For Zaph regarding your most recent blog post

[Amphiprion]

Hey Zaph,

Since I can't email you, you evil, evil man I thought I'd post this here. It's regarding your impedance measurements of tall stuffed cabinets.

Years ago I made a pair of Tempest subwoofers that were, externally, 23x23x37. 1.25" MDF, dual 4" flared ports. I took an impedance sweep of one of them with no stuffing and with way overdone 50% fill. Please see below for pics and results.

Obviously, it shot the Ql to crap, but it certainly killed out almost all of the resonances. This was done to show how dramatic an effect it could have, obviously the proper amount was somewhere in between combined with a low and steep crossover. Thanks,

-Someone who would have otherwise emailed you this.

Build Page

Measurement Zoomed In (taken with a friend's LAUD setup and the exported text files plotted in an old software package that used to be offered by Adire).

[owdi]

I was thinking of starting this exact same thread after the latest blog entry at zaphaudio.com :-) My Dayton 12" sub, before and after.





The dip/peak was fixed with a very small amount of stuffing, about 1/4 of a small pillow. The enclosure is 24" deep.

EDIT: If you are reading this Mr. Krutke, I apologize for sucking up bandwidth by refreshing your blog 6 times a day, in anticipation of the results for the latest batch of tweeters.
Dan

[Paul Ebert]

So, does the extra damping at the bottom of the enclosure reduce the Vb substantially? That is to say, do I need to increase the size of the box to make up for the extra damping. If so, do I increase it one for one to the volume of the damping material?

[owdi]

So, does the extra damping at the bottom of the enclosure reduce the Vb substantially? That is to say, do I need to increase the size of the box to make up for the extra damping. If so, do I increase it one for one to the volume of the damping material?

Just the opposite, you need to decrease box size if you add stuffing.

http://www.moodym.com/audio/fiber.html

Dan

[ThomasW]

That is to say, do I need to increase the size of the box to make up for the extra damping

Just the opposite, you need to decrease box size if you add stuffing.

Generalizations like this aren't all that helpful.....

Good box modeling programs have settings that compensate when an amount of damping is chosen. This means the box size remains the same unless a there's large deviation between the amount of damping spec'ed in the model and the amount actually used.

It's been a while since this was posted so here it is again....

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 different amount.

[---k---]

Zaph,

Interesting. Makes me want to get some impedance measurements of my Khanspires. At times, I have been able to feel a little of the beat when I place my hand on the top of the box. Right now, they are lined with 1" OC703. One of them does have some light stuffing in the bottom. I've never noticed any muddy base, but now you got me wondering.... dang you!

[Amphiprion]

So, does the extra damping at the bottom of the enclosure reduce the Vb substantially?

In my case, it did not significantly do so. If you look at the impedance minima between the two large impedance peaks, it shifts downwards by 1Hz at most. That's the tuning point. The most dramatic effect the stuffing had was the damping it introduced, as evidenced by the severe reduction in the height of the two large impedance peaks (as well as absorbing the HF resonances).

[JonW]

The impedance plots in Zaph's blog...

If I didn't know any better, I'd have to guess the plots are from a Scanspeak 18W/8531 woofer. 8)

[jkrutke]

Yup. I guess the point of my blog blurb was that the required thickness of damping is proportional to the longest dimension, with the node based on the wavelength. Then of course, the noise reduction coefficient of the damping for a given thickness comes into play. If you take a high enough resolution impedance curve, you can figure out exactly which wall needs more damping. This is why often, side walls in narrow enclosures only require 1/2" of damping because the wavelength to the other side is so short, and the driver surface is spread out across that distance.

Interestingly, a 2.5 way in a tall open volume (no separate chambers) enclosure does not need quite as much damping, particularly if the lower woofer has a bit of space between the upper woofer. Essentially, you have two different wavelengths to the bottom canceling out each other's reflections. Something to consider with MTM floorstanders or monster WMTMW systems.

So, for tall single woofer enclosures, putting the same amount of damping on the bottom as the sides and back is not enough. Whispermat works wonders in the midrange but in tall enclosures it's not enough. You can see it in this chart they published.

[jkrutke]

The impedance plots in Zaph's blog...

If I didn't know any better, I'd have to guess the plots are from a Scanspeak 18W/8531 woofer. 8)

Good guess, made obvious by the little 900hz blip, which is not enclosure related.

[todd95008]

Interesting blurb in S-pile blog on along these lines.

http://www.stereophile.com/thefifthelement/608fifth/index2.html

I now have about 4 inches of dense stuffing on the bottom and another 6 inches at the top of a 38" tall Zaph waveguide TMM.


Todd

[jkrutke]

Just for the record, this item has been moved from my blog and placed in the Tidbits section where all the other oddball measurements live. It's under the heading "Tall enclosures..." right at the top for the moment, but probably will go down the list over time. There's some discussion of it in the ZRT project also.

The article suggests that 6" may not be enough for a 42" high enclosure. What was not mentioned was the density of the insulation and it's absorption curve. That is ultimately what matters. In the ZRT design, it took 8" densely packed.

It's also not mentioned that multiple drivers at different places in the long enclosure functionally cancel out lengthwise pressure nodes. For example, the 2.5-way version of the ZRT will not exhibit this problem quite as much as the 2-way, which has a single woofer close to one end.

There's really no problem with tall enclosures, this is just something that should be addressed for midbass clarity.

[---k---]

It's also not mentioned that multiple drivers at different places in the long enclosure functionally cancel out lengthwise pressure nodes.

I've measured my Khanspires impedance with a WT3 (plot at the begining of the thread). I also don't get any wiggles in the impedance or colouration with my 50" tall enclosure. I only have 1" of OC703 in the top and bottom. I suspect it is as you describe, the multiple drivers are canceling out the lengthwise pressure nodes.

[todd95008]

The article suggests that 6" may not be enough for a 42" high enclosure. What was not mentioned was the density of the insulation and it's absorption curve. That is ultimately what matters. In the ZRT design, it took 8" densely packed.

I was thinking the same thing as I read the article !!!
There are some high density fiberglass products (O/C 703 & rigid 705) that have very good absorption down to 125Hz.

It's also not mentioned that multiple drivers at different places in the long enclosure functionally cancel out lengthwise pressure nodes.

I think this is true only to a certain extent unless the internal modes are spaced just right to cancel each other ??

The extra stuffing (second round of it that is) did in fact clean up the mid bass (or mud-bass).

Todd