DIY Isolation cab - Revisited

It's been quite a while since I built that little box. It works reasonably well, but could definitely be better.

The box now belongs to a buddy of mine who lives in a small flat. I don't know how much use it's been put to, but it couldn't come with me to the new property.

While reserching methods for building a studio, I realised that I could have done the iso-cab so mch better.

The box was made with 9mm ply, glued to a pine frame and some earth wool was placed inside.
Performance was ok, but it could still be heard and was only any good for a small amp.
I think maybe even the 15 watt Fender Princeton was too hot for it.
Something like a 5 watt Valve Jr, Black Heart, Tiny Terror is more suitable.
This is due to two factors, the size of the cabinet and the density of the walls.

The cabinet I built was basically a 600mm cube. This was due to size restrictions with the flat I was living in.

What I've read is that the small volume of air can lead to speaker and output transformer damage, due to back voltages being generated in the voice coil.
Indeed on one test of a 10" driver, I was alarmed to find that the magnet was quite warm to the touch.
This is not something I have ever noticed before. I stopped that test fairly quickly. Every other test I did was at a lower volume and on a speaker with a larger magnet (better heat dissipation, among other properties).

Despite the small sound however, the sound was pretty good, no "boxiness" was present, but I think that was likely due to the mic being pressed right up against the speaker grille (no room for ambient sound).

A better design would be to double the length of the box, leave out the rock wool (it really did nothing in terms of blocking the sound path), use 18mm ply and a couple of layers of plaster board (with an air gap between the inner and outer layer). Essentially building a tiny version of a room within a room. Plaster boards properties lend itself to be quite the good little insulator. Not what I expected when I started my research.

Making a sound proof box it seems, is entirely dependent on just a few factors. Density, Mechanical Isolation and a Perfect Seal.

That means Thick Walls, Rubber & Air isolation between inner & outer boxes, and Blocking the movement of air through any gaps that may appear during assembly.

Anywhere air goes, sound goes.

2011 - Weber Z-Matcher

Weber Z-Matcher 50W version

I've had this red box for a few months now, and have used it exclusively with my Fender Princeton Reverb RI and Epiphone 12" cab, which resided in my isolation cab.

The Princeton is engineered for a nominal load of 8 Ohms, but the Epiphone cab is 16 ohms. Everything I've read about the subject is that the amp down't handle the higher load very well, and that it would be better to run two 8 om speakers in parralell(gets you 4 ohms), rather than an 8 and 16. It's all to do with math and ratio's and running mismatched loads is a no no. Sure it's going to get you a load of 6 ohms or so, but since one speaker has less resistance, most of the power will go through there and will naturally be louder than the other as a matter of course. Also this doesn't fit the ratio that the amp requires.
First I tried using a resistive dummy load to match everything up, but since a speaker doesn't have a fixed resistance (due to the combination of frequency and inductance of the voice coil), the results weren't exactly pleasing.

I'd read all about inductive loads being the way to go, and pretty much the only one that I could find with any reviews, was the one from Weber. The manufacturers of fine speakers for guitar amps.
After a lot of deliberation and indecision, I decided that having a variable inductive load would be the way to go. so I bought the 50w Z-Matcher.

The purchasing process is a little odd, it's all via email and can take some time to get your quote back, but the staff were helpful when arranging the postage. But the transaction itself was done via Paypal and they were happy to send it Via USPS when asked. Weber build all of their stuff to order, so there's a bit of a waiting period, but being in Australia, I've gotten used to waiting for my stuff to arrive. Hey, if I can't get what I want locally (or locally at a competitive rate), then I have to go offshore for the products I want/need.

They estimated up to six weeks for assembly, but it was at my door in less than a month (not bad in my opinion). Anyway, back to the box. It came in a simple postage container wrapped in paper and bubble wrap. It's quite a heavy unit, so I delayed taking it home for a few days.
There's no manual to speak of, but there's no need if you're familiar with the basics of speaker matching (all us amplified guitars and bassists should be!).

