Noise from HVAC in the church! How do we reduce?
Noise from HVAC in the church! How do we reduce?
(OP)
Hey guys,
Thanks for the help with my last problem, Designing a Chime for my church. Now I am preparing to address another problem. The return for the HVAC system in the nave <sp?> of my church is at the rear of the room. The two openings are something like two feet by four feet with louvered grates over them. The duct work runs about ten to fifteen feet, almost straight back, to the HVAC machinery. As you can tell, it makes a lot of noise when it comes on and is the greatest distraction during the service. I don't know if almost all of the noise is due to the fans or if it is also turbulance from the air passing through the louvers. Would anyone like to take a stab at some advice on this problem for me? The machinery is probably close to 20 years old. Do you think purchasing new fans would substantially change the amount of sound? Do you think another louvered cover design would help out significantly? Does anyone know where I would go to get a db meter? Thanks. Ed the Dinosaur
Thanks for the help with my last problem, Designing a Chime for my church. Now I am preparing to address another problem. The return for the HVAC system in the nave <sp?> of my church is at the rear of the room. The two openings are something like two feet by four feet with louvered grates over them. The duct work runs about ten to fifteen feet, almost straight back, to the HVAC machinery. As you can tell, it makes a lot of noise when it comes on and is the greatest distraction during the service. I don't know if almost all of the noise is due to the fans or if it is also turbulance from the air passing through the louvers. Would anyone like to take a stab at some advice on this problem for me? The machinery is probably close to 20 years old. Do you think purchasing new fans would substantially change the amount of sound? Do you think another louvered cover design would help out significantly? Does anyone know where I would go to get a db meter? Thanks. Ed the Dinosaur





RE: Noise from HVAC in the church! How do we reduce?
There's many things that can go wrong. Here's some:
1st, look wether there's a silencer installed on the duct line. If not, get one!
2nd, the air speed is too high. A high velocity air flow would generate a high frequence noise in the louvers.
3nd the fan isn't operating correctly and creates air pulses. That would generate a low frequence noise that appears and disappears in cyclus.
4th Could be some vibration transmitted from the fan to the wall wich in turn would radiate noise. check if the fan/duct are properly vibration isolated.
If all this is OK: right fan operating point, right air velocity, and you've installed the best silencer possible,no vibration transmission problem, then you could think about installing an axial fan. This type of fan has a noise spectrum wich is "poorer" in low frequencies than radial fans. That makes it easier to reduce noise from the fan.
At last, Radio Shack sells sound level meter that should be good enough for that purpose.
OkdB
RE: Noise from HVAC in the church! How do we reduce?
http://www.reliabilitydirect.com/soundlevelmeters/EXT-4...
RE: Noise from HVAC in the church! How do we reduce?
If the spectra are dominated by wideband noise, then the liklihood is that duct wall vibrations are the culprit being excited either by turbulent boundary layer flow through the ducts or by wake shedding excitation from the
airfoil or plate louvers. Match any discrete peak frequencies to fanmotor running speed times numbers of periodic features like fan blades, rotor slots, etc.
Presuming you won't find this measuring/analyzing equipment in your church's basement workshop, then try some practical things with onhand devices such as 2x4's to prop up the ducting at various places, wire mesh screens instead of louvers, etc. using your ears as the quality control
device. If you find it's the ducting that's the real problem, then buy lots of duct tape and a few acoustic tiles.
RE: Noise from HVAC in the church! How do we reduce?
Sometimes you can reduce the blower speed if the motor is the multi-speed type, and have the wheel (blower cage) balanced.
The other choice is to set the blower for continuous operation during the service.
RE: Noise from HVAC in the church! How do we reduce?
RE: Noise from HVAC in the church! How do we reduce?
You will need to design for NC20-25. Another option is to fit a speed controller and ventilate at full speed prior to occupancy and reduce to low speed when in use. 1/2 speed will give you half volume flow and around a 15db reduction in sound level.
If the fan is out of balance, you will know it by feeling it. Either get a new one or have the one you've got balanced in position. Speak to the fan manufacturer and if they a good one, they will know who can offer that service. I've done it loads of times.
If re-newing, get a decent fan. Centrifs are quieter but take up more space than an axial.
You could also try lining the duct with foam. That helps a lot.
Friar Tuck of Sherwood
RE: Noise from HVAC in the church! How do we reduce?
Then, if you find you've got a high-frequency airflow noise or fan blade-pass noise, you can try using ductlining materials such as those made by Owens-Corning for this purpose without excessive pressure drop.
Also make certain the fan and fan plenum are isolated from the frame of the building, preferably with sufficiently compliant rubber mounts.
RE: Noise from HVAC in the church! How do we reduce?
If you find the freeware version anywhere, download it now.
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
RE: Noise from HVAC in the church! How do we reduce?
RE: Noise from HVAC in the church! How do we reduce?
Google finds things like this better than say Yahoo.
The FFT analysis capability of Cool Edit will be much more useful in determining the source of your noise than something like the octave band filter on a sound level meter.