Mylar stretch formula

merovingio

Goodmorning everyone!
I'm building a 30cm X 80cm cell.
Is there a formula to calculate how much to pull mylar (in Kg) ?
Thank you all.

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RogerGustavsson

Depends on many things. What are you building? Electrostatics or magnetic planars? 30x80 cm will not work for electrostatics without further supports of the diaphragm. For a magnetic planar, you may want to divide the diaphragm in smaller areas in order to reduce the moving mass. The tension is also depending on what thickness the Mylar has.

WrineX

yep, for planarmagnetics it depends as well if they should play low end and top end from the same foil. verry hight tensions are required to be able to use the mylar down low and up top. the high tension also makes it possible to exclude for instance 20 hz because that would limit max spl greatly when using small magnet to foil spacings.

thickneses of 12 (magnepan) 24 or even 30+(like B&G does for tweeters and midranges) micron are used. using less the 12 will result in not being able to stretch it tight enough without breaking it. at least i could not

RogerGustavsson

A friend of mine built a few electrostatics, the largest one built to drop below about 100 Hz. I think he told me that he streched the 6 µm Mylar type C to 85% of the break limit. In an electrostatic there are so many factors to consider to get a stable diaphragm.

It is different for a magnetic planar. For lower frequencies the tension is not that high, depending somewhat on what mass the conductors have and the desired response. Remember, the bass should have a peak to compensate for the short around the baffle. Sometimes the tension is varied from top to bottom of the driver in order to spread the resonances. For mid or tweeter drivers, the suitable tension is different. To get the best high frequency response the Mylar need a higher tension and that increases the lower cut-off frequency. Like WrineX writes, the BG Neo drivers use a thicker plastic film to allow for high tension. They can do so because the diaphragm is so small compared to e.g.Magnepan. That way the moving mass is still low for the BG Neo.

Disco-Pete

Paid Member
yep, for planarmagnetics it depends as well if they should play low end and top end from the same foil. verry hight tensions are required to be able to use the mylar down low and up top. the high tension also makes it possible to exclude for instance 20 hz because that would limit max spl greatly when using small magnet to foil spacings.

thickneses of 12 (magnepan) 24 or even 30+(like B&G does for tweeters and midranges) micron are used. using less the 12 will result in not being able to stretch it tight enough without breaking it. at least i could not
Would a double mylar diaphragm result in an effectively stiffer diaphragm? I mean if you stretched say a 6mil diaphragm first and then a second one but also glued it to the first. Could you not then make a larger panel without additional support? At least for a bass panel? I'm thinking just because you could then tailor/design the appropriate compliance.

merovingio

Voorei realizzare un'elettrostatica di 60X25 cm, tipo Acoustat.

RogerGustavsson

Would a double mylar diaphragm result in an effectively stiffer diaphragm? I mean if you stretched say a 6mil diaphragm first and then a second one but also glued it to the first. Could you not then make a larger panel without additional support? At least for a bass panel? I'm thinking just because you could then tailor/design the appropriate compliance.

Why not use a thicker diaphragm from the beginning? For bass it can be thick, the wire on the Mylar is a lot heavier. For bass the tension is lower, really low compared to ESL. Bass and mid/tweeter are really different! The higher the frequecy, the stiffer the diaphragm. For bass, the tuning cannot be very low, maybe just above 30 Hz after running them in (the fundamental will sink 5-12 Hz with use). The lower the tuning, the lower the maximum possible SPL. A tuning similar to the large bass drivers of the Magnepan Tympani will limit the useable frequency range to about 300-400 Hz (or even lower) if you aim for high quality reproduction.

Calvin

Hi,

thicker diaphragm material can be stretched harder than thinner material.
As such you can reach higher resonance points -even though it may appear counter intuitive at first.
The peak of the higher resonance point is less pronounced, but still may easily reach +10dB and requires either filtering out or some means of damping.

jauu
Calvin