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.
Regards,
Bill
gedlee said:
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.
john k... said:
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.
john k... said:
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.
gedlee said:
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
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.
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.
I knew my question in my last post was rough but when I typed it it was the last minute before I left work and picked up my kids from school yesterday so I had no time to define what was transparent and what was opague, etc.
Anyway, I have to say that JohnK's way of doing things is very impressive. Unlike some of us, me included of course, using assumptions, guesses, pseudo-science, (execuses us, we don't have time, at least I don't.) JohnK uses thorough scientific and engineering approach to look at even small problems.
In this instance, I guess (oh yes, guess again) JohnK's result was that the cone is rather opague (oh yes, guess again, no dB level defined) so it may be like what Gedlee said "needle in a haystack", but JohnK still "worked out a complete approach" to let us have a more accurate understanding of what is going on.
Regards,
Bill
Anyway, I have to say that JohnK's way of doing things is very impressive. Unlike some of us, me included of course, using assumptions, guesses, pseudo-science, (execuses us, we don't have time, at least I don't.) JohnK uses thorough scientific and engineering approach to look at even small problems.
In this instance, I guess (oh yes, guess again) JohnK's result was that the cone is rather opague (oh yes, guess again, no dB level defined) so it may be like what Gedlee said "needle in a haystack", but JohnK still "worked out a complete approach" to let us have a more accurate understanding of what is going on.
Regards,
Bill
I don't think cove transparency is a black and white issue. For example, a few years back I was working with a guy who had a small 2-way with Eton midrange. When measured in the enclosure there were obvious spikes in the frequency response and corresponding glitches in the impedance. Taken out of the box and mounted in an IEC baffle the response was smooth. So here is an obvious case where the driver was "transparent enough" that the builder's poorly damped enclosure was coloring the sound. Rethinking the enclosure design and adding sufficient damping solved the problem. So where is the threshold? This driver was obviously so transparent that the problem was obvious in the impedance curve. I don't think that the issue can just be swept under the rug under the guise that all drivers are acoustically opaque or all are transparent. Either way, OB mounting potentially eliminates the problem from consideration.
