Soooo. If you take a gander at the "woofer" me thread, you'll see that I'm thinking about doing a subwoofer that's basically the same as the "Critical Q" subwoofer on the web. I went back and took another look at that site, http://members.ozemail.com.au/~joeras/sub_index.htm
Joe Rasmussen (the Critical Q dude) measured his Peerless 83500 XLS, and he came up with a higher Qts than the published spec (.23 rather than .20), and a smaller Vas (124L rather than 138L. He figured an effective volume of 42L. How'd he get that number? I get 33L.
He also describes a very different method of modifying the plate amp than the manufacturer documents.
Can anyone tell me what kind of EQ is really needed? (Assume a "normal" amount of room lift.) It looks to me like a boost of 6dB in the low 20's would be indicated.
Joe Rasmussen (the Critical Q dude) measured his Peerless 83500 XLS, and he came up with a higher Qts than the published spec (.23 rather than .20), and a smaller Vas (124L rather than 138L. He figured an effective volume of 42L. How'd he get that number? I get 33L.
He also describes a very different method of modifying the plate amp than the manufacturer documents.
Can anyone tell me what kind of EQ is really needed? (Assume a "normal" amount of room lift.) It looks to me like a boost of 6dB in the low 20's would be indicated.
Dunno, Dave. I plugged Rasmussen's own numbers into WinISD and came up with the same thing you did-the box is Qtc=0.50 at 33 liters.
On the other hand, if you put the Peerless into a 42 liter box, (using Joe's measured Thiele-small parameters), your Qtc=0.46.
However, at Fc, that difference is only 0.7 dB, so maybe not a major issue.
On the other hand, Joe Rasmussen somehow says that the Fb of the box will be at 39 Hz. Using Joe's own measured parameters, I came up with an Fb of 35.7 Hz, instead of 39.
Rasmussen's calculations seem to be off, but not by much, and his measurements of the Thiele-Small parameters seem to indicate a woofer that conforms pretty close to the published specs.
I honestly think you can go either way on this and still have what you are looking for. I would think that it would be wise to measure the Thiele-Small parameters of whatever XLS you buy and go for a Qtc of between 0.5 and 0.46. I think your numbers are going to be different from Joe's although they might be reasonably close.
PS: measuring the WinISD simulation, at 42 liters you are -6 dB at 39 Hz and -6.7 dB at 35.7 Hz.
On the other hand, if you put the Peerless into a 42 liter box, (using Joe's measured Thiele-small parameters), your Qtc=0.46.
However, at Fc, that difference is only 0.7 dB, so maybe not a major issue.
On the other hand, Joe Rasmussen somehow says that the Fb of the box will be at 39 Hz. Using Joe's own measured parameters, I came up with an Fb of 35.7 Hz, instead of 39.
Rasmussen's calculations seem to be off, but not by much, and his measurements of the Thiele-Small parameters seem to indicate a woofer that conforms pretty close to the published specs.
I honestly think you can go either way on this and still have what you are looking for. I would think that it would be wise to measure the Thiele-Small parameters of whatever XLS you buy and go for a Qtc of between 0.5 and 0.46. I think your numbers are going to be different from Joe's although they might be reasonably close.
PS: measuring the WinISD simulation, at 42 liters you are -6 dB at 39 Hz and -6.7 dB at 35.7 Hz.
Dave, here is a chart of step responses for various values of Qtc. As you can see, there are only minor differences between Qtc=0.5 and Qtc=0.71. If you ever saw the step responses of Vented Boxes, which some people, (including myself), think sound just fine, then you would see there is no real reason to get nervous about this. Regardless of what you do, you are going to be much, much closer to Qtx=0.5 than Qtc=0.71.
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Now here are the step responses for various vented alignments. Generally, the a=1 and a=4 are the most useful-the other two are fairly extreme setups. I don't know about the Keibs and SC4 alignments, because Small does not go into them into detail. but you get the idea-vented boxes give bigger output, but not as good step response. And step response would be a good indicator of transient response, I would think.
Both closed box and vented charts are by Small, with the colored printing added by myself.
When you compare these charts, now you know why Rasmussen says that building a closed box with a Qtc=0.5 is like adding a servo to a box with a different alignment, especially a vented alignment-even one of the better ones.
It should also be pointed out that the lower the frequency, the less distortion is audible. Considering the advantages in output, cone excursion requirements, etc., that is why the vented box is popular. So the argument goes on. But here are the charts to show how well each follows a single pulse.
Both closed box and vented charts are by Small, with the colored printing added by myself.
When you compare these charts, now you know why Rasmussen says that building a closed box with a Qtc=0.5 is like adding a servo to a box with a different alignment, especially a vented alignment-even one of the better ones.
