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[Irvin Pribadi]

Got a question on the application of bass traps.

My church has room amplified bass response at the back of the sanctuary anywhere from 5-15 ft from the back wall.
The sanctuary room is approx 28'x40'x10' with commercial carpeted floors.
My sub (LPF @70Hz) is on the floor up front 10.5' from front wall 6in from side wall.

I've researched the theory of why this happens (standing waves, etc) and have gathered that there are two solutions:
1. Use bass traps to reduce reflected sound waves
2. Move listeners away from that area
I'm open to any other solution inputs.

So about the bass traps, in my situation due to windows on the wall I can only mount these high on the back wall against the ceiling. The side walls are not possible either.

Do the bass traps need to be at ear level to be most effective?
Thanks!

[Shane Flowers]

Our sanctuary is a little bigger, but I have twin 18" Yamaha subs since I have all of the instruments running through the PA.  In the corners (like where my booth is) the bass is so loud it hurts.  In the middle of the sanctuary, it's almost non existent. 

I've been looking at these bass traps or I may just build my own.  I've played with the position of the subs and it has improved quite a bit, but I think perfection can't be achieved without traps.  If they just reduce the buildup in the corners (and my booth!) I'll be happy.

http://www.sweetwater.com/store/detail/EliteCT45Sa?utm_source=CSE&utm_medium=PPC&utm_campaign=none&utm_term=EliteCT45Sa&SID=-1&zmam=89202644&zmas=1&zmac=1&zmap=EliteCT45Sa

[Irvin Pribadi]

I've been looking at these bass traps or I may just build my own.  I've played with the position of the subs and it has improved quite a bit, but I think perfection can't be achieved without traps.  If

Those acoustic panels are only 2" thick. Unless the bass you mention is high bass around 200Hz, those acoustic panels won't be effective for anything lower. Low freq just have too long of a wavelength (several feet) to be able to get trapped but such thin material.

I'm looking for a trap to attenuate around 50-80Hz, so I need 6" thick ones to be effective.

My question is still where do I mount these? ear height? ceiling corner with back wall?

[Irvin Pribadi]

Btw, my church uses modern pop/rock music for praise & worship.
Things are preferably loud around 95 db.
Problem is mostly with the bass guitar when it hits the lower notes (low E, F, G).

[Jordan Wolf]

Btw, my church uses modern pop/rock music for praise & worship.
Things are preferably loud around 95 db.
Problem is mostly with the bass guitar when it hits the lower notes (low E, F, G).

Irvin,

You may want to browse the Install forum to see if there are any similar queries to your own.

[Tim Weaver]

Irvin,

You may want to browse the Install forum to see if there are any similar queries to your own.

Chances are you will need to build some helmholtz resonators to deal with it. I'm guessing that they'll need to be closer to the floor to do any good for you.

Something like this...
http://www.audioholics.com/education/acoustics-principles/helmholtz-resonant-absorber/helmholtz-resonant-absorber-page-2

[Brad Weber]

Irvin and Shane, one critical piece of information not provided are the existing wall and ceiling constructions.  A common wall construction of a 1/2" or 5/8" layer of sheetrock on studs 16" to 24" on center will allow some of the energy to be absorbed through diaphragmatic action, at 100Hz the absorption coefficient of such a construction can easily be greater than 0.5.  In addition, a good amount of the low frequency energy will pass right through the wall.  However, brick, CMU, poured concrete, etc. do not react the same.

One common approach to bass traps using traditional acoustical panels, or at least ones with no solid backing, is to use them to 'cut the corner', placing them in the corner at an angle to both walls.  This creates a cavity if varying depth behind the panel, increasing the apparent thickness of the panel and with the varying depth, perhaps generally affecting a wider bandwidth.

If you have specific frequencies you want to absorb there are a number of tubular, slotted, perforated. diaphragmatic and other tuned absorber options.  They all takes some design to tune for the desired frequencies and getting absorption over a broader frequency spectrum may take multiple variations, but they can be quite effective.

[Arnold B. Krueger]

Those acoustic panels are only 2" thick. Unless the bass you mention is high bass around 200Hz, those acoustic panels won't be effective for anything lower. Low freq just have too long of a wavelength (several feet) to be able to get trapped but such thin material.

Spacing absorbtive material several inches from a reflective wall significantly increases its effectiveness at lower frequencies.

My question is still where do I mount these? ear height? ceiling corner with back wall?

Bass in a room is usually greatest where two or more surfaces join.  So, the junctions of walls, floors, and walls are preferred locations for absorbers.

Also, smaller absorbers spaced in say a checkboard or striped pattern on a wall are more effective than the same area covered contiguously.

Exposing the edges of absorbtive material can also increase its effectiveness.

