Carpeted Walls and Rear Ports

[Russell L]

Maybe a dumb question, but could a carpeted wall behind my amp have any effect on my cabs projection or response in any way? Last night we played on a small stage in a corner where the walls were carpeted. Seems like I had to turn the volume up more and add some bass. Then, there is also another room I play in regularly in which there's a carpet hanging right behind my cab. That room works differently, though. But, I'm wondering what anyone's thoughts are about carpet behind a rear-ported cab.

Thanks in advance.

[dog1]

Hmmmm. Never considered such a thing. And I am not sure whether front or rear porting makes a difference...but the hanging carpet, or carpeted walls are sure to suck up sound.

[billfitzmaurice]

Originally Posted by Russell L

Maybe a dumb question, but could a carpeted wall behind my amp have any effect on my cabs projection or response in any way?

Yes and no. The carpet will absorb midrange waves that normally wrap around the cab and reflect off the wall. The lower in frequency the less will be absorbed. By the time you get down to the low frequencies passed by a port the carpet will not absorb anything, so no, it won't affect a rear ported cab any differently than a front ported cab.

[Philonius]

If I just keep reading Bill's posts; sooner or later will I qualify for a degree? But... slightly off topic; distance from a rear wall has significant effect, no? Better to be reasonably close if possible, if I recall correctly.

[billfitzmaurice]

Originally Posted by Philonius

If I just keep reading Bill's posts; sooner or later will I qualify for a degree? But... slightly off topic; distance from a rear wall has significant effect, no? Better to be reasonably close if possible, if I recall correctly.

Never 1/4 wavelength within the low frequency bandwidth, otherwise rear wall reflections will cause phase cancellations. That means either closer than 2.8 feet or more than 7 feet, and that's to the front of the cab.

[darius8]

The two rooms that the OP played in may have two different wall constructions.

[billfitzmaurice]

Originally Posted by darius8

The two rooms that the OP played in may have two different wall constructions.

The shapes of the rooms would have more effect than the wall constructions, assuming one wasn't concrete and the other a canvas tent.

[darius8]

What about playing on a stage vs. concrete floor?

[billfitzmaurice]

Originally Posted by darius8

What about playing on a stage vs. concrete floor?

That depends how 'soft' the stage is. If poorly constructed it can resonate, causing boom.

[Russell L]

Originally Posted by Philonius

If I just keep reading Bill's posts; sooner or later will I qualify for a degree? But... slightly off topic; distance from a rear wall has significant effect, no? Better to be reasonably close if possible, if I recall correctly.

Heh, heh, yeah, I'm hoping for the same. (Tip o' the hat to ya, Bill).

I'm usually within the distance Bill mentioned from a wall, although there are times when it's impossible.

Last night my cab was at a 45-degree angle to the carpeted wall in order to face the audience. Well, not exactly, I did turn it a bit toward center stage. But, it may have just been this room (first time there). The other room I play in a lot is a more normal set-up in which the wall is just straight behind me. But, it just so happens that where I have to set my cab is right in front of a carpet hanging like a drape. I always think about it every time. But, I never have a problem with hearing lows or having enough volume in that room.

So, I'm assuming from this discussion that it's more about general room acoustics than about the carpet, right?

[billfitzmaurice]

Originally Posted by Russell L

So, I'm assuming from this discussion that it's more about general room acoustics than about the carpet, right?

The two are inseparable, as both the room dimensions and materials combine to give the end result.

[Russell L]

Originally Posted by billfitzmaurice

The two are inseparable, as both the room dimensions and materials combine to give the end result.

Ok. Then I guess the effects can vary widely depending the particulars.

[KJung]

Originally Posted by Russell L

Heh, heh, yeah, I'm hoping for the same. (Tip o' the hat to ya, Bill).

I'm usually within the distance Bill mentioned from a wall, although there are times when it's impossible.

Last night my cab was at a 45-degree angle to the carpeted wall in order to face the audience. Well, not exactly, I did turn it a bit toward center stage. But, it may have just been this room (first time there). The other room I play in a lot is a more normal set-up in which the wall is just straight behind me. But, it just so happens that where I have to set my cab is right in front of a carpet hanging like a drape. I always think about it every time. But, I never have a problem with hearing lows or having enough volume in that room.

