Cabinet Dampening Materials

[Ian Mackenzie]

Rich,

My post was meant to be a primer for some hands on stuff.

Be great if you can arrange some comparisons.

Ian

[richluvsound]

Ian,

I have components. I gather we are gearing this experiment toward newer designs. I would prefer to develop something that be of some practical use to a Member of the forum. I will need some parameters set for me .

Rich

[Ian Mackenzie]

Build two "outhouses" about the same dimensions as the Tardis using difference materials and go for your life...LOL

[Andyoz]

I think it will be very interesting to build A-B boxes just to hear the difference.

I'm gonna say something contoversal here, but don't be surprised if you like the box with less damping.

I often wonder if the reason I like the JBL bass "sound" is their enclosure designs generally have large flat panels that are generally not very well damped. I got sick of listening to the newer, "cleaner", highly damped boxes a while ago. There was no balls to the sound and maybe box resonances is something to do with the "balls".

If you have a live cello player in your living room, you are hearing sound emitted by almost every surface of the instrument. This sound interacts with the room in a complex way at the lower frequencies as the multiple sources are spread out spatially (different parts of the instrument link into different modes of the room). Probably not a good example, but you see what I'm getting at.

[richluvsound]

Build two "outhouses" about the same dimensions as the Tardis using difference materials and go for your life...LOL

yeah, lock her in it and set in on fire !!!!!!

[cvengr]

.. but don't be surprised if you like the box with less damping.

I often wonder if the reason I like the JBL bass "sound" is their enclosure designs generally have large flat panels that are generally not very well damped. I got sick of listening to the newer, "cleaner", highly damped boxes a while ago. ...If you have a live cello player in your living room, you are hearing sound emitted by almost every surface of the instrument. This sound interacts with the room in a complex way at the lower frequencies as the multiple sources are spread out spatially (different parts of the instrument link into different modes of the room).

Along these lines, I've also been curious as to the best design of the enclosure chamber in a ported enclosure. I would think an egg shaped interior might be better suited but it really depends upon response function of particular compnents employed, along with the harmonics of the program being reproduced, IMHO.

In an attempt to quantify and qualify these impressions, I've paid attention to the near field measurements employed by many to measure particular transducer response functions, and single frequency measurements at near and far field responses for particular transducers/(single speaker assemblies), but I don't recall any studies or test results showing various port resonances for various harmonics.

For example, Consider the following description posted at https://carnot.physics.buffalo.edu/a.../msg00103.html
Pianos are subjected to what is known as "stretched tuning",
because the "strings" used for the low notes are complex structures
with small wire wrapped around another wire to increase the linear
density. The result is that they don't vibrate like simple strings,
and the overtones occur at frequencies that are higher than those for a
simple string. When notes are sounded on the piano, most of the energy
is in the high overtones, and when notes are played together the beats
between the overtones give the sound its character. Since the
overtones become progressively sharper for the nonideal low pitch
strings, the fundamentals have to be tuned to be more and more flat
relative to the middle strings in order for combinations of overtones
to sound "pleasant". To compromise, the higher strings tend to be
tuned sharp relative to the middle, although the degree of "stretch"
for the higher strings is much smaller than for the lower strings. The
degree of "stretch" also depends on the physical size of the piano--
a 9-foot grand piano can be built with longer, more massive strings
than a small upright spinet, so the grand piano has less "stretch" than
spinet (and therefore tends to sound better). You can read about
stretch tuning in most texts on musical acoustics.
The 256 Hz "C" was something that puzzled me when I first began
teaching a musical acoustics course. While doing some background
reading in preparation for classes on construction of musical scales,
I ran across a reference to a "scientific" scale that was proposed
either late in the 19th century or early in the 20th century. The
idea is that since musical intervals (pitch differences) are defined
as frequency ratios, only one "standard" pitch needs to be defined.
Historically, this has been done with the A above middle C on the piano
having a frequency somewhere in the neighborhood of 440 Hz, although
the actual standard has varied somewhat. When (around the end of the
19th or beginning of the 20 century) it was decided to have an official
standard of pitch, one suggestion was to make all the C's have
frequencies that were multiples of 2. However, this turned out to be
too far from musical preferences of that time, because a 256 Hz middle
C puts the next A at about 431 Hz. At this time, even wind band
instruments were built to be tuned either to a 440 Hz A or a 435 Hz A
(I have one of those old instruments at home), so adopting a 256 Hz
standard for middle C would make all existing instruments very sharp.
Thus the 440 Hz A was settled on as a standard, and the scientific
scale persists only in the form of tuning forks for physics labs. (I
can't remember where I read this little history, but if anyone is
desperate for a source, I'll try to track it down.)

Steve Luzader


--
Stephen Luzader
Frostburg State University
Frostburg, MD

---

Now lets consider the sound reproduction of middle C from a
piano, reportedly one of the more difficult instruments to
reproduce.

The sound being reproduced is not only the 440Hz freq, but a
series of convoluted harmonics as the above article suggests.