Front Panel:
amp in
amp impedance (2-16 ohms)
speaker impedance (2-16 ohms
speaker out

Rear Panel
Line Volume
Line out - Balance
Line out - unbalanced
Parallel Speaker out

It was put to work matching the Epiphone Cab in the isolation box.
The first test I ran it through was the line out. Honestly I wasn't really impressed. Clean guitar was fine, but overdriven just sounded fizzy (pretty much like any amp with line out ever).
But hey, I wasn't exactly expecting a miracle there.

The next test I did was mic (Shure SM57) up the speaker and A/B that with the line out. Guess what? The mic even carelessly placed, sounded a lot nicer than the line out. I left it like this for a few months, and just playing with it when I had a couple of spare hours. After a while I noticed a bit of a weird distortion when using reverb on the Princeton. Especially if I was driving the reverb hard. I figured it was either a dodgy cable or maybe a quirk of the speaker cab inside a fairly small volume of air (there's a lot going on in an iso cab with a large speaker. Standing waves and other funky things can send voltage back to the transformer of the amp if you're unlucky). Also my headphones are a little rattly in some frequencies.

Upon further investigation, it turned out not to be a problem with anything mentioned above, and I haven't got definitive proof yet, but I have a hypothesis which makes sense (need to find an engineer wiling to experiment).

I decided to test the whole rig outside of the cabinet and directly with my ears, reducing the variables where interference could occur(cab, mic, lead, mixer, earphones). First up, I tested the amp by itself on a rubber mat to damp vibrations into the floor. Aside from making the house rattle when it was barely past two on the dial, the amp sounded as it should with the reverb cranked).

The next thing I did was to put the Z-Matcher between the amp and it's stock 10" speaker and set for 8 ohms per side. The distortion came back. Next I plugged in the 16 ohm cab. Again, fine on straight guitar signal, weird distortion with reverb. Last up I placed the speakers in parallel. The 16 ohm cab directly into the amp, and the 8 ohm stock speaker via the Z-Matcher (the amp side of the Z-Matcher was set to 16 ohms in order to present and 8 ohm overall load to the amp). As I'd already had a suspicion that the distortion was happening inside the Z-Matcher, it was no surprise to find that I had the distortion coming through the 10" speaker and not the 12".

The Z-Matcher works flawlessly with any signal presented to it as long as there is no reverb in the mix.
What I think is happening is that there are weird things happening inside the inductors of the Z-Matcher, due to the complex reflections generated by the reverb.

Reverb adds to those complexities and since the coils of the Z-Matcher aren't on the move as they would be with a speaker/magnet combination, there is a bit of a inductive choke effect going on and the result is the distortion that I'm hearing. It's subtle and I don't think that it would matter much in a band setting. The sound does stand out for me now as I'm aware of it. But I don't think it's a deal breaker on the Z-Matcher. It's just a quirk of electromagnetism and I can live with it as the unit serves it's purpose.

2011 - DIY Isolation Cabinet

Anyone who lives in an apartment, townhouse or just generally close to their neighbours and plays guitar (or any other instrument) knows that noise can be a point of contention for those who don't appreciate your style or tones. My home is old, and pretty much everything that can rattle, does rattle when my amps are turned up to the sweet spot for tone.

Actually, the house starts to rattle a lot lower than that.
To give you an idea of what that equates to on the dial, it's about 30% on an Epiphone Valve Jr, position 2 on a Fender Princeton and about 10% on a Delta Blues. What might sound great for recording, doesn't translate to a good relationship with one's neigbours.

To get the maximum tone out of my amps and to keep things friendly, I decided to build an isolation cabinet to place a speaker cabinet and microphone inside. Simple enough idea, go get some plywood or mdf and something to dampen the sound waves. I totally over-engineered the box in terms of my level of skills.
I'm no carpenter, and it's fairly obvious by the end product, that maybe I should have gone down a much simpler path. But I've made my bed so to speak, and must lay in it!

The materials cost around $120 Australian all up.

Had I just built it out of 18mm ply or MDF it would have cost around 30% less, and taken about 80% less time too!

Anyway, the bill of materials is as follows.
5 x lengths of  19x70x1800mm cut into 600mm sections x 12
1 sheet 9x1200x2400mm Plywood cut into 8 equal panels (leaves two spare to make a pedal board from).
45 x12mm screws
Wood glue
One bag of R3 "Sound Screen" Rockwool

I wanted to get some skills with making joins, rather than using a bunch of brackets to hold the frame together.
My plan was to make a finger joint for the front and back frame, connected by the side frames which were made as lap joints to the front and back. This arrangement in theory would create a perfect 600mm cube.

