Influence of holes in a wall (theoretical)

[ThomasT]

Hi,

just a theoretical academic question.

Assuming you have a cubic room (room modes doen't matter here, its just for easy calculation) of sidelength a = 3m.
All walls have ideal 50dB sound isolation. So the whole room should have 50dB sound isolation.

And now I drill a hole in one wall. How does this effect the sound isolation?
E.g. a hole with 1cm diameter (or 1cm²) result in sound isolation loss of 20dB, 30dB, 5dB, 49dB?
One with 1mm diameter, or 1mm²?

At first I thought I can ratio the surfaces and add the weightes intensities (50dB = 0.00001).
But this results in unrealistic low sound isolation loss. And I assume that wave effects matters here, too.

Are there any calculations or some rule of thumb?

[Soundman2020]

Good question. I don't have a theoretical answer for you (I'm looking for one), but I do know from first hand experience that it is a LOT that you lose, even with a small hole. I witnessed exactly this situation recently in a studio where the cable conduit (20mm) between LR and CR was still open, not yet sealed, and the isolation was terrible. After the installation was complete (all cables in place and the conduit sealed), the isolation was really good.

Putting numbers to that is hard, since I didn't actually measure it, but subjectively I'd say that it was about a 20 dB loss due to the conduit. However, that conduit wasn't really just a hole, but rather a long tube, which probably affected things in some way, and I suspect the loss in isolation would have been even greater for just a simple 20 mm hole.

Put it this way: that small hole made the LR totally unusable.

I seem to recall a post somewhere here, where someone posted actual figures, but I don't recall who / when / where. You could try searching for it!

Suffice it to say that even a very small hole will seriously compromise your isolation.

AFAIK there is no simple mathematical relationship between hole size, surface area, and isolation loss. My guess is that it is also frequency dependent, to a certain extent, but I do know that the hole does not have to be large at all in order to have a drastic effect on isolation.


- Stuart -

[jbassino]

There are fancy acoustic softwares (and expensive) like Insul that can predict such questions for you

[xSpace]

...-drilled a 2" hole in one side of the wall, and another hole across from this hole and up about 3 feet (so the sound would have to pass through the fiberglass insulation to reach the other side)... STC goes down dramatically, but look at the graphs: low frequency performance, even as high as 200hz is hardly affected...

[ThomasT]

If I understand this correctly, than there is no influcence on sound isolation of low frequencies.

Looking at them as waves with large wavelengths it makes sense that holes are nealy invisible as well as small objects. But from a practical point of view it seems not correct. And from the point of air pressure, too. But the hole has a kind of resitance.

Confusing.

[Soundman2020]

If I understand this correctly, than there is no influcence on sound isolation of low frequencies.

I saw that, and I'm having a hard time believing it is true. If a 2" hole has no effect because it is "too small for the bass waves to get out", then how the hell do bass ports work on speakers???? Low frequency waves seem to have an easy time getting out of those 2" holes.

But from a practical point of view it seems not correct. And from the point of air pressure, too.

I agree totally!

The fact that the STC rating didn't change speaks volumes, but NOT because it means that cutting a 2" hole in your wall has no effect! Rather, it speaks volumes in the sense that it highlights just how screwed up the STC method is for rating walls! Obviously from the graphs, there is a HUGE loss in isolation. The fact that the STC rating hardly changed at all simply underlines what a lousy system it is for measuring isolation.

In any event, getting back to small holes in large boxes: Sound waves move as pressure fronts, so they should have no problem getting through small holes: Change the pressure on one end of a pipe, and the pressure will change on the other end, sooner or later. I reckon the issue is that the hole acts as some kind of resonator, which probably attenuates certain frequencies but not others. That would not surprise me. Plus, there are actually two such "resonators" in series in the experiment described there (2 holes, staggered a couple of feet apart) so who knows what effect that might have in the outcome. For example, if the two holes happened to be roughly a quarter wavelength apart for certain low frequencies (very likely!), then those frequencies would probably end up NOT making it through, since the second hole could be in a null with respect to the first. After all, the MSM wall itself is ALREADY a tuned system, so poking holes in it a specific distance apart can only serve to re-tune it in some fashion, the way I see it. To me, it looks like what the guy actually accomplished is to build two back-to-back low-Q Helmholtz resonators, in a third tuned system, and got results that he didn't have a clue how to interpret. (But that's just my opinion!)

I just don't think that the "experiment" proves anything at all, to be quite honest! Now if they would have taken the trouble to repeat the experiment hundreds of times, using different size holes and different spacing between them, THEN there might be some interesting conclusions that one could draw. But from one single experiment with one single hole size at one specific distance, to me, doesn't show anything useful at all.

Maybe one of the real acoustic experts can comment here: Andre? Ro? Rod? Eric? Ethan? John? Anyone-else-I-forgot-to-mention?

- Stuart -

[xSpace]

Stuart,

You are a very gifted person in your ability to retain information. You have a few blind sides. The man that did the tests, if you will spend the time to integrate the information, is a highly regarded scientific technician. Not to diminish anyone of the names you suggested but Brian is far above and beyond those guys capabilities as it pertains to scientific ability.

He is an expert.

The black and white that you adhere to is cumbersome at best. There are things that can happen outside of the cookie cutter acoustics that happen around this forum, and this is a prime example.

You should read it and understand it. You will get better from this information, it is that important.


Brien

[AVare]

I have little to nothing to add to the information already given

Not to diminish anyone of the names you suggested but Brian is far above and beyond those guys capabilities as it pertains to scientific ability.

He is an expert.

I find this quite amusing. Eric has a Ph.D. in acoustics, and has designed the world's quietest studio, for which he did several tests to confirm design theories. I seriosuly doubt anyone has greater abilities that he does overall.

Andre

[xSpace]

Andre,

I missed Eric's name in the line up. I may have missed all of them at 4AM when I was trying to post.


The information is supported by http://www.nrc-cnrc.gc.ca/index.html from Brian Daytons' sig file and that is "expert" enough for me.