Closed back Vs others enclosures & DC offset (Xmax & voicecoil rest position stabili)
Hi all,
I've not seen it talked that much here, but i was intrigued by BCspeakers talking about DC offset care. I found today a interesting 2013 patent from Harman here : US20160173972A1 - Sealed Speaker System Having a Pressure Vent
- Google Patents. It has the advantage of beeing intelligible enough for me
.
What do you think about this ? Would a closed back with driver mounted reversed would be better imune to this by transfering heat outside ?
I first would have thought that a closed back would have been more linear than accounting solely on suspension for keeping coil around rest position. TH are know for good heat transfer, but i'm still suspicious about it keeping DC pressure equal each side of the cone to keep voicecoil working centered around the gap.
As with a balance, i would want to "tare" closed back pressure to "tare" voicecoil position
Graph in the patent are probably extreme cases, but still stiff suspension seems the way to go... Do you know of others documentation about it ?
Damien
Hi all,
I've not seen it talked that much here, but i was intrigued by BCspeakers talking about DC offset care. I found today a interesting 2013 patent from Harman here : US20160173972A1 - Sealed Speaker System Having a Pressure Vent
- Google Patents. It has the advantage of beeing intelligible enough for me
.What do you think about this ? Would a closed back with driver mounted reversed would be better imune to this by transfering heat outside ?
I first would have thought that a closed back would have been more linear than accounting solely on suspension for keeping coil around rest position. TH are know for good heat transfer, but i'm still suspicious about it keeping DC pressure equal each side of the cone to keep voicecoil working centered around the gap.
As with a balance, i would want to "tare" closed back pressure to "tare" voicecoil position
Graph in the patent are probably extreme cases, but still stiff suspension seems the way to go... Do you know of others documentation about it ?
Damien
"Would a closed back with driver mounted reversed would be better immune to this by transferring heat outside?" Yes!
How about this?
Aperiodic Loudspeaker Enclosure Design
How about this?
Aperiodic Loudspeaker Enclosure Design
Still not understood ^^.
I'm not interested at all with aperiodic box. I'd like to talk about how avoiding DC offset voicecoil, keeping closed back contrary to the patent. In fact, the "benefit" shown of the patent would not have been needed if the driver have been mounted magnet to the other side. The patent show that the problem is taht small rising temperature cause air expansion in closed back volume and somewhat push cone so that voicecoil isn't working anymore at his rest position around the gap.
I would much prefer real closed back with no leak to use this closed box volume as way to keep subwoofer working around it rest position because of elasticity of this volume, avoiding voicecoil offset.
If there's no closed back chamber, no matter the topology (offset/reflex/TL/8th order/6order....) the DC working position is set only by suspension.
I'm not interested at all with aperiodic box. I'd like to talk about how avoiding DC offset voicecoil, keeping closed back contrary to the patent. In fact, the "benefit" shown of the patent would not have been needed if the driver have been mounted magnet to the other side. The patent show that the problem is taht small rising temperature cause air expansion in closed back volume and somewhat push cone so that voicecoil isn't working anymore at his rest position around the gap.
I would much prefer real closed back with no leak to use this closed box volume as way to keep subwoofer working around it rest position because of elasticity of this volume, avoiding voicecoil offset.
If there's no closed back chamber, no matter the topology (offset/reflex/TL/8th order/6order....) the DC working position is set only by suspension.
It should be easy to use a thin and long tube to equalize the pressure between the outside and the inside the box. A 2 meter long 5 mm diameter steel pipe or so. It should not effect the closed box above a fraction of a Hz or so. As long as the box is truly closed the air expanding due to heat will effect the DC offset of the driver.
I would not lose too much sleep over these kind of minor issues. If it bothers you so much, just drill a 1mm hole thru the enclosure to relieve any pressure difference. Your typical "closed box" does leak slightly, too.
If you are generating so much heat by the driver motor to heat the internal air in the box and drive up pressure, causing a D cone displacement, then you have some other more bothersome changes to worry about (power compression, TS parameter drift, etc.).
In the end, the technique of mounting the driver "backwards" so that the motor (magnet side) is on the outside will move the heat source out of the box and the problem will disappear.
It s not only for practical application but it interest me too. The thing is that appart from the graph in the patent that show too huge to be real ( Brian :
around 5mm ) offset, I ve never seen quantified DC offset. Maybe because it needs Klippel kind measurements tools. Brian, I wouldn't have thought at aluminium cone as heat transfer, good point.
I wonder too if voicecoil offset could happen from pressure differences too in tapped horn.
