Bass attenuating performance for ventilation silencers

[Jedanor]

Tlindaas,

This is a little bit of a loaded question. It will depend on the allowable amount of static pressure drop the system can handle. There are silencers on the market (google IAC LFS) capable of providing in excess of 15 dB dynamic insertion loss at low frequencies (63 Hz). The tradeoff is significant increase in pressure drop to your system. Bear in mind if the system operates more inefficiently it may produce more noise than you have now, so this is also a tradeoff.

What are you using the room for where you need low frequency isolation from silencers? That will help assist in providing your solution, assuming you need one.

[tlindaas]

Originally Posted by Jedanor

It will depend on the allowable amount of static pressure drop the system can handle.

Sadly I don't have that information. Have to talk to the developer of my building to get it, and he's on vacation.

Originally Posted by Jedanor

There are silencers on the market (google IAC LFS) capable of providing in excess of 15 dB dynamic insertion loss at low frequencies (63 Hz). The tradeoff is significant increase in pressure drop to your system. Bear in mind if the system operates more inefficiently it may produce more noise than you have now, so this is also a tradeoff.

Are you talking about something like this: http://www.industrialacoustics.com/u...prod%20lit.pdf

It's a silencer that attenuates 6db @31.5hz, 16db @63hz, 32db @125hz and 46db @250hz for an air velocity of -1000 fpm.

But it's '10 long... is this the rule, that to get any substantial low frequency reduction you have to use really long silencers? How do muffle/baffle/attenuation boxes at the intake and outtake compare, spec wise?

Description of a design Rod suggests here: Why do I need roof ventilation?!

Picture of a design he suggests: HVAC Baffle Box Construction Plans

(Picture of similar type of box attached.)

Originally Posted by Jedanor

What are you using the room for where you need low frequency isolation from silencers? That will help assist in providing your solution, assuming you need one.

I need to reduce noise leakage via the ventilation ducts to the neighbours from a tracking/mixing room yet to be built. Sound attenuating solutions both inside and outside the tracking room is an option.

[avare]

Originally Posted by tlindaas

What kind of bass attenuating performance can one expect from the different types of ventilation silencers?

I'm particularly interested in solutions that make sense in thick, heavy MAM partitions.

(Obviously) I have read the enterie thread. Effective low frequency isolation requires space. I recommend getting Rod's book for starters. It has an excellent chapter on HVAC. One of the key methods to obtain good isolation with ventilation is the layout of the ducts and vents. There are numerous sources with information on teh attenuation of various standard duct designs. Downlaod the McQuay HVAC Acoustic Fundamentals Application Guide . It provides detailed information on calculating perfomance. Unfortunately the McQuay site link is not working at the moment. If thise are not sufficient, ASHRAE has detaield calculations for plenum attenuators in its applications manual.

Andre

[Jedanor]

Yes. That was what I was talking about.

The labyrinth style attenuators Rod proposes do work and are well known in the literature. I used to have a data sheet based upon empirically derived data for expected insertion loss for each "bend" in this style attenuator. Most attenuators, including certain silencers, attempt to insert a torturous path in the air stream to reduce sound. Unfortunately, they just are not very effective at low frequencies and you will need to have a clear understanding going in on how much attenuation you actually need. If you do not know the allowable pressure drop you can safely assume 1/4" as acceptable assuming it isn't a prop fan. You will quickly find that low frequency silencers (which are effective acoustically) significantly increase system pressure. All attenuators introduce pressure drop so you must be very careful you do not make your problem worse. A mechanical engineer should be able to provide you the info you need as a starting point.

I wish I had better news for you.

Jed