Strange problem in a hall.

Posted: Sat Mar 15, 2008 4:00 pm

Today I did a choir concert in the local community college auditorium. I used an ORTF setup with two RODE NT-2A microphones. I was back about 20 feet from the first row of the choir and up about 3 feet over heads of the singers in the back row. Pretty normal setup for us. The problem is the auditorium, which if you look at its floor plan is a big Mickey Mouse, with the two ears that can be rotated to other venues located behind the main concert hall. The stage is enclosed by a shell and the ceiling for the shell is dropped from the stage house. It is basically a shell with in the stage. Any way our problem with that we were getting bounce off something which made the main sound image seem to swing back and forth between the center and left right. I can fix it but this was strange. Our hosts and the people on the stage crew were great and they made my job very easy and it is one venue I like to do because of the stage people and their can do attitude.

Has anyone else run into this problem?

Posted: Sat Mar 15, 2008 5:14 pm

Tom

From your description it sounds as if there were curved walls in this space. Is that correct. If so focused reflections will do all manner of crazy things with localisation.

Posted: Sun Mar 16, 2008 6:47 am

Almost every wall was curved or beveled, you are correct,

Posted: Mon Mar 17, 2008 2:48 am

And so this is why I like MS. Don't like the stereo soundscape? Knock the S out of it. Then add some stereo reverb. Problem solved. Solid imaging. Mono is good when you add the stereo reverb. So collapse your ORTF to Mono. Add some stereo reverb/ambience.

Tricky bitch
Ms. Remy Ann David

Posted: Mon Mar 17, 2008 2:55 am

RemyRAD wrote:

Mono is good when you add the stereo reverb. So collapse your ORTF to Mono. Add some stereo reverb/ambience.

Tricky bitch
Ms. Remy Ann David

A great idea - shame about the hall though - one of the major design issues we promote as acoustic designers is to avoid concave surfaces in a hall - especially a music hall.

Posted: Mon Mar 17, 2008 3:59 pm

I talked to the director and she knew about the acoustical problems before the concert but just figured that we would take care of it (it would have been nice IF she mentioned it) The hall is really weird. This is the first recording that this kind of thing has happened to me.

I don't know who the acoustical consultant for this hall was but I think he was asked to do the impossible and make the hall suitable for all its different uses.

They had acoustical tile on the ceiling when the hall was built but it started falling down and so they took is all down. This may have exacerbated the problem(s) I should have taken RemyRAD's advice and done it M/S. There are some really nasty peaks and troughs though out the audio where the wave fronts amplified or canceled as they got to the microphones.

The recording microphones were in the center of the stage and I think we were in a parabolic focus point from the walls and ceiling and floor. Even a M/S rig might have had problems in this situation.

One nice thing is that they just installed a $100,000 sound system using L-Acoustics speakers and amplifiers and a new Soundcraft board. IMHO I think they should have spent the money on acoustics especially since this hall is used for orchestra concerts and choral concerts as well as classed and the occasional rock or folk concert.

My motto is as always has been "live and learn and try not to make the same mistake twice" so I guess I learned my lesson.

Thanks for all the insights.

Posted: Wed Mar 19, 2008 4:58 pm

Concave curves, particularly parabolas, are rarely good news in performance spaces.

Apart from working regularly at the Royal Albert hall in London, which is a barely tamed acoustic nightmare needing a lot of practice and 'working around', another hall in London, Methodist Central Hall in Westminster, has a series of parabolic arches supporting an amazing parabolic dome reaching across all four sides on top of a more or less square room. The combination of the dome and arches creates multiple reflection paths with pairs of focal points at different places in the room, some closely coinciding with others.

The most awkward manifestation of these pairs of focal points is that anything happening in one is 'transmitted' to the other with very high efficiency. If you're 'lucky' enough to put a mic in one of these focal points, it'll pick up what you expect it to, plus exactly what's happening in it's reciprocal point. Since these points tend to be geometrically opposite each other in the parabola, you get, for example, a beautiful recording of one very close sounding cello from the right hand side popping up on the left of the mix through an otherwise problem free violin outrigger. And perhaps vice versa. Or in my case, a main pair picking up an oboe, through an entire orchestra and choir balance, as if it were inside the mic! It's a similar basic principle to that used by a parabolic reflector on a mic intended to pick out distant sounds with great directivity in wildlife or surveillance recording.

