Sound transfer through duct wall?

[TSN79]

The attached image shows the top of a residential air handling unit, and how a flexible silencer (right) and a spiral duct (left) is attached. The duct is steel, and the silencer aluminium. The owner proposes that fan noise will transfer to the room through the centimeter wide piece of exposed aluminum at the connection of the silencer - supposedly twice as much as it is aluminium, and not steel. I for one have a hard time believing that this can notably add anything to the room's total sound level. The thickness of both is 0,5 mm. What do you think?

 

[Randy Beikmann]

Sound radiation is proportional to surface area exposed, and that is quite a small surface. Plus, it looks like the exposed part is not thin sheet, but a heavier flange. If so, it would vibrate, and radiate, very little.

It looks like there is a whole lot more exposed, vibrating material on the top of the HVAC unit that would produce more sound than the exposed part of the duct.

 

[TSN79]

Sound radiation is proportional to surface area exposed...

Thanks. Could you explain to a layman in some way what this means exactly?

 

[Randy Beikmann]

You could call sound radiation the total amount of sound "given off" by the surface. To produce a certain amount of sound, you could have a large surface vibrating a little (in millimeters), or a small surface vibrating a lot. (It's not quite that simple - but a decent start).

Since you have a small exposed duct surface, it would need to vibrate a lot to produce significant sound, compared to the much larger surface area of the HVAC unit.

IF there's any doubt, you could easily put some sound barrier material around the exposed part, and see if it makes any difference in sound.

 

[TSN79]

You could call sound radiation the total amount of sound "given off" by the surface. To produce a certain amount of sound, you could have a large surface vibrating a little (in millimeters), or a small surface vibrating a lot. (It's not quite that simple - but a decent start).

Since you have a small exposed duct surface, it would need to vibrate a lot to produce significant sound, compared to the much larger surface area of the HVAC unit.

So you're saying that in order for sound to pass through a material, the material needs to vibrate to a certain extent? In this case it's the fan-noise inside the unit the owner proposes radiates through the duct. The unit itself is somewhat insulated on the inside to prevent sound radiating from the unit. You say radiation is proportional to surface area - if the area was to double, is it possible to say what that would do in terms of change in dBs?

 

[Randy Beikmann]

As a rough estimate, doubling that small area would change its sound radiation by 3 to 6 dB. That's a small increase from (my guess) a very small number. The amount of sound from the rest of the unit would (my guess) be much larger, and would dominate the dB level. In other words, you are doubling a number that is 1% of the total, perhaps.

Incidentally, adding two dB levels (from two surfaces) is tricky if you're not familiar with it. dB levels don't add directly. dB levels are log functions, and you need to take anti-logs of each surface's level, add them, and then take the log. Doubling a small source that is combined with a much larger source won't increase the total by 1 dB. Again, I'm making assumptions here.

So you're saying that in order for sound to pass through a material, the material needs to vibrate to a certain extent?

Yes, in fact if a panel doesn't vibrate, it doesn't make sound at all. Compare the sound of an acoustic guitar to an electric guitar that isn't plugged in. In the acoustic guitar, the vibration of the strings feeds into the body of the guitar, and its light, thin surface vibrates. It acts as a sounding board, or speaker.

In contrast, the body of the electric guitar is relatively dead, because it is stiff and heavy. The string vibration produces little vibration of its surface, so very little sound.

Another example is when you're in a hotel next door to a loud neighbor. When they make noise, it vibrates the wall between you (like a microphone does when you talk). Then the wall vibrates on your side (acting like a speaker). You don't directly hear the noise the neighbor makes. You hear the result of the vibration their noise caused. Hope that helps!

 

[TSN79]

As a rough estimate, doubling that small area would change its sound radiation by 3 to 6 dB. That's a small increase from (my guess) a very small number.

The HVAC unit is rated at 65 dB(A), which I guess is at some set distance and at a certain fan speed. How can one best estimate how this small exposed area will affect this number?

 

[Randy Beikmann]

There are too many unknowns to estimate that without measurement data. If you have the equipment, I would measure the sound without, and with, a sound absorber and barrier wrapped around the currently exposed surface. If you look under the carpet of today's cars, you'll see a soft absorber next to the floor pan (maybe foam rubber), covered by a dense rubber barrier. The barrier, as the name implies, is to block most of the sound from the floor pan. The absorber absorbs much of the sound that echos back and forth between the floor pan and the absorber. It also keeps the barrier from touching the vibrating floor pan, and turning into a speaker itself.

Even without instrumentation, you should be able to just listen to the unit with and without such blanket wrapped around the duct. My bet is you won't hear any difference. If you do, why not leave the wrap on? You just need to make sure there are no temperature or environmental issues with it.

 

[TSN79]

My bet is you won't hear any difference.

That's pretty much my prediction as well. Also there is the spiral duct at the other connection that in my mind contributes much more sound-wise that this small area of exposed aluminium. Or?

Really appreciate your help by the way :)

 

[Randy Beikmann]

No problem. Good luck!