acoustic coupling

[fatback]

I've been raising my Midget to chest height by putting it on a keyboard stand. The idea is to stop my upright feeding back, and that's working a treat. However, I seem to be losing a significant amount of volume.

How much volume, at what frequencies would I be likely to lose at that elevation?

This is a pretty important question for me, as I've now found myself overdriving the amp (EA Doubler) at louder gigs, and that's nasty.

Also, if a cab on a stand wobbles a bit cos of an uneven stage, does that cause any volume loss?

I really don't want to have to use a second cab, and the doubler / midget combo was well powerful enough until I started raising that cab.

Any ideas, cab gurus?

Edited September 11, 2011 by fatback

[Bill Fitzmaurice]

[quote name='fatback' post='1369701' date='Sep 11 2011, 02:09 PM']I've been raising my Midget to chest height by putting it on a keyboard stand. The idea is to stop my upright feeding back, and that's working a treat. However, I seem to be losing a significant amount of volume.[/quote]
If the volume you're losing is at the frequencies where feedback occurs it's moot, because if you don't lose the volume you don't lose the feedback. An exact figure is difficult to arrive at, as there are a number of variables. To vastly oversimplify you'll lose about 6dB, centered at the frequency where 1/4 wavelength is equal to the distance between the cab and the floor.

[fatback]

[quote name='Bill Fitzmaurice' post='1370030' date='Sep 12 2011, 01:07 AM']If the volume you're losing is at the frequencies where feedback occurs it's moot, because if you don't lose the volume you don't lose the feedback. An exact figure is difficult to arrive at, as there are a number of variables. To vastly oversimplify you'll lose about 6dB, centered at the frequency where 1/4 wavelength is equal to the distance between the cab and the floor.[/quote]

I think it cures the feedback just because the cab is not so near the body of the bass rather than because of lost frequencies. 6db is quite a lot though.

The cab is a metre off the stage, which would give a freq centre of 85.75hz, is that right?

I wonder if the problem might have been made worse as I tend to eq out a lot of mids and boost the bass, leaving the amp with a lot of work to do.

I might be able to get away with raising the cab somewhat less; i'll maybe try that.

[Bill Fitzmaurice]

[quote name='fatback' post='1370248' date='Sep 12 2011, 06:36 AM']I think it cures the feedback just because the cab is not so near the body of the bass rather than because of lost frequencies.[/quote]The distance would not make any difference. It's the notch filtering that accounts for it.

[TomTFS]

[quote name='fatback' post='1370248' date='Sep 12 2011, 11:36 AM']I think it cures the feedback just because the cab is not so near the body of the bass rather than because of lost frequencies. 6db is quite a lot though.

The cab is a metre off the stage, which would give a freq centre of 85.75hz, is that right?

I wonder if the problem might have been made worse as I tend to eq out a lot of mids and boost the bass, leaving the amp with a lot of work to do.

I might be able to get away with raising the cab somewhat less; i'll maybe try that.[/quote]

If you're only using it for personal monitoring, just re-eq it so you can hear it? Also try something a bit more stable than a keyboard stand. Especially with lower frequencies you want the driver to be as secure as possible to maximise output.

[roonjuice]

I would certainly look at your EQ, and if possible, for more percieved low end, stick your amp in a corner where possible. Remember the mids are where the money is;)

[fatback]

Thanks for the replies.

Looks like it is raising the cab that's cost me output. I didn't realise the volume drop could be so great.

I've probably made things worse by cutting so much mids, although in my defense my upright with steel strings produces a very middy attack that I need to reduce if I can. Sounds like I have to live with more mid though.

Unfortunately, putting the cab against a wall hasn't been an option on the stages we've had, and a keyboard stand is the only way of raising the cab that is guaranteed to find enough space, so I reckon I'm stuck with that.

Definitely the last couple of gigs I've had stability problems with the stand and no time to wedge it better, so maybe that's not helping. I wonder how bad the effect of a wobbly stand could be?

Edited September 12, 2011 by fatback

[markstuk]

Have you thought about using a lower/angled amp stand? But I suspect what Bill says is the core point - the quieter you are the less feedback you're going to get...

