For the active membrane of an ESL ... is there an overall better alternative to Mylar? Therefore, any approach to tame the unwanted artefacts emerging from this type of film is welcome. My approach would be first to heoretically test for the effectness of each option in isolation from other influences. So a testbed such as the frame described in this test might be helpful also to test any damping measure. Instead, the complex structure of a perforated stator, partially meshed sandwich might have a masking effect. And then for the gut feeling? Every gastroenterologic specialist can tell you that guts have the potential to betray. And most people might tell from their own experience.
The dust cover films instead, as intended to be tested in this thread, have other properties to match than the active membrane. So obviously besides Mylar many other materials are used for this purpose. I am really eager to test my bunch of rolls by the time I will be ready to do so.
My delay has a simple cause: I have only a limited (=precious) quantity of film, and therefore I want to have the film sheets confectionned in a way that they can serve several times on the test frame, without being destroyed by having to be "torn" off. Furthermore, changing the films has to be quick, easy and reversible. This is why I want to first figure out the best way how to handle these different materials and tasks. An example: In order to test for the material properties of each sheet make in isolation from other influences, I want to hold the film in place and tension it by the four corners only for a first test series. All four borders of the sheet should be freely floating in ambient air, while a vast aera in the center should be reasonably and evenly tensionned. Tensionned from test to test always by more or less at the same, but also variable forces. So no taping to any frame. I want to avoid uncontrolled damping (which would be part of the "therapy"). No standing waves and the like. To figure out how to archieve these goals now turns out to be a more delicate process of trial and error than I imagined before. I e.g. already ripped twice a corner piece away from the Mylar 4.2 sheet while probing tensioning with different approaches to anchor the four stretcher strings into the Mylar sheet corner. But things seem to slowly settle in direction of a useable setup, in the end. All I have to do is to be patient also to myself ...
The dust cover films instead, as intended to be tested in this thread, have other properties to match than the active membrane. So obviously besides Mylar many other materials are used for this purpose. I am really eager to test my bunch of rolls by the time I will be ready to do so.
My delay has a simple cause: I have only a limited (=precious) quantity of film, and therefore I want to have the film sheets confectionned in a way that they can serve several times on the test frame, without being destroyed by having to be "torn" off. Furthermore, changing the films has to be quick, easy and reversible. This is why I want to first figure out the best way how to handle these different materials and tasks. An example: In order to test for the material properties of each sheet make in isolation from other influences, I want to hold the film in place and tension it by the four corners only for a first test series. All four borders of the sheet should be freely floating in ambient air, while a vast aera in the center should be reasonably and evenly tensionned. Tensionned from test to test always by more or less at the same, but also variable forces. So no taping to any frame. I want to avoid uncontrolled damping (which would be part of the "therapy"). No standing waves and the like. To figure out how to archieve these goals now turns out to be a more delicate process of trial and error than I imagined before. I e.g. already ripped twice a corner piece away from the Mylar 4.2 sheet while probing tensioning with different approaches to anchor the four stretcher strings into the Mylar sheet corner. But things seem to slowly settle in direction of a useable setup, in the end. All I have to do is to be patient also to myself ...
While tweaking a method to anchor the fixing/tensioning strings at the four corners of the film, I made a set of quick and dirty measurements with the Mylar 4.2 C film under different tensions.
On my actual iteration of the test frame, the film gets hold in place and eventually also tensionned crosswise diagonally, and the the frame allows for different amounts of tensioning. The tension is generated simply by adding the weight of a 1.5l PET bottle weighting either 500g or 1600g onto each of both of the diagonal tensioning string loops. If tension is applied, then a seemingly near uniformly stretched aera in the center region of the film gets built up.
Red: No Mylar film
Green: Mylar 4.2 fitted, relaxed and nearly without any diagonal tension
Brown (hidden by and as same as Blue): Diagonal tensioning with 500g weight
Blue (hiding Brown): Diagonal tensisoning with 1600g weight ... and inconsistently labeled, should be labeled M42_16 instead of M42_15
These are preliminary and partial results only. E.g. I did not yet assess the basic resonance frequencies of the films at the different tensions. Anyway, in terms of the 1ms artefact the result shows that it seems to play practically no role wheter the film is relaxed or tensioned. Furthermore, the two different tensioning forces show exactly the same artefacts at 1ms (f_res will be different).
What does this mean now? E.g. there is this the nice little story/idea/concept that a dust protective film will quasi compliantly move along with the sound waves, quasi surfing on top of the wave. And therefore a dust protective film might have no sonic impact. At least with this Mylar 4.2 C this is not true. Even the completely relaxed film shows a strong artefact which most probably will correspond to a reflection. And in this very measurement series, the relaxed film's artefact amplitude is even greater that the one when the film got tensionned. Aha.
There are two other aspects on this graph worth pointing to: The green graph's 1ms artefact peaks a bit before the brown/blue artefact. This is because while relaxed, the film was bending with a small convexity toward the mic. The other goodie is the fact that naked red 2ms reflex shows a higher amplitude than the green, brown and blue one. Logically: The reflected energy by the film will no more reach the reflecting plane at the back of the film.
It seems to become a really fun playfield ...
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