Node ring

[buggyboy]

Boy, the acoustic threads are dead here.


Anyway, here is the question.

Standing at the pulpit, wearing our lav, ringing starts at a frequency of c#.

Step back 1 foot...ringing stops. Step to either side by 1 foot, ringing stops.

This screams out to be an acoustic problem. Could it possible be anything other than acoustics?


Speakers are about 12 feet away on each side. 7 feet to the side and about 7 feet forward.

mic is shure 839.

I moved the pulpit to see if it was contributing to the ring. the ring is the same even when the pulpit is moved away.



Second question. If I play a frequency generator through the foh speakers, and walk through the room, and find hotspots and cold spots...this would indicate bad acoustics? correct?


Last question. A line of nodes directly on the north/south centerline of a room would indicate that the walls needing treatment would be the east and west walls?

Especially if those 2 walls were hard plaster.

Thoughts?

[cmchamp]

1: Yes, it's possible to get similar results in any situation where an open mic is placed dead center between two loudspeakers amplifying that open mic. Not necessarily just an acoustic problem, but a gain-before-feedback AND louspeaker choice/placement issue combined with room acoustics. In this instance, acoustics would be last on the list. You can duplicate the same thing in an anechoic chamber (no reflections).
2: How many loudspeakers are being used to amplify the sound source for this? If more than one, then you're experiencing comb filtering AND mode/node.
3: Not necessarily, if you are using, say 100Hz as your tone, using both speakers (in L/R configuration), then you're most likely experiencing the LF power alley down the center of the room.
Again, speaker choice, placement, voicing, lack of or presence of a sub and it's placement would come to play more here than just acoustics.

[Brad Weber]

Let's start by saying that yes, all of the things you noted are acoustic issues. However, they may not be or only partially be a factor of room/architectural acoustics.

There are some simple things you can do to help determine the cause. For example, what happens if you turn off one speaker? The room will not have changed but any interaction of the two speakers will no longer be involved. The situation will also no longer be symmetrical, so if moving slightly to one side was moving you in or out of the speaker pattern the effects should then differ to each side.

[buggyboy]

Excellent!
Thank you for creating testable hypotheses!

Notes:
2 speakers, Bose 802, no sub.

Regarding speaker voicing. Just tonight I tried out the bose controller. I found it introduced a high frequency ring. I’ve setup our system to cut loads of mid-low (250hz – 1khx, largest cut at 400hz). In it’s current configuration it can put out unity + about 10db before feedback. After routing the output through the bose controller, I would start to hear crunchy, electronic-feeling, ringing (higher pitched than normal) at about +4db.

You mentioned three problems, mode/node, comb filtering, and GBF.
The solutions I’m aware of for these issues are acoustic treatment for node issues and single point speaker to eliminate comb filtering. I’m unaware of a “solution” for GBF.

I will disconnect one speaker to see if that changes the pulpit hotspot.

If this hotspot remains with only one speaker in use, what will this indicate?


Also, if I play a tone generator through that one speaker (no open mics) and continue to find hotspots, what will that indicate.

Question: in an anechoic chamber, with 2 speakers, would the comb filtering be heard as individual hotspots, or as a line of equal “heat”.

Thank you!

[Brad Weber]

Originally Posted by buggyboy

Notes:
2 speakers, Bose 802, no sub.

Regarding speaker voicing. Just tonight I tried out the bose controller. I found it introduced a high frequency ring. I’ve setup our system to cut loads of mid-low (250hz – 1khx, largest cut at 400hz). In it’s current configuration it can put out unity + about 10db before feedback. After routing the output through the bose controller, I would start to hear crunchy, electronic-feeling, ringing (higher pitched than normal) at about +4db.

In this, http://www.audiorail.com/802_controller.gif, the yellow trace is the response of a Bose 802C controller. It pretty obviously provides significant boost, about 4dB around 250Hz and 17dB or so around 15kHz. That could certainly relate to your experience.

Originally Posted by buggyboy

You mentioned three problems, mode/node, comb filtering, and GBF.
The solutions I’m aware of for these issues are acoustic treatment for node issues and single point speaker to eliminate comb filtering. I’m unaware of a “solution” for GBF.

