Noise from HVAC in the church! How do we reduce?

[Dinosaur]

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

 

[okdb]

Hello,

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 &quot;poorer&quot; 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

 

[dgallagher]

okdb makes some good points - here is also a link to purchase an inexpensive sound meter on-line:

http://www.reliabilitydirect.com/soundlevelmeters/EXT-407706.htm

 

[vanstoja]

Before you can solve an excessive noise problem, you need to know what component(s) are the source of the annoyance. Ideally, you need a microphone, a good tape recorder and a noise analyzer to run airborne noise traverses or point measurements along the ducting, in front of the intake louvers and in the vicinity of the fan and its drivemotor. An accelerometer on the latter two components to measure structureborne noise would also be nice. Playing the taped data into the noise analyzer can get you frequency-amplitude spectra (preferably from 0-10KHz) which will reveal any high level discrete tones like fan blade passing, motor shaft 1 per rev and rotor slot passing.
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.

 

[hacksaw]

typically what you do is line the duct with sound absorbant material and install baffles that break up the flow path, without excessive drop) between the louvers and the blower.

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.

 

[dpeters13]

First depending on the type of equipment by 20yrs old most HVAC equipment is on its last leg of life anyway. All the things the people are indicating, silencers, baffles, etc. increase the pressure drop of the system and will thus reduce its performance. If all of the other items are not there such as low velocities through both the grille and ductwork and acoustically lined duct adding a silencer will be a waste of your money.

 

[friartuck]

If you go to IAC Ltd (Industrial Acoustics)and give them a sketch and the fan details, they will select a silencer for you.

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

 

[Rob45]

First, you can get an indication of the noise frequency spectrum with an inexpensive Radio Shack microphone and using the Soundrecorder app found in MS Windows; play the sound file back using CoolEdit 96, a nice bit of freeware, and the "Analyze" function found therein. Compare a file with the HVAC noise to one without to determine the problem frequency(ies).

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.

 

[MikeHalloran]

CoolEdit Pro is now Adobe Audition (USD299).

If you find the freeware version anywhere, download it now.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[lilliput1]

The sound level meter should be capable of measuring to minimum 25 db and also must have switcheable frequencies to allow sound level measurment at each octave(63 hz, 125 hy, 250, 500, 1k, 2k, 4 k & 8k)band. This is to allow design of rooms to NC or RC 25 and also allow proper selection of sound attenuators which have different attenuation property for each frequency band.

 

[Rob45]

As of a few minutes ago, the freeware version of Cool Edit 96 was available from among other sites

http://www.threechords.com

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.