Maybe specifically closed back with reverse driver without metal cone got a advantage on others topologies from elastic pressure of the back volume helping keeping voicecoil centered, the effect is clear around DC in sims and should apply whatever is on input. A good point for Volt speakers... For my own application, I think of low qts sub in small back volume with some front chambers...Of course, the bigger back volume, the less centering effect there is. I clearly lack of knowledge and documentation on the subject.
I wonder if ipal pressure sensor have some averaging process to slowly correct misequilibrium between front and back DC pressure of the cone. I'm probably totally wrong.
EDIT : just had to google klippel dc offset in fact...lol But, it becomes too hard ^^ https://www.klippel.de/fileadmin/klippel/Files/Know_How/Application_Notes/AN_21_Bl_Shift.pdf
around 5mm ) offset, I ve never seen quantified DC offset. Maybe because it needs Klippel kind measurements tools. Brian, I wouldn't have thought at aluminium cone as heat transfer, good point.
I wonder too if voicecoil offset could happen from pressure differences too in tapped horn.
Maybe specifically closed back with reverse driver without metal cone got a advantage on others topologies from elastic pressure of the back volume helping keeping voicecoil centered, the effect is clear around DC in sims and should apply whatever is on input. A good point for Volt speakers... For my own application, I think of low qts sub in small back volume with some front chambers...Of course, the bigger back volume, the less centering effect there is. I clearly lack of knowledge and documentation on the subject.
I wonder if ipal pressure sensor have some averaging process to slowly correct misequilibrium between front and back DC pressure of the cone. I'm probably totally wrong.
EDIT : just had to google klippel dc offset in fact...lol But, it becomes too hard ^^ https://www.klippel.de/fileadmin/klippel/Files/Know_How/Application_Notes/AN_21_Bl_Shift.pdf
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is that patent saying the offset is from asymmetric pressures? (some Youtube "excursion" videos look that way - but does it really happen at that magnitude ?)- in home use I've see "port rectification" where the woofer's path and center will move forwards from zero input center at higher levels. What would one use software wise (if anything) to simulate the effect? A damped vent/aperiodic enclousure
can alter input Z vs plain sealed box
can alter input Z vs plain sealed box
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From the few i manage to understand, Klippel documentation seems to show that it mostly come from asymetric BL, and mostly increase H2. So it cannot be simulated from thiele parameter which are only small signal parameters and don't say anything about how a speaker work for higher power. The only thing our standard sims show is that small closed back limit excursion (in each way) by it pressure. It would require FEM sims...which are only few times encountered here.
with vents having asymmetric flange, the offset seems to be from air in the port moving easier in one direction than the other, so kinda like rectifying AC without a cap filter and getting AC superimposed on top of DC.. The cone's new center position tends to move forwards from normal.
"port rectifcation" is mentioned in this paper
http://mariobon.com/Articoli_storici_AES/jbl/JBL_2002_AES_Reflex_Ports.pdf
"port rectifcation" is mentioned in this paper
http://mariobon.com/Articoli_storici_AES/jbl/JBL_2002_AES_Reflex_Ports.pdf
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That's basically what the Bose AM5 Series II bass module looks like inside.
I see why they call it a bass module instead of subwoofer. It would be interesting to see like a 12" version with an F3 of 20-30hz. I can't believe those 5.25's handled 790 watts without bottoming out.
Bose Acoustimass 5 series II Speaker System Review price specs - Hi-Fi Classic
Bose Acoustimass 5 series II Speaker System Review price specs - Hi-Fi Classic
The Bose system uses in-line compression (if I remember correctly, lamps and I think PTC devices). These ramp up the impedance above a certain voltage level. This makes it almost impossible to blow them them using your typical stereo amp. The lamp protection approach is poo-pood by audiophiles, but it works, and you do find it quite a bit in the pro audio industry too, though usually to protect tweeters, not woofers.
Using lamp compression with with bandpass bass modules does offer some unique advantages, in that the the increased resistance just causes the response to get a little more lumpy, and as it's instantaneous I'll bet that few would notice it, and the acoustic bandpass action of the subwoofer filters out a good part of any distortion produced anyway. End result - you can turn up the volume and not not worry about blowing the drivers. The system just stops getting louder.
BTW, SR's quoted response for the AM module seems very similar to what I measured. "There were two points of maximum output in the range of the bass module, at 120 and 55 Hz. Between 50 and 250 Hz the output varied only ± 3 dB. It fell off at about 24 dB per octave below 50 Hz and at a remarkable 50 dB per octave from 250 to 330 Hz. "
In red is the measured response of the Bose AM7 module, and in blue is the predicted response of "Enigma 2", my intended rebuild.
In red is the measured response of the Bose AM7 module, and in blue is the predicted response of "Enigma 2", my intended rebuild.
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