Coupling & Decoupling- What?

[Michael B]

TLDR
What practical giggling advice On coupling/decoupling do you have?

Lately I have seen posts regarding:

- lack of coupling of the cab/combo resulting in poor bass sound in a room
- lack of decoupling of the cab/combo resulting in poor bass sound in a room

I imagine every situation is different, so do you need couoling / decoupling? - it depends.

I wonder though if people are actually dogmatically in one camp or another rather than approaching it as a case by case thing.

I see the patented decoupling pads and I think maybe I need one of those. Then I see someone using 2 layers of foot pads and achieving ok results. Then another post lamenting the lack of coupling at a show.

Any reading on the subject sounds convincing but not practical.

Why spend $89 when you can get almost the same effect with a milk crate or cheap mat. Or embrace the floor and reposition as needed.

What practical giggling advice On coupling/decoupling do you have?

 

[Michael B]

Here is a great post which talks about two issues (not in contradiction).

Here's something interesting to add to our knowledge base.

and this from Alex Claber designer and maker of Barefaced speaker cabinets
"

Thanks to Bill Fitzmaurice (horn speaker guru) for mentioning this and getting me googling...

The Allison Effect was named after Roy Allison who realised that room boundaries seriously affect bass reproduction, not only in terms of boundary reinforcement (generally good for us) but also boundary cancellation.

"The best bass response is achieved by having the bass source either very close to or very far away from reflective surfaces. This generally means close to the rear and side walls and floor and far from the far wall and ceiling."

Why is this?

If you place your speaker 1/2 a wavelength away from a boundary then, at the frequency that corresponds to that wavelength, the sound will be reinforced significantly. We're talking the kind of boost you get from cranking a parametric EQ with Q narrow. If this particular frequency is a boomy/muddy one, it will not be fun. And if you're playing an acoustic instrument, feedback city...

The flipside: If you place your speaker 1/4 wavelength away from a boundary, then at the frequency that corresponds to that wavelength (twice the previous frequency if the speaker hasn't moved) the sound will be cancelled significantly. A typical amount would be -15dB at the null frequency. Nasty. Truly tone sucking.

The solution? Place your cab so it's less than 1/4 wavelength (of the upper bass range i.e. 100Hz) away from any boundaries OR more than 1/2 wavelength (of your lowest frequency) from any boundary.

How does this relate to the real bass playing world? A 41Hz (low E) sound has a wavelength of 27.5 feet (I'm sorry, I'm still hanging onto the Imperial system). A 31Hz (low B) sound has a wavelength of 36.5 feet. So, fellow four stringers, you need to place your cab at least 13'9" away from any walls (fivers, 18'3"). So that's how to avoid peaky boundary reinforcement.

To avoid notching boundary cancellation you need to be within 1/4 wavelength of a boundary in the range of 31 to 100 Hz. So really you need to get your cab within 2'10" of a boundary. Which part of the cab, you may ask? The bit that's making the sound, i.e. the bass speakers themselves and the ports. So given that most bass cabs are 14"-20" deep, you need to get the cab really close to the back wall, but not so close that the ports' operation (if rear-ported) is affected. Think 6"-12" away.

So what's the best solution on the gig? Place your cab on the floor or on a short stand. Place it close to the rear wall. Place it close to one side wall. The far wall and the other side wall should be far enough away unless you're playing a boxroom. The ceiling will often be a pain, but one boundary causing nulls and peaks is a lot worse than all six getting in on the action.

Alex"

 

[s0c9]

I'll be honest... unless I have a rocky/boomy floor on which the amp/cab sits.... it NEVER crosses my mind..

 

[PaulNichols]

I'll be honest... unless I have a rocky/boomy floor on which the amp/cab sits.... it NEVER crosses my mind..

Likewise, my practical advice is to not worry about it too much. There are so many factors that can come into play with stage sound including things like phase/time alignment with the FoH rig, monitors, etc, that worrying about this one small thing makes no real sense. Classroom theory is all very well, but the real world practicalities of 99.9% of gig situations mean classroom theory is all it will ever be. Realistically in most situations you're not able to get a measuring tape out and ensure your cab is a certain distance away from walls so why worry about it?

 

[Michael B]

Likewise, my practical advice is to not worry about it too much. There are so many factors that can come into play with stage sound including things like phase/time alignment with the FoH rig, monitors, etc, that worrying about this one small thing makes no real sense. Classroom theory is all very well, but the real world practicalities of 99.9% of gig situations mean classroom theory is all it will ever be. Realistically in most situations you're not able to get a measuring tape out and ensure your cab is a certain distance away from walls so why worry about it?

I don't disagree at all about your premise except that it might be interesting to read and discuss. And it would be easy enough to guess 6-12" and estimating 20 vs 40'. At least that's free and quick.

I would only bother though if a problem was obvious. No sense looking for trouble.

If I become the idiot with the tape measure, punch me in the face.

