acoustics theory behind an overhead dome

[DonnyThompson]

Hi Rod, Space, et al...

A few weeks ago, I was hired to do a remote for a band - actually, to mic up drums and get tones and levels on location.

This band had made arrangements with a local nightclub to use their room during the day when the club was empty.

Over the dance floor, there is a large domed ceiling. The circumference of the dome was 44'. The height of the ceiling was 12', However, the apex of the dome was 15' . (I didn't measure these heights myself, as no ladder was available, but the club owner happened to be there, I asked if he knew the heights off-hand, and he did.)

The material of the dome was mirrored. I could not determine what the material behind the mirrored surface was. I can't even be exact enough to tell you the actual mirror material, it did not appear to be true glass to me, but more of a polished mylar covering of sorts.

The dance floor was solid, no empty space underneath, material was wood planking, finished. Throughout the club are various materials, such as glass, brass, wood, metal, etc. The room itself is circular, but there are areas with corners and nooks for various VIP areas, bars, etc.

In short, I ended up getting a very nice overhead drum sound, using a pair of 414 EB's in an XY array. The sound was full, highs were silky, and there wasn't any apparent tonal weirdness occurring.

I did not 'mic the dome", as no ladder was available to get me high enough, and truthfully, I'm not sure I would have done so anyway. The client was very happy, especially the drummer.

I was wondering if one of you guys could explain to me the acoustics theory behind this - if the dome was a factor - or not.

Also, your thoughts on might have resulted had I been able to mic the dome?

Just curious...

d.

 

[RemyRAD]

Donny, that's a good one LOL? Just your description. The mirrored dome. How do you MIC the dome? Okay here is how. Since you didn't put a microphone on the dome. You should have? You put a cardioid on a tall stand, directly under the apex/Apogee. Keep it a few feet away. It makes a great parabolic microphone. So you'll be able to hear people FART, directly below the dome, with music and noise in the background.

I think I would mic the dome, like a Leslie cabinet? No! Wait! Hang a ceiling fan and put the mics on the ceiling fan. It would be awesome!

Haven't you done other gigs at churches? This thing sounds like a decommissioned Jewish synagogue? They are frequently round. Though I never really went much.

I have found, while working for the Foundry United Methodist Church (Lincoln's former church) I was in the left rear corner, sound system desk. Domes are fascinating. I could plainly hear, whispered conversation, on the total opposite side of the sanctuary. Sometimes after the sermon, I'd walk over to them and continue the conversation I was listening to, eavesdropping on LOL. It blew peoples minds! It was creepy.

The concept behind the dome was structural. They found that out before they had the word physics. And it helped to Bounce & Balance, the sound around without amplification, back in the day. Ya know before electricity. That's why they use it on football fields. Parabolic microphones that is. It's like a satellite dish. You substitute a microphone for the feed horn. And voilà. A much tighter directional microphone than a shotgun microphone. You can focus them better.

Design concept? It wasn't a design concept. It was a converted synagogue to a nightclub. Have you been hitting the Budweiser again?

My beer gets stale in the refrigerator if I don't remember to drink it.
Mx. Remy Ann David

 

[paulears]

It can also work the other way around when the curve of the dome is visual for aesthetics rather than following a real curve. One venue I use has a sort of curve, but made from tile sized flat panels and the curve tightens and releases to clear the steelwork it's hiding. The really weird thing is that there is one position dead centre where sound almost cancels completely. If you stand there it's like a PA system has suddenly switched off. Your voice seems to come out of your mouth and not make it to your ears. A foot left or right and normality returns, and if the mic is dropped from six foot man head height to 5" 6" head height female it's fine. If you record with a normal pair of mics, anybody in the dodgy area just vanishes. If it's something you are close miking they tend to shout, thinking they can't be heard. Very, very strange.

 

[MadMax]

Physics is Phun!

 

[dvdhawk]

Observations from a non-expert:
I work in a lot of old churches that were built before there was electricity, much less amplification. I've noticed that in old churches that have domed or generally concave chancel areas (generally behind the altar / pulpit), that the curves basically do what they were intended to do - provide a little extra projection to the back of the sanctuary by a parabolic effect. I'm sure the results would all depend on the radius of the curves and the location of the sound source relative to the focal point. There are no free rides though, even a whisper (you didn't want anyone to hear) will be easily audible to a small area out there. And for every sweet spot where it's focused, there are usually lots more little areas of voids. I know several of the churches have relocated the pulpit, and/or choir, outside the curving ceilings and walls because the sound from under the dome was just too weird for the pastor or choir in the chancel area - very pingy and phasey. Not only do the curves project sound out, they collect sound. There are examples of 'whisper galleries' in old-world cathedrals and the Capitol rotunda design.

I hate it when I find a 150 year old church, and they're trying to do a loud-ish electrified contemporary p&w band from an area what was designed for a small choir. The room was built to naturally amplify sound toward the pews, and cramming a band in there is like putting them in a big megaphone. On those visits, you will eventually hear, "the band is great, but they're just too loud. Is there anything we can do about that?"


I've played a couple outdoor stages with big old-fashioned band shells too. On those occasions, you will find me standing as far forward as the stage will allow. And if you have to play from the back of the stage, and manage to find a sweet spot deep in the bowl, you'd better not move. A few inches can be the difference between hearing everything, just one thing, or nothing. Mercifully, the PA stacks are well outside the dome, or you'd be pummeled to death by some frequency or another.

Sounds like you got a good one for your project. I'd guess putting a mic in the dome would have been a total 'hit or miss' situation.

 

[DonnyThompson]

I think I would have put a mic in the dome if I'd been able to, Hawk.

I was curious about what part it played with the other overhead mics in the overall sound, if any.
Had I been able to, I would have probably miked it just out of curiosity... it's not as if I didn't have enough mics or inputs, although I didn't have a ladder so it's pretty much a moot point.

They have a parabolic area at the Great Lakes Science Center in Cleveland. It's amazing. I stood at one end while my wife stood at the other - we were separated by a distance of at least 25', and we could talk in normal conversational tones, and across a loud and crowded lobby filled with visitors and shouting kids, we were able to hear absolutely everything that each other was saying. We were even able to talk in loud whispers and both of us were able to make out most of what we were saying..

This parabolic area was very large though. I don't recall the exact size, but my memory is that the each parabola was at least 15 ft in diameter.