In Harry F. Olson’s book, “Acoustical Engineering”, originally published in 1947, he discusses acoustical room treatment for the broadcast studios of this time period. It is always nice to look back at what used to be and compare it to what is now. Maybe, we can learn something.
In his section on “Broadcasting Studios”, he says that the acoustic goal “in the early days” was for a very low reverberation time. Obviously, more absorption type material used, the more difficult it is for singers and instruments to come through consistently and predictably all the time. He goes on to say that now (1947) the microphones are used are directional. This use of more directional microphones (1947) ” has eliminated the necessity of extremely dead studios.” As a result of this, ” the quality and artistic effects of the collected sound are materially enhanced.” Once again we are in 1947. I wonder what sound engineers today, would say about this microphone technique.
He then focuses on the current thinking (1947) of acoustic room treatments for broadcast studios. He talks about reflections from room boundary surfaces and “standing wave systems”. These “standing waves systems” produce uneven pressure distributions throughout the room and their value must be reduced down to the lowest level. This can be accomplished by leaving the walls very absorbent, thus producing a very “dead” room. Olson goes on to say that a better balance is a blend of diffusion and absorption. He discusses four different diffusion/absorption blends that they used in his time.
The first room treatment absorption/diffusion product combination was a series of different size rectangular patterns containing some type of absorption material. The absorbing material was to be placed at certain positions on the room walls. These positions were based on a paper written for the Journal Of Acoustics by Potwin and Maxfield. I am guessing that direct opposite walls had opposite treatment arrangements. Anyway, this method involved a series of arranged sound absorbing panels hung on a standard wall and the wall itself that was not covered by the absorbing panels provided the “diffusion” for this scenario and the rectangular panels the absorption.
The second blend of treatment was a series of poly cylindrical devices with a curved surface placed side by side throughout the studio walls. This states the author provides diffusion. Today, we now know that a curved surface such as found in a poly cylindrical unit is really sound redirection and not actual sound diffusion. No matter what the nomenclature, Olson figured out one had to use both diffusion and absorption to achieve some sort of sonic reality in a small room. This is still not known today, both at the professional and the consumer level.
Method three is a series of panels once again, but this time the flat panels are placed next to each other, but at about 30 degree angles from each others corners The surface of each panel is serrated to provide “diffusion”. Once again, more sound redirection instead of real diffusion as we know today. Method number four is identical to number one, but with spherical shaped absorbing panels instead of rectangular shaped ones. I am assuming the same described layout in number one is preferred.
It is great to see that absorption and “diffusion” (sound redirection) room acoustic technologies were alive and well in 1947. They were a solution to “dead rooms” in 1947. This combination of actual sound diffusion and sound absorption is used today with many variations on this standard and time tested theme for room acoustic treatments.
Reverberation times can be balanced throughout the church with proper treatments placed on the correct surface areas.
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