OHSC's WetSac fans emitted excessive noise.
Complaints about noise from the co-generation facility at the Ottawa Health Sciences Centre started when the unit went into operation in 1992. Owned and operated by TransAlta Energy Corporation, the non-utility power generating station supplies power and steam to the Ottawa Health Sciences Centre Complex, and is located about 400m from a quiet residential area.

Although active noise cancellation, in the past, has been relatively unsuccessful for mitigating outdoor noise problems, AERCOUSTICS Engineering Limited designed and installed such a system during the summer of 1994. This project overcame significant engineering constraints to complete one of the first known applications of this new technology - A technology which uses active noise to cancel the offending noise electronically by generating a mirror image or out of phase noise signal. The combination of noise and anti-noise results in an energy efficient solution that is less than one-fifth the cost of silencer baffles, is also cost effective.

Aercoustics Engineering Limited was retained by TransAlta Resources in 1993, to mitigate excessive low frequency noise from high speed operation of two wet surface air condensing or WetSAC, fans. These fans are axial, 26 ft. in diameter, have flow rates of 1,000,000 cubic feet per minute; the 15-foot exhaust stack is constructed of a stiff, lightweight quarter-inch fiberglass shroud shaped for aerodynamics.

The low frequency noise was a result of a 23.8 Hz pure tone due to the blade-pass frequency of the Wet SAC exhaust fan(s). The blade-pass frequency is the product of the number of fan blades and the fan rotational speed. The resulting 23.8 Hz pure tone is both evident in the exhaust sound spectra and in the sound radiated by the stiff, lightweight fan shroud that behaves much like a loudspeaker.

Complaints of low frequency noise from the neighboring community at OHSC generally occur between 7 pm to 11 pm under receptor downwind and temperature inversion conditions. The indoor low frequency noise level, in fact, is the same as the outdoor level. The low frequency noise is perceived as a distant rumble or vibration that causes rattling in floors, walls, and windows. Research indicates that people have an increased sensitivity to low frequency sound in the 20 Hz to 75 Hz region. This point is significant since the human ear is very sensitive to slight increases in low frequency energy. Even though it is believed that human hearing is not discriminating in this range as defined by the `A' weighting sound curve, which de-emphasizes low frequency sound thereby giving more weight to mid and high frequency sound, the changes remain evident.

In cooperation with the Ontario Ministry of Environment and Energy, state of the art low frequency noise criteria was proposed for the project. The selected criterion correlates the unweighted or `flat' sound level with the dBA sound level. Sound measurements taken on site verified that the design target of the active noise cancellation system is to attenuate low frequency sound emissions in the 25 Hz 1/3 octave band by at least 6 dB.

Extensive research and cost benefit analysis of several noise control measures were undertaken prior to selecting active noise cancellation which was demonstrated to be the most practical and cost effective solution. Other mitigation methods analyzed included:

• Aerodynamic redesign of upstream fan supports which were deemed to be one of the sound propagating mechanisms causing flow disturbances by promoting flow separation and propagating pressure fluctuations. This approach revealed structural constraints and uncertainty in the performance of potential modifications.
• Silencer baffles were also investigated, but their installation and support would have been a difficult, costly structural operation. The WetSAC fans' backpressure capability was found to be insufficient to withstand the sizeable pressure drop. As for the baffles, they would introduce a drastic reduction in efficiency and premature mechanical failure would be inevitable. And long-term capital costs would be well over $1 million, more than 5 times the total cost of the active noise cancellation system; energy savings inherent in the use of the active noise cancellation system are especially significant.
• Helmholtz resonators - several tuned chambers built around the fan shroud and vented into the fan exhaust flow were also reviewed, but this method was deemed high risk and experimental due to the susceptibility of the resonator performance to flow conditions, and the significant construction effort required.

Aercoustics carried out an active noise control demonstration to determine the feasibility of the active noise control method  in mitigating WetSAC low frequency noise emissions. Low frequency noise in the 24 Hz frequency range was reduced by at least 6 dB with the use of a horn loaded, compressor driven electro-pneumatic transducer and signal controller. Since the demonstration system equipment proved to be costly and maintenance intensive as a permanent installation, a study into achieving the same performance from a practical and cost-effective alternative was conducted.

A permanent active noise control system was engineered in the spring of 1994 and integrated into the overall operation at the OHSC facility that same summer. Several innovative and unique design features in the installation combine fundamental engineering principles of electrical, mechanical, acoustical and audio engineering with signal processing, instrumentation design and manufacturing quality control. These principles resulted in custom designed and fabricated high precision loudspeakers and microphones (electrodynamic transducers). The loudspeakers and micophones were manufactured in Canada, and designed to operate year round in the very humid WetSAC fan area over a temperature range of -40 to +40 degrees C. In particular, the microphone array has special built in provisions to prevent condensation and humidity problems which can severely impact transducer performance.

Kevlar diaphragm loudspeakers are housed in heated enclosures built to withstand the hostile outdoor environment. Each loudspeaker system was designed and specifically tuned to the frequency range of interest to meet a target sound level of 125 dB at 1m at 24 Hz. In total, two sets of four loudspeaker enclosures housing 16 18-inch drivers were placed circumferentially around each respective WetSAC fan to optimize cancellation.  In this configuration the axial acoustic centres of the source and anti-noise source are nearly identical, a condition necessary to maximize noise cancellation.

Reference microphones are mounted on each fan shroud.

The active noise controller was designed to ensure that the offending noise source and cancellation sources would always be out of phase at the 23.8 Hz cancellation frequency. The system requires an absolute frequency reference for reliable operation. The frequency reference is supplied by a tachometer that is mounted at the fan drive shaft.  There are also two reference microphones mounted on each fan shroud which provide the amplitude and phase reference of the fan noise. An error microphone mounted on each pair of loudspeakers monitors the output and phase of the signal from the canceling noise source. The controller processess this information such that the error signal is always out of phase with the reference signal. The loudspeaker output is modulated with up to 9000 Watts of available amplifier power to optimize cancellation of the fans' low frequency noise. Long term viability issues have also addressed - the unit has been designed to shut down automatically if it malfunctions.  In the event of problems with hardware, the system transducers all operate in pairs to ensure that a backup unit exists; microphones and loudspeakers are inspected daily and the system's performance is monitored live from the plant control room to ensure optimal cancellation.

Sound measurements performed around the perimeter of the plant property indicate an 11 dB reduction at 23.8 Hz (about twice the reduction possible through traditional methods) and a global cancellation pattern, since there are no regions where the anti-source is adding to the offensive noise. Fan low frequency noise is about four times louder without the active noise cancellation system. This acoustic performance not only surpasses the six dB target but also confirms the expectations of high performance as a result of aligning the acoustic centres of the anti-noise and offending noise sources. The Aercoustics active noise cancellation system has been operating reliably since August 1994, and with the facility four times quieter than it was, there have been no noise complaints since its installation.

Ideally suited for the control of low frequency noise sources such as fans, transformers, wind turbines, arc furnaces, baghouses, combustion exhaust systems, and for reduction or elimination of excess vibration, active noise cancellation has many potential noise and vibration control applications.

Aercoustics Engineering Limited has demonstrated the benefits of active noise cancellation by effectively mitigating environmental low frequency noise emissions. The significance of this unique solution is supported by the long term cost savings and the superior acoustic performance of active noise cancellation over conventional noise control methods.