[g'bye, Dick Rees]

Btw, my church uses modern pop/rock music for praise & worship.
Things are preferably loud around 95 db.
Problem is mostly with the bass guitar when it hits the lower notes (low E, F, G).

Rather than try to corral the horse after it's left the barn, why not treat it in the stall?

You should start by examining the positioning of the subs in the first place.  That combined with some judicious use of EQ and LPF may well solve the problem without building things.

[Shane Flowers]

Rather than try to corral the horse after it's left the barn, why not treat it in the stall?

You should start by examining the positioning of the subs in the first place.  That combined with some judicious use of EQ and LPF may well solve the problem without building things.

Just what I was thinking.  I have my bass guitar, keyboard and drums on a channels that have a compressor.  The compressor knocks down the peaks, but adds a little gain everywhere else so the musician is happy with the volume but it's not overpowering everything else on the platform.

I've also found that a lot depends on the guitar.  Our old Ibanez never gave me this problem.  Our new Yamaha with active pickups can REALLY add some bottom end when turned up.  Of course, the bassist always turns it up so they can hear, but the curve is just too heavy on the bottom end.  The compressor has done a great job of giving him the volume he wants while keeping the low end in check.

[Brad Weber]

Rather than try to corral the horse after it's left the barn, why not treat it in the stall?

You should start by examining the positioning of the subs in the first place.  That combined with some judicious use of EQ and LPF may well solve the problem without building things.

Ironically, before I read the second paragraph I interpreted your first comment almost exactly the opposite, that whatever you do is not going to change the room.  The reality is that the gear, its location, any processing, etc. will not change the room or eliminate problems caused by the room, however they can minimize or exacerbate them and can also add their own additional issues.  So it may be necessary to look at the system application, the processing and the room to see what is the most effective solution.


Irvin, to give a simple answer, in general bass buildup is worst in corners so that is where any treatment may be most effective.  That can actually also be true for higher frequencies as treatment on one surface of a corner may not just affect the sound hitting it directly but also have any sound hitting it as a reflection off the other nearby surfaces.

You mentioned that your sub has a 70Hz low pass filter.  That is a little bit lower than common for many systems, what are the speakers involved and what are you using for the crossover?  Also, where are the mains located, what is the high pass for the mains and what slopes are the high pass and low pass filters?  You mentioned wanting to trap 50-80Hz and I'm just wondering if part of the problem is due to sound being reproduced by both of the mains and the sub.

[g'bye, Dick Rees]

Brad....

That's pretty much where I was headed.  Before going to any great lengths and expenditure of time and money, I'd want to know exactly what is happening and if the existing gear/placement can be adjusted and/or improved prior to radical reconstruction.

Bass traps aren't small.

[Irvin Pribadi]

Thanks all for the inputs so far.

I'm running an aux fed sub setup where only the kick drum, floor toms and bass guitar goes to our 18" folded horn subwoofer. Those 3 also go to the main where the main has roughly a 60Hz HPF.
I'm running two 15" cabs as mains left and right of front stage, 7ft high, 14ft from front wall delayed by 2ms, sub is 11ft from front wall, below left main on the floor, no delay.
I'm trying to EQ the sub and mains to blend well and here's where I discovered the problem.

Haven't tried positioning the sub elsewhere as choices are pretty limited. Would this make much difference?

What I've noticed from just my own hearing and running a reference mic thru REW software is that LF response <200Hz is very uneven with peaks and lows differring up to 20db or so. Running a sine sweep from 30 to 200Hz u can hear clear pulsing which matches what is seen in the REW measurement graphs.
I know it's the room coz changing listening position by several ft can totally reverse the peaks and valleys seen/heard. My EQ attempts are useless as it can only fix one small position only.

Sorry missed one important thing: our building used to be a bank. (doh!!!) 15% of the room is concrete wall (covered with dry wall), another 15% is bullet proof windows (1" thick glass), rest being normal drywall and standard windows. The ceiling is gypsum, floor is commercial carpet on concrete slab.
During Sunday service there is not much sound leakage outdoors which is good for our neighbors but keeps me wondering where all the sound energy goes.

[Samuel Rees]

I'm running an aux fed sub setup where only the kick drum, floor toms and bass guitar goes to our 18" folded horn subwoofer. Those 3 also go to the main where the main has roughly a 60Hz HPF.

