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Cabinet Positioning and Feedback


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My question is really meant for discussion about front of house rather than on stage monitors. Apologies for the long introduction.

 

When using our own PAs, most of us will be using the  "virtual" point source source cabinets. The type that have a woofer and a tweeter in a cabinet on a pole, with or without a subwoofer RCF Art series etc. Some are now going over to the sub on a stick/tea chest bass systems characterised by the Bose systems that have a miniature line array of multiple small speakers, all mounted at slightly different angles horizontally, allied with a subwoofer. The effect of multiple small drivers, as i understand it is firstly that the sound can be steered into the audience, and that the changes in angle help reduce feedback.

 

Going back to the the two per side speakers, is there any benefit to angling the two speakers asymmetrically to try to cancel reflected waves?  What I mean is say to point the speakers at opposing corners but not at the same angle. I will add some diagrams later to explain what I mean.

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We tend to set up with FOH PA cabs slightly 'toe-in' to spread the sound to audience but not so far that they bleed onto the band area/vocal mics.

 

Toe-in so that sound is also less reflected off side walls and beamed more at the audience... if that makes sense.

 

Edited by warwickhunt
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3 minutes ago, warwickhunt said:

if that makes sense.

It does and my thought is that the Toe in on one cab is slightly different to the other so that any reflections do not add. Of course the other thing is to angle the cabs down slightly.

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17 hours ago, Chienmortbb said:

Bose systems that have a miniature line array of multiple small speakers, all mounted at slightly different angles horizontally, allied with a subwoofer. The effect of multiple small drivers, as i understand it is firstly that the sound can be steered into the audience, and that the changes in angle help reduce feedback.

 

Strictly speaking, Bose systems, which work very well, especially in irregularly shaped rooms in my experience, are not a true line array. In a line array, drivers all point in the same direction laterally. The main benefit of a line array is that they throw sound very effectively, so those furthest from them hear clearly.

 

In larger venues, you will see multi-cab line arrays arranged in a 'J' shape, to direct some of the sound to those nearest the stage.

 

It's certainly worth experimenting with angling PA cabs to try to improve sound spread and minimise feedback. Depending on the shape/size of the venue, you may have to resort to notching out troublesome frequencies if feedback is an issue, though.

 

 

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The Bose configuration is intended to widen high frequency horizontal dispersion. They did it that way instead of the right way, which is to use tweeters rather than just midbasses alone.

 

Quote

The main benefit of a line array is that they throw sound very effectively, so those furthest from them hear clearly.

That's true of large arrays, 3 meters or more high. With shorter arrays the main benefit is the reduction of the vertical dispersion angle, especially in the high frequencies. This sends more energy to the audience where you want it, less to the floor and ceiling where you don't. The result is much cleaner sound, especially in acoustically poor environments, which is probably 95% of the clubs we play. Even a short array of tweeters only 40cm high can work much better than the typical point source horn used in most PA speakers.

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4 hours ago, Bill Fitzmaurice said:

With shorter arrays the main benefit is the reduction of the vertical dispersion angle, especially in the high frequencies. This sends more energy to the audience where you want it, less to the floor and ceiling where you don't. The result is much cleaner sound, especially in acoustically poor environments, which is probably 95% of the clubs we play. Even a short array of tweeters only 40cm high can work much better than the typical point source horn used in most PA speakers.

 

Absolutely. I use a pair of Fohhn LX150s, which contain 12x4" midbass drivers, with a centrally placed tweeter. They are very clean, project well and are pretty resistant to feedback.

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 A line source projects an equally high wavefront, with the sound intensity at any single point of that wavefront only a portion of the whole. With a point source all of the intensity lies within the much shorter wavefront. That being the case even when a mic near the speaker is within the wavefront the lower intensity at the mic with a line versus point source reduces the potential for feedback.

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Hi John, the secong part is that no I don't think that's a good idea generally. Note the generally.

 

Most of your feedback issues if you are talking about high frequencyhowlround comes from what is going on close to the mics. Sound falls away with distance so the shortest route is the one you need to suspect first. Toeing in the speakers will probably make things worse. The horns (all speakers in fact) don't have a cut off point at which nothing spills to the side. The sound radiated sideways falls off slowly and starts to lobe off axis. the further off axis generally the less high frequency content but remember those off axis lobes. Turn your speakers in and you decrease the angle to the mic increasing high frequencies and you may even reach the mic with one of the lobes so statictically you'll increase the chance of howlround. It might be better to toe out but moving the speaker forwards is going to be your best first move.