What I forgot to account for was the thickness of the plywood and my crappy skills getting in the way.

I'd bought a book on woodworking and gone out and bought a cheap set of chisels.
Only one of the chisels was sharp enough to do any cutting, and by the time I got through making one pocket for the finger joint, it was immediately apparent that I was not going to finish the project in a short amount of time. I'd recently bought a plunge router attachment for my Dremel, and wanted to use it, but had never actually played with a router before. So I put together a jig and fooled about until I had a working setup.

The jig worked ok, but wasn't exactly at the right height to work effectively. Also I think the Dremel itself isn't quite up to the task of making the depth of cuts required for an accurate join. I experienced a bit of shuddering in the slot which caused uneven fingers and pockets as well as at one point, the depth guide failed and I plunged too far into one pocket. After all of this I was getting the hang of the tool. Close to completing the routing and having a set of fingers that would work ok, the Dremel stopped dead in it's tracks.

I ended up reverting to a saw and chisel arrangement that was less than ideal. I lacked to tools to do the job accurately, and often I lacked the patience and just rushed the cutting, only to have very uneven joints at the end of the day. Somehow I managed to get the front and rear frames glued square. But at some later point, I forgot to do this when trying to get the cube assembled. I'm a family man and have responsibilities, so I was often called away from the job. Due to my impatience to get on with it when I returned to the task, errors began to creep in. The final product means that it's mostly square, but not quite. Front, back, sides and bottom are all ok, but for whatever reason, the lid is not.

It was all glued up in a day, and while that set, I put the panels on. Everything seemed fine, and after letting the wood glue set for an hour or so, I began to implement the rockwool. This is an itchy business. The insulation is quite thick and difficult to compress via hand. I'd also run out of timber, so that I couldn't add any mechanical force to press the rockwool against the sides of the box.

Out of desperation to complete the job before the daylight faded (I have to work outside) I placed my Epiphone Valve Jr cabinet within the frame, inside the exposed rockwool. It was obvious right away that this had the potential to tear the insulation on insertion and removal. Since the shops were closed, I decided to cut up the tarpaulin that covers my work bench. I'd cut it in roughly 700mm sections that I could place over the rockwool and  stapled it around the opening in the cabinet. The tarp is plastic and made the cabinet slide in and out of the box easy and reduced the amount of particles that would be kicked up, reducing the skin irritation caused by handling the stuff.

After sitting about for a week, due to weather and having to work my day job, I decided to change out the tarp for a rough hessian weave. This isn't completely impervious to the dust and itchiness from the Rockwool, but it doesn't make a crinkling sound when moved about, nor is it reflective like the plastic of the tarp.

I tested the cabinet early in the afternoon, with my Epiphone 12" extension speaker driven by the matching 5w head. It did ok, but there was a lot of bass being let out at maximum volume, which was a little dissapointing to say the least. If it didn't cope with five watts, what chance did it have when three times the power was put to the test? As it turned out, I needn't have worried. Insulation in the lid, wasn't a perfect fit, so I augmented it with a picnic blanket! (Simply because I didn't want to mess around with the leftover Rockwool at this point). I'd adjusted this and then plugged in my Fender Princeton Reverb, the results were more in line with what I expected.

The bass trap absorption in the cabinet isn't great, but even when the amp is at full volume, you can have a conversation at a pretty normal level. The house does vibrate a bit, and I'd done the initial test in the kitchen which has a tiled floor. I spoke with one of my neighbours about it and he told me that he could only just hear it whenhe opened his front door. A good sign! The tone from the speaker and microphone is good too, not boxy compressed sound, it just sounds like the mix is up against the grille cloth(which it is). The box isn't air tight, which might actually be a good thing from what I have read.

Were I to do this over, I'd just get a 1200 x 2400 slab of 18mm thick ply cut to size and glue that together, rather than messing about with crazy joinery that I clearly wasn't up to doing well.

Sound samples and more pics to come.