 

[Downunderwonder]

Likewise, my practical advice is to not worry about it too much. There are so many factors that can come into play with stage sound including things like phase/time alignment with the FoH rig, monitors, etc, that worrying about this one small thing makes no real sense. Classroom theory is all very well, but the real world practicalities of 99.9% of gig situations mean classroom theory is all it will ever be. Realistically in most situations you're not able to get a measuring tape out and ensure your cab is a certain distance away from walls so why worry about it?

meh. Theory can become second nature very quickly when you understand it, even if you don't understand it but hear what happens when you put your amp in the corner and next time 4ft away from the corner.

Besides, a mat on the floor, or gardening pads at 5% of the cost, still acoustically couples 99.9% of the bass. Even a short stand.

I assume any audible effect of a mat is from insulating the cab from the floor mechanical vibrations which were started acoustically.

A short stand doesn't even decouple acoustically. But pointing the cab more toward your ears than the back of your calves lets the directional mids be heard far better. A lot of people hear more mids as less bass.

This last is why you really get some milage out of guitar players using smaller amps on stands pointed at their own heads. There's your tone bud, mic that!

 

[PaulNichols]

meh. Theory can become second nature very quickly when you understand it

Of course it can but that wasn't my point, my point was that realistically in many situations you'll end up in a place dictated by things other than how far away from the wall your backline is. There's nothing wrong with knowing the theory, it just can't really be put into realistic practice in many situations.

This last is why you really get some milage out of guitar players using smaller amps on stands pointed at their own heads. There's your tone bud, mic that!

I seem to spend half my life tilting people's guitar amps to fire at their ears rather than their ankles. As someone who's primarily a live engineer I'm a massive advocate of smaller amps mic'd and driven properly, even the 1 watt Blackstar valve amps can sound pretty big mic'd if they're used sensibly.

 

[Michael B]

Not in depth but it is on topic.

Invalid Link Removed

A bit more. At least it says why you would want to couple or decouple.

Tech Bench: Sound Good Live

 

[seamonkey]

The "pads" are a gimmick, placebos, they do nothing unless you happen to have a really badly designed thin wall cabinet.

Instead look up: boundary effect
Here's one: Is Speaker-Boundary Interference Killing Your Bass?
Here's a handy chart, print it out and keep it with the cabs
[Invalid or Expired Link Removed]

 

[tfer]

The only time I worry about coupling is one venue we play at - it has a plywood stage. Sounds fine until you play a G note. Then you'd swear the world is falling around you!

F# or Ab are fine, but that plywood has a resonant frequency of G, and it almost doubles in volume!

Off the stage, it's noticeable, but only slightly. On stage, it's like standing on a speaker cone.

 

[4Mal]

One room I play a lot has a serious G issue. It took my a while to find the right place for my cab, the right amount of cut and the freq to cut at...

 

[s0c9]

The only time I worry about coupling is one venue we play at - it has a plywood stage. Sounds fine until you play a G note. Then you'd swear the world is falling around you!

F# or Ab are fine, but that plywood has a resonant frequency of G, and it almost doubles in volume!

Off the stage, it's noticeable, but only slightly. On stage, it's like standing on a speaker cone.

Hence my original post, 2nd from the OP.
In this scenario, one deals with it by adjusting, etc.

 

[bucephylus]

First of all, here is a link to one of the more intelligent posts on the topic that I have found in TB searching:
Stage Coupling and Bass Response

What you will see there is that two kinds of coupling effects can happen. The earlier BFM reference I believe describes acoustic coupling. I would tend to agree 100% IME that I have not had issues with that. I have had issues with mechanical coupling.

Which is why I started making devices for this problem. I initially started making related devices because the micros at high volume gigs had some stability questions on the cab; and my initial efforts were focused on that issue (not related). As at least one earlier poster stated, some cabs are definitely more problematic than others. And there are other issues (handles etc); but, that's a different thread.

My experience which led me to work on cab/stage decoupling occurred with a Big Band on one of the standard 5/8" plywood portable stages, and where I ran into apparent coupling issues with the stage mics at sound check. I solved that issue with $5 gardening pads from the WalMart across the street. And those work; but, I was looking for something that was more portable, as in easily carried in the cable bag. I have patented my design; but, only because I would rather avoid seeing some offshore knockoff in GC's stealing the idea. I do not in my wildest dreams foresee trying to build a business from it. LOL. By all means, if you want to drag gardening pads around with you for that one in 25 gig when something like this might happen, be my guest. Or, Greenboy suggests making a board for yourself with Sorbothane feet; which is a fantastic idea. Except you will find the Sorbothane is not altogether cheap. In any case, my design is essentially aimed at eliminating the board.

IME, this problem does not occur on most stages. But, it can occur on the common portable 5/8" plywood stage. How often each of us encounters that situation really depends on the gigs you do. Though, I suspect such devices could also be useful for helping with isolation in an apartment. But, if you never play in such an environment, then I can see where you might regard such devices as a placebo, or some other derogatory term; and in that case, you would not need one.

I can't control the stages I play on; hence, I try to be prepared. This weekend, I am gigging on a concrete slab at a backyard wedding reception. I don't like that stage; but, don't anticipate coupling problems. LOL.

YMMV