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Help line array system at a school


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18 hours ago, Mallette said:

but don't get why you think a 180 degree pickup isn't virtually ideal

This isn't my opinion - physics tells us that it's not ideal for GBF.

 

You are very keen to point out that you've had success in the past - I won't argue with that perception at all (it would be insane actually). You're an honest dude and I fully believe your experience. You had enough GBF in your application.

 

However....

 

You had enough GBF despite your approach - not because of it. The people in your example were were simply projecting enough to not require much gain from the system. The thing about GBF is either you have enough or you don't. If you have enough, then it doesn't matter if one approach gives you more - once you don't have feedback, then it's not like you can have less than no feedback. This doesn't mean your approach is the best approach - it just means it worked for your application.

 

And that point is important because your approach doesn't work for the other 90%. If you were giving advice, would you recommend something that worked 10% of the time, or 90% of the time? And if you saw someone offering up crap advice, then wouldn't you speak up? 

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5 hours ago, dwilawyer said:

Why couldn't you mike for choir without elevation (flat stage) and use one wireless mike for any solos?

 

He wants an improvement, not perfection, and he has a $200 budget.  There is no way to get any improvement?  

The difference between a choir and a theatrical performance is the number of people speaking (singing) at the same time. The distance from the acoustic source to the microphone is offset by the number of acoustic sources. A single person talking would need about 10dB more gain versus 8 people covered by the same mic. Ever heard a solo from the middle of a choir that wasn't using a separate mic for the solo?

 

The other problem is the amplitude variation as the talent moves around. That effect gets masked a bit when multi-mic'ing a choir. It's much more noticeable with a single voice.

 

Perhaps it's because I'm a professional, but I simply won't "attempt" something that won't confidently meet a minimum quality standard. It's also been my experience that inconsistent sound quality is a lot more frustrating than consistently bad. It's also been my experience that a consistent bad scenario is the best way to drum up the resources to actually fix the problem correctly. When you go in trying to make a less than ideal setup work, then you waste all this time distinguishing between user error and the actual demerits of the approach. But hey, you armchair quarterbacks won't be on the receiving end of the scowls generated by these crazy ideas.

 

It has been my experience that doing something poorly is much worse than doing nothing. When doing nothing sounds bad, then you can say "hey, here's the proper way to solve this problem". If every parent of these kids donated $200 like this one parent wants to, then the camp is going to be set for a very long time with a proper solution. This burden shouldn't be carried by the one dad with enough experience to know that it's a solvable problem.

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1 hour ago, DrWho said:

This isn't my opinion - physics tells us that it's not ideal for GBF.

I do not debate PE's.  Why?

 

"Do not meddle in the affairs of dragons, for thou art crunchy and good with ketchup."

 

But I still don't get why a 180* pickup pattern isn't as good as my own experience with it, as well as the many installations "back in the day" who reported excellent results that sparked my own interest.  The only thing I found close was ribbon, with the rear node angled up.  But it still got more audience noise than PZMs. 

 

I would like your thoughts on this just so I understand your position.

 

Dave

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The polar pattern tells you the relative pickup of the microphone in different directions. Directly in front is normalized to 0dB. As you rotate around the polar pattern, the sound pressure turned into voltage decreases as the polar bubble moves towards the center. The steps in these charts are generally 5dB.

 

When you place a microphone in a system, the GBF benefit of the microphone is the on-axis response subtracted from the off-axis response.

The on-axis response is defined by the angle of the mic relative to the desired source (the person talking).

The off-axis response is defined by the angle of the mic relative to the undesired source (the speakers or reflection points in the room).

 

If you want to be hardcore, then you need to sum the energy from all of the undesired sources. We don't need to run real numbers though - just point the rejection lobe of the microphone towards the source of feedback.

 

Here are pictures illustrating the concept.

 

polar-patterns.jpg

 

 

The PZM on-axis and off-axis response is the same - so the GBF benefit is 0. With a cardoid, you could be as high as 20dB, but in practice it's usually more like 12dB (because there is usually more than one feedback path).

 

Using plexiglas, there is a lot of complex frequency dependent behavior happening - even if you position the PZM so that the 180 degree pickup is pointed away from the speaker. It's because the plexiglas isn't infinitely large. The plexiglas also lets sound travel through it, as well as vibrates at certain frequencies (it becomes it's own reradiator). At the end of the day, the cardoid is already providing a 180 degree pickup in the forward lobe, and provides better attenuation in the rearward lobe.

 

Now if you wanted to use some 4" thick plexi in a 4x4 ft sheet, then maybe you'd get enough blocking - but why stop there? Why not put everyone inside a fishbowl?

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6 hours ago, DrWho said:

At the end of the day, the cardoid is already providing a 180 degree pickup in the forward lobe, and provides better attenuation in the rearward lobe.

Dang.  Wish I had time to go back an be an engineer so I could respond to this as a peer.  Your images are the basic patterns, but no PZM pattern.  The are not cardioid, at least according the Crown.  But I'll leave it at this.  I hate cardioids and they sound awful. All of them.  My ears, for whatever reason, like only natural patterns.  I'd like to know the source of needing 4" of plex to achieve with PZM's.  Crown's materials claim the pattern derives from the microphone itself and the square meter mounting is relevant only to achieving full response when the microphone is on a flat surface.  You appear to suggest this is bullshit and the microphone is actually Omni...or something.  Looking for clarification and whether my ears are lying or not. 

 

Dave 

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The PZM pattern is the same as an omni-directional with the bottom half cut off - just a single hemisphere. In fact, the construction of a PZM is really just an omni-directional capsule placed as close to the boundary as possible. This means comb-filtering above the 1/4 wavelength distance - and that's where the special sauce happens. Your rejection pattern (the side opposite the hemisphere) is limited by the attenuation properties of your boundary. A thin piece of plexi doesn't offer consistent attenuation and gets less as you go lower in frequency. And diffraction is the reason the panel needs to be large - again, something that gets worse as you go lower in frequency.

 

Guess what - polar control of the speaker gets worse as you go lower in frequency too.

 

And cardoid mics don't sound awful. When are you going to start recommending magic microphone cables and special power cords?

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Btw, here are some STC for Plexiglas:

http://www.eplastics.com/Plastic/plastics_library/Plexiglass-Noise-Reduction

 

But note the caveat below the charts:

Quote

* Noise reduction obtained in enclosures depends on the completeness of the enclosure, tightness of joints, etc. The above as dB(A) noise reductions were obtained in a completely enclosed, tightly joined structure. These conditions are seldom achieved in the real world; however, even under more realistic conditions, the use of Plexiglas sheet barriers can reduce noise levels enough to protect against heavy damage. The main purpose of this table is to indicate the relative noise reduction capabilities of commonly used materials in terms of dB(A).

For free-standing plexi, you can subtract a good 10 to 20dB from these charts.

 

4" may have been an exaggeration, but the point is you can't get it thick enough to pull off numbers equivalent to a good cardoid without other implications.

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