Acoustic Liners for Air Handling Units

I think you should be talking to the customer.

If I'm correct in my assumption, you are still fishing for a way out of your problem of missing internal acoustic liners.

Do you have any kind of spectral analysis of the sound? Where is the 97 dB, 50Hz or 5,000Hz? What type of instrument did you use. How close to the actual origin of the sound was the sensing microphone placed? Orientation?

I think you should either fix the problem or get a waiver from the customer.

Have you considered active noise cancellation?

Your guess is partly correct in the sense that we would want to remove the minimum amount of ducting that is sufficient for sound attenuation.

Talking to the customer will be the last step after the issue has been thrashed out technically.

The 97 dB is the fan noise measure in an anechoic chamber. The figures are given in the Air Handler computer selection. Furthermore, the AHU is double skin with insulation of course.

With this scenario and the given airflow, we would like to know if like lenghts of both supply and return must be insulated or just supply and what might be the minimum lenghts we should worry about disassembling?

Thanks

"The 97 dB is the fan noise measure in an anechoic chamber" does not mean anything in the present application.

You aren't in an anechoic chamber. You have reflected power, and a multitude of other problems, to complex to enter into here.

There are no ducts in an anechoic chamber, so the data is invalid.

As always, it boils down to where the microphone (or sound intensity probe) is located relative the the source of the noise.

Hire an acoustics consultant.

Velocity of air in the ducts also need to be considered. Higher velocity ducts create more sound. Usually up to 5 meters from the plenum acoustic treatment is done.

For achieving NC 30 even the fan selection need to be with low noise and the ducts need to be designed for 1000 feet per minute velocity and acoustically lined for the entire duct. For maintaining NC 30 even the conditioned space also need to be acoustically treated to keep the noise level even when the fan is not operating.

I'm an HVAC contractor and what you really need to know is the duct layout. How far is the AHU from the room, the further the better. How many elbows are there, elbows are your friend. They need to be square elbows with no turning vanes. You may need a sound attenuator at the discharge of the AHU. You also need to keep the velocity low that will help you sound levels, but that will not help with transmitted sound from the fan. The best way to kill that sound is to use lined square elbows and a sound attenuator. Lined straight duct will mostly help with noise coming from the duct being transmitted into the space the duct is running through, it will only moderately help with fan noise in the space. 97dB is pretty loud was that measured at the discharge of the AHU? If it was then I'm guessing that you will need a sound attenuator at the AHU. Find a supplier for Dynasonics or Ruskin sound attenuators, they can help you with finding the right attenuator for you project. Good luck and I would be glad to answer any other questions you might have.

fishpipes,

I don't think they have any on-site sound data. I understand that the 97 dB is just the fan in free space. I still think they need someone with knowledge of acoustics which they do not have at the moment.

I also get the impression that the OP is over their head with this project.

FISHPIPES: Your answer speaks for itself that you are an excellent HVAC Contractor and your offer to entertain more question is equally decent and much appreciated. I will certainly have you in mind if there are any further questions.

I appreciate your response and of all those who shared their knowledge and experience.

Without knowing the whole duct configuration i can not say. Where are you try to achieve an NC of 30? Is it at the diffusers? If it is at the diffusers what size and type are they. In order to minimuze the sound you need to look at the whole system, not just one part. Lining the duct will help but without knowing more I really can't help more you with more then it will help.

One last statement before I go. I understand that you are worried about 97dB.

Remember that, simply stated, sound decays with distance. I think as the distance doubles the sound is reduced by 6dB. Now, you have reflected noise to deal with in a duct so it gets complicated.

What you really need to know is the sound level at the vent, not at the fan.