What do you do when your Blower Disappears??
by Lloyd B. Smith and Chet Rataski

When The Piedmont Theatre Organ Society (PTOS) removed the Wurlitzer Bal1A (Opus 2170) from the Elon College Gymnasium in 1985, it was put into storage at Burlington,NC. In 1989 when we started moving the organ to Bristol, TN-VA to install it in it's present location at the Paramount Center for the Arts (ex Paramount Theatre), we found no sign of the 7-1/2 HP blower. It had disappeared!! A blower had to be found fast. Through our contacts in ATOS, we had heard that Larry Donaldson of Birmingham, AL, Alabama Theatre fame had obtained several industrial stack blowers. These monsters had been used to provide a draft for the tall brick smoke stacks often seen in bygone days sprouting into the skies of some of our larger industrial cities (before the EPA). The one we got from Larry is a three phase 20HP, 3200CFM, 3600 RPM single stage blower manufactured by North American Mfg. Co. The static pressure is rated at 28" of water but ours is putting out 32". We now had the blower we needed but then the question was - could we reduce the pressure to a useable 17" and make it quiet enough to use in the theatre.

First we tackled the pressure problem. Most of the Paramount Wurlitzer is voiced on 10" except the Tuba and Tibia which are voiced on 15". Most blowers designed for 15" will deliver around 17" static so we had to reduce our blower pressure by 15". We considered several options to reduce the pressure but the least expensive and simplest solution to implement was to provide a static regulator at the blower. The regulator is approximately 2' X 4' and uses a curtain valve. Pressure is maintained by 8 - 40 pound Organ Industries regulator springs. The input is fed directly from the blower at 32" and the output feeds the two chambers at 17" of wind through 15" ducts. The head of the static regulator is fairly stable and even under the most strenuous demands, moves less than 2 inches from its static position.

The second task was not so easily completed. Fortunately, one of our PTOS members, Lloyd Smith of Bristol, TN, is a mechanical engineer and is very well acquainted with blowers. He kindly agreed to help us solve our noise problem. The first thing that Lloyd did was to disassemble the blower and check it over. If he had not done that, we probably would have lost the blower within the first weeks of use. He found that most of the rivets that held the vanes to the rotor were loose. The blower was sent to a repair shop in Chicago for a complete rebuild. When we got it back, it was like new. Here is Lloyd's explanation of what he did to quiet the blower.

In early 1991, the newly installed Bristol Paramount 3/11 Wurlitzer was receiving finishing touches in preparation for its inaugural concert. During this work, the installation crew became aware that the sound pressure level from the organ blower was several dB higher than the ambient noise in the seating area. It was decided that this objectionably high noise level must be brought down to a level no greater than that of the theatre heating and air conditioning system. In addition to the usual type of blower noise, there was also an annoying whine which appeared to be traceable to the blower.

The whining sound was the subject of the initial investigation. The pitch of the sound was determined to be about 1,060 Hz. Further examination proved that this was the pitch caused by the rate of the successive blades in the blower rotor passing by the portion of the fan known in the trade as the "cut-off" point. The cut-off point is the part of the fan scroll which is closest to the rotor. In this industrial service blower, as in most blowers, the cut-off is perpendicular to the blades in the rotor.

In order to cure the whine, a sloping "cut-off plate" was bolted inside the blower. With the sloping cut -off plate in place, each successive blade encounters a sloping surface instead of the siren like cut-off which is more typical in industrial fans. Experience in industry has shown that, in addition to reducing noise, the use of the sloping cut-off plate increases the efficiency of the blower in the order of at least five percent.

After the blower modification (see figure above), the 1,060 Hz whine was gone and measurements showed that the sound pressure level in the auditorium had been reduced by 6 dB. This greatly improved the situation for the theatre audience. But there was still a way to go in achieving the goal of rendering the organ blower inaudible above the heating and air conditioning system. Further measurements determined that at least another 6 dB of noise reduction had to be achieved in order to reach the goal.

Most of the remaining noise emission was found to be from the inlet of the blower. This pointed to the need for an inlet muffler. So a "king size muffler" was designed and built of chip-board. This muffler was designed according to the classic theory applied in most automobile mufflers (see figure to the left). The noise reduction effect is achieved by forcing the air through a labyrinth with a number of turns but with minimal air resistance. Final tests with the muffler in place (see Figure to the right) , indicated that the combined effect of the modified cut-off plate and the muffler, was a 14 dB noise reduction which is approximately 1/8 as loud as it was when he started.

Soon after the work on the organ blower was completed, we were honored to have Lee Erwin play our inaugural concert. The moment of fulfillment came for the crew when Lee Erwin asked that we turn the blower on. He was told that it was already on.

Notes:
dB is a measure of sound pressure. A change of ~6dB = 2 X the pressure, ~12dB = 4 X the pressure , etc.
Hz = Hertz or cycles / second