Absorption coefficient of perspex

[Jens Eklund]

Not sure what you mean but this stuff is reflective and possible absorption (then acting as a membrane) will depend on how it´s mounted. See this thread for more info on how reflective materials absorb depending on mounting:

Flooring question

[Laidlaw]

thanks for the link Jens, i'll check it out.

it's actually for an experiment i'm doing regarding pzm's on various size boundaries. basically the panels are hanging of a mic stand with the pzm in the middle. I wanted to put some info in the report about AC and what the properties of perspex are but cannot find anything.

thanks for the reply mate

[andrebrito]

You can check on Deamp stuff although their stuff is microperforated it could give you an idea... I would say due to is weight and density the absorption is around 0.10 to 0.20

[Dange]

Originally Posted by Laidlaw

I wanted to put some info in the report about AC and what the properties of perspex are but cannot find anything.

Well you have found something, in your case it most probably has an absorptive effect but that is not quantifiable and is probably negligible. At low frequencies especially it will be dependent on how the panel is mounted, there is unlikely to be data out there that you can use, i.e. that someone has done a test with the same mounting

It might have the opposite effect, rather than absorbing sound, the panel might resonate and boost the measured level. As the Pzm mic is in the centre of the panel then you might see the fundamental mode having an effect.
Do you see anything in your results that suggests the perspex panel is having an effect? E.g. large peaks (or dips) in the response suggesting resonance of the perspex? You should then look at the decay time of the peak

[Laidlaw]

thanks for the replys

i'm having a hard time wrapping my head around the whole acoustics side of things, i get the real basic stuff but then it all gets a bit tricky.

i'm don't know about fundamental modes, or resonant frequencies... would that mean that the wavelength would be the same as the width of the panel?

i posted some frequency charts of the results on another thread as i was having a hard time figureing out exactly what these graphs tell me... they are available here - PZM frequency response depending on boundary size - frequency analysis questions

[Jens Eklund]

Do a search on "baffle diffraction".

[Dange]

I've reproduced your results here:

85mm panel in red, 255mm in green and 425mm in blue

1m

2m

4m

[Dange]

What software is this measured with? I think you might see clearer results if you can filter/window out other reflections. e.g. from the walls. You can do this in some software by only analysing the first bit of the measured impulse response

[akebrake]

Originally Posted by Laidlaw

thanks for the replys

i'm having a hard time wrapping my head around the whole acoustics side of things, i get the real basic stuff but then it all gets a bit tricky.

i'm don't know about fundamental modes, or resonant frequencies... would that mean that the wavelength would be the same as the width of the panel?

i posted some frequency charts of the results on another thread as i was having a hard time figureing out exactly what these graphs tell me... they are available here - PZM frequency response depending on boundary size - frequency analysis questions

Hi Laidlaw,

The boundaries you tried for your experiment (perspex) seem to be very small...

Are you sure the area are only 85mm2 , 255 mm2, 425 mm2 ?
(info from the above link) or are the sides of the perspex 85mm, 255 mm and 425mm (square plates) ?

The microphone you used (U843r) seem to have 3 cardioid mic-capsules inside which make conclusions from measurements impossible.. This mic is specially designed for conference table use.

Plus..I think you need to study acoustical measuring technique some more before your next measuring session.

Of coarse we will support you

Cheers

[Laidlaw]

Originally Posted by Dange

What software is this measured with? I think you might see clearer results if you can filter/window out other reflections. e.g. from the walls. You can do this in some software by only analysing the first bit of the measured impulse response

i was using fuzzmeasure for the graphs, but the sweep was generated in protools using mcdsp sweep generator. perhaps the sweep was too long (15sec) i know the sweeps in fuzzmeasure can be shorter but it wasn't available where i was recording the results.

[Laidlaw]

Originally Posted by akebrake

Hi Laidlaw,

The boundaries you tried for your experiment (perspex) seem to be very small...

Are you sure the area are only 85mm2 , 255 mm2, 425 mm2 ?
(info from the above link) or are the sides of the perspex 85mm, 255 mm and 425mm (square plates) ?

The microphone you used (U843r) seem to have 3 cardioid mic-capsules inside which make conclusions from measurements impossible.. This mic is specially designed for conference table use.

Plus..I think you need to study acoustical measuring technique some more before your next measuring session.

Of coarse we will support you

Cheers

they were square panels. eg 85mm each side.

unfortunately that microphone was the only one available, i would have preferred one like the crown models with the backing plate.

hahaha i have a LOT to learn as far as acoustics goes without having to analyze results!

[akebrake]

Originally Posted by Laidlaw

they were square panels. eg 85mm each side.
.........

OK, that makes more sense.

In order to investigate a microphone, by means of acoustical measurements you have to "isolate" the device under test (DUT) from disturbances. This is done by acousticians in anechoic chambers or out of doors. If you don't have that possibility you have to figure out ways to work around the problems. E.g. Using time selective measurements. Like FM.

How will different time windows affect the measurements? Do you need a larger room? Do you need to place the boundary microphone differently?
Or use another kind of microphone? (pressure)
Can difference measurements help?

After solving that, is it possible for you to make another set of measurements? May be post an IR file.

Best
Ake

BTW. Long sweep times increases signal to noise ratio = good.