"an very"?? how silly - it must have been a fraudian typing slip ;)
Greetings Folks
I stumbled across an interesting phenomenon today in the studio build.
It's nothing new, and acousticians will no doubt readily explain things. but to a layman like myself, it was somethign really interesting to observe.
So what was it?
Well, I having a break from studio building, and I thought I'd refresh my memory on what kind of isolation the Main Studio Door was providing. For those who read My Studio Diary, you'll know that I have only currently installed one door. It's a room-within-a-room, so eventually there will be two doors. And this installed door still needs the seals tweaking. FYI, the door is 5 layers of 18mm+1 layer of 15mm drywall, and uses double half round neoprene seals, bank vault style, with a compression latch.
OK, there's the background...
So I turned up the Boombox in the studio so it was ear-splitting, and went out the studio and shut the door as tightly as I could.
Bear in mind the current state of this door. I could hear the music pretty well, but you WOULD describe it as being quiet. I would guesstimate the TL is currently around 40 to 45dB.
Now... I know one one of the seals is leaking ( due to slightly warped door - to be fixed later ), so I looked for a quick way of covering the gap between door and frame to see how much I could improve the TL. The first thing I saw was a largish sheet of 9mm ( 3/8") MDF which was about 6 foot high and about 2.5feet wide. It's area was about 3/4 of the door area.
And here's the interesting bit.....
As I offered up the 9mm MDF to the door to cover part of the frame gap, I immediately noticed an INCREASE in Level of the music from the boom box!! And it was quite an increase. As soon as I placed the panel flat against the door, the TL returned to original state.
This cought my attention, and I forgot about testing the door seals and wanted to test this increase in TL instead ;)
I then held the MDF panel flat against the door again, and took a mental note of the music level. I then moved the panel slowly away from the door.
Amazingly I only had to move the panel a couple of millimetres away from the door to hear a distinct increase in level.
I observed these effects.
1] When the panel was about 50mm (2") from the door the music was at it's loudest. i.e maximum loss in isolation. This was quite considerable, and I would guess from experience we're looking about 6 to 8dB
2] The increase in level was freqeuncy dependent, and was a lower midrange boost ( roughly 500Hz to 1kHz )
3] As I moved the panel further away from the Door, I could hear the decreased isolation centre frequency drop. - wow!
The conclusions of a layman with a little bit of education are...
I'm observing the effects of amplification at the resonant frequency of a mass spring mass system. Increasing the air gap obviously lowers the resonant frequency and it was real cool to hear this in the real world.
And even a layman like me can easily visualise a sheet of light 9mm MDF, 1" to 2" airgap, thick door ( as specified above ) having an MSM resonant freqeuncy well in the midrange.
And it also proves that air acts as a spring whether it's in a sealed void or not. Eric! Experimentation proves you right
Now all this raises some questions about Isolation Projects being talked about on the various Acoustic Design and Construction Forums there are on the Net.
You see, in the 3 to 4 years, I've been lurking and posting on these forums, I've seen an awful lot of people floating floors, and decoupling speakers without ANY regard for the resonant freqeuncy of the Mass-Spring-Mass system. Even most of the people advising them, through no fault of their own ( i.e simple naivety ), don't know about calculating such freqeuncies. Let alone their importance.
Since learning about MSM from "Eric The Master!" I've been trying to help on various forums, where there are floating floor threads. Hopefully I've saved people hassle and money
But now I've actually observed the negative effects of an MSM resonance that's too high in a double leaf 'wall', I'm thinking the same situations is occouring on Acoustics Forums regarding wall and ceiling isolation, as is/was occouring with floating floor ignorance.
Now the general advice being given on walls and ceilings on forums is great. It all points ( if indirectly ) to getting the MSM resonant frequency down as low as possible, and preventing a silly coincidence dip, etc,etc, but there's RARELY any mention of actually calculating the resonant frequency.
People are sometimes a bit aghast ( in a nice way
) at my zillion layer drywall studio, but the very reason for all those layers is I wanted to get the resonant frequency as low as possible.
But even for people who haven't the need for a bomb shelter like mine, but do need a moderate amount of Isolation, what's the point if the Isolation Design and Construction actually amplfies some frequencies. Particularly important if your monitoring in the control room while tracking in the studio next door. Not to mention the annoyance factor for those neighbouring the studio.
Eric has an MSM calculator on his website. After my enlightening experiement today, I would say that more Studio Builders should use it.
There ya go, some observations and a thesis from a progressively educated acoustic layman
Any comment welcome of course
Paul
