Sealed Subs: Barometric Pressure Changes & Heat Affecting Excursion

[RandyMidd]

Just finished one of my two subs and after 30 minutes of playing, the air inside of the sealed sonotube had heated and expanded to the point that the driver cone was protruding outward (with the power shut off).

Funny because it occurred to me as I was building a shraeder valve might be nice to compensate for barometric pressure changes.

Thomas, have you gotten sealed units shipped to you at your altitude with the driver cones protruding out?

This must be a common occurrance and it must affect excursion & performance...what does everyone do about this phenomena?

[ThomasW]

Thomas, have you gotten sealed units shipped to you at your altitude with the driver cones protruding out?

This must be a common occurrance and it must affect excursion & performance...what does everyone do about this phenomena?

Nope, never seen it, never heard anyone talking about it.

The only situation where I've see anything like this occur is with companies that use inflatable 'air-packs' instead of standard peanuts or foam pack to protect stuff being shipped. Frequently the air-packs are either flat (burst) or hype inflated

[RandyMidd]

Nope, never seen it, never heard anyone talking about it.

Now you have...am I the only one who takes the word sealed literally?

Picture of sub's cone protruding forward, with no power to driver.

After a while, I suspect because the trapped air cooled and pressures went back down, the cone returned to the neutral position.

I'm worried now because Thomas has never seen or heard of this???

Anyone else???

I'm going to have to put a shraeder valve on this, evacuate with a vacuum pump and charge it with some inert gas...OMG another reason to go ported.

[ThomasW]

Must have gotten pretty hot to cause that much expansion... 8O

[RandyMidd]

My sealed sub is sealed...I thought they were supposed to be...check out the Teflon tape and o-ring seals for the driver hold-down bolts in the picture.

I even glued the driver gasket ends together.

I'm starting to think I need a small air leak...if no one has any solutions for me I'll take out one of the hold-down bolts' Teflon tape and o-ring and see how that goes.

FWIW, looking at the amount of music I played it must have been more like an hour of high volume and material with decent bass.

[ThomasW]

I understand that, but the reality is for enough heat to be generated that would result in the increase in pressure needed to move the cone out that much, things were HOT

[RandyMidd]

Maybe I've stumbled upon a new type of superfiberglass...that converts energy to heat as never before...it's the nice fine white kind without the nasty fiberglass shards, and the kraft paper removed of course...

[dyazdani]

I understand that, but the reality is for enough heat to be generated that would result in the increase in pressure needed to move the cone out that much, things were HOT

Yes, you'd have to have a significant temperature increase I think to cause that. I never really thought of that though. I'm not sure why you'd inject it with an inert gas though...hydrogen would be cool! :twisted:

[ThomasW]

Randy,

By weight how much insulation is in your tubes?

[RandyMidd]

3 pounds and 5 ounces exactly in an enclosure that's 2.6 cubic feet of actual free air....so 1.28 lbs. per cu ft.

[Davey]

Randy,

Put a small pinhole somewhere in your enclosure to equalize static pressure. It won't effect the operating performance of the sub in any way.

Cheers,

Davey.

[cjd]

A 100 degree temperature swing (in C or K) is about a 20% increase in volume...

If I do some sloppy math I see a bit under .1cu/ft change in volume. That's a bit under a 4% increase. That's about a 20 degree temperature increase (in C) very roughly speaking.

I do not think this is out of the realm of possibility at all, especially because I rounded to the tall side so it could just as easily be looking for a 15 degree temperature change.

Heck, seasonal change could do that.

This is IF I'm remembering to do all the math right.

BTW, any driver with a phase plug can be "sealed" and yet will never really be sealed since there is always VC gap and the back has to be vented... You really went all out! My math teacher in HS had a story she liked to tell... imagine a boy over there and a girl over here, and they approach each-other by halving the distance between them... then half again, and half again. Mathematically they will never reach each-other. But there is a point at which they're close enough... :B

C

[ThomasW]

Randy,

Put a small pinhole somewhere in your enclosure to equalize static pressure. It won't effect the operating performance of the sub in any way.

Cheers,

Davey.

That's Linkwitz' recommendation.

The RL-15" doesn't have a phase plug. It's does have an alu cone and a thick rubber surround so not much to leak there...