Not to say that this is going to make or break your problem, but it sounds to me like your crossover settings are not working for you. Your mains and your subs are sharing some frequencies and are pretty darn low. Especially if your crossover has a gentle slope, the mains (hpf @ 60hz) will still be pushing at a bit all the way down @ 50 hz and full signal a very low @ 62 hz and your subs (lpf @ 70 hz) will still be pushing out some 80 hz. Depending on your slope settings, worst case scenario you've got both mains and subs producing substantial signal in the 50-80 hz range; best case scenario you've still got both subs putting out full range 60-70hz. I suggest matching your hpf/lpf up at one frequency with a steep slope, if you have gear where you can change that, and moving them up 10 or 20 hz. IMO 60 hz is way to low to let into the mains in most subwoofer equipped systems, and the 70-85 is totally applicable for subwoofers. You should consider your venue's needs and look at this as just an example, but I have a portable rig I work on with 2 x 12"s over 2 x 15" subs and a medium slope crossover @ 100 hz. A club venue I work at has 4 x 12"s and 4 X 18" subs with a steep crossover at 80 hz.

[Brad Weber]

I know it's the room coz changing listening position by several ft can totally reverse the peaks and valleys seen/heard. My EQ attempts are useless as it can only fix one small position only.

I do not doubt that your space is a factor in the results, however your observations would also be applicable to the interactions between the single sub and two overlapping mains, which because the relationships to the speakers vary by location could also result in the variances noted between different listening locations.

It could be coincidental but with a 60Hz high pass of some slope on the mains and a 70Hz low pass some slope on the sub, the very 50-80Hz range that you noted seems to be the problem appears to closely match the frequency range being reproduced by both of the mains and the sub.  A very simple test but what happens if you use just one main and no sub?   Just the sub?  Just the other main?  Basically, are the perceived issues reduced if the multiple speaker interactions are eliminated?

Particularly if that does make a difference then I agree that pushing the crossover up so that more is handled by the single sub and less going to multiple speakers may make sense.

[Irvin Pribadi]

Thanks Brad and Sam,

I'll investigate this some more (sub only, mains only, both) to see if there is any correlation with the uneven bass issue I have.

Raising the x-over point to 100Hz may require me to add the keyb to the aux fed sub as well. I'm concerned that my keyb won't have that low end girth.
I'll try a few things out and see.

Any input on sub location anyone?
I read that distances between 9-3ft to the front wall (backline) would cause cancellations at 30-100Hz respectively?

[Samuel Rees]

Raising the x-over point to 100Hz may require me to add the keyb to the aux fed sub as well. I'm concerned that my keyb won't have that low end girth.

In any case you at least need to sync them up at the same frequency and a steep slope, or the mains a little higher than the subs if it is a gentle slope. Otherwise, certain frequencies will be getting extra power being amplified by both the mains and the subwoofers. From what you described, your system should be putting out a bunch of extra signal between 60-70 hz at least, possible more.

Personally, I've never crossed a main/subwoofer system below 80 hz.

[Mike Allgeier]

Any input on sub location anyone?

Don't know if this would be feasible but could you try (maybe during a rehearsal) moving the sub to the location where the problem is, then walking around the room to see where the problem disappears (or is attenuated a good bit)? Move the sub there and see how that works. It could produce the same problem somewhere else in the room.

Coincidentally, I just spent 1 1/2 days with an acoustician/systems guy assisting him in tuning up our main auditorium. We have another room, our FLC, that is much larger than the one you're talking about (it's 55'X100'X22' IIRC) and we have some problems with low frequency hang (reverb) in it. We don't use instruments, only acapella singing but I asked him about taking some of those really large vases (like old pottery wine storage vessels) and loosely filling them with insulation and placing them around the room. He was intrigued with the idea and thought that it might actually help. The big question in that equation is how many of those would you need for your situation?

[Ervin Hyatt]

My sub (LPF @70Hz) is on the floor up front 10.5' from front wall 6in from side wall.

Irvin,
 I am Ervin.

You need to work on four things before you do any traps. First low cut (high pass) on everything but kick bass some keys maybe. second Move sub away from wall as much as you can. 6 inches will couple yor sub to wall and the wall will add what you want to cut.  thrid work on your crossover between mains and sub. I would experiment with LPF on sub set much higher try around 120 to 130. Cut mains at 100 to 120 hz. work on curve until sounds naterual. If you can aux feed sub so you can control levels on each source. Play with these feq. until room sounds its best. The sub set at 70 is the problem you do not have your sub set for instruments but hip/dance. If you have eq or contorller also put low cut at 34 hz with steep roll off. May need to go higher to 40 hz. I have found I do need much power below 80 hz in church and we have a very contempory sound execpt on Kick and bass is all and some on keys at times. I also put some sub on pastor with low cut (high pass) becuase he does not have much low in countryman mic. Fourth bass traps are very difficult to get to cut much below 120 hz because the ladies will not want huge things hanging on walls and you can get much deal with before going there.

Hope it helps.
Ervin

[Mac Kerr]

You need to work on four things

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Mac

[Ervin Hyatt]

Done sorry.

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