Subs

[Jakester]

19 hours ago, Bill Fitzmaurice said:

Lots of things aren't particularly noticeable if you're not able to make side by side comparisons. When you put subs under tops you not only lose boundary loading, by not have them close to a wall, you also can have cancellation notches when they're 1/4 wavelength out from a wall. If they're split by less than two wavelengths you will have power alley comb filtering. That's the science, and the science is never wrong.

There are three reasons why people put subs under mains.

#1: "That's how everybody does it".

#2: "It's the only way we know to do it".

#3: "We don't know any better way to do it/we don't know why it's the wrong way to do it".

 

Now you know.

 

Well, the science may not be wrong, but in practice if it isn't objectively noticeable, then what's the issue?

 

There's 'ideal in a laboratory setup' and then there's 'shoving all the gear in the corner at the Dog and Duck'. Most gigs tend toward the latter than the former. 

[vsmith1]

19 hours ago, Bill Fitzmaurice said:

Lots of things aren't particularly noticeable if you're not able to make side by side comparisons. When you put subs under tops you not only lose boundary loading, by not have them close to a wall, you also can have cancellation notches when they're 1/4 wavelength out from a wall. If they're split by less than two wavelengths you will have power alley comb filtering. That's the science, and the science is never wrong.

There are three reasons why people put subs under mains.

#1: "That's how everybody does it".

#2: "It's the only way we know to do it".

#3: "We don't know any better way to do it/we don't know why it's the wrong way to do it".

 

Now you know.

#4: "There's no space to have the sub and top separate." [File under pragmatism at the Dog & Duck]

 

[Bill Fitzmaurice]

I've never encountered a situation where the sub and top could not be separated, as the sub can go pretty much anywhere. They don't even need to be facing the audience, or even able to be seen, because they're omni-directional, and the long wavelengths they produce aren't blocked by obstacles.

[Al Krow]

24 minutes ago, Jakester said:

 

Well, the science may not be wrong, but in practice if it isn't objectively noticeable, then what's the issue?

 

There's 'ideal in a laboratory setup' and then there's 'shoving all the gear in the corner at the Dog and Duck'. Most gigs tend toward the latter than the former. 

 

I do have to agree with you. We were playing at a function gig recently going through the DJ's PA set up: he had two RCF 705s with tops on poles directly above. Can't recall our band sounding better overall - we normally don't use subs. I particularly loved the drums and audience were loving it / very complimentary. 

 

And, in practice, you're never going to be able to A/B your set up because it will only be truly compared once the audience is there absorbing your sound waves - hugely different to an empty venue as we all know. 

 

I think tbf from some of Bill's recommendations (eg paying someone to be sound engineer at gigs), he's coming at this from the more pro end of the market. We're doing pretty well as a band, but certainly wouldn't be able to afford to bring a sound engineer on board for our gigs. I totally recognise the much greater expertise from Bill and others when it comes to this stuff - it's just that it won't necessarily translate into the semi-pro or hobbyist band set up many of are involved with. 

 

So if it works and works well, I'd say crack on! Perfection can be the enemy of the good. And a neater, potentially more secure set up taking up less floor space will often trump a marginal improvement in sound quality. If it means you're using a sub to enhance the kick that's likely to be a noticeable improvement to your sound, certainly in larger venues, than not having one at all. 

 

Edited September 19, 2023 by Al Krow

[Pirellithecat]

Couple of comments. 

I suspect US Pubs are larger than the Dog and Duck.   It is not the case (in the UK at least) that there is room for a sub as well as the drummer at most of our gigs.   Last weekend, there wasn't room for the neck of the bass without hitting the Alabaster Lion's Head on the wall!  Ditto at the previous gig (without the Lion's Head).

The second point is that, in such small spaces it is common (routine) to have the PA close to the wall so I guess the sub under the Tops works in terms of boundary loading? 

A couple more comments

Whilst our "on Stage" sound needs work, the sound out in the  "audience" is pretty good and is definitely better with the sub (so says our Drummer's tutor).  It may be that, given the low level of anticipated quality (in't Dog and Duck), the sub, used under the PA tops, works.

And Finally!   The issue with Subs behind the Mics (or even mic'd kick via drum monitors) is feedback, as the mic's (especially the Drummer's) pick up the low frequency "noise" giving very problematic deep resonant feedback.  Difficult to work out what it is at first (omnidirectional?) but by a process of elimination (turning the drummer OFF!) that's what it is.   

