Hard membranes such as aluminum, magnesium and Hexacone (Eton's Kevlar honeycomb membranes) have a much higher stiffness than polypropylene, so their movement is much more like an ideal piston than that of polyprop that can bend. However, most of these membranes exhibit strong resonances in the multi-kHz region.
One will usually hear that this is acceptable in a woofer or woofer/midrange application if a sufficiently steep low pass filter is used.
On the other hand, these membranes are like bells that can ring at certain frequencies. Imagine striking a bell with a well-padded hammer or striking it very slowly. You will probably not convey energy with spectral content at the resonance of the bell, yet you can excite the overtones.
I wonder if the same can happen with a metallic membrane even when care is taken not to drive the motor with high frequencies.
Is this why paper is still used so much in high end speakers like Scan Speak? Does anybody know what the hard paper is that they use e.g. in their 21W 8555/00? It is not the carbon or kevlar filled material that they have in some other speakers.
Eric
One will usually hear that this is acceptable in a woofer or woofer/midrange application if a sufficiently steep low pass filter is used.
On the other hand, these membranes are like bells that can ring at certain frequencies. Imagine striking a bell with a well-padded hammer or striking it very slowly. You will probably not convey energy with spectral content at the resonance of the bell, yet you can excite the overtones.
I wonder if the same can happen with a metallic membrane even when care is taken not to drive the motor with high frequencies.
Is this why paper is still used so much in high end speakers like Scan Speak? Does anybody know what the hard paper is that they use e.g. in their 21W 8555/00? It is not the carbon or kevlar filled material that they have in some other speakers.
Eric