The solution to both avoiding comb filtering and improving gain before feedback is proper system design and operation.

Originally Posted by buggyboy

I will disconnect one speaker to see if that changes the pulpit hotspot.

If this hotspot remains with only one speaker in use, what will this indicate?


Also, if I play a tone generator through that one speaker (no open mics) and continue to find hotspots, what will that indicate.

It will indicate that the issue is probably not interaction between the two speakers. It may not clearly indicate if the issue is the room, the interaction of the speaker with the room, the speaker itself (pattern, aiming, etc.) or some combination of these.

Originally Posted by buggyboy

Question: in an anechoic chamber, with 2 speakers, would the comb filtering be heard as individual hotspots, or as a line of equal “heat”.

Yes. Seriously, it could well be both. Combfiltering is the result of the relative phase of multiple sources at a point in space. If the sources are in phase or coherent at any point they sum and the resulting level is up to 6dB higher for two sources (6dB if they are also of equal level, less if they differ in level). If two sources arrive 180 degrees out of phase at a location they cancel (Again, if also of equal level, otherwise less attenuation occurs). The relative phase is determined by the relative path length (the distance the sound has to travel) and the frequency.

So what typically happens with multiple speakers is that at any point in the room there will be a relative path length and level differences that can cause certain frequencies to cancel, other to sum coherently and many to be between those two extremes. But at other points throughout the room the relative path lengths differ and thus the results differ.

If you have a symmetrical layout, there is a point midway between the speakers where the distance to the speakers and the level of the speakers will be equal. You can extend a line out where that situation applies, typically down the center of the room. Along that line the two speakers will be summing coherently. And at lower frequencies it will take some movement off that to introduce sufficient difference in level or arrival time (relative phase) to start getting any destructive interference (where the signals interact to attenuate the resulting level). That is the 'power alley'.

At the same time, at other points in the space different frequencies can be in or out of phase, thus creating summation and cancellation at different frequencies throughout the space. Adding to that, the level from each source is also going to vary throughout the space and likely with frequency as well. The net result is variations in level and frequency response throughout the space.

Good system design will try to minimize those variations as well as the additional factors of interactions resulting from the interactions of the sound with the room.

[buggyboy]

I had been looking for a good graph to show the action of the bose controller. The one you provided works well. I was afraid of this, the controller works by boosting the high and low, rather the attenuating the mids. Hence the crunchy high ringing I heard of electronically enhanced highs.

The solution to both avoiding comb filtering and improving gain before feedback is proper system design and operation.

This, while true, is not actionable for me. It's not something I can test, try, or do.

Is GBF a condition? It seems to be a symptom rather than a cause. That is, any number of things may cause poor GBF, and there are as many solutions.


Nodes detected with only one speaker...

It will indicate that the issue is probably not interaction between the two speakers. It may not clearly indicate if the issue is the room, the interaction of the speaker with the room, the speaker itself (pattern, aiming, etc.) or some combination of these.

If only one speaker is connected, would it not be physically impossible for the issue to be caused by the interaction between the 2 speakers.
I'll grant you, the bose is not directive enough to be worth 2 cents. I could test if this was the problem by changing the angle of the speaker.
What would the issue be if it was "the interaction of the speaker with the room"?
I'd like to test my way into a single conclusion.

So, I'll test:
disconnecting 1 speaker to test for comb effects.
changing angle of remaining speaker to test for pattern effects.
I've already established by moving the speakers that the issue is not direct feedback (aiming).

What else must I test to back my way into a single cause, and thus a single solution?
(Or a definite multiple cause, and thus a definite multiple solution).


What material would be useful for blocking sound going in undesired directions. I would assume mass is the largest player in this use. So, with a non-directive speaker, could a wall be built (of concrete) that would guard against sound going backwards?


thanks

[buggyboy]

Update!

Disconnecting one speaker basically killed the node at the pulpit.
ringing the happens as I increase gain, but the "stepping into a hotspot" at the pulpit is gone.

There is still ringing in the room before I have sufficient spl... but this one issue was comb filtering. I knew about comb filtering from my reading, but didn't even think about it with this latest issue.

