In 1984 and 1985 our crew spent a year rebuilding and requipping Studio C at Hyde Street Studios in San Francisco. In addition to acoustically and cosmetically reworking the studio room itself, we installed a 40-input API console and a set of speakers that were an imitation of the large monitors in a famous Hollywood Studio. Near the end of the installation, we engaged the services of Terry Delsing, at that time chief tech at The Plant in Sausalito, to help up set up the speakers and address some flutter echoes in the control room.
Here’s what we did:
First, feeding white noise through the speakers and utilizing a spectrum analyzer, we noted the frequencies that exhibited notable peaks. We took an 8-foot long and approximately 20-inch wide strip of Celotex (“sound board”), a commonly available acoustic absorber found at any Home Depot. We hung that strip by fishing line or nylon rope from a mic stand. We put a 15-inch speaker immediately next to the surface of the strip of Celotex and fed that speaker with an oscillator set to the problem frequency. We attached a Frap guitar to the Celotex, and fed the output of the pickup into our spectrum analyzer.
By removing one or two-inch wide strips from the bottom of the Celotex, one at a time, we were able to ascertain when the hanging Celotex resonated at the problem frequency. We then made three more matching Celotex pieces, covered them with some fabric, and hung them at the back of the control room, leaving them free to move. I’m not an acoustician, but I assume that by modifying the hanging Celotex to resonate at the appropriate frequency, the movement of the hanging panels transformed the problem frequencies into kinetic energy, thereby improving the linearity of the control room frequency response.