Amp facing forward or rear-ward...?

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noplanb
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Amp facing forward or rear-ward...?

Post by noplanb » 11 Sep 2018, 21:24

This might've been commented on in passing before.
I saw a school band project presentation on the weekend, and the overall volumes were certainly not typical gig volumes, but near the stage was starting to get uncomfortable (for me) - over 90dB according to my android app.
The bass player was all of 16 years old, and seemed to be 'there' sometimes, and other times not - I put it down to him still being a beginner. Anyhow, he started using a pick confidently - and I heard just mush/boom.
Then I realised it could be because he was either DI'ed or mic'ed, and anything could've happened with the EQ on the FOH desk - but more importantly, his amp was facing rear toward him and pointing up.
So I started thinking about what frequencies, cancellations and phase shifts there could be to stuff up his sound. I assume if he was using a pick, he at least would be getting some crispness through his own speaker - I could hear the notes stopping, but still rather dull.
So, for a rear-facing bass amp, here are my guesses:
1) the very bottom, ie 80Hz or so and down, will come through unchanged - the wavelength is too long to be affected much by anything
2) 80Hz to maybe 200Hz will be resonated and phase-shifted, and be the prime maker of muddy boom typical of many auditoriums
3) 200Hz to say 500Hz may vanish! - in parts - due to it being reproduced strongly by the bass amp, but going out of phase with the FoH and thus cancelling - this maybe the other source of an incoherent sound - it is this lower mid that has the definition
4) 500Hz and up is now to do with what the desk person *thinks* a bass should sound like - and if he just thinks it should be a dull boom, then these freqs get turned down
5) The brain-ear biological-software is an amazing powerful clever thing - it will phase correct certain frequencies that arrive at your ear at slightly different delays, depending on the frequency. This could be why moving speaker positions and directions can make such a difference - one moment is terrible, then shift something 500mm, and it just sounds great. It adds to the effect of 'correcting' your bass sound in everyone's ears if you get near enough to right.

In summary, point 3) (and 5)) would indicate why it is not good to use your bass amp as rear-facing foldback for quieter venues. It might have to get quite loud before it doesn't matter - maybe huge outdoor venues (also much less reflections and standing wave resonances)
I'd be interested in your observations. What do you think?
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Bill Fitzmaurice
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Re: Amp facing forward or rear-ward...?

Post by Bill Fitzmaurice » 11 Sep 2018, 22:10

Cancellations between the back line and PA are real, but they don't occur in the mids and highs, they occur in the lows. This explanation is specific to PA subs being split left/right, a major no-no, but it also applies to interactions between the backline and PA:

http://www.prosoundweb.com/article/in_s ... wer_alley/

With a rear facing cab what's heard out front is what's through the PA. If it's not right it's the fault of the person mixing the PA.

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noplanb
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Re: Amp facing forward or rear-ward...?

Post by noplanb » 11 Sep 2018, 23:10

what's heard out front is what's through the PA
Always great to see your replies! :)
Nice article - I think the cancellations in the comb effect is what I was trying to express - I can see that above the very low you don't get 'alleys' of cancellation, but maybe a difference between left and right ear - which I thought is still not ideal, and may contribute to 'mudiness'.
My story refers to lower power PA's where the foldback volume coming through to FoH is not much less than the PA FoH, so some frequencies are 180degrees out of phase, and hence may disappear or just be attenuated, thus changing the tonal signature of the bass guitar.
Any real world examples folk have come across?
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Re: Amp facing forward or rear-ward...?

Post by Bill Fitzmaurice » 12 Sep 2018, 00:35

Those frequencies out of phase will be in the longer wavelengths only, as shorter wavelengths won't wrap around the cab to merge with the PA output. If anything there will be less interaction between the amp and PA when the amp is rear facing than forward facing. If it sounds bad out front it's all on the hand on the PA console.

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Re: Amp facing forward or rear-ward...?