 

I am not in any way technically competent and do not question the theoretical aspects of this debate.   However, in the real world there are many factors which frustrate technical expectations ... I suspect this is one. 
Oh and Al said it much better and is far more emotionally intelligent than I (or is that "me"?) 🤪

[Phil Starr]

This is a really interesting debate, I hope it doesn't get testy. I sit in the middle on this and I don't think there is real disagreement. Nobody is flying in the face of the physics and it is really, really helpful to understand as much as possible. When you are stuck with 20 mins to fix a problem and surrounded by punters who just want you to get on and play it helps to have a bit of theory to tell you where to start looking, and after all we do all face the same problems. I also suspect that performance spaces in the UK are often smaller and less suited to live music than in the USA. I'd say that the average age of the pubs that I play in is close to 200years old in this rural part of the UK. The hint is in the word Pub or public house, many of them are just houses adapted a century ago or more to sell alcohol.

 

That said subs are effectively omnidirectional at all of the frequencies they are used for (without splitting hairs) so positioning and feedback isn't about in front or behind the mic line. It is more about how close to boundaries the sub is positioned. Each boundary reflects the sound increasing it's volume and changes it's phase at wherever point the listener is standing or where the mic is situated. You are setting up a complex series of bass peaks and troughs all around the room. You are also boosting the bass by quite a lot  so that placing the sub in a corner as opposed to just on the floor is pretty much the same as suddenly turning the bass eq on kick and bass from flat to full up. A lot of bass feedback is because the subs are just too loud. How close to the boundaries is important too as different frequencies are emphasised depending upon the spacing.

 

Mic-ing up a kick drum isn't straightforward either, you have the angle and height of the mic to consider and the kick drum is a highly resonant space, so how close to the hole do you put it? All of these things will affect gain before feedback as well as the tone of the kick through FOH. If you are going for a nice full tone you are emphasising those bottom frequencies which in turn are going to acoustically feedback to those resonant skins on the drums. Add in a floor monitor nearer the kick drum than the drummers ears and you have a nightmare to deal with.

 

If you are having bass feedback problems then use the subs sparingly, keep them away from side walls if possible, expect to have to turn them down or up depending upon the venue and if your mixer allows it filter the lowest frequencies if you have issues. keep any floor monitors well away from the kick mic and even be prepared to steeply roll off the bass on the drummers monitors. You don't really need anything below 80Hz in the monitors anyway all the musical information you need to play is above this frequency.

 

The issue though is time, I always think of the witty riposte after my friends have gone and the solution to last night's PA problem the day after the gig 

[Bill Fitzmaurice]

You absolutely neither need nor want anything below 100Hz in floor monitors. Whatever is down there you'll hear from the mains or subs, because they're omnidirectional below roughly 200Hz. Having content below 100Hz in the monitors can even reduce overall low frequency output, because the distance between the various sources can result in phase sourced cancellations. All you want from monitors is the directional mids and highs that you can't hear from the mains.

The main advantage to knowing the science behind low frequency production and dispersion is you can walk into a room and know where you'll get the best placement results just by looking at it. If you don't understand the science then your sub placement will be like playing darts...while drunk...and blindfolded. If you don't care about the results just hope that the audience doesn't either.

[Woodinblack]

2 hours ago, Bill Fitzmaurice said:

 If you don't understand the science then your sub placement will be like playing darts...while drunk...and blindfolded.

 

To be fair, I don't think my ability playing darts would be an different under those circumstances

[Al Krow]

3 hours ago, Bill Fitzmaurice said:

You absolutely neither need nor want anything below 100Hz in floor monitors. Whatever is down there you'll hear from the mains or subs, because they're omnidirectional below roughly 200Hz. Having content below 100Hz in the monitors can even reduce overall low frequency output, because the distance between the various sources can result in phase sourced cancellations. All you want from monitors is the directional mids and highs that you can't hear from the mains.

The main advantage to knowing the science behind low frequency production and dispersion is you can walk into a room and know where you'll get the best placement results just by looking at it. If you don't understand the science then your sub placement will be like playing darts...while drunk...and blindfolded. If you don't care about the results just hope that the audience doesn't either.

 

But is it fair to say if you're dispensing with floor monitors altogether because you're using IEMs, and also using just one sub rather than two eg because that's all the size of the venue warrants or your budget/boot space allows (delete as applicable), then the phased source cancellation is another thing you don't need to worry about?

 

At which point does it not become a choice of: do want the additional bass reinforcement by placing your sub adjacent to a wall, or do you prefer a neater floor footprint and more stable top placement by mounting your top above the sub?

 

And if the latter is a significant step up over not having a sub at all, have you pretty much not hit bull's-eye by doing so? 

 

Just asking for a fellow darts player I know. 

[Bill Fitzmaurice]

Wall or corner loading doesn't just give more output in the lows, it also prevents boundary reflection sourced cancellations. If your sub is two meters out from the wall you will have a cancellation notch centered at 42Hz. That's non-negotiable.  As for the notion that wall or corner placement will give too much low end that's what volume controls are for. Some 75% of the energy required of PA comes from the subs. Anything you can do to improve the efficiency of the subs via placement will give cleaner output from both the subs and the amps driving them. Aiming the subs at the wall or into a corner makes for even lower distortion, as distortion manifests as harmonics in the directional frequencies above the sub pass band. They're attenuated by aiming the speaker backwards, while the output in the sub pass band is unaffected.