I intend to stick with a single speaker going forward to eliminate the power alley. I see no way of using 2 speakers and compensating for the comb effects.

I'd like to test-fly the speaker and see what happens, but I haven't thought of a way to do that yet.

Interesting note: The frequency of the ringing is most apparent as I bring the lav near my chest.

Example. I attach the lav to a space mic stand at normal height and in it's normal location. I bring up the system level to the edge of ringing. I go to the stage and clip the lav on my shirt... the system will begin to ring.

Take the lav away from my chest and the ringing decreases.

Sounds like "proximity effect"...but I'm not sure if there is a solution for it.

I should probably close this thread and start a new one with whatever the next issue is.

Thanks!

[Frank DeWitt]

Originally Posted by buggyboy

Update!


Interesting note: The frequency of the ringing is most apparent as I bring the lav near my chest.

Example. I attach the lav to a space mic stand at normal height and in it's normal location. I bring up the system level to the edge of ringing. I go to the stage and clip the lav on my shirt... the system will begin to ring.

Take the lav away from my chest and the ringing decreases.

Sounds like "proximity effect"...but I'm not sure if there is a solution for it.

I should probably close this thread and start a new one with whatever the next issue is.

Thanks!

It is proximity effect but it is not all bad. The mic is also more sensitive at picking up your voice. Being more sensitive to feedback from the speakers is just more of that no free lunch part of physics. (Grin) I think I remember this coming up before but the easy way to get more GBF is to move the mic closer to the talker and the easy way to do that is with a head worn mic.

Frank

[pdc]

Originally Posted by buggyboy

Boy, the acoustic threads are dead here.


Anyway, here is the question.

Standing at the pulpit, wearing our lav, ringing starts at a frequency of c#.

Step back 1 foot...ringing stops. Step to either side by 1 foot, ringing stops.

This screams out to be an acoustic problem. Could it possible be anything other than acoustics?


Speakers are about 12 feet away on each side. 7 feet to the side and about 7 feet forward.

mic is shure 839.

I moved the pulpit to see if it was contributing to the ring. the ring is the same even when the pulpit is moved away.



Second question. If I play a frequency generator through the foh speakers, and walk through the room, and find hotspots and cold spots...this would indicate bad acoustics? correct?


Last question. A line of nodes directly on the north/south centerline of a room would indicate that the walls needing treatment would be the east and west walls?

Especially if those 2 walls were hard plaster.

Thoughts?

Sorry for getting in on this so late, but I thought a clarification is in order. Unless someone beat me to it, let me say that a node is a cancellation created by speaker placement. A mode would be a standing wave...the source of the ringing.

[Jay M]

My first thought is that your pulpit is causing the feedback. Is it hollow sounding like a drum? Is it covered in carpet? Make sure there is no resonance in the pulpit, then cover it in carpet to reduce reflections.

Second thought is that you should use a Countryman E6 instead of a Lav. When we switched to the Countryman it made the single biggest improvement in sound that we have ever seen...eerr... make that heard. The reason is simple, not only more gain before feedback, but also you can EQ for sound quality rather than feedback elimination.

There obviously no substitute for a properly designed system, but in a pinch, a feedback destroyer can work wonders. The Driverack PA is fairly inexpensive and the auto EQ and feedback destroyer in it can make a radical improvement. Just understand that auto EQ is in no way as good as a real human being who knows what he is doing and has proper measurement tools.

~Jay

[pdc]

There is never a good reason for a feedback destroyer. They eat system gain and make noise. If your system is not right, fix it. It could be a matter of moving your speakers or adding some treatment. You cannot put a bandaid over a tumor.

[buggyboy]

Thanks again for the helpful replies.
My node (thanks pdc) is gone.

the modal ringing is still present, and will remain an issue until the room is treated, which I'm working towards now.

I'm hoping to audition an earset mic in the near future. although I doubt the pastor will much like the idea. I'd love to be able to eq for sound quality rather than gain!

I'm convinced that treating the room is the right way to go I'm preparing a post at another site hoping to get some practical suggestions for the treatment itself.

I think that in all (<millions spent) rooms ringing will happen at some spl...the issue is making sure that point is past the spl you need to use.