[cjd]

Thomas, assuming your phase plug comment was aimed at me, I was only trying to point out how leaky things could be and still be considered "sealed"

C

[dyazdani]

A 100 degree temperature swing (in C or K) is about a 20% increase in volume...

How did you figure this? Just curious as to the assumptions you made...

[ThomasW]

Thomas, assuming your phase plug comment was aimed at me, I was only trying to point out how leaky things could be and still be considered "sealed"
C

I'm with you on this..... :T

I look a phase plugs and can't imagine what keeps them from leaking?.

Must be magic...

[RandyMidd]

I did a little looking around with regards to air's pressure temperature relationship... and found to expect 1/273 psi of pressure change per degree C.

I assembled this sub the night before in my garage... a 22C or 40F temp change inside the enclosure, between the time it was assembled and after playing it hard is a comlete guestimate but seems within the realm...so the pressure change would be .081 psi not taking into account barometric pressure changes...that's per square inch mind you, so multiply times the cone (and surround) surface area....roughly 150 square inches for the total force applied.

.081 x 150 = 12.15 lbs of force pressing on the backside of the cone. :E

[cjd]

I did a little looking around with regards to air's pressure temperature relationship... and found to expect 1/273 psi of pressure change per degree C.

Same numbers I used, I just worked volume vs. pressure. Pretty basic variables.

C

[ThomasW]

.081 x 150 = 12.15 lbs of force pressing on the backside of the cone. :E

If that's the case, take out some damping to increase the efficiency of the system. That will keep the motor cooler

[Scott Simonian]

so 1.28 lbs. per cu ft.

That sounds like a heavily stuffed enclousure! I'd take the advice from Thomas and remove some if not half of that stuffing.

[dyazdani]

Same numbers I used, I just worked volume vs. pressure. Pretty basic variables.

C

Yes, but these factors usually assume relationships at constant pressure/volume, etc. I was just curious as to what conditions apply for the numbers you found...

[Mark Seaton]

Hi Randy,

Your first picture looks like an excessive amount of offset for an RLp-15. If you didn't back the T-Nuts, just remove one screw to vent and equalize the pressure. To me that looks more like a driver with +DC on it, but I've been surprised before.

Within your room you shouldn't have much shift in temp to worry about, so a one-time venting should do the trick. If someone has spare time and wants to check the worst case offset, you could convert the pressure difference to a total force on the cone area, and then look at the Cms of the driver and determine a much worse case offset. If you see more offset than this, something is wrong. It will be less than this because the pressure reduces as the driver offsets and the driver's suspension will stiffen somewhat as it moves further.

[Kevin P]

I guess this is the first time that someone took "sealed" to mean completely airtight (toss it in the pool and it floats) sub.

In reality, it doesn't need to be completely airtight. Just airtight enough so that the pressure takes a second or three to equalize. This way bass frequencies don't cause air to leak, but air can still get in and out to compensate for temperature and barometric pressure changes.

A pinhole would work. Or just do a reasonable amount of effort into "sealing" the enclosure without going all-out and making it 100% airtight. A tiny amount of leakage from the mounted edge of the driver or from the terminal cup will do the trick.

[cjd]

Yes, but these factors usually assume relationships at constant pressure/volume, etc. I was just curious as to what conditions apply for the numbers you found...

Yeah, you're absolutely right, of course. And I have no idea, because I threw those numbers together while suffering one of those "I'm jetlagged and I don't know it" states. Which I think I'm nearing again now (it's about midnight-ish maybe, for me)

C

[atm98]

Pressure and volume in air.

Hi guys,
I will chime in here on the math.
When dealing with air, we will not be able to calculate exact values without altitude and humidity information. However, we can make some assumptions and get close.

Assuming an Ideal gas law P1*V1/T1 = P2*V2/T2. Assume that pressure equalizes as the speaker is allowed to move. Equation becomes V1/T1 = V2/T2. So as a ratio, V2/V1 = T2/T1. Now simply convert temperatures to Kelvin or Rankin.

Example: Temperature change from 70F (294K) to 100F (311K) increases volume by 311/294 = 5.8%

All gases have different thermal expansion characteristics. Adding a new gas will only slightly improve things. Looks like your best bet is some type of small hole.

