Combos, Cabs, and Coupling
 

I've known about acoustic coupling as a principle for a long time, but I had a surprising encounter with it last week, and thought it worth sharing and comparing.

My practice amp is an EBS Drome combo. 150W, 112, killer EBS front end compression, EQ, drive. It keeps up with a moderately loud drummer and 100W or so of guitars (there've been 2x50W and 1x100W formations). Great sound, amazing bottom for such a little amp...when used correctly! I've played bars and small clubs with the thing, putting it up on a little 6" plastic step stool to get a little better dispersion, and it was great. Last week, there was a 30" tall Fender combo (BX300C or whatever) in the practice space, so I put the Drome on top of it, giving me easier access to the controls (I'm tall, it's small) and, I thought, more sound for my ears as opposed to for my ankles. But the whole rehearsal, I was playing with the EQ on the amp and on my bass, desperately wondering where my bottom end had gone. Set the Drome up back on the floor and, voila, my bottom returns. The little thing was clearly designed to be used on the floor, and 6" is, empirically, not far enough to break the acoustic coupling that the design assumes. I'm probably not going to bother doing the experiment of mounting it on a variable height stand to crank it up and down and find where things break down, but the difference between floor level, where it's a great amp, and 30" up, where it sounds embarassingly thin, was really surprising.

Is this a dirty little secret of the Drome, or is this more broadly observed with cabs and combos? I keep hearing of people setting up stacks but only powering the top cab. Could they be screwing themselves at the bottom end?

It can lose coupling with the floor between 2-3 feet high. What also happens as you said is better dispersion and because the amp isn't pointing at your feet you hear highs and mids better. So less lows and more highs is a double whammy.

I definitely noticed this when I was using a Hartke 210XL on an amp stand. It had way better low end on the floor.

How high was the amp stand? As I was saying, 6-8" had no negative impact on sound, at least for the Drome.

Unless it's up higher than 2 1/2 feet, it was stronger mids and highs and not weakened bass. The bass response is still there, just not perceived as loud because of the highs and mids being more prominent to your ears.

This is the nice thing about Genz Benz shuttle combos I own. They have a kickstand built into the bottom of the cab that aims it at your ears but still allowing it to couple with the floor.

Jimmy is correct. You've effectively turned up the mids and highs to your ears, that's all.

You'll lose a little low end - 3 dB tops - unless the cab is at least 3 to 6 feet away - when you get phase cancellations (huge dropouts).

Yeah, that could be, I did have it pretty high though - I'd say 2 feet anyway. The 210XL sounded much better when I stood it on end (so the 10's were vertical) and left it on the floor. I have since sold that cab.

I'm more of a fan of leaving cabs on the ground but angling them up now. Just bought one of those Ampwedge things to replace my hi-tech chunk of 2x4.

My little SWR Workingman's 10 has a spring loaded metal prop handle on the bottom to angle it up.

Ya, when I use a single 210, I wedge it tilted up as well. Same difference and a lot easier to carry than an amp stand.

The cancellation takes place at 1/4 wavelength of whatever frequency is being affected. The 30" would've put the right around 120-130hz or so. Exactly the area needed to hear some "oomph" out of a small cab. Take that away, in addition to putting the mids in your ear, and it'll be drastically thin.

Also, the speaker in the other combo, not beimg used or hooked up to anything, would've been vibrating a little in sympathy with yours, adding even more to the bass cancellation problem.

This is why I have always "bit the bullet" and used a full stack. It gives you floor coupling while also getting a speaker up near your head. A hybrid full stack works very well too. For bass, a 15" combo atop an extension cab; for guitar, a 12" combo atop a deep 2x12" extension. Not that much more to move, and the sonic benefits of floor coupling are clearly worth it.

I find some time that it also works the other way. There are some hollow, raised wooden stages that just bring too much boom, and the solution is to raise and dis-couple the cabinets from the stage.

I believe that what you're describing is a mechanical coupling effect. You can get around that, as you say, by reducing the contact area between your cabinet and the stage. But that's independent of height. My 6" plastic step-stool does this admirably without, apparently, breaking the acoustic coupling on which the little combo apparently depends.