Introduction
A neighbor adjacent to one of the Country Club’s golf green complained about the audibility of golf course fan noise on his property and within his house. At your request, David Coate Consulting (DCC) measured fan noise at the site on July 19, 2013 to assess both the severity of the issue as well as to identify preliminary noise control methods.
Noise Measurement Summary
The following conclusions can be drawn from the noise measurement data and analysis:
• Fan noise levels are in compliance with Richmond’s Noise Ordinance (Chapter 38, Article II).
• A tone at 125 Hz (the blade passage frequency) is very prominent -8 to 10 dB higher at 125 Hz compared to the adjacent frequency bands (100 Hz and 160 Hz). A tonal condition is defined as when the level at a particular frequency is higher than its adjacent frequency bands by 3 dB, so this tone is very prominent. As a result, the tonal component of this noise is much more annoying than would otherwise be assumed based only on overall A-weighted noise level.
• This tone is audible and substantially above background ambient sound levels on the terrace of the house and in many rooms inside the house.
Audibility of sounds is governed by the extent to which a tone or a portion of the frequency spectrum is above the ambient sound frequency spectrum.
Noise Control Options
Fan modifications
It is likely that the elimination of reduction of the tone at 125 Hz would cause this sound to be much less objectionable which suggests some type of fan modification is in order. In addition, noise control at the source, if possible, is usually the most effective and cost effective approach.
The data collected thus far indicates that it may be possible to modify one of the 50” 5 HP fan units to reduce fan speed while maintaining adequate CFM and throw, and orient the fan itself so that the side of the fan housing faces the house instead of the face or inlet of the fan. Noise measurements of this fan indicate that the side of the fan is 6 dBA lower than the face/inlet sides. Reduction of fan speed usually results in significant noise reduction.
Other fan modifications, if later deemed necessary, could include the installation of fan blades designed for noise control. Curved fan blades with higher solidity than the current fan blades can be much quieter than conventional fan blades. Some research would be required for this option to determine if manufacturers of this type of fan blade would have a suitable size/configuration for this application.
Noise Barrier/Louvered Barrier
A noise barrier could reduce fan noise levels by 5 dBA if the line-of-sight from fan to the house were completely blocked both vertically and horizontally. Higher noise reductions may be possible by increasing the height and length of the barrier. Accurate predictions of barrier noise reduction can be made using software programs such as CADNA.
A louvered noise barrier also may be an option if air circulation is an issue. The depth of the louvers is determined on the basis of both overall noise reduction and airflow requirements.
Visual aesthetics of noise barriers could be an issue in this case.
Other Noise Control Options
Other noise control options exist including sound masking and building sound insulation. Sound masking refers to introducing a new broad-band and pleasing sound, such as a water fountain, at the listener location. If the noise level of this new source was sufficiently high, it would mask the fan noise. For example, a water fountain could be placed at the terrace for this purpose.
Building sound insulation treatments, such as the installation of acoustical replacement windows, can provide up to 10 dBA improvement inside the house. Acoustical replacement windows typically have two layers of glass, separated by a large airspace. The disadvantage of this approach is that only building interior noise levels will be affected and it can be costly especially if in the future additional buildings need such treatment.
RESULTS OF VARIABLE FREQUENCY DRIVE TEST
The previous tests showed that a 125 Hz tone being generated by fan #14 and fan #8 was quite noticeable both outside and inside the home. DCC conducted a test of a Variable Frequency Drive (VFD). A VFD can be used to reduce fan speed, assuming that the larger 50" fan unit would be installed. The VFD allows fan speed reduction while maintaining adequate CFM. Noise level is exponentially related to blade tip speed, so significant noise reduction is possible using a VFD. In addition, the previous tests showed that the 50” fan produces approximately 6 dBA lower noise levels when the fan is oriented sideways with respect to an observer. The following presents noise data for a series of tests conducted on 8/16/2013 designed to test the viability of this mitigation strategy.
Figure 1 shows the results of the most important test which was the 50” fan oriented sideways relative to the house with the measurement conducted at the terrace. Four tests are shown: full fan speed (60 Hz), two lower speeds (55 Hz and 50 Hz), and with all fans off. (The peak at 6300 Hz was associated with insect noise and therefore can be ignored.) It is clear from the data that the 60 Hz tone at full fan speed is much higher than ambient noise levels and therefore would likely be audible at certain times. However, this lower frequency sound –even if audible—is not nearly as objectionable as the 125 Hz tone associated with the smaller fan. The 55 Hz tone at that fan speed is lower (due to fan speed reduction), but is still above the ambient. At 50 Hz, there are some peaks just above ambient levels, but subjectively it was very difficult to hear any fan noise at the terrace at this fan speed.
As a result of these tests, I recommend the lower 50 Hz fan speed while maintaining a sideways orientation of the fan relative to the house. Based on the manufacturer’s spec sheet, the 50” fan produces 47,000 CFM while the 36” fan and 30” fan produce 24,000 CFM and 14,400 CFM respectively. This data suggests that the speed of the 50” fan could be reduced even more and would still produce airflow greater than that of the smaller fans. Having additional margin for further noise reduction—via further fan speed reduction-- may be beneficial. At certain times ambient noise levels could be lower than what was measured, thus making the fan noise more audible at those times.
SUMMARY
• The 50” fan oriented sideways to the house and powered by a VFD appears to be a good “first cut” noise mitigation strategy for this situation.
• The lowest fan speed at 50 Hz would be the recommended setting. At this setting it was difficult to detect fan noise at the terrace.
• Fan speeds lower than 50 Hz appear to be viable based on scaling the CFM of the 50” fan to the CFM of the smaller fan. It may be necessary to explore this option for time periods in which ambient noise levels are lower than what was measured.
Other Considerations
As discussed, the same mitigation strategy employed for Fan #14 could be employed for Fan #8. The exact placement of this fan has not yet been determined, but the sideways orientation to the house should be maintained. Noise levels for Fan #14 and Fan #8 combined will be slightly higher than for Fan #14 alone. Those effects can be acoustically modeled, but it may be more expedient for you to subjectively evaluate this effect by listening to both fans on and then each fan on by itself.
Should additional noise reduction be needed at some point, I would suggest the following potential mitigation strategies be investigated:
• Line the interior of the fan housing with an absorptive material. The effect of this could be evaluated by first using standard fiberglass batting, which would be replaced with a more environmentally robust material after confirming the effectiveness of the approach.
• Extend the length of the cylindrical fan housing. Theoretically this should provide even more attenuation in the sideways orientation. On the other hand, this approach may not provide meaningful additional attenuation at locations far from the fan.
• Replace the current fan blades with higher solidity blades designed for low turbulence. Other fan projects have realized substantial improvements via this approach.
• Install louvered noise barriers.
• Employ a combination of timers and VFD’s on fans.
• Install a water fountain at the terrace location to provide additional sound masking of fan noise.