Please don't let this stop you from playing with hydrogen.
:B

Hope this helps.
Austin

[cjd]

Well, Austin gives the formula I hacked through tossing rough numbers and instinct around...

C

[dyazdani]

Hi guys,
I will chime in here on the math. When dealing with air, we will not be able to calculate exact values without altitude and humidity information. However, we can make some assumptions and get close.

Assuming an Ideal gas law P1*V1/T1 = P2*V2/T2. Assume that pressure equalizes as the speaker is allowed to move. Equation becomes V1/T1 = V2/T2. So as a ratio, V2/T2 = T2/T1. Now simply convert temperatures to Kelvin or Rankin.

Example: Temperature change from 70F (294K) to 100F (311K) increases volume by 311/294 5.8%

Hope this helps.
Austin

This is exactly what I did on my figuring...

[---k---]

Ahhhhhhh.

I come here to excape math!!!

[RandyMidd]

I read and appreciate everyones comments and sage advice.

...take out some damping to increase the efficiency of the system. That will keep the motor cooler

That sounds like a heavily stuffed enclousure! I'd take the advice from Thomas and remove some if not half of that stuffing.

I don't understand the fill amount...I thought a 1.25 lb per cf fill would increase my enclosure size by about 25 to 27% based on this . Don't I want to increase my enclosure size to lower my Q towards .5?


I'll try the suggestions in my second sub...and compare the two...this may take a few days because I'm also going to try more bracing to see if that helps the pulsing table top...it's difficult to see it move but you can sure feel it.

[ThomasW]

Our usual recommendation is to size the box as close as possible to the desired "Q", and fine tune the performance with damping. Doing this avoids the issue you're having now, which is the need to remove damping to increase the efficiency, then having the "Q" rise as a result.

This is one reason I lobbied for the larger diameter tube back in July when you were in the design stage.

Don't fret all that much about the change in "Q", this probably won't be as audible as you might imagine

The other thing to consider is that the output level was only one sub. So you were driving it really hard. With the addition of the second sub the workload will drop.

So yes experiment with less damping in sub #2 and compare the performance between them. Eventually the amount of fill will be a compromise between efficiency and SQ. And then you have the LT potential of the DEQ to factor into the equation.

[Ilkka]

If someone has spare time and wants to check the worst case offset, you could convert the pressure difference to a total force on the cone area, and then look at the Cms of the driver and determine a much worse case offset. If you see more offset than this, something is wrong. It will be less than this because the pressure reduces as the driver offsets and the driver's suspension will stiffen somewhat as it moves further.

If using a force of 12.15 lbs per cone area (as calculated earlier), one gets ~8.7 mm offset (assuming D2 driver was used).

[RandyMidd]

assuming D2 driver was used

Correctly assumed.

This is one reason I lobbied for the larger diameter tube back in July when you were in the design stage.

The rebel ego prevaled that day and now I'm learning his lessons.

Eventually the amount of fill will be a compromise between efficiency and SQ.

Would an ammeter in series with the sub be helpful for this or do you just do it by ear?

[ThomasW]

Would an ammeter in series with the sub be helpful for this or do you just do it by ear?

Do it by weight. Cut the amount of damping 1/2 for the next box, then compare the performance.

Here's some just FYI data

These are snips of text from the old Bass-List where Ken Kantor talks about damping. Ken has a reasonable amount of knowledge regarding this... The last paragraph is what I do..... :T

How does damping work?

In a sealed enclosure the driver moves in and out some distance. Multiplying the displacement distance by the surface area allows us to determine the volume of air pushed. While the front side of the woofer pushes air molecules which fill our listening areas with sound, the air in a sealed enclosure is compressed and expanded with the inward and outward motion of the driver. To more simply describe the situation in the box, we see that the quantity of air molecules in the box is fixed (it's sealed), while the motion of the driver's cone changes the volume of the sealed box.

Going back to the ideal gas laws, when you compress a volume of air, the temperature will increase. Similarly, when you increase the enclosure volume of a sealed enclosure, the temperature will reduce.