Post by PilbaraBass » 12 Sep 2018, 13:21

I always used my amp as a foldback in a tllt-back configuration (it is designed to do this) and then I went to FOH. My volume on stage was only enough for me to be confident of what I was playing. Since we used electronic drums, not much stage level was required (80dB probably).
.
While I did care about what was going to house, it wasn’t my job to make it sound good in the fronts. I could tell a bit from other foldbacks what it sounded like and that was generally OK.

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Re: Amp facing forward or rear-ward...?

Post by david1234 » 12 Sep 2018, 14:27

Sounds like a sound desk or sound person problem to me, too.

I've come across plenty who can't tell one deep sound from another and turn the bass guitar down because the keyboard is too boomy....
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Re: Amp facing forward or rear-ward...?

Post by PilbaraBass » 12 Sep 2018, 16:14

david1234 wrote:
12 Sep 2018, 14:27
Sounds like a sound desk or sound person problem to me, too.

I've come across plenty who can't tell one deep sound from another and turn the bass guitar down because the keyboard is too boomy....
One good way to deal with this is to get a bit of gaffer tape and tape the keyboard player's left hand to their side... :D
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Re: Amp facing forward or rear-ward...?

Post by slowlearner » 14 Sep 2018, 09:17

^ A sax player buddy of mine trained the keyboard player in his jazz band...
"Take your left hand and stick it under your left buttock. Then just leave it there... forever." :D

Should be part of the AMEB syllabus. ;)
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Re: Amp facing forward or rear-ward...?

Post by Bill Fitzmaurice » 14 Sep 2018, 12:33

You wouldn't get far telling Billy Joel or Elton John to do that. This is also a case of sound man competence. Those who know what they're doing will high pass keys around 100Hz. For that matter they'll high pass the bass between 60 and 80Hz, to compensate for the totally different response of PA speakers and electric bass speakers.

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Re: Amp facing forward or rear-ward...?

Post by Petebass » 14 Sep 2018, 15:04

Bill Fitzmaurice wrote:
14 Sep 2018, 12:33
You wouldn't get far telling Billy Joel or Elton John to do that. This is also a case of sound man competence. Those who know what they're doing will high pass keys around 100Hz. For that matter they'll high pass the bass between 60 and 80Hz, to compensate for the totally different response of PA speakers and electric bass speakers.
....... And the really smart ones won't tell the musicians what they've done.
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Re: Amp facing forward or rear-ward...?

Post by petersemple » 15 Sep 2018, 08:31

Yes. Used to run a desk occasionally and the guitar player would go for really boosted screechy trebly solos... over the top of the singer. Quick fader adjustment later and "sure, you sounded great!!!"
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Re: Amp facing forward or rear-ward...?

Post by narcdor » 15 Sep 2018, 10:43

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noplanb
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Re: Amp facing forward or rear-ward...?

Post by noplanb » 16 Sep 2018, 21:25

Maybe a harder sound to tame as you now have 3 sources of different phased sound - monitor front, monitor rear and FoH. You've gotta want the FoH to be loud enough for the stage monitor not to affect the audience experience. But I notice it is marketed for 'tighter' stages - which may not have FoH for bass?
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Re: Amp facing forward or rear-ward...?

Post by Bill Fitzmaurice » 16 Sep 2018, 23:22

On that. Below the baffle step frequency (google it) radiation is omni-directional, so a rear firing woofer won't make any difference. Above that midrange reflections off a wall behind the cab won't be audible in the audience. On stage it could help the rest of the band hear your mids, but it might not. The main reason for a rear firing woofer would be to help the drummer hear your mids. This design isn't the way to do it, unless you're in the habit of putting your cab directly in front of the drums. The way to do it is with a pair of identical cabs, stacked vertically, with the lower cab aimed at the drummer, the upper cab aimed at you.

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Re: Amp facing forward or rear-ward...?

Post by narcdor » 17 Sep 2018, 12:31

I suspected it was just to be able to fit all the speakers in the box, I was also kinda kidding. I do wonder what Jim is thinking on this one.
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