It should also be pointed out that the lower the frequency, the less distortion is audible. Considering the advantages in output, cone excursion requirements, etc., that is why the vented box is popular. So the argument goes on. But here are the charts to show how well each follows a single pulse.
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Here is Subwoofer Simulator, written by our own member, F4ier, and how it simulates a Peerless 830500 in a 42 liter box. I am using the Peerless numbers here-didn't feel like changing them to Rasmussen's numbers. But Rasmussens's and Peerless' numbers are close, so it makes little difference.
These curves are for an amp driven at 28.3 volts-which translates into 200 watts for a 4 ohm woofer. The 830500 is essentially a 4 ohm woofer-okay, maybe a 5 ohm woofer. Not much difference.
The upper green line is the response using typical room gain-the same curve that Rasmussen uses, only added to the simulation chart. This was made possible by a file by Serow, to be added to Subwoofer Simulator.
The lower green line is the response without room gain. As you can see, room gain adds quite a lot.
The red line is the impedance, with the peak right where Rasmussen says it is, more or less.
Room gain of course varies room to room, and between various parts of room. These are averages.
But this shows why SY is saying position the subwoofer before you EQ-room gain is like an EQ in and of itself. And it will vary in different parts of the room.
PS: About that dropoff over 100 Hz-Subwoofer Simulator takes into account the speaker's response if the cone were perfect. The high frequency rolloff due to inductance is possibly over emphasized-that is why the program is called Subwoofer Simulator.
These curves are for an amp driven at 28.3 volts-which translates into 200 watts for a 4 ohm woofer. The 830500 is essentially a 4 ohm woofer-okay, maybe a 5 ohm woofer. Not much difference.
The upper green line is the response using typical room gain-the same curve that Rasmussen uses, only added to the simulation chart. This was made possible by a file by Serow, to be added to Subwoofer Simulator.
The lower green line is the response without room gain. As you can see, room gain adds quite a lot.
The red line is the impedance, with the peak right where Rasmussen says it is, more or less.
Room gain of course varies room to room, and between various parts of room. These are averages.
But this shows why SY is saying position the subwoofer before you EQ-room gain is like an EQ in and of itself. And it will vary in different parts of the room.
PS: About that dropoff over 100 Hz-Subwoofer Simulator takes into account the speaker's response if the cone were perfect. The high frequency rolloff due to inductance is possibly over emphasized-that is why the program is called Subwoofer Simulator.
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kelticwizard said:
To my untutored eye, that still doesn't look too flat. Consider also that I want to use an off-the shelf box if possible. I found one expensive one that's about 30 liters. Otherwise, there's just 1 cu ft (28L) boxes. Subtract about 6 liters (?) for the woofer itself and a plate amp if I go that way, and thats 22 liters. Even if I can get 25% back by stuffing it, I'm coming up short.
There is an apparent sparcity of woofers with Qts below .4 or so, making it difficult to build a subwoofer with a Qtc of around .5. Other possibilities are some 8" or 10" woofers by Peerless and Scan Speak. Using those might require multiple drivers, so we are talking about some bucks.
GM said:
GM: You are right, the inductance does take it's toll. But the factory plot shows the response rolling off 2 dB-from 91 dB @ 120 Hz to 89 dB @ 280 Hz-instead of several dB's as Subwoofer Simulator shows. It would not surprise me if the rolloff is somewhat greater than the factory plot, since most subs really are not designed to be used much abobe 100 Hz. but I don't think the rolloff is as great as Subwoofer Simulator shows. F4ier, SubSim's creator, has said that SubSim shows the response if inductance were the only determining factor in rolloff.
PS: One more thing. Unlike most subwoofers, Peerless includes a Faraday ring in almost all their better speakers. I am not certain how it works, but I believe the shorting ring has the effect of making the speaker act less inductive than you would think. Or something like that, I never can quite understand it completely, only that the shorting ring affects inductive behavior.
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How about the “XLS Car” 12” woofer from Peerless?
http://www.partsexpress.com/pe/pshowdetl.cfm?&PartNumber=297-656&DID=7
That one has a Qts of .30, a very low Vas of 2.85 ft^3, and Le of 1.9 mH. Fs is kinda hight at 28 Hz, however.
Is there some reason (other than the tacky logo on the dust cap) why the speaker is unsuitable for stereophile music?
http://www.partsexpress.com/pe/pshowdetl.cfm?&PartNumber=297-656&DID=7
That one has a Qts of .30, a very low Vas of 2.85 ft^3, and Le of 1.9 mH. Fs is kinda hight at 28 Hz, however.
Is there some reason (other than the tacky logo on the dust cap) why the speaker is unsuitable for stereophile music?