We generally stuff low frequency enclosures with insulating materials, most commonly fiberglass. The idea is that the fibers or other material helps to absorb some percent of the heat generated by the compression of the air in the enclosure. This heat is then released during the expansion half of the cycle. This is known as isothermal operation (temperature stays the same). As stated earlier, all heat is not absorbed, but the degree to which this occurs is referred to as isothermal operation.

With some percentage of heat absorbed the trapped air does not change in pressure as much as an unstuffed volume. In fact, it behaves like a slightly larger volume. In theory, this phenomenon could provide an apparent increase in volume of ~40%. As noted by Vance Dickason in the Loudspeaker Design Cookbook, practical limits with real materials tend to max out at about a 20% apparent gain (going from memory here-number may be slightly diff). With a sealed system this results in a lower Fb and lower Qb.

When stuffing is placed in a sealed box there is also resistive damping to varying degrees depending on the material. In a very old message to the DIY Bass List by NHT founder/designer Ken Kantor he noted his observations that in a real box it is common to see roughly equal parts resistive damping and isothermal operation. (check the bottom of this page for an archive of one of these posts)

The following is an E-Mail addressed to the bass list on stuffing an enclosure. Ken Kantor is the former CEO & co-founder of NHT.

From: Ken Kantor
Date: 04 Apr 95 03:41:37 EDT
Subject: Stuffing Stuff

"In light of recent discussions, let me share some thoughts regarding cabinet stuffing. I'll do this from a practical point of view, partly because the physics side has been well articulated by Doug. The other reason I'll stay away from theory in that, in the matter of cabinet fill, theory has proven over the years to be of only limited help in real-world speaker design. I'll also confine most of my comments to issues related to sealed systems. Vented systems do share a
few of these same issues, but really the goals and the physics of stuffing a vented box are different.

Most professional designers would agree that practical experience, combined with trial and error, is best way to find the optimum stuffing material, quantity and method for a given design.
This is why good designers routinely experiment with fill in the development of a new system, ala Vance's data cited here. This particular information is a valid data point, but it is important
not to over-generalize. If you are designing a system that differs substantially in shape or volume or source impedance (passive crossover) from a known you will need to iterate for best
performance.

In my practice, adjusting the filling is the last step in getting the bass right, and is used mostly to fine-tune the system Qtc and resonance. As increasing amounts of polyester are added to a
sealed box, the resonance and Q gradually go down. This can be shown mathematically to be due in roughly equal parts to the effects of simple resistive damping and isothermal conversion. At some point, a minimum is reached, and further material simply reverses the trend by taking up volume. During the filling process the impedance curve is constantly monitored, and
convergence to optimum usually takes only a short time. Filling also has the important effect of reducing internal reflections, to reduce standing waves and comb filtering. However, the amount of filling has comparatively little effect on its efficacy in this regard.

[Side Note- it is a common misconception, I believe, that professional designers rely heavily on LEAP and SPICE and CALSOD to define their designs a priori. On the contrary, professional designers use these modeling tools mostly to guide and optimize revisions. Unlike DIY designs, a typical commercial 2-way will go through perhaps 3 revs of each driver, 2 to 4 box trials, and easily a dozen+ crossover changes.]

Lining the walls of a vented enclosure to reduce internal reflections, or filling a transmission line to absorb the back wave, highly absorptive wool or fiberglass are ideal. However, these materials will not generally provide the desired results in a sealed system. It is true that they will provide more reflection absorption than polyester, but the later is quite good in this regard in the critical midrange. In a sealed system you don't want absorption at lower frequencies anyway; you want damping and isothermal conversion. I have tried "all-out" efforts using fiberglass lining and
polyester fill to achieve the best of both worlds. I found the results to offer little practical benefit over polyester alone, but its worth looking into.

All NHT systems now use polyester fill, of one variety or another. We used to use fiberglass in our vented designs, but found a Danish polyester that mimicked the properties of
fiberglass very closely. I don't know if this kind of polyester is available to hobbyists. Excluding this special poly, there are essentially two kinds of fiber available: pillow stuffing,
and audio-spec polyester. The later type allegedly has hollow core fibers, but I have been unable to verify this with my keen eyesight! Sorry, but forget the pillow type. Sure, it's easy
to get. If you use enough, it will damp the midrange, and that's better than an empty box (by a lot). But it will have little effect on the lower frequencies.

