Harry Olson wrote a book entitled “Acoustical Engineering”. It was published in 1991 but with only a new introduction from a 1957 original publication. The book “Acoustical Engineering” was based on an earlier work entitled, “Elements of Acoustical Engineering” copyrighted 1940 and 1947. I like to look at older acoustic books and read what the current thinking was at the time. It is amazing how some things have really changed and some things have not at all. I like one section entitled, “Orchestra and Stage Shell”. It talks about acoustic treatment in outdoor theaters.
The article focus states, “When orchestra and stage productions are conducted in outdoor theaters it is desirable to provide a shell to augment and direct the sound to the audience, to surround the orchestra with reflecting surfaces and to protect the performers and instruments against wind, dew, and other undesirable atmospherics” I like the use of, “undesirable atmospherics”. It is a nice way to say bad.
In this necessary “shell”, we must focus on the acoustical treatment that will line our shell with, in order to maximize the sound quality for all parties concerned. It appears from this section, that most of the shells in those days were concave in shape and design. This concave shape produced what the book calls, “intense and sharp concentrations of reflected sound in both the shell and audience area”. It also goes on to say that “these acoustic effects are particularly undesirable when the sound is picked up by microphones on the stage for sound reinforcement and broadcasting”. This reflected energy produced by this concave shell can not achieve a balance sound for the conductor’s position. Therefore, without the orchestra leader hearing what the audience hears, we have a definite acoustical issue when it comes to the treatment used inside our shell.
Poly-cylindrical shell or a concave shell lined with poly-cylindrical structures will produce the best sound for all parties concerned concludes this section of thought. It will be good at the microphone positions, the orchestra leader, and finally the audience. A poly-cylindrical structure is shaped by an 180 degree arch and then a flat surface for mounting. The arch is not a diffusor. It is a sound re-director. It uses the angle of incident equals angel of refraction physical law. Numerous poly-cylindrical devices installed in a wall, would redirect the energy that strikes them in different directions opposite to their original striking direction, thus creating a sound redirected sound field.
I have never heard a shell lined with poly-cylindrical devices, but I wish I could get that chance. Most concert shells I have heard are lined with quadratic diffusors which are usually positioned in both vertical and horizontal planes thus, providing two dimensions to our sound field which would be directed at the conductor and audience. It would be interesting to compare quadratic diffusion sound with poly-cylindrical sound just to hear the difference. It could be different in many ways.
With two dimensions of sound created by quadratic diffusors positioned both “vertically” and “horizontally” I would know that sound because I have created it on numerous occasions. It is characterized by a smooth and equal frequency spread across the room plane it is focused on. The diffused sound field would be equal parts “air” and equal parts sound. If it were a solid, it would look like a very loosely woven tapestry spread across a room boundary surface. I hope sound redirection through a poly-cylindrical lined shell retains this feature but adds something of its own. I do not know what that would be, but I would sure like to hear it.