One way to demonstrate the effect is to locate a pair of points, and have a person stand in one point and whisper something so as to be inaudible to someone a foot or two away. A person standing in the reciprocal point will hear what's said as if it's coming from inside his own head. I don't know whether it's still possible but you used to be able to try this in the Capitol building in Washington DC. They discovered the problem many (maybe hundreds of) years ago when the main room was used by politicians for meetings and discussions. It became known that some people were eavesdropping private conversations taking place elsewhere in the room and steps had to be taken to prevent the problem. When I visited it as a tourist some years ago, there were two brass plates on the floor marking a sample pair of focal points. When standing directly over one and stooping to whisper at the floor a person with his head at the same height over the other plate on the opposite side of the room can hear every word as clear as his own thoughts, even thought the room itself is full of people chattering away. It's uncanny.

Anyway, monoing a balance can help by removing stereo imaging problems but if you've got a mic or mics in one of the focal points you'll still get the 'inside head' super close miced effect causing things to leap out of even a mono mix. Since it's common to get multiple focal points, and particularly exciting in rooms with multiple parabolic surfaces, you can get get a kind of acoustic multipath distortion with specific instruments or frequencies appearing to swing in and out of focus and to fly up and down in level. The only solution to this is to move the mic(s) to minimise the effect. On the bright side, these points usually are very tightly focused, in both the vertical and horizontal planes, and you don't normally have to move far to make a big difference. In the case of my rogue oboe, moving the main pair a little under 12" to one side fixed it completely. OK, that gave me a stereo image a little off centre but it wasn't a big problem and much better than Handel's Messiah with permanent non-optional oboe soloist!

Posted: Fri Mar 21, 2008 6:16 am

0VU

Thanks for your thoughtful reply.

Posted: Fri Mar 21, 2008 8:02 am

The "whispering gallery" in the US Capitol building is in the National Statuary Hall.

Posted: Fri Mar 21, 2008 8:41 am

There's a spot like that in Fairmount park in Philadelphia; the civil war memorial. It has two parabolic bench seating areas, (can't recall if it's marble or granite) and you can sit at one end, more than 50-60 away from the other person, and whisper to them. It's really a lot of fun and freaks people out - in a good way. The curved stone in a 90 degree arc makes for a perfect sonic reflector.

It's kind've a forgotten tourist spot nowadays, and due to its location, it's rare to see anyone there anymore. Still, I like to bring out of towners there and give them a little sonic treat.

I just finished a big choral recording in a church with too many round/arch-like ceilings, and had similar troubles to Tom. I mic'ed as much as I could up-close, and did M-S on the main wind/brass ensemble, which was right in their own little zone, but it was still WEIRD.

Also, one of the solo spot mics just sounds odd, and I'm sure it had a lot to do with the curved backdrop and the rest of the hall's acoustic. Actually, I ALWAYS have problems in this one venue; between the odd way the ensembles are always forced to set up and the weird acoustical enviro; I HATE it. I'm actually considering suggesting these groups do NOT record there, but then I'd be out of a gig whenever they go there. No easy choices. Evil or Very Mad

Posted: Fri Mar 21, 2008 7:02 pm

I have not experienced this phenomena in a recording situation, but a local park path in our town has something called 'The Whispering Stone', which is a piece of granite hollowed out so that one can sit sit in a curve and listen to the amplified sounds of the stream flowing about 80 feet away.

If a person speaks softly down by the stream then you can hear them as if they were next to you.

Posted: Sat Mar 22, 2008 6:34 am

BobRogers wrote:

The "whispering gallery" in the US Capitol building is in the National Statuary Hall.

Thanks Bob - that's the one Smile

A beautiful room but I'd hate to record in there.

I was trying to remember when I was there and it was about twenty years ago during a sightseeing trip on a tour with a youth orchestra. Now I feel old. Embarassed

I forgot another classic 'Whispering Gallery' at St Paul's Cathedral in London which has several other ...er... 'interesting' acoustic properties, which make for a fun recording venue, particularly when the favoured position for performers in public concerts is directly under the dome. Rolling Eyes

Posted: Sat Mar 22, 2008 7:33 am

There's one (a whispering Gallery) in my home town of Bardstown, KY, in the ST Joseph's Proto Basilica, but it's from the Organ in the choir loft up and around the ceiling down to the Alter up front, so basically you have to crank the Organ to get a nice sound in the rest of the church, but you're blasting the Priest & co out of the water whenever you're playing. You hit a couple of stops, land on the first chord and watch them jump Smile

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