So, I'm assuming from this discussion that it's more about general room acoustics than about the carpet, right?

The key here is it has nothing to do with the location of the port. As long as you have the port far enough away from a wall so that it is not blocked (i.e. 6" or so), a rear ported cab will pretty much behave like a front ported cab.

However, carpet, people, drapes, etc. all suck up a bit of upper midrange and treble. Combine that with a large room, and especially if you are on a hollow stage, which adds some mechanical coupling issues (boomy bass), you are in what we call in the business 'hell'. regarding generating an even, articulate, full range tone out to the audience with backline.

[Russell L]

Originally Posted by KJung

The key here is it has nothing to do with the location of the port. As long as you have the port far enough away from a wall so that it is not blocked (i.e. 6" or so), a rear ported cab will pretty much behave like a front ported cab.

However, carpet, people, drapes, etc. all suck up a bit of upper midrange and treble. Combine that with a large room, and especially if you are on a hollow stage, which adds some mechanical coupling issues (boomy bass), you are in what we call in the business 'hell'. regarding generating an even, articulate, full range tone out to the audience with backline.

Yeah, it was kinda like hell, in a way. I also had the cab sitting in a chair so I could hear it better. The chair was on the 8" high hollow stage while I stood on the carpeted floor (on a concrete slab) about 6' in front of it. I kept wishing I had two 15s instead of one. The cab was on its side with the slot port running vertically, if that makes any difference (which I don't think it does).

[billfitzmaurice]

Originally Posted by KJung

However, carpet, people, drapes, etc. all suck up a bit of upper midrange and treble. .

That's a good thing. It may be counter-intuitive, but good clean bass is easier to get in a dead room than a lively one. Damping surfaces have little effect on the low end, but they have a major effect on the mids and highs, and it's the mids and highs that provide our ear/brain with direction, pitch and timber cues. A lively room with lots of early reflections jumbles up those cues, so even though they don't actually affect the low frequencies they do affect how we perceive them.

I kept wishing I had two 15s instead of one

Taller sources will sound much better in tough rooms, as taller sources reduce vertical dispersion, which reduces early reflections that create muddy sound. They work better in a good rooms too, but it's the tough rooms where their superiority is most obvious and beneficial.

[Russell L]

Thanks ya'll, I'm learning stuff every day (and night) here. Been a musician all my life, but never studied the technical side much, just played. I know music theory, but not this stuff. But, I'm trying now.

[1958Bassman]

If you can get the largest, squishiest people to congregate in the corners, they can act as bass traps, which will smooth the bass response.

Next time you experience this perceived loss of output, walk around the room when you're setting up and you'll notice live and dead spots. if you can move your rig (amp and speakers- just wanted to be clear) to one side or the other and forward/back a bit, you may be able to find a happy medium.

[billfitzmaurice]

Originally Posted by Russell L

Thanks ya'll, I'm learning stuff every day (and night) here. Been a musician all my life, but never studied the technical side much, just played. I know music theory, but not this stuff. But, I'm trying now.

If you want to sound really good without understanding basic acoustics you have to play guitar.

[Russell L]

Originally Posted by billfitzmaurice

If you want to sound really good without understanding basic acoustics you have to play guitar.

LOL, Bill. Been there. Did it for a living for awhile. Now, bass has been my passion for the past 24 years. And yeah, I'm having to think more about acoustics much more than when I played guitar.

Thanks to all for helping.

[will33]

Some interesting reading on the subject here:
Lenard Audio - Education - Acoustics

That really has more to do with sound treatments for studios, concert halls etc. that your local gig obviously doesn't have but the principles are the same. Of course, most of us don't have time to take measurements and consult charts for every gig we play but you can use the basic concepts, look around the place, and probably find a thing or two you can do to help your sound.

Most (all?) of the usual gigs most of us play weren't built with acoustics in mind, but you do the best you can.

More cool stuff in the "education" section of that site but for whatever reason, you have to use the little "site map" link at the bottom to find most of it.