 

The other cause of feedback are room resonances. Moving the speakers away from reflective surfaces may help here but it's really difficult to read a room in a few minutes whilst setting up in a new venue. that's much more appropriate for permanent installations or a touring band with sound engineers and time to set up properly. Speaker placement does make a difference but a set toe-in is a bit like setting your graphic to a smily eq.

 

I'd also be looking at the microphones first. What is their pickup pattern, cardioid or super cardioid? The former may be picking something up from the sides but not from down the barrel. the latter is more likely to be getting it from along the axis of the mic and not from the sides. Watch your vocalist too, they tend to wave the mic around and point it into corners at random. You can often spot the direction that creates the feedback issue. they will blame you for the feedback though :)

 

Place your speakers to give the audience the best coverage, not to anticipate feedback you might not get. Good coverage and you might be able to turn down a notch. We've played places with L shaped rooms before now or even two separate rooms. You just have to deal with what you have when you get there.

 

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On 25/03/2024 at 19:44, Chienmortbb said:

is there any benefit to angling the two speakers asymmetrically to try to cancel reflected waves? 

My 2 cents worth, no. Unless you're in a room with completely parallel walls, the chances of getting any cancelation or comb filtering in a pub/club full of chairs, tables, punters, bikes on walls etc, I'd have thought were 0. Not to say that it will probably sound different in different areas, but I wouldn't have thought that was due reflected cancellation. 

 

@Phil Starr interesting video, but unless I've missed something, you're man seems to miss that an audience will stand in the middle of the room, not in "middle middle", so on his model they get no mid/top at all. 

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2 hours ago, Buddster said:

My 2 cents worth, no. Unless you're in a room with completely parallel walls, the chances of getting any cancelation or comb filtering in a pub/club full of chairs, tables, punters, bikes on walls etc, I'd have thought were 0. Not to say that it will probably sound different in different areas, but I wouldn't have thought that was due reflected cancellation. 

 

@Phil Starr interesting video, but unless I've missed something, you're man seems to miss that an audience will stand in the middle of the room, not in "middle middle", so on his model they get no mid/top at all. 

I am actually subscribed to that channel but somehow missed that  video. I think the centre fill  speaker is meant to help although the height, on top of the subs, does not get through to the middle. However, as he says do the best you can and don't fret.  

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Thanks to everyone that has replied. Regarding mic/speaker placement, almost every gig I tell the band that they have the monitors in the wrong place, to cardioid and two hyper-cardioid mics. Try to tell them that the cardioid pattern is not flat but in fact more like and apple and the smoke starts coming out of their ears.

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37 minutes ago, Chienmortbb said:

Thanks to everyone that has replied. Regarding mic/speaker placement, almost every gig I tell the band that they have the monitors in the wrong place, to cardioid and two hyper-cardioid mics. Try to tell them that the cardioid pattern is not flat but in fact more like and apple and the smoke starts coming out of their ears.

It's imbecilic. They don't need your word they can check this themselves in seconds by just speaking into the mic from the front, sides and back.

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12 minutes ago, Phil Starr said:

It's imbecilic. They don't need your word they can check this themselves in seconds by just speaking into the mic from the front, sides and back.

Have you met  my band? The singer was convinced we had a faulty cable causing feedback once!

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The video is informative, but completely ignores wall loading subs, which is far more effective than placement in front of the stage, not only in terms of how well the subs will work but also because in most clubs center placement is impractical, if not impossible.

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1 hour ago, Phil Starr said:

It's imbecilic. They don't need your word they can check this themselves in seconds by just speaking into the mic from the front, sides and back.

 

1 hour ago, Chienmortbb said:

Have you met  my band? The singer was convinced we had a faulty cable causing feedback once!

You mean it's not just me ???? ...... Phew, cheered me up no end! 

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17 hours ago, Phil Starr said:

I'd also be looking at the microphones first. What is their pickup pattern, cardioid or super cardioid? The former may be picking something up from the sides but not from down the barrel. the latter is more likely to be getting it from along the axis of the mic and not from the sides.

 

At a recent gig, the second guitarist's mic (for BVs) was feeding back rather readily. I noticed that although it was behind the FOH speaker, due to his pedalboard he'd got it angled at coming on for 45 degrees towards that side - he was quite close to the speaker too. At mid-session interval I got him to adjust the mic position so it was pointing about 45 degrees away instead (cardioid mic) and that reduced the feedback issue considerably. I'm not sure that would have come to mind as a possible cause had the issue of cardioid and hypercardioid polar patterns not come up in here recently.

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In terms of eliminating feedback there seems to be a consensus that cutting the pre EQ input gain on the desk is better than cutting the post EQ faders vol.

 

Is that the correct approach? If so, what's the thinking behind it given the volume per channel ends up being the same ?