Here is the thread on bandwidth, inductance, etc. Post #7 gives Dan Wiggins', (of Aidre Audio) "take" on subwoofers, inductance, etc.
Dan did not answer my question about the listed Le of a subwoofer acting the same in a sub with a Faraday ring as a sub without one, and I don't blame him. Adire's fine subwoofers do not choose to take the Faraday ring route, and I doubt Dan wants to get into arguments about the magnetic behavior of a feature that he does not feel is necessary to begin with.
Dan did not answer my question about the listed Le of a subwoofer acting the same in a sub with a Faraday ring as a sub without one, and I don't blame him. Adire's fine subwoofers do not choose to take the Faraday ring route, and I doubt Dan wants to get into arguments about the magnetic behavior of a feature that he does not feel is necessary to begin with.
I thought the faraday ring was intended to achieve a symmetrical magnetic field. When woofers are in a bass reflex enclosure and operating at higher excursion, they will start to have part of the waveform cut off and the driver will only work on the part where it moves to the rear or something like that.
BAM:
Yes, the Faraday ring is used to achieve a symmetrical magnetic field, for the reasons you state. That is why they are a good thing.
I don't understand the mechanism, despite several people trying to explain it to me, but apparently they do this by limiting the inductance in some manner. From this, I am given to understand that a speaker with a Faraday ring and an inductance of say, 3 mH might not behave the same as a speaker with the same inductance and no Faraday ring.
There is another way to make the magnetic field symmetrical, or more symmetrical, and that is by means of an extended pole piece which puts more magnetic material in front of the voice coil to counterbalance all the magnetic material in back of the pole piece. This seems to be the route that good subwoofer manufacturers use when they don't use the Faraday ring.
The two subwoofer manufacturers I can think of that use the Faraday ring are Peerless and Scan Speak. The Peerless seems to have high inductance listed but the CSX 10" model is flat up to 1500 Hz. Similar results for their smaller woofers, all of which in the CSX line have a Faraday ring.
On the other hand, the subwoofer manufacturers who don't use the Faraday ring seem to have response rolling off at the high frequencies-which might be a factor in their choosing not to use a Faraday ring in the first place. I believe the reasoning is that the extended pole piece makes the magnetic field symmetrical enough to prevent clipping off half the wave form, plus the designer gets a nice 6 dB/octave crossover built in to augment or even constitute a crossover in and of itself.
I am given to believe that the Faraday ring gives somewhat less distortion than the extended pole piece-even though the extended pole piece works well enough to prevent "suck-in" or "oil-canning" which clips off half the waveform. But it might be questionable how much of the difference is really hearable in a speaker essentially designed to be used under 125 Hz.
Yes, the Faraday ring is used to achieve a symmetrical magnetic field, for the reasons you state. That is why they are a good thing.
I don't understand the mechanism, despite several people trying to explain it to me, but apparently they do this by limiting the inductance in some manner. From this, I am given to understand that a speaker with a Faraday ring and an inductance of say, 3 mH might not behave the same as a speaker with the same inductance and no Faraday ring.
There is another way to make the magnetic field symmetrical, or more symmetrical, and that is by means of an extended pole piece which puts more magnetic material in front of the voice coil to counterbalance all the magnetic material in back of the pole piece. This seems to be the route that good subwoofer manufacturers use when they don't use the Faraday ring.
The two subwoofer manufacturers I can think of that use the Faraday ring are Peerless and Scan Speak. The Peerless seems to have high inductance listed but the CSX 10" model is flat up to 1500 Hz. Similar results for their smaller woofers, all of which in the CSX line have a Faraday ring.
On the other hand, the subwoofer manufacturers who don't use the Faraday ring seem to have response rolling off at the high frequencies-which might be a factor in their choosing not to use a Faraday ring in the first place. I believe the reasoning is that the extended pole piece makes the magnetic field symmetrical enough to prevent clipping off half the wave form, plus the designer gets a nice 6 dB/octave crossover built in to augment or even constitute a crossover in and of itself.
I am given to believe that the Faraday ring gives somewhat less distortion than the extended pole piece-even though the extended pole piece works well enough to prevent "suck-in" or "oil-canning" which clips off half the waveform. But it might be questionable how much of the difference is really hearable in a speaker essentially designed to be used under 125 Hz.
Okay, you guys have talked me out of all the others. Now talk me out of this one:
http://www.northcreekmusic.com/Drivers/Poseidon1.htm
What's wrong with putting this in a Qtc = 0.5 box and giving it some EQ on the low end? (I notice they did not list the Le parameter.)
http://www.northcreekmusic.com/Drivers/Poseidon1.htm
What's wrong with putting this in a Qtc = 0.5 box and giving it some EQ on the low end? (I notice they did not list the Le parameter.)
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