A Monster Box Construction Methods Project

8/10/2020 Initial post. PLEASE DONATE TO MY GOFUNDME!

9/26/2020 added results of Rounds 1 & 2

Which wood to use? Which fill? What does bracing do? Is CLD best? I'm starting this project to start quantifying some answers to these questions. A lot of work has been done before in fits in starts by others (BBC, Kef, etc.) but they were often narrow investigations and you couldn't compare results because measurement conditions were different among the different studies. Some have cited manufacturer's damping specs, but I have no idea how that translates to the real world. Ditto on accelerometer data. So here I will be doing SPL measurements, that is after all, what we actually hear.

 

The cabinet is ordinary, large enough for the panels to have some radiating area and magnify differences between changes. I will be using a Peerless 830970 firing into the cabinet using MLS signals. When I do the fill/lining I will mount the SB15 as a dummy woofer and measure what radiates through it. Mic placement is 1/8" from the SB15 dustcap when doing lining/fill testing, and 1/2" from either the side or rear panel when doing wall radiation testing. I use MLS signals and have settled on a 50ms gate and 1/24 octave smoothed.

To discuss the results, I've started at a thread at diyaudio.com

In the plots where "bare driver" is shown, this is the far field measurement of the Peerless 830970 (levels are adjusted so relative SPL is correct, if you see a resonant peak near the bare driver level, it really is that loud outside the box) as shown here:

ROUND 1 

First up, tests of 6 cabinet materials. Note: given the relatively low SPL of the sound radiating from the cabinet, the ambient noise floor can start to show up below 200hz, so consider this the cutoff for useful information.

1/2" JUNK PLYWOOD

Got this stuff from Home Depot. Looks like 3 layers plus thin veneers.

1/2" HIGH QUALITY PLYWOOD

Good quality 7-ply birch plywood

1/2" MDF

Note: I started harmonic distortion testing after I had cut a hole for the SB15 dummy woofer in the 1/2" MDF and 3/4" mediocre plywood, so I capped the hole for HD testing. Because of this, I don't really think it is strictly comparable to the other boxes that never had a big hole cut in them.

3/4" MEDICORE PLYWOOD

AC Radiata pine plywood from Home Depot. 7 plys, occasional small void. Note: I started harmonic distortion testing after I had cut a hole for the SB15 dummy woofer in the 1/2" MDF and 3/4" mediocre plywood, so I capped the hole for HD testing. Because of this, I don't really think it is strictly comparable to the other boxes that never had a big hole cut in them.

3/4" HIGH QUALITY PLYWOOD

Very high quality Russian birch plywood, 13 plys

3/4" MDF

Following is a comparison of 1/2" and 3/4" materials respectively:

Fill and Lining testing

Next we test filling and lining, and measure how much sound radiates through a woofer cone to the outside. The baseline is an empty box, no lining, no bracing. Dummy woofer is an SB Acoustics SB15, terminals shorted. The wood going forward will be the 3/4" mediocre plywood. 

Monacor MDM3 wool batting 
Looks like this isn't available anymore. About 1/2" thick batting of wool and IIRC poly. Given the performance of 2 layers (1" nominal) I would like to find a new source of wool batting, since my big complaint with the denim insulation is the amount of fuzz and fiber it gives off. Makes me worry it will get into the voicecoil of a driver. Some lining will be multiple layers, so note the description in the plots

1" Sonic Barrier

From Parts Express

1.8" Ultratouch Denim insulation
Available at HD (and other places), reasonably priced, performs well. No wonder the popularity. Now if would just stop shedding so much stuff!

Meniscus Audio bonded dacron batting

This is interesting stuff. Not as dense as wool or denim insulation, but for exactly that reason it used by the transmission line designers to completely fill the first half of the line behind the woofer with a consistent, know density material. The plot labeled "stuffed" is filling 1/2 of the box, just past the top of the dummy woofer.

Polyester pillow stuffing

1/3 of a pillow I bought from Walmart

Following are comparisons I find informative:

ROUND 2

Round 2 results! Couple issues make these results not exactly comparable to the first round. First, repeatedly disconnecting the source driver ended up with a terminal coming off, lead and all. I bought a couple more drivers, and of course the response isn't exactly the same. So I've taken a new baseline empty box measurement with the 3/4" mediocre ply identified before (AC Radiata pine plywood). Second, I already cut holes in that box for the previous fill testing, so built a new box. So new baseline is a new driver in a new 3/4" mediocre ply box.

On measurement conditions, remember that the amount of signal escaping the box is relatively low, so low frequency noise can start to show up <200hz. So take that as the lower limit. The one exception is the fill testing where the amount of bass leaving the dummy woofer is actually pretty high, so you see a typical low frequency roll off there.

For the CLD enclosure tests, I used 1/4" MDF as the primary material. Why so thin? Well it seems to me that a proper CLD should be doing more with less - design smarter, not harder. I built box-within-a-box, minus the baffle, which was a solid piece of MDF left for last. I used a fine tooth trowel that made 1/16" beads to try to keep a consistent application of adhesive. See pics below to get an idea. For the Nidacore and 1/2" XPS foam I used Loctite PL300 adhesive, since it is made to use with foam.

First plot is the new baseline of an empty box for CLD testing, using the new driver and 3/4" mediocre plywood used before. Second plot is a baseline with the new driver of the denim insulation, identified in round 1 as a top performer. 

1" Melamine foam

.80" 100% wool for quilts from homeofwool.com.

This looks the same as other wool batting for quilts, so I think any 100% wool batting for quilts you can find will behave similarly as long as the nominal thickness is the same:

Comparison of the top 3 performers:

Bracing

Next is bracing. I used 3/4" x 1 1/2" oak braces from Home Depot. The control is just a simple cross brace. Then test CLD braces using Sikaflex 292i or Weicon Flex 310M Classic as the adhesive. There is a 95% overlap between the braces.

Weicon Flex 310M Classic

A construction adhesive made from MS polymer, a new-ish family of adhesives that supposedly has vibration dampening also. Very creamy and easy to work with:

Sikaflex 292i

This is a construction adhesive with claims of vibration dampening. Rather thick and tacky:

Following are some comparisons of the results. I was surprised that the CLD braces didn't really seem to do much, at least in my implementation. I'm guessing with the 95% overlap, the bracing is effectively solid, again, in this implementation. Next round I will try 50% overlap and see what that does.

And now the full CLD construction! The first two boxes are simply 1/4" MDF joined by either the Sikaflex or Weicon. The last pair use either 1/4" Nidacore or 1/2" XPS foam as an interior layer, glued with Loctite PL300, and again using 1/4" MDF as the inner and outer skin. I built box-within-a-box style, but the baffle was a single piece of 1/2" MDF.

And last, Resonix CLD squares applied to the 3/4" mediocre ply box above. The story of these is interesting. A hobbyist at diymobileaudio.com had done a very popular CLD test and one product rose to the top in overall performance and price: Sundown Solutions CLD tiles. The surprise was that there were other similarly constructed CLD tiles, but performance was surprisingly variable. Some years later SDS went out of business the hobbyist started his own line with the aim of being the same (or better) as the SDS tiles. I write all of this to alert you that just any butyl rubber and aluminum tile may not behave this well.

Some comparisons:

Soma
Sonus