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Probably because the Aux sends for the monitor is Pre-fader (and as most feedback is due to loud monitor volumes) turning the pre Eq gain (ie pre fader) would turn down the monitors too, helping reduce feedback. 

However, turning monitors down using the gain isn't really the way to do it, it should be turned down via the Aux send. 

Channel gains would normally be set at the sound check and not really touched again as it effects everything on that channel (other aux sends, post fader volume etc) 

 

Note. Eq is pre-monitor mix (normally) so adding top eq to get the FoH sound will effect the monitors and this can cause feedback, so taking the top off can help reduce f/b. Even turn the hi eq down on the physical monitor helps. 

Edited by Buddster
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15 minutes ago, Buddster said:

it should be turned down via the Aux send. 

Channel gains would normally be set at the sound check and not really touched again as it effects everything on that channel (other aux sends, post fader volume etc) 

This is a basic principle. 

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1 hour ago, Al Krow said:

In terms of eliminating feedback there seems to be a consensus that cutting the pre EQ input gain on the desk is better than cutting the post EQ faders vol.

 

Is that the correct approach? If so, what's the thinking behind it given the volume per channel ends up being the same ?

I can't see it making a difference so long as the gain isn't set so high that you've introduced some clipping. some of the cheaper mixers back in the day struggled for gain and everything was set to 11. Modern digital mixers have oodles of dynamic range and really quiet mic pre's so you don't need to set the initial gain high and gain staging becomes less critical. As Buddster says you wouldn't normally touch the channel gains once you'd done an initial check. There's a lot of folk lore from the old days still kicking around. I just recall the settings from memory for all our gigs now, except for the drummer who is always fiddling with her send on the elecric kit. She insists she doesn't but the meter says she does :)

Edited by Phil Starr
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21 hours ago, Bill Fitzmaurice said:

The video is informative, but completely ignores wall loading subs, which is far more effective than placement in front of the stage, not only in terms of how well the subs will work but also because in most clubs center placement is impractical, if not impossible.

To be fair that was for a particular hall and given that space and access to top quality kit and lots of it most of us would do something similar. The useful thing about this video is the heat maps which illustrate the dispersal patterns. A picture is worth a thousand words I guess :)

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36 minutes ago, Buddster said:

Probably because the Aux sends for the monitor is Pre-fader (and as most feedback is due to loud monitor volumes) turning the pre Eq gain (ie pre fader) would turn down the monitors too, helping reduce feedback. 

However, turning monitors down using the gain isn't really the way to do it, it should be turned down via the Aux send. 

Channel gains would normally be set at the sound check and not really touched again as it effects everything on that channel (other aux sends, post fader volume etc) 

 

Note. Eq is pre-monitor mix (normally) so adding top eq to get the FoH sound will effect the monitors and this can cause feedback, so taking the top off can help reduce f/b. Even turn the hi eq down on the physical monitor helps. 

 

That's helpful thanks. We're actually using pretty limited stage monitoring, guitarist has a small Behringer B205D one in the main line up, but otherwise we use IEMs.

Is it the high frequencies that are usually the main culprit for feedback? We've certainly had low end boominess too, which I suspect is not uncommon for many bands at some point.

 

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23 minutes ago, Phil Starr said:

The useful thing about this video is the heat maps which illustrate the dispersal patterns. A picture is worth a thousand words I guess

I often wish there was such a thing as an audio version of a thermal imaging camera. Real time display. 

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We really struggled with feedback initially, and certainly getting the gain sorted on the mics helped.
Surprisingly though it was the monitor/mic positioning/monitor volumes which proved to be the key. i.e The Monitors were much more important than I thought and once this was sorted feedback is no longer an issue. 
Things that helped
1)  Matching the stage position of the monitors  in accordance with the type of Mics has really helped, Cardiod/Hypercardiod, etc
2) Getting the monitor volume down.   Once the band's in full cry there's a surprising amount of sound for FOH on stage, so getting the idea that the monitor is just supplementing the sound that's needed really helps.  However, this it the most difficult aspect, as people never say " the Monitors Too loud" .      

3) Taken the tack of... "Yes, we can sort out the volume in a minute ....... for now it's just the relative level of each element not the overall volume", if the "balance is right we can adjust as the set goes on IF it's not loud enough.  So if someone asks to be louder they may be saying " I can't hear myself over the other vocals (etc.)".  So the balance could be achieved by reducing the other elements in the mix.        

Still need to remove all the knobs on the monitors to stop the band member deciding to tweak their monitors tone/volume during the gig!!  

Or just say "sod it" and concentrate on bass playing.

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