Once the sound has been recorded, (assuming that feat has been
successfully accurate) the electrical signal of those frequencies
passing to the speaker system which might very accurately reproduce
the signal into phonetic energy in near and far field measurements
of the transducer. When response function attenuate, they might be
increased with the use of ported enclosures tuned to particular
resonance frequencies thereby increasing the response of the tuned
frequencies and harmonics thereof.

The sound being reproduced though is also convoluted with the
harmonics generated by the enclosure. So constructive and destructive
interference of sound waves within the enclosure may also generate
1st, 2nd, third order harmonics which would be enclosure dependent.

Dampening the enclosure would assist in reducing the magnitude of those
enclosure dependent harmonics perhaps by several orders of magnitude.

Meanwhile, it appears to me that the quest to reproduce the original
harmonics accurately is more largely the challenge of the recording
engineer.

This unfortunately leads me to the unending quest of next desiring to
unmix the recordings and remix to my taste. Go figure.

The JBL bug never ends.

[terryblulite]

Picked up some "Thermacels" additive from the local hardware store.I have a couple of exposed ducts downstairs I'll try it on,and see if I get an SPL reduction or not.This mostly a thermal ceramic micro-sphere thing,but the website has sound damping products,too. www.hytechsales.com Interesting reading-not sure if I'd buy a used car from them-but interesting!

Didn't mean to hijack a train of thought-For 12 bucks plus tax at the Tru-Value hardware store,it's going to be interesting to experiment with.I might mix a bit up with Duratex or another coating just to see what happens.

[Robh3606]

I am going to resurrect this thread. I finally got some MDF cut for a B380 for my new sub. Got a BX-63A off E-Bay and decided to put a spare 2235 to good use.

I think this is a good cabinet to try the Mascoat out on so I will post how it goes. I was hoping to have an accelerometer by now but that didn't work out so it will be some what subjective. At least we will find out if it's compatible with MDF. Should be coating in a couple of days.

Rob

[Mr. Widget]

Hey Rob,

An impedance curve should show any differences in cabinet resonances... Why not run an impedance curve with no coating, with fiberglass, then take out the glass and put the Mascoat in there and measure it again?

It would also be interesting to build a series of identical boxes with different bracing and check out the impedance curves.


Widget

[Robh3606]

Hey Widget

Good idea.

I want to run a curve to verify the FB as it is before I glue the port together. BBPro has been very accurate. I checked my 4344 boxes and they were right on the money so expect these to be close to the 26hz target for the tuning.

Rob

[JBL 4645]

Guys,

I really think you are heading down the wrong track with the thin brush applied layers referenced in those links. They will work on relatlively thin metal sheets but not on the much heavier (and inherently better damped) MDF or plywood boards.

You should be looking at something like this (constrained layer damping) - the 32mm layer, not the 15mm one:

http://www.cdm-uk.co.uk/CDM-UK_SDS/M...ORE-MARINE.pdf

We deal with this company in Ireland and have found them one of the few in the industry to have realistic pricing. These products actually are not rocket science.

Rich...not sure how it would go on your curves (p.s. if it works, can I get a discount of a pair of your new creations!!).

CDM-ISO-CORE-MARINE

Sound insulation Boards

I’m going to ask if Columbia Timbre has this in stock and see what the price is, I wonder if I can use that to cover over the window to reduce traffic, aircraft and people sounds down to a reasonable minimum SPL db figure.

Hey Widget

Good idea.

I want to run a curve to verify the FB as it is before I glue the port together. BBPro has been very accurate. I checked my 4344 boxes and they were right on the money so expect these to be close to the 26hz target for the tuning.

Rob

Rob

That’s not a bad range 26Hz does it shake well for movies soundtracks.

http://www.lansingheritage.org/images/jbl/specs/pro-comp/2235/page1.jpg

[Robh3606]

This software newer ceases to amaze me how accurate the sims are.


Hz Ohms Phase


24.2103 7.2714 -3.4455
24.6604 7.2722 -1.8418
25.1189 7.2261 -0.0304
25.5859 7.2369 2.0117
26.0616 7.2083 4.3396
26.5461 7.2784 5.8329
27.0396 7.2831 8.0947
27.5423 7.3698 9.8611
28.0544 7.3942 11.3920
28.5759 7.5352 13.6985
29.1072 7.6858 15.8429
29.6483 7.8460 17.6673

Zmin should be the box tuning frequency and sure enough it's at 26.1hz with a 26hz target tuning point. Not bad at all.

Rob

[Robh3606]

Here's a photo of the glop still wet. It brushes on, has what smells like a latex base, cleans up with water and dries in a couple of hours. So it's really easy to apply and clean up after. It's very thick and for the most part stays where it's but with a little sagging of vertical sides if you go too heavy with the application. There also seems to be no issues with the water content and MDF. No visible reaction during the wet application.

Does it work?? Well have you ever done the knuckle rap test and heard the pitch change slightly when you get close to a corner or brace?? The coated panels don't do that. They are the same and don't change when you move the place of impact. Does that mean anything?? It tells me the panel is acting as a single uniform mass. Is that good of bad?? You tell me.

Final tuning with the box glued closed ended up being 25.6Hz

Rob