[Bill Fitzmaurice]

[quote name='markstuk' post='1370764' date='Sep 12 2011, 12:07 PM']Have you thought about using a lower/angled amp stand? But I suspect what Bill says is the core point - the quieter you are the less feedback you're going to get...[/quote]Feedback is caused by the body resonating in concert with the cab output. Reduce the output and you reduce the feedback. Increase the output and you increase the feedback. Want to get rid of the feedback without reducing the level? Get a solid body.

[Delberthot]

[quote name='markstuk' post='1370764' date='Sep 12 2011, 05:07 PM']Have you thought about using a lower/angled amp stand? But I suspect what Bill says is the core point - the quieter you are the less feedback you're going to get...[/quote]

I've lost count the number of times I've posted this picture:

[fatback]

If the feedback reduction is about the notch filtering then I should maybe just use the notch filter on my amp

Naw, too fiddly, I found. One big advantage of the stand is that it's easy to find a spot on a very crowded stage where it can be clearly heard. And it does work. No feedback at loud volumes. I was just puzzled at the losses.

The angled stand is a nice idea. Seems a good compromise, although at the height in the picture, it wouldn't be much less lossy than the one I've got - it's really only a small amount closer to the floor.

Here's a related question - is a bass sound omnidirectional from the cab when you're up close, like within 5 feet?

[Bill Fitzmaurice]

[quote name='fatback' post='1371580' date='Sep 13 2011, 06:03 AM']- is a bass sound omnidirectional from the cab when you're up close, like within 5 feet?[/quote]It's omnidirectional below roughly 200Hz at any distance.

[TheRev]

I spent quite some time experimenting with putting my cab on a stand to eliminate feedback - the only way it worked was to get it well up above the body of the bass and even then I had to stand stock still and usually at a specific angle to present as little of the front of the bass to a speaker (mine or a monitor) as possible. It was a useful technique though, especially in my old swing blues band where the onstage volume could get silly and a double bass onstage was more about image than actual notes.

However, I dont use a stand anymore as the following two actions seem to have solved the problem for me.

1) Less volume on stage, more out the front where it belongs. Can be a bit of a problem when you're in a small bar using the PA for vocal only and everything else on backline, but in those type of venues you don't need to be stupidly loud. In my jazz/soul trio, our singer always has his vocals coming through the monitor at full tilt, which is what usually sets my bass feedback off. I used to reduce my backline to a point where the feedback stopped - and to where I could no longer be heard above the vocal monitor. Now, if he gets too loud at soundcheck, I just stand there and let the bass feed back until he gets fed up and turns his monitor down. It's all in the training really.

2) I fixed four litte feet to the front corners of my Omni 10.5 (great cabs - thanks Bill! ;-)) and set it facing down so that the speaker fires into the floor like the Acoustic Image combos. Now, I'm sure there's a wealth of technical reasons why I shouldn't do this, but I get a sound and volume that both the drummer and I can live with. I have heard it said that a downward firing speaker will give you a good onstage sound but a crap sound out front where the punters are. This could well be the case, but I've had more negative comments regarding uncontrollable feedback or poor intonation (due to not being able to hear myself above the vocal monitor....obviously *ahem*) than I have about the quality of my bass sound 10 feet from the stage.

Just my thoughts...

Dave

Edited September 13, 2011 by TheRev

[fatback]

Interesting points. Thanks.

I think I've been lucky on the feedback front anyhow, as the combination of bass and pickup (full circle) seems very resistant. The only time I suffered was when I had the cab behind me on the floor within about 6ft.

Unfortunately, most of our gigs are in very cramped spaces with vocal only PA. When the cab was on the floor it was hard for the band to hear, especially for the drummer. Maybe that's because as Bill points out it's not omnidirectional above 200hz?

Anyhow, the stand solution was excellent bar that loss of volume. I reckon what i have to do is cut less mids so the amp doesn't have to work so hard. Presumably the amp has to use lots more watts to push out bass.

btw, intuitively you'd think a slight wobble in the stand would cause problems (newton and all that), but how severe could that be?