Well, that's pretty much all I know about stuffing speakers."

_________________________________________

In another message Ken Kantor added (excerpted):
"Exact enclosure volume is not critical, and stuffing can be added or subtracted to fine tune the response. I recommend adjusting the stuffing by monitoring the impedance versus frequency of the sealed box system. Add stuffing to lower the frequency where the impedance is highest. When that impedance peak starts to rise in frequency, you have added too much. The NHT/SW3p uses 820g of acoustic polyester stuffing with the 1259, but your enclosure may do better with slightly

[RandyMidd]

For the second sub I reduced the fiberglass fill by 43%, I only used enough glass to line the inside without reducing the batt thickness...see the picture below.

Initially I thought SUCCESS because I could SEE the second driver moving more than the first and it was LOUDER too.

Having two subs makes comparing easy and the first one with the HEAVY fill won the sound quality contest...the bass was more controlled and sounded more like a musical instrument and less like an earthquake...even after compensating for the extra dB's from the second one.

So yes experiment with less damping in sub #2 and compare the performance between them. Eventually the amount of fill will be a compromise between efficiency and SQ.

I think two of these play loud enough even with the extra fill...I'd rather have the SQ than the efficiency...so unless anyone has any suggestions for me I'm going to fill-'er-up.

As for allowing SOME ventilation in the sealed enclosure...I found that leaving out the o-ring and teflon tape on one of the bolt threads should be adequate for pressure EQ (second picture).

[ThomasW]

The one in the picture is the stereotypic example of 'heavy fill'. More than that is overdamped.

If the driver your first post was driven hard enough to heat the enclosure enough to bulge out the suspension, the amp was being abused. Regardless of SQ benefit, continuing that situation will destroy both the amp and the drivers.

If as Mark Seaton suggested it's a DC offset issue, pounding the amp will result in the melting of the VC's

[RobP]

I have to say that I have never seen a driver do that in a sealed enclosure 8O. I have done alot of air tight sealed enclosures in automobiles where the temperatures can reach fairly high levels and have never had one do that.

[RandyMidd]

I should have disconnected the sub from the amp to be absolutely sure it was pressure but I'll bet no one has seen an EP2500 do that when it's turned off either.

[RandyMidd]

If the driver your first post was driven hard enough to heat the enclosure enough to bulge out the suspension, the amp was being abused. Regardless of SQ benefit, continuing that situation will destroy both the amp and the drivers.

As you pointed out I'll be using two now instead of just one AND right now I'm crossing to them at 150Hz using little bookshelf speakers until my 10" 3-ways are finished...then I'll probably be able to cross at 50 or 80Hz...I'm hoping those two items will take some load off of them... :scratchhead:

[RandyMidd]

Just now I shut off the Behringer EP2500 and noticed that the green power light and the red clip lights SLOWLY fade but remain lit for about a minute after the switch has been turned off...is this normal on these? :unsure:

No unusual offset from the drivers...

[RobP]

if I am reading correctly, you have two different sub enclosures, right?

[ThomasW]

I don't know about the clip lights, it's common for green power indicator lights to slowly fade as the caps in the power supply discharge.

[RandyMidd]

if I am reading correctly, you have two different sub enclosures, right?

Yes, I just got the second one put together today.

I don't know about the clip lights, it's common for green power indicator lights to slowly fade as the caps in the power supply discharge.

Thanks...my other proamp doesn't behave like that...I'll assume it's all normal.

[RobP]

Do they both behave this way or just one?

[RandyMidd]

Just one but the other is a different make...

I tried the Behringer just now and noted on shutdown...
FIRST the green light remains lit but is dimming...
THEN the red clip lights light up and begin to dim as well...
The clip lights are the last to completely go dim...see picture.
All this takes about a minute maybe more.

[BigJim_inFLA]

Just one but the other is a different make...

I tried the Behringer just now and noted on shutdown...
FIRST the green light remains lit but is dimming...
THEN the red clip lights light up and begin to dim as well...
The clip lights are the last to completely go dim...see picture.
All this takes about a minute maybe more.

My EP2500 does the same thing. Its just the caps discharching, nothing to worry about.