 

I've been stand mounting my tops for thirty years and have yet to have a mishap. Perhaps it's because I have my Rottweilers stationed below them?

 

If you have in ears exclusively there's no need for the precautions required with floor and/or side fill monitors.

Edited September 22, 2023 by Bill Fitzmaurice

[Al Krow]

2 hours ago, Bill Fitzmaurice said:

Wall or corner loading doesn't just give more output in the lows, it also prevents boundary reflection sourced cancellations. If your sub is two meters out from the wall you will have a cancellation notch centered at 42Hz. That's non-negotiable.  As for the notion that wall or corner placement will give too much low end that's what volume controls are for. Some 75% of the energy required of PA comes from the subs. Anything you can do to improve the efficiency of the subs via placement will give cleaner output from both the subs and the amps driving them. Aiming the subs at the wall or into a corner makes for even lower distortion, as distortion manifests as harmonics in the directional frequencies above the sub pass band. They're attenuated by aiming the speaker backwards, while the output in the sub pass band is unaffected.

 

I've been stand mounting my tops for thirty years and have yet to have a mishap. Perhaps it's because I have my Rottweilers stationed below them?

 

If you have in ears exclusively there's no need for the precautions required with floor and/or side fill monitors.

 

Bill some very useful points there, thank you.

 

Just picking up on one point if I may - given the consensus is that the main benefit from using a sub is to enhance the kick drum which is typically going to be in the 50Hz to 80Hz range, why should it matter if there is a cancellation notch centred at 42Hz? Ok I appreciate that is very close to the fundamental of the low E string on bass but a lot of us will be HPF'ing our basses below 80Hz, in any case, to deliberately avoid a boomy low end, and certainly wouldn't want the 40Hz region to be being boosted by the sub.

[Downunderwonder]

9 hours ago, Phil Starr said:

bass feedback problems

Ime it's incredibly difficult to get the bass to feed back even if trying. Yet the first place everyone stares is the bass player whenever there's a problem in the low end.

 

Acoustic guitars

Drums

The drummer's tummy rumbling.

 

These are all things to check on before the bass guitar.

 

[Bill Fitzmaurice]

7 minutes ago, Al Krow said:

 why should it matter if there is a cancellation notch centred at 42Hz?

That was just an example.  There will always be a cancellation notch where the speaker radiating plane is 1/4 wavelength out from the wall. The frequency depends on the distance, the longer the distance the lower the frequency. There are two ways to eliminate it. One is flush mounting the speakers in the wall, which isn't an option for the most part, although it is common in theaters. The other is to not have the speaker 1/4 wavelength from the wall below the speaker baffle step frequency (where the baffle is 1 wavelength in dimension, and dispersion goes to less than 180 degrees). For the 30 to 300Hz frequency range that's roughly between .3 and 3 meters. That's more or less impossible to do with large 3 way full range speakers, and is one reason why they've pretty much been replaced by separate subs and mains. You can place subs close enough to the wall so that it's not 1/4 wavelength out at any frequency within its pass band, and tops far enough away from the wall so that they're not 1/4 wavelength out at any frequency within their pass band. But you can't if they share the same footprint.

[JPJ]

Our gig yesterday was a large'ish social club concert room with a proper stage. Our PA comprises two EV ZLX15 'tops' and two old Peavey 400w powered 15" bass bins. Nine and a half gigs out of ten I will place the bins under the tops simply because of space limitations. As I mix from the side of the stage (Behringer XR18) I also commit what some consider to be a cardinal sin in live sound and run a dBX Driverack. 

For yesterdays gig, I placed the bins on the floor in front of the stage on the centreline of the room (approximately) and side by side. The tops were on stands either side of us on stage. I had time to run the setup utility for the Driverack (pink noise anybody?) and save this new config for future use. 

What I noticed from this setup was: 1) bloody hell the bins were loud, and I was able to get some of that chest pumping kick drum from them (albeit) only over the first ten metres or so. 2) the overall mix was much better, with the vocals coming through loud and clear. 3) the Driverack was making much more use of the bins, with the level indicators on both the tops and the bins both regularly hitting four bars (with the normal subs under tops, the subs are lucky to see one bar). 4) Our nominal 1900w PA coped admirably with the room, yes we were not the loudest, but we were loud enough and I received several compliments on the sound. 

I must confess that I previously thought the gain from running this configuration would be marginal but actually it was significant. There is only one other regular gig where I could use this setup, but I definitely will be using it whenever I can. 

[Bill Fitzmaurice]

36 minutes ago, JPJ said:

 I also commit what some consider to be a cardinal sin in live sound and run a dBX Driverack.

I can't imagine anyone with half a clue saying that, but I guess there's plenty of PA operators who don't have half a clue. 🤥