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Natural Science Forum / Physics / Acoustics / July 2007Fan Noise - roof instalation
Hello group, I am up against a factory ventilation noise problem. Vent outlets on the roof are very close to a residential building. Noise measurements completed by the inspector revealed a 200 Hz tonal component. Could this be originating from the fan or airflow in the plastic duct (the duct is rectangular, outlets on the roof contain no elbows)? Is it better to invest into silencers or to build a barrier between the outlets and the residential building? Thanks Milan > I am up against a factory ventilation noise problem. Vent outlets on > the roof are very close to a residential building. Noise measurements > completed by the inspector revealed a 200 Hz tonal component. Could > this be originating from the fan For 200 Hz, it's almost always the fan blade passage frequency. This is virtually impossible to eliminate without changing the fan, which is nigh on impossible. > or airflow in the plastic duct (the > duct is rectangular, outlets on the roof contain no elbows)? There is a minor effect of the length of the duct being a little selective as to which frequency is better propagated. > Is it > better to invest into silencers or to build a barrier between the > outlets and the residential building? Two strategies come to mind: 1- Reduce fan RPM; using different sheave diameters if belt driven, or a variable speed drive if possible. Reducing the fan speed by 50% will reduce fan noise emission by 20 dB. 2- Build a barrier wall high enough to break the line-of-sight from the fan outlet to the nearest residence window by several feet. Good Luck. Angelo Campanella Ang is probably right that it is the blade passage frequency of the fan. Multiply rpm by number of blades and divide by 60 and see if you get 200. I am not sure why Ang would not recommend silencers. Silencers can be effective at 200 Hz though they are typically more effective at higher frequencies. The ideal silencer is one tuned to silence a band right around 200 Hz as long as the fan speed remains constant, but broad band silencers can provide benefit. >> I am up against a factory ventilation noise problem. Vent outlets on >> the roof are very close to a residential building. Noise measurements [quoted text clipped - 27 lines] > > Angelo Campanella Thank you both for the replies. The inspectors measurements were a little misleading, claiming that noise is due to the airflow stream from the outlet. I will go out on the site next week and measure for myself and see what the conditions for installing the barrier are. Should the barrier be tuned any specific way to react to the 200Hz tone? This tone adds 6dB penalty, being mostly responsible for the noise nuissance. Is aiming the outlet with an elbow away from the noise sensitive property likely to help? At the moment, the outlet is aimed vertically, with a trapezoid hood to protect it from weather. Thank you again for a prompt reply. MIlan Sorry I missed the attenuator. It's a possibilty, to be weighed ainst oher options. > The inspectors measurements were a > little misleading, claiming that noise is due to the airflow stream > from the outlet. He's right to the extent that the airstream is also the location of te source. > I will go out on the site next week and measure for > myself and see what the conditions for installing the barrier are. > Should the barrier be tuned any specific way to react to the 200Hz > tone? There's no way I know of to tune the barrier to any single frequency. Barriers act like a low-pass filter. Stopping 200 Hz is one of it's lesser capabilities, but it can help. > This tone adds 6dB penalty, being mostly responsible for the > noise nuissance. Aha! a tone; shurely the blade passage frequency. > Is aiming the outlet with an elbow away from the noise sensitive > property likely to help? That will make one or two dB difference; but not tnough to rid the nuisance. If you can provide me the following data, I can tell you how high (above the outlet height) it should be: 200 Hz level at the house 200 Hz level desired to be a the hourse (hence the 200 Hz dB attenuation you want) disance from house to barrier. Distance from barrier to exhaust outlet. The height above existing terrain of the existing line-of sight. The character of the ground reflection (1.0=reflect; 0.0= absorb). (May I presume the reflection to be midway from receiver to barrier?) Then I can tell you the height above the direct linbe-of-sight (you determine the appropriate elevations all around) that the top of the barrier must achieve. NB: The closer to the fan that the barrier is located, the lower it may be. Angelo Campanella > Sorry I missed the attenuator. It's a possibilty, to be weighed > ainst oher options. [quoted text clipped - 54 lines] > > Angelo Campanella An acoustically lined duct silencer incorporating a Hemlholtz resonator chamber tuned to absorb frequencys encompassing the 200Hz band will certainly help. Slowing the fan down will indeed lessen the intensity of the tone, but will also move the tone lower in the spectrum. |
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