resonant frequency in a boomy hall

[craigie]

We played a friend's wedding at a community center last night and it was a typical hall: long with hard floor and ceiling and very boomy. I run a TC electronic RH450 with an Eden 212XLT. I run a post EQ line into the board. This allows me to make adjustments to suit the room. We mix our own sound from the stage. Everything was set before I got there and my amp was in the corner. I moved it out of the corner and did my usual bass cut (80hz I believe is the preset of the RH450 bass control). I also always keep my amp on castors which helps decouple it from the floor. I use it for side fill and let the PA do the heavy lifting. That cleared up a lot of the boominess but there was a crazy resonant frequency at Bb to C. Twice as loud as other notes.

I compensated by playing those notes much quieter. Barely even plucking or picking. I then boosted up the low mid control and swept the frequency center until that note seemed to increase in volume. Then I cut it quite a bit. Really an unusual EQ setting with quite a bit of low and low mid cut. I boosted gain to bring the volume back up and add back a bit of fulness. Seemed to work Ok and was the best I could do with the time and equipment I had. I'm making a mental note in a hall like that to search for resonant frequencies.

I wish I had a chart of what frequencies (and fundamentals) the RH450 EQ corresponds to and the width, "Q" of the parametric EQ. I'll try contacting TC electronic but I guess I'll have and figured it out by "crank and sweep".

 

[Stumbo]

With side fill and in the PA you were bouncing off the side and back walls.

Possibly facing your amp forward could be an option in reducing the resonance.

 

[craigie]

With side fill and in the PA you were bouncing off the side and back walls.

Possibly facing your amp forward could be an option in reducing the resonance.

Good idea. My amp volume is pretty low. Next time I'll just unplug the speaker cable to troubleshoot mains vs amp without having to change the master volume on my amp which is really sensitive. I also use a strange system of IEM'S to block out my buddy's wedges which he likes loud and I just put some vocals, a bit of guitar and enough mix to get the pulse. I'll try bringing up the bass in the IEM'S next time too and dropping the amp. I'm wary of overloading my ears so I try and put the minimum in the IEM'S. I seemed to have a jump in my tinnitus a number of gigs back..... that's a different issue though and I'm working through it.

 

[tfer]

Was it a raised plywood stage?

I've got a ton of experience on those. The resonance is from the floor. Fortunately, from the audience, it's normally not noticeable, provided the PA speakers are on stands, and not also sitting in the stage.

It seems incredibly loud from your position because the plywood stage is acting like a driver, and you're standing in the middle of it, but if you're wireless, go out to the audience area, and it's not nearly as bad as where you are on stage.

 

[craigie]

Was it a raised plywood stage?

I've got a ton of experience on those. The resonance is from the floor. Fortunately, from the audience, it's normally not noticeable, provided the PA speakers are on stands, and not also sitting in the stage.

It seems incredibly loud from your position because the plywood stage is acting like a driver, and you're standing in the middle of it, but if you're wireless, go out to the audience area, and it's not nearly as bad as where you are on stage.

No stage. Just a hard floor. I leave my amp up on Castors and pa speakers are on stands. The length of the hall just happens to coincide with around B,C so those frequencies set up standing waves. The amplitude seemed to be doubled so that makes sense if the sound waves are bouncing off the far wall, then back wall, then they synch up to form constructive interference. My guess.

 

[drpepper]

I wish I had a chart of what frequencies (and fundamentals) the RH450 EQ corresponds to and the width, "Q" of the parametric EQ. I'll try contacting TC electronic but I guess I'll have and figured it out by "crank and sweep".

I made this a while back. TCE confirmed it, and I think they even used it or referred to it.

http://www.rccvideo.com/rh450centerfreqs.pdf

I think I read that the width is two octaves. I'm about 99% sure it's in the manual. Edit - The manual says "Fixed 2.1 oct."

 

[Downunderwonder]

No stage. Just a hard floor. I leave my amp up on Castors and pa speakers are on stands. The length of the hall just happens to coincide with around B,C so those frequencies set up standing waves. The amplitude seemed to be doubled so that makes sense if the sound waves are bouncing off the far wall, then back wall, then they synch up to form constructive interference. My guess.

you seem to know most of what you need to know. It could also be there is a ceiling to floor distance. You get resonance when there is a multiple of a wavelength of a fundamental or overtone.

One room I played, with solid floor and low solid ceiling, thundered a C. It was the guitar as much it was me but guess who copped the blame.

 

[craigie]

I made this a while back. TCE confirmed it, and I think they even used it or referred to it.

http://www.rccvideo.com/rh450centerfreqs.pdf

I think I read that the width is two octaves. I'm about 99% sure it's in the manual. Edit - The manual says "Fixed 2.1 oct."

