4 WL resonators clamped at one end (like a 'whip' antenna), so high aspect ratio ones (long) need to be supported at both ends. If PVC, it may be necessary to wrap them in insulation also. For long vents I used cardboard tubes, so didn't have to insulate them.
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Raoul:
>I think 1kw is excessive. I also don't think the amp can handle a 1 ohm load (Brahma wired in parallel), so the driver will probably see something in the region of 700w (probably still excessive, depending on the box configuration). I am not sure if wiring the driver in series or with seperate sources will alter the parameters.
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Define excessive. To reach DD/DTS reference levels requires 115dB/listening position. Assuming that room gain/boundary loading ~offsets the loss over distance (BIG assumption unless it's a small room), then with series wired VCs the efficiency falls to ~88dB/W/m in the mid 20s where there's often some strong signals in action/sci-fi movies, so to get there requires ~2^(115-88/3.01) = ~501.5W, and for parallel wired = ~2^(115-94/3.01) = ~126W.
This also assumes that the other channels are all flat to 20Hz and can hit 105dB/listening position. This isn't likely in most HT systems, so bass management is used to route the other channel's LF to the LFE channel. Now it has to produce 115+(10*log(5)) = ~122dB/listening position. Now we need ~2513.8W/series (there's enough Xmax BTW) or 631.4W/parallel.
Since few amps can handle a ~1.3 ohm nominal load for any length of time at any appreciable power, the VCs should be wired in series if the amp isn't rated for <2 ohm loads.
That said, I've read that many folks don't listen at reference, but as much as 10dB lower, so amp requirements drop accordingly. Still, I take the view that you don't want to ever clip the amp, so recommend sizing it based on a 'worst case' scenario, which usually means there's a bit of dynamic headroom reserve.
For instance, In my room there's no appreciable gain until ~13Hz and SPL falls at a measured ~4dB/octave (I've never measured a typical room that lost 6dB/octave), so to reach reference level at a 14ft listening distance I need dB(peak) = 115+(13.29*log(14/3.2808)) = ~123.37dB, or ~130.37dB if I need to use bass management, so if I were to use this driver I would need up to four to get the efficiency up sufficiently to 'get by' on 1kW.
It's academic for me though, I had two Contrabasses for awhile and they tore my old stick built house apart in short order reproducing explosions and 16Hz organ pipes.
Then there's the problem of adequately isolating the equipment, but that's another story.......
Bottom line is that until you know how much room/boundary gain is available and the loss over distance is combined with your desired SPL level, and whether or not the amp can handle low resistance loads, saying 1kW is 'inflated/excessive' is a bit premature.
GM
>Seems to me it would make more sense to look at the amount of power that is needed to reach xmax in the box in question, even 1kw seems inflated to me.
Another question that seems relevant to which I don't have the answer, is at which point does a vent become too long? I have a 4" vent that is 1m long and it tends to ring - it would be better as a digeridoo than a vent!!!
====
If you bother to check with a reasonably accurate program you'll see that it takes considerably more than 1kW to reach Xmax in 3ft^3/20Hz. This driver is very inefficient, potentially requiring mass quantities of power to reach HT reference levels depending on the app., so 1kW isn't necessarily 'inflated' (see below).
Sometime back, DW derived this somehow: 13560"/(20*Fb), which appears to work well enough. Some folks use considerably longer ones though and claim they sound fine, so as always, YMMV.
WRT 'ringing', vents are 1/4 WL resonators clamped at one end (like a 'whip' antenna), so high aspect ratio ones (long) need to be supported at both ends. If PVC, it may be necessary to wrap them in insulation also. For long vents I used cardboard tubes, so didn't have to insulate them.
====
Raoul:
>I think 1kw is excessive. I also don't think the amp can handle a 1 ohm load (Brahma wired in parallel), so the driver will probably see something in the region of 700w (probably still excessive, depending on the box configuration). I am not sure if wiring the driver in series or with seperate sources will alter the parameters.
====
Define excessive. To reach DD/DTS reference levels requires 115dB/listening position. Assuming that room gain/boundary loading ~offsets the loss over distance (BIG assumption unless it's a small room), then with series wired VCs the efficiency falls to ~88dB/W/m in the mid 20s where there's often some strong signals in action/sci-fi movies, so to get there requires ~2^(115-88/3.01) = ~501.5W, and for parallel wired = ~2^(115-94/3.01) = ~126W.
This also assumes that the other channels are all flat to 20Hz and can hit 105dB/listening position. This isn't likely in most HT systems, so bass management is used to route the other channel's LF to the LFE channel. Now it has to produce 115+(10*log(5)) = ~122dB/listening position. Now we need ~2513.8W/series (there's enough Xmax BTW) or 631.4W/parallel.
Since few amps can handle a ~1.3 ohm nominal load for any length of time at any appreciable power, the VCs should be wired in series if the amp isn't rated for <2 ohm loads.
That said, I've read that many folks don't listen at reference, but as much as 10dB lower, so amp requirements drop accordingly. Still, I take the view that you don't want to ever clip the amp, so recommend sizing it based on a 'worst case' scenario, which usually means there's a bit of dynamic headroom reserve.
For instance, In my room there's no appreciable gain until ~13Hz and SPL falls at a measured ~4dB/octave (I've never measured a typical room that lost 6dB/octave), so to reach reference level at a 14ft listening distance I need dB(peak) = 115+(13.29*log(14/3.2808)) = ~123.37dB, or ~130.37dB if I need to use bass management, so if I were to use this driver I would need up to four to get the efficiency up sufficiently to 'get by' on 1kW.
It's academic for me though, I had two Contrabasses for awhile and they tore my old stick built house apart in short order reproducing explosions and 16Hz organ pipes.
