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Author Topic: A quick note on the ortho wall  (Read 1458 times)

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anetode

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A quick note on the ortho wall
« on: August 17, 2014, 09:16:52 PM »

With Marv's talk about radiation patterns I went back and had a look in the Loudspeaker and Headphone Handbook (I've a small digital library for quick reference, PM me if you'd like a copy of a book or something). Anyway, I noticed a peculiarity mentioned in the section on isodynamics:

Quote (selected)
The influence of the membrane on the acoustics of such a headphone is determined
primarily by the acoustic mass of the membrane, LM. The low mechanical
tension results in a low membrane resonance of less than 200 Hz which is, however,
not seen in the resulting frequency response, since the dominant acoustic compliance
CO of the air volume is much lower than that of the membrane. The result is effectively
a series resonance circuit of LM with CO, having a resonance at about 4 kHz.
Figure 14.11 (curve 1) shows the calculated response for the equivalent circuit (Fig.
14.12) leaving out all acoustic resistances except for the leak impedance R0, L0. To
produce a good frequency response, this resonance must be almost critically damped.
The natural damping effect of the radiation resistance RR is shown in curve 2. This
being as yet insufficient, a porous resistance RH is also provided to give curve 3.
Above 5 kHz simulation with lumped elements is inaccurate, due to the inhomogeneity
of the sound field in the cavity.



Hmm...







So as mentioned in prior discussions there are many approaches to damping that acoustic resonance through adjusting the tension, seal, etc.
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arnaud

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Re: A quick note on the ortho wall
« Reply #1 on: August 18, 2014, 12:17:41 PM »

This is pretty cool anetode, isn't this the same book you used to post figs on HeadFi about various driver loading types and the influence of pad porosity?

So, if I understand the current snippet correctly, the equivalent circuit model can predict the first acoustic resonance in the depth of the earpad which is actually more of a coupled diaphragm as mass / earcup cavity as spring type (for light tensioning case that is)?

I would not mind getting the full reference to the book. Only one I ever bought for audio is the loudspeaker design cookbook, which I find a bit lousy for actual understanding of the physics (it really is more of a cookbook).

I come from a background of numerical simulation. For headphone model, I would just mesh the heck out of it and compute a response, it doesn't provide all the clues about the physics unless maybe doing sensitivity studies. For this, electrical circuit analogy is pretty neat to understand driving parameters quickly (in the frequency range  where they're valid).

For the ortho wall, isn't it typically much higher in frequency, e.g 8-10kHz? I had this thought that it was the periodic pattern of the magnet structure that cause multiple diaphragm and / or acoustic resonance in the slits occurring all at once (which we refer to as pocket modes I'm periodic structure jargon).

Arnaud
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anetode

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Re: A quick note on the ortho wall
« Reply #2 on: August 18, 2014, 07:59:52 PM »

So, if I understand the current snippet correctly, the equivalent circuit model can predict the first acoustic resonance in the depth of the earpad which is actually more of a coupled diaphragm as mass / earcup cavity as spring type (for light tensioning case that is)?

Yup, the tensioning is then a trade off between damping this resonance and losing amplitude in the bass frequencies. I'm guessing that this is why the LCD-3 has a noticeable dropoff in bass vs. the LCD-2. Also the resonance can be toyed around with to help it contribute to the HRTF curve compensation. I'm also guessing that the result doesn't always correspond to the model, being much more sensitive and forcing much back and forth in variations between Audeze, who chose to release revisions of existing models, and HiFi-Man, who released like a half dozen models before hitting their stride.

For the ortho wall, isn't it typically much higher in frequency, e.g 8-10kHz? I had this thought that it was the periodic pattern of the magnet structure that cause multiple diaphragm and / or acoustic resonance in the slits occurring all at once (which we refer to as pocket modes I'm periodic structure jargon).

This was an explanation of what I think has been referred to as the "ortho wall" and the usage of the phrase varies, so I may be wrong. I've no idea about higher frequency modes relating to magnet structure.
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Marvey

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Re: A quick note on the ortho wall
« Reply #3 on: August 18, 2014, 08:09:20 PM »

Also, keep in mind those early measurements were on the foam - not felted/foamed solid plate like the current ones. So some of those ortho walls are of greater magnitude than they would be once placed on the ears. Nevertheless, the ortho walls are always there even if a good seal.
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