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Old 7th April 2009, 12:50 AM   #5121
gedlee is offline gedlee  United States
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Quote:
Originally posted by JohnG
A quick point on acoustic transparency of cones:

When doing the transparency test suggested by Earl, or something similar, the speaker should be connected to the amplifier of interest, with the input to the amp shorted or otherwise set to zero with the amp on. This will account for the damping effect of the amplifier on cone motion.

John G

I thought of this, but you could easily just short the leads and this would be fairly accurate. But there will still be a peak at resonance, only lower Q.
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Old 7th April 2009, 03:20 AM   #5122
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Quote:
Originally posted by JohnK
What I will say is that cone drivers make very poor microphones.?
That suggests that when sound waves bounced off from the box enclosure hitting the cone, it does not generate much current in the voice coil. From this, we can derive this: the cone barely moves.

If the cone material is sound "transparent", like light passing through glasses or water passing through paper filters, of course, it would barely move.

If the cone material is sound "opague", but if it absorbs or reflects much of the sound waves, again, it would still barely move.

In other words, JohnK's hint is no hint. He is keeping us in the dark about his findings.

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Bill
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Old 7th April 2009, 11:45 AM   #5123
mige0 is offline mige0  Austria
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Old 7th April 2009, 02:05 PM   #5124
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Quote:
Originally posted by gedlee



Why would you want the driver to be a microphone?
I don't want to. But as think about what HiFi Nut said. "That suggests that when sound waves bounced off from the box enclosure hitting the cone, it does not generate much current in the voice coil. From this, we can derive this: the cone barely moves."

Looking at the back EMF generated by a driver used as a mic gives an indication of of the acoustic energy absorbed and then retransmitted by the cone.



Bill, an acousticly transparent medium is one which absorbs all the acoustic energy incident upon it and then emitts it from the other side without internal reflections or losses. It does not imply that the medium doesn't move. Just the oppsite, the medium must vibrates in the same nanner as the incident wave.
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Old 7th April 2009, 02:34 PM   #5125
gedlee is offline gedlee  United States
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Originally posted by john k...

Looking at the back EMF generated by a driver used as a mic gives an indication of of the acoustic energy absorbed and then retransmitted by the cone.
This seems like looking for a needle in a haystack to me. Unless you are talking about driving the system with another source. If self driven then the very small back EMF when compared to the driven voltages would be hard to detect reliably. And then there is the fact that energy absorbed by the cone does not mean its all "reradiated", it could just be absorbed. How do you sort that out?

I was thinking about this whole situation of why people look to such small things like box resonances and cone transmission when there are such major problems, like polar response, left uncontrolled. It seems to me this comes about because its easily measured. Its like THD, which nobody really believes anymore, but because its easy to do its still done all the time and given a significance way beyond its importance. Polar response is not easy to do, so its basically never done and never looked at, therefor it must not be very important. I get asked all the time - "If THD is so meaningless, then why do people measure it all the time?" Wow! Thats a world class question! Wish I knew why people always seem to do the wrong thing.

I had a long talk with Laurie Fincham of THX some time back about THD. He said that he is very close to elliminating it from all THX specifications because no one could PROVE its relavence to perception. This was enlightening since for the first time I found someone who saw the issue as it is. It's not the burdon of the sceptic to prove that THD isn't relevent, its the burdon of the believer to prove that it is. According to Laurie, THX has not been able to find any solid evidence that THD is meaningful. Only that its easy to do.
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Old 7th April 2009, 02:44 PM   #5126
gedlee is offline gedlee  United States
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Quote:
Originally posted by john k...

Bill, an acousticly transparent medium is one which absorbs all the acoustic energy incident upon it and then emitts it from the other side without internal reflections or losses. It does not imply that the medium doesn't move. Just the oppsite, the medium must vibrates in the same nanner as the incident wave.
John, I'm don't think that I would agree. "transparent", just like in glass, is a continuum and does not imply that it "absorbs all the acoustic energy incident upon it" It could reflect some, transmit some and absorb some, all in highly complex ways depending on the mediums characteristics. One might define "transparent" as a minimum of 50% of the intensity being transmitted to the other side, or 90% (very rare), but there is no such standard defintion. And "the medium must vibrates in the same nanner as the incident wave" isn't right either, since there are mediums where the "medium" itself does not move, like a porous medium. Of course it will move a little, but this is not the dominate mechanism of sound transmission, the wave basically moves right through without the structural part of the medium moving significantly.
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Old 7th April 2009, 03:24 PM   #5127
Pano is offline Pano  United States
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Quote:
Originally posted by gedlee
I was thinking about this whole situation of why people look to such small things like box resonances and cone transmission ....
'Cause they're so easy to hear and don't sound good? Gee, I don't know.
They only become "small things" if you work hard to make them small.
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Old 7th April 2009, 03:48 PM   #5128
dlr is offline dlr  United States
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Quote:
Originally posted by gedlee


John, I'm don't think that I would agree. "transparent", just like in glass, is a continuum and does not imply that it "absorbs all the acoustic energy incident upon it" It could reflect some, transmit some and absorb some, all in highly complex ways depending on the mediums characteristics. One might define "transparent" as a minimum of 50% of the intensity being transmitted to the other side, or 90% (very rare), but there is no such standard defintion. And "the medium must vibrates in the same nanner as the incident wave" isn't right either, since there are mediums where the "medium" itself does not move, like a porous medium. Of course it will move a little, but this is not the dominate mechanism of sound transmission, the wave basically moves right through without the structural part of the medium moving significantly.
A bit of an interesting discussion. I think that those who make general statements that driver diaphragms are "transparent" are implying that they allow 100% of any reflected energy to pass unimpeded, hence the only perfect solution for that issue is an open baffle. It's dogma to them, there's seldom any nuance or qualification as you suggest, it's practically heresy to claim otherwise.

Dave
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Old 7th April 2009, 04:20 PM   #5129
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I just trying to keep it simple. We are talking about driver cones. We don't need to get all esoteric here. The point was that if sound is to be transmitted through a cone the cone must be excited to vibrate. Bill indicated that if the cone was transparent sound would pass through it without exciting the cone to vibrate. We don't really need to worry about what % of transmission defines transparent. In the sense we are discussing this transparent and opaque are just words.

If the SPL at 1 M is 90 dB and the component of that sound from transmission of reflected wave inside the enclosure is 40dB below that level is the cone transparent or opaque? Lynn's definition of transparent may fall into your range of opaque. But if the transmission loss is 40dB then that's what it is what ever it is labeled.
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Old 7th April 2009, 04:23 PM   #5130
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And it would be different for different cones, surely....
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