Speaker isolation cabinets

Using a speaker isolation cabinet as just a load, with a tap and cabinet simulation filter
Speaker isolation cabinets require powerful equalization
How to quickly make a speaker isolation cabinet
Speaker isolation cabinets are a legitimate form of the traditional cabinet
Supplemental gear you will need for a speaker isolation cabinet
The importance of a good mic
A better approach to "emulation" of various tube amps

Using an iso cab as just a load, with a tap and cabinet simulation filter

It might sound best to use a speaker isolation cabinet as just a load, and tap the power-amp output (such as via Marshall head's DI Out jack) and send it through a cabinet simulation filter. That approach allows consistent settings without worrying about mic placement within the cabinet.

In Guitar Shop Feb 95 page 38, the rig diagram for Queensryche's Michael Wilton's live setup shows *two* of these: "Enclosed Marshall 4x12 Load Box Speaker". But he only seems to be using it to load the amp; no mic hears the speakers in these boxes; the line-level signal is electronically tapped at a "Line Box" that is driving the load speakers; that tapped signal is sent to time-fx. Note that this works in conjunction with final Wizard 4x12 cabs, thus avoiding running the signal through a series of 2 guitar speakers, which would muddle the tone; you only want a guitar-speaker curve in there one time, not two. The alternative, which I would do, to capture overdriven speaker tone before time fx, would be to mic the iso cabs, and use full-range monitor speakers at the very end of the chain.

Iso Cabs require powerful equalization

If you place a mic inside a sealed cabinet (rather than tapping the input and sending it to a cabinet response simulation filter), as you close the door, even listening directly to the guitar speaker, you can hear a drastic, very bad shift and closing of the sound; it's not that the mic hates being enclosed; it's that the speaker and cabinet hate being sealed all the way -- it's like going from a large open-back 1x12 cabinet, to a large closed-back 1x12 cabinet, and then again to a closed-back *and* closed-front 1x12 cabinet. To compensate, it's practically mandatory that you use a parametric equalizer to open the sound back up again. Ideally, you should sweep the speaker-and-mic with a frequency analyzer, comparing a standard cabinet, the isolation cabinet when open, and the isolation cabinet when closed -- then use a powerful equalizer to adjust the signal coming out of the sealed isolation cabinet. You can try various mic positions, but you will still *have* to use an equalizer, and really, an analyzer too. Like earbud headphones, there *is* potential here, but it requires active electronic compensation through powerful, detailed equalization.

Someone needs to try building a very long or very large isolation cabinet or isolation closet. If the front has a depth of 10 feet, this should be enough space to reduce the coloration sufficiently, reducing the need for post-mic equalization. The ideal would be a soundproofed full-sized room, but then you are back to the scale of a full-on recording studio.

How to quickly make a speaker isolation cabinet

It's easy to improvise a speaker isolation cabinet! But it requires a pair of speaker cabinets, which can easily run $500, while the Demeter speaker
isolation cabinet (about the only one available so far, unfortunately) is
$500 and already set up.

Take two closed-back guitar speaker cabinets and fasten them together
front-to-front using standard case fasteners. Use foam strip to seal the
gap. You only need to install 1 speaker, in 1 cabinet. You can leave out
all the other speakers, if the power handling matches. Mount a
microphone inside, facing the speaker. Use a rubber-band mounting
system to isolate the mic from vibrations.

If you already own two matching guitar speaker cabinets, you can
quickly experiment with a speaker isolation cabinet by lying the cabinets
horizontally on a bed. Even to do this *quick* test right, you'll need:

o a decent dynamic mic
o a mic cable
o maybe an XLR-to-1/4" adapter
o some form of monitor amp and speaker.

Make sure the speakers are connected with the right ohms and power
rating (about 50% more power handling than the tube amp produces).

You can already guess the results of this improvised speaker isolation
cabinet: it basically sounds great, because it uses actual speakers
connected directly to the tube amp -- it sounds more real, complex, pure,
and physical than an attenuator or inductive load. But some eq beyond
adjusting the mic placement would be nice, especially since it will less
convenient to adjust the mic placement, compared to an open-front guitar
speaker cabinet. And this improvised isolation box leaks low bass -- too
much to play thumping muted chords in the middle of the night.

Speaker isolation cabinets are a legitimate form of the traditional cabinet

A more complete list of types of guitar speaker cabinets:

o open back
o closed back
o closed front (and back)
o half open back, half closed back

A speaker isolation cabinet is not a mutant, inferior cabinet, but just a different type of cabinet than open back or closed back. However, it does practically require powerful equalization, if you use a mic rather than tapping the input and sending it through a cabinet simulation filter. The speaker isolation cabinet is a completely legitimate type of speaker cabinet. You can have feedback, dynamic feedback, and room response sounds as well. To get feedback, turn up the final power amp to a few watts. To get room reverberation, add some room reverb in effects unit b. It gets between you and the speaker, which is good as well as bad. It's like only being able to hear your guitar in the control room monitor speakers, rather than directly through your guitar amp's speaker cabinet. This requires you to buy additional equipment: mic, cable, final power amp, and at least one monitor speaker.

Supplemental gear you will need for a speaker isolation cabinet:

o Good dynamic mic (Peavey, $200)
o Mic cable ($25)
o Mic stand or holder, and rubber-band isolation mic holder ($50)
o Mic adapter ($15)
o Second effects stage ($250)
o Power amp (or you can use a stereo, for testing)
o Monitor (or stereo speaker, for testing)
o Speaker isolation cabinet or pair of guitar speaker cabinets
o Speaker ($100)

Instead of the mic, you can tap the input of the iso cab, reduce the level, and send that through a cabinet response simulation filter.

The importance of a good mic

I tried a $150 Shure Beta 57 mic, and it had too much treble roll-off for a
Celestion Greenback, which is a warm-sounding speaker. My tone was
like *warm* British, when not using post-speaker equalization. Buy a
higher-quality dynamic mic, with good treble response, if you care about
great tube amp tone. I'm told the $200 Peavey mic was specifically
designed to be better than the Beta 57.

You can put multiple mics in the cabinet. If you are only using a 3 or 5 watt tube power amp, you can put a good condenser mic inside the cabinet.

A better approach to "emulation" of various tube amps

Use an actual tube amp to emulate other tube amps. Don't attempt to emulate multiple tube amps using something that isn't even a real amp in the first place. The Tone Engine (MIDI eq and preamp, low-watt power tube, iso cab, mic, MIDI eq) raises the stakes on what it means to "emulate" amps.

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