Then there's the problem of adequately isolating the equipment, but that's another story.......Bottom line is that until you know how much room/boundary gain is available and the loss over distance is combined with your desired SPL level, and whether or not the amp can handle low resistance loads, saying 1kW is 'inflated/excessive' is a bit premature.
GM
GM you really are living up to your blurb "loud is beatiful ..."!!!
I assume that you mean wrap the PVC to dampen vibrations of the pipe itself, right?
Both ends would be supported and flared.
Why did you get rid of your Contrabasses and why did you have two? I can imagine that would be hard
Everyone has a different view on what is excessive, depending on their expectations mainly. To some, anything more than a TV speaker for HT might be excessive, to others a pair of high excursion 15" subs is bare minimum.
GM said:
I assume that you mean wrap the PVC to dampen vibrations of the pipe itself, right?
Both ends would be supported and flared.
GM said:It's academic for me though, I had two Contrabasses for awhile and they tore my old stick built house apart in short order reproducing explosions and 16Hz organ pipes.
Why did you get rid of your Contrabasses and why did you have two? I can imagine that would be hard
GM said:
Everyone has a different view on what is excessive, depending on their expectations mainly. To some, anything more than a TV speaker for HT might be excessive, to others a pair of high excursion 15" subs is bare minimum.
>GM you really are living up to your blurb "loud is beatiful ..."!!!
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I try.
====
>I assume that you mean wrap the PVC to dampen vibrations of the pipe itself, right?
====
Correct.
====
>Why did you get rid of your Contrabasses and why did you have two? I can imagine that would be hard
====
Greedy medical system. Mainly because I got them cheap. I bought four basslist kits so I could get efficiency up to basshorn levels, wound up having to sell the two I built cabs for (along with some other stuff), so still have two kits, but for a variety of reasons have little enthusiasm and no $$ to get them up and running. While there's life, there's hope, so no plans to sell them short of another financial catastrophe.
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>Everyone has a different view on what is excessive, depending on their expectations mainly. To some, anything more than a TV speaker for HT might be excessive, to others a pair of high excursion 15" subs is bare minimum.
====
Understood, but I fail to see the relevance to whether 1kW is too much power for his app. since we don't have all the info. It's been my experience though that folks with inefficient subs grossly underestimate their power requirements initially and wind up having to buy a second, more powerful amp, ergo suggested 1kW as a minimum.
GM
====
I try.
====
>I assume that you mean wrap the PVC to dampen vibrations of the pipe itself, right?
====
Correct.
====
>Why did you get rid of your Contrabasses and why did you have two? I can imagine that would be hard
====
Greedy medical system. Mainly because I got them cheap. I bought four basslist kits so I could get efficiency up to basshorn levels, wound up having to sell the two I built cabs for (along with some other stuff), so still have two kits, but for a variety of reasons have little enthusiasm and no $$ to get them up and running. While there's life, there's hope, so no plans to sell them short of another financial catastrophe.
====
>Everyone has a different view on what is excessive, depending on their expectations mainly. To some, anything more than a TV speaker for HT might be excessive, to others a pair of high excursion 15" subs is bare minimum.
====
Understood, but I fail to see the relevance to whether 1kW is too much power for his app. since we don't have all the info. It's been my experience though that folks with inefficient subs grossly underestimate their power requirements initially and wind up having to buy a second, more powerful amp, ergo suggested 1kW as a minimum.
GM
Well, I guess it's time for an update. I tuned a cabinet to 28Hz with a 4" vent as suggested on the Adire audio website. It was an easy and inexpensive way to get the Brahma moving. I powered the monster with a Hafler 9300 wired to the voice coils as seperate sources. I am not sure why, but wiring each vc with its own channel seemed to sound better than bridged, series wiring. Anyway, the sub is very enjoyable to listen to with music, but I wanted a little more bottom end for movies. Sounds a little anemic/ congested during some of the blasts and thuds in the "Hulk". So, with an excellent deal from Adire on a B-stock PR15 and a little generosity on another purchase, I now have two Adire audio PR15's. I also purchased a BK Electronics OMP1000.
I plan to wire the OMP1000 in series, which should provide about 725W (don't worry GM, if needed I can always add another OMP1000 and wire one to each v.c. for a total of 2100W). I have been modeling boxes on LSPCad and it looks like a 3ft^3 box, roughly an 18" cube, will due the trick. I plan on adding 1000g to each PR15 to reach a tuning around 20Hz.
The only concern I have is the group delay. When I model the box on LSPCad, I am getting a group delay above 40ms at 20Hz, which appears to peak at about 48ms at 18-19Hz. The rise is sharp; group delay is below 20ms at 25Hz. These figures model for a bare box -- stuffing only increases the group delay. Are these figures acceptable? Can I do anything to lower the frequency at which the group delay peaks without completely destroying my box response?
BTW, thanks for all the help on this project so far. This is my first sub and I really appreciate all the help you guys have offered. I have really learned a lot.
I plan to wire the OMP1000 in series, which should provide about 725W (don't worry GM, if needed I can always add another OMP1000 and wire one to each v.c. for a total of 2100W). I have been modeling boxes on LSPCad and it looks like a 3ft^3 box, roughly an 18" cube, will due the trick. I plan on adding 1000g to each PR15 to reach a tuning around 20Hz.
The only concern I have is the group delay. When I model the box on LSPCad, I am getting a group delay above 40ms at 20Hz, which appears to peak at about 48ms at 18-19Hz. The rise is sharp; group delay is below 20ms at 25Hz. These figures model for a bare box -- stuffing only increases the group delay. Are these figures acceptable? Can I do anything to lower the frequency at which the group delay peaks without completely destroying my box response?
BTW, thanks for all the help on this project so far. This is my first sub and I really appreciate all the help you guys have offered. I have really learned a lot.
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