I can't answer your questions, George, but I'll be very interested to see what people come up with. I'm a guide at a somewhat older building: a neolithic chamber tomb in Orkney, Scotland. It also has an acoustic "hot spot", but I've never known enough about acoustics to explain why.

Generally speaking, this happens because sound bounces around, and the location of specific reflectors (walls, ceilings) means the various reflections will either reinforce each other or cancel each other out, or interfere in some other way. It's analogous to the shape and placement of a lens affecting the focus of a picture taken with a camera.

More specifically:
Can you provide a floor diagram?

Thank you for your responses

I'll can answer some of your question. The church is in a cruciform shape with a 54 foot high dome over the transept or crossing point. The single center aisle nave has a barrel vault. The present floor is all stone. All the other surfaces are plaster over stone. Originally the nave floor had wood planks and there may have been tile between the altar rail and the sanctuary (altar area).

The church is oriented on an east-west axis with the altar at the east end and the doors at the west end. The hot spot is in, I think a curious location. It is on the center line of the nave and about two feet inside the altar rail. It is almost directly under the western rim of the dome. It is nowhere near the altar and not under the center of the dome.

We know from a 1772 inventory that the pulpit was located at the southwest corner of the transept reached by a short flight of stairs and had a sounding board.

Blueprints of the church are available as part of the online HABS (Historic American Building Survey)

Engineering is not my background. I have an undergraduate degree in German and an MBA. Algebra was as far as I got. I was hoping that someone knew of an acoustical analysis of a similar structure and I could pick up a few general principles.

Yes, the acoustics of a space can be very complicated. Pictures would help. However, one common way to collect or project a sound or signal is with a parabolic concave surface. When sound comes in to the dish it is relfected to the focal point F'. Similarly, one standing at the focus F and singing toward the dish would have the sound reflected outward all in one direction. In essence, rather than the sound waves spreading out in all directions, they are directed toward one desired direction. I would look for a curved, concave wall near the "sweet spot".

Two parabolic dishes facing each other are a popular science museum demonstration.

Yes it is Concepcion.

More for me to think about. Will try the east rim next Monday. any books on the subject for a non-engineer?

Last time I went, we didn't have to pay!

George,

I would be surprised if the effect were accidental, but changing the tile from wood to stone may have moved the "sweet spot". Both wood and stone can be hard surfaces, but they are of different hardness.

(If I'm insulting the intelligence of our regular crowd at this point, the regular crowd may stop reading).

Church buildings, when cruciform, have a long axis and a short axis. Organs, and choirs for that matter, were often placed in whatever gallery one could create - precisely so that the sound would travel up and down this long axis. Choirs would be above the congregation not because they were held to be superior but because they could more effectively fill the building if the sound they created were, literally, above the heads of the congregation.

The Spanish knew how to build church structures for maximum accoustical effect, but had to adapt their techniques to the materials available in the New World. I wouldn't be at all surprised if they built this mission with unaccompanied voices as the main instrument. (In our own day, the history of Spanish organ builders is less well known than that of, say the French or the English, or even the Germans, but I'm sure that, with a little digging, you might find out what instrument(s) if any were used at the mission.


As to the proportions of nave and dome, the question isn't so much one of merely size, but also of "orientation", for lack of a better word. Think of a billiard table. A cue ball can strike another ball, which then goes in a predictable direction. (If the direction isn't what the player wanted, that's because he didn't hit it correctly to achieve the desired result.) Sound travels this way in a large building, at least when carpet and such things doesn't get in the way. Now imagine a billiard table with one end rounded. That round end will have the effect of focusing the sound. Miniature golf is an apropos example here. How do we get the ball to go exactly where we want? Mostly we just hit it and hope for the best, but if we had better control and more patience, we could hit our shots to make them go exactly where we wanted them to, by using the curvature of the surface to our advantage. Once upon a time we banked corners on roads that anticipated high speeds. (Not so much anymore, but I digress). It's the same principle at work.

I hope that's some help for a non-engineer.

Oh, and one anecdote. Some years ago my choir director asked me to sing the Litany at the Easter Vigil. I had almost no voice, since it was the height of allergy season in the Ohio River valley, but I thought I had to try. Rather than face the congregation from some point near a microphone, I planted myself on a prie-dieu and sang directly in the direction of the tabernacle, which was behind the high altar. I tried to sing
by looking up to heaven. No one had trouble hearing me. The sound bounced off the hard surface and sufficiently filled the building, though I had little voice.

In some similar form, that's what your sweet-spot is.