Plastic wrapped broadband absorbers: measurements

[latestflavor]

i'm going to take some measurements of some old broadband absorbers i have lying around.

They are already fully wrapped OC 703 or 705 (cant remember which) 8" thick: two 4" panels wrapped with 0.5 mil plastic each. The plastic is painter's dropcloths from big box hardware store.

So thinking I will take measurements and unwrap and re-do. That would kind of be a one-time deal and i don't want to eff it up.

I'm considering forming a circle around my desk with them, measuring and removing the face on them w/o moving, re-measuring, then perhaps a best efforts with removing all plastic (there will be some minute movement due to removing full wrap).

Any comments on what measurements you would like to see? I have acoustisoft RplusD and an Isemcon mic and usb card. Gotta get my microsoft windows skills back and read my old notes on the software.

[latestflavor]

i was envisioning something like this that would easily capture early reflections.

[localhost127]

Originally Posted by latestflavor

i was envisioning something like this that would easily capture early reflections.

if that's the design goal then you shouldn't be using any reflective surface on the broadband porous absorber.

[latestflavor]

no design, just trying to capture the effect of the plastic vs no plastic.

[localhost127]

Originally Posted by latestflavor

no design, just trying to capture the effect of the plastic vs no plastic.

Originally Posted by latestflavor

i was envisioning something like this that would easily capture early reflections.

[latestflavor]

people have been stating that plastic under 1 mil thick around their broadband absorbers are transaprent. since mine were wrapped and i believe the myth to be quite untrue - i have not seen any measurements to support or dispel it.

furthermore i also have my own questions as to whether frequencies lower than what would be intuitively affected (very HF) might be affected due to impeding the porosity of the absorbent. Many people just recommend to keep plastic out of first reflections but is overall efficiency being sacraficed with the plastic?

thats where i am going with this

[7thbass]

I think this is a helpful exercise. Subscribed.

[localhost127]

Originally Posted by latestflavor

people have been stating that plastic under 1 mil thick around their broadband absorbers are transaprent. since mine were wrapped and i believe the myth to be quite untrue - i have not seen any measurements to support or dispel it.

furthermore i also have my own questions as to whether frequencies lower than what would be intuitively affected (very HF) might be affected due to impeding the porosity of the absorbent. Many people just recommend to keep plastic out of first reflections but is overall efficiency being sacraficed with the plastic?

thats where i am going with this

why not generate a simple ETC response?

[latestflavor]

These measurements were taken in R plus D and imported into REW. SPL has 4 measurements with plastic, 4 without (no averaging) and they are all fairly identical. The measurements without plastic had to be offset (raised in level) by 3.8 (db i assume) to match those with plastic.

For some reason, there is a level difference and i can't determine why. If it was a propagation effect of being completely surrounded by plastic or if i accidentally lowered the volume on my monitor controller. The IR when normalized are not scaled similarly either. Aside from annoying, i am wondering if anyone has any insight on that.

Caveats: The mic may have moved ever so slightly although the computed mic/spkr distance was the same in R Plus D. Mic was pointed at speaker, and it was not calibrated (at the factory i think they calibrate to a DPA). The "cave" did move during plastic removal, but i had detailed tape lines on the floor and carefully reconstructed.

My initial analysis:

No effect on low end through 500hz. From 600hz-3khz there are some minor difference, (some phase changes around 3k) but the glaring difference is 5k and above, the much greater comb filtering with the plastic.

I didn't throw up waterfalls - they were looking fairly similar as i was playing with them, no change in the lows but in the highs yes, i just don't know how to level offset those.

if someone wants to see some other data let me know and i'll try to upload the source files.

Thoughts?

[localhost127]

generate the envelope time curve (check ETC) - also, only measure 1 speaker at a time.

it would be best if you would post the source files,

[latestflavor]

Here are some 10ms ETC's. This is my first time using REW. If it doesn't look right I'll have to wait to post the acoustisoft measurements.

I overlooked mentioning that all measurements are single speaker no sub, and desk(top) was removed and racks that supported it were moved aside (not far, but as much out of the direct first reflection as my wiring would allow).

I will try to post two PCM source files.

[latestflavor]

PCM files:

NoPlastic12v2.PCM

Plastic1v2.PCM