Acoustic damping

[opt-e]

I understand that damping materials are used to damp cabinet vibrations and absorb unwanted backwaves from the driver.

Why, however, do people tend to only address certain frequency ranges and ignore others. I see that, for example, it is recommended for a ported NatP/Modula MTM, the walls are lined with Sonic Barrier or acoustic foam. I have also seen recommendations for some sort of loose fill like dacron to be clumped behind the woofers to absorb midrange reflections. What about the lower bass reflections?

It would make sense to damp all internal reflections across the entire frequency band, but this doesn't seem to be done.

I am also unclear on the differences with regard to stuffing in ported vs sealed designs.

How critical are these decisions in the whole scheme of things?

Anthony

[joecarrow]

If you put stuffing in a ported box, it tends to take away from the bass output. That's the reason why people say not to put dacron in a ported box- the resonance of the air in the box is desired, since it's what's creating your low bass.

In a sealed design, the resonance of the air inside the box is bad. Ideally, the air inside the box would just at like a massless spring acting on the cone. You can determine which frequencies need to be damped based on box geometry and the wavelength of the sound at a given frequency.

Damping all reflections could be achieved in a large box (like an infinite baffle- at least 5 to 20 times Vas) with loose stuffing that gets progressively better at blocking bass as it gets closer to the wall. Specifically, you'd have an anechoic chamber behind your driver.

All of that has to do with air resonance. Cabinet vibration is different, and if you build the cabinet in a reasonably stiff manner (at least 3/4" thick, with braces), it shouldn't be a major issue in a small box.

As I understand it, standing waves (internal air resonance) affects cone motion, and cabinet resonance is just the sides of the cabinet moving in a sympathetic resonance causing secondary acoustic output.

[opt-e]

Excellent post, this the exactly the information I was looking for :T
Thanks!

[joecarrow]

I'm glad it was helpful :T

I hope to have more to post in the future- something along the lines of the folks who post hard numbers and graphs. As far as I can tell, standing waves and internal cabinet resonances have a fairly strong modeling tool in Martin King's software on www.quarter-wave.com.

There seem to be less resources on cabinet resonance. I have been considering a test rig for measuring the relative stiffness of different cabinet construction materials- but it seems to me that somebody has already had a good start at this. I guess it's time for me to do some searching after this!

[BretH]

From what I have read here dampening has another purpose: to keep vibrations from leaking back out the front through the larger driver's cones.

[opt-e]

From what I have read here dampening has another purpose: to keep vibrations from leaking back out the front through the larger driver's cones.

As I understand it, this is how internal resonances in a sealed enclosure are transfered to the 'outside' world, assuming the cabinet walls are well braced and damped.

[joecarrow]

That's a good point. On a side note, I found another thread on cabinet resonance measuring here:

http://htguide.com/forum/showthread.php4?t=16564&highlight=cabinet+resonance

[ThomasW]

Snips of info from Ken Kantor....

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.

----------------------------------------------------------------
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."

----------------------------------------------------------------

"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."