Thanks for that! Now I see why my approach was pretty much doomed to failure. C is 65.4 Hz and I didn't shift the bass center frequency. If I went -12 it might have made a difference. In fact I cut at 280 Hz with the bass and maybe around 180 Hz with the low mid lol.

Curious that your chart doesn't interpolate smoothly between numbers. I'd imagine that they set up the controls in a linear fashion. Rounding on your part perhaps?

 

[drpepper]

Thanks for that! Now I see why my approach was pretty much doomed to failure. C is 65.4 Hz and I didn't shift the bass center frequency. If I went -12 it might have made a difference. In fact I cut at 280 Hz with the bass and maybe around 180 Hz with the low mid lol.

Curious that your chart doesn't interpolate smoothly between numbers. I'd imagine that they set up the controls in a linear fashion. Rounding on your part perhaps?

Doomed? Hmm, that might be a bit strong. Yes. A 2 octave width is certainly not going to give any type of surgical precision or a notch filter effect. From that perspective, if your intent/hope was to narrowly cut a particular narrow range of frequencies peaked precisely on a specif target, you're correct that the amp isn't capable of that. But where you are able to, I'd think you can capture the predominant frequency by simply being close to it at the peak that you are cutting. The bass and treble bands don't technically have a "center", because they are shelved (all frequencies, below for bass and above for treble, are cut or boosted at the same level as the set point...the amp's inherent rolloff is operative beyond that). Going to 70 Hz for the bass shelving point is going to capture 65 Hz.

I'm can't claim to have a lot of hands-on success in trying to deal with these kinds of issues using the amp alone. Live and in the moment, it's been more of a mixed bag, but you might also try targeting octaves as mid or shelving points? e.g., if 65 is your identified target, hit 130 and/or 260...just spit-balling.

The controls specifically are not linear, and being on a curve was the reason that the map is of any value. The manual provided the low, middle and high frequencies. That gave the math necessary to interpolate the frequencies at each detent.

 

[ga_edwards]

A couple of thoughts for you, as I have battled similar issues before, and people will frequently tell you to move your amp to combat the standing waves, but this isn't always possible.

Is the problem just on stage or is it our front too? The problem with sending a corrected signal to the PA is it may make the cut frequencies sound fine on stage but sound weak out front.

A simple single band parametric pedal and put it in the effects loop of your amp could help - I believe Artec make quite a good one, otherwise, try and find a zoom MS60b - this has a 2 band parametric model but crucially tells you what frequencies you are dialling in, in (IIRC) commonly used 1/3 octave bands. This way you can send either a 'pre' (if the problem is just on stage) or 'post' (if it's out front too) signal to the PA from your amp. The caveat is that on your amp, if set to 'pre' will mean sending a completely unaffected signal (no compression or tube tone) to the PA.

Also, you have have a smartphone, download RTA Lite. This is a free real time frequency analyser. Play a few notes and watch the bars - the offending notes should leap out and you will be able to indentify them and cut as appropriate with a parametric eq.

 

[pigpen1]

Why not cut frequency from foh and compress, maybe duck based on freq?

 

[jnotes]

We played a friend's wedding at a community center last night and it was a typical hall: long with hard floor and ceiling and very boomy. I run a TC electronic RH450 with an Eden 212XLT. I run a post EQ line into the board. This allows me to make adjustments to suit the room. We mix our own sound from the stage. Everything was set before I got there and my amp was in the corner. I moved it out of the corner and did my usual bass cut (80hz I believe is the preset of the RH450 bass control). I also always keep my amp on castors which helps decouple it from the floor. I use it for side fill and let the PA do the heavy lifting. That cleared up a lot of the boominess but there was a crazy resonant frequency at Bb to C. Twice as loud as other notes.

I compensated by playing those notes much quieter. Barely even plucking or picking. I then boosted up the low mid control and swept the frequency center until that note seemed to increase in volume. Then I cut it quite a bit. Really an unusual EQ setting with quite a bit of low and low mid cut. I boosted gain to bring the volume back up and add back a bit of fulness. Seemed to work Ok and was the best I could do with the time and equipment I had. I'm making a mental note in a hall like that to search for resonant frequencies.

I wish I had a chart of what frequencies (and fundamentals) the RH450 EQ corresponds to and the width, "Q" of the parametric EQ. I'll try contacting TC electronic but I guess I'll have and figured it out by "crank and sweep".

Sounds like you've thought of about everything. Some rooms, have impossible acoustic challenges. I sometimes forget how different the bass can sound from being on stage or standing in the room at various locations. More volume is usually not an improvement.