1.5g Mms and 1.8Tesla Magnetic circuit: What do you think?

nasubi

Member
2016-05-01 4:06 am
Japan
Hi;)

3 years ago, I designed and built the magnetic circuit for Fostex Fe83en whose flux is 1.8 tesla using three of neogymium magnets and pure irons.
Now the unit is going to be mounted on alminium pipe which is about 1m long like "Timedomain Yoshii9".
Fe83en's corne mass is only 1.5g so I think it will become absolutely strict time-coherent system.

Though I guess you all think amount of low frequency level won't enough, It is aims only for the quiet music played by small number of people like Piano Trio, Ambient music, not for Rock, Techno, or EDM, so I think it's ok.

But I'm not sure whether my planning is correct. I'm not familiar with Acoustic engineering.
So What do you think of my plan? Any suggestion?

Here is my blog URL(written in japanese):nasubi‚̃y�[ƒW

Thank you.
 
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nasubi

Member
2016-05-01 4:06 am
Japan
My post disappeared because of server error. So I reposted here. Sorry.
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Hi;)

3 years ago, I designed and built magnetic circuit for Fostex Fe83en whose flux is 1.8tesla using three of neogymium magnets and pure irons.
Now the unit is going to be mounted on alminium pipe which is about 1m long like "Timedomain Yoshii9".
Fe83en's corn mass is only 1.5g so I think it will become absolutely strict time-coherent system.
It aims for the quiet music played by small number people like Piano Trio, Ambient Music, not for Rock, Techno, orEDM.
Though you may think the amount of low frequency will not be enough, I think it's ok.

But I'm not sure my idea is correct. I'm not familiar with acoustic engineering.
So What do you think of my idea? Any suggestion?

Here is My blog URL(written in japanese):nasubi‚̃y�[ƒW

Thank you.
 
This is something that is going to be down to personal taste & perception. It certainly wouldn't have anything even approaching LF output, so it wouldn't suit me, even with very simple music. But that's just my taste -if you're happy, that's all that matters.

Leaving aside the absense of any LF output (I would assume steady rolloff below about 1.4KHz), you essentially sound like you've got it in the end of a modified plane-wave tube, which are often used for testing compression drivers. It won't be absolutely time coherent, since nothing is in our current engineering reality (just more or less compromised) but it will probably be as close as you're likely to get using drivers of this type.
 

nasubi

Member
2016-05-01 4:06 am
Japan
Thanks Scottmoose.
>I would assume steady rolloff below about 1.4KHz
I have similar simulation data. Actually, I will try to compensate for the loss using equalizer.

>you essentially sound like you've got it in the end of a modified plane-wave tube, which are often used for testing compression drivers.
I had known it but I've never associated Yoshii9 with it before. I've always thought of Yoshii9 as kind of damped QWT. Its point of view is new to me and has become a great help. I'll try to examine the details closely.

Thanks again,
nasubi
 
A driver mounted on the end of a pipe is about the worst possible arrangement for bad diffraction effects. Try adding a rectangular baffle around the driver (or even better, a trapezoid). You want to avoid having equidistant driver to baffle edge distances which can cause severe 6 to 10dB diffraction dips and peaks.

If I am understanding you right, you want to put a driver on an open ended pipe as a resonator? A long pipe on the back of a driver will have a natural 1/4 wave frequency. 1m pipe has about 86Hz 1/4 wave frequency but will be kind of a one note resonance. Either add progressive stuffing or dampening material inside or even better, taper the pipe to a smaller exit terminus for smoother response and deeper extension.

See this nice write by SL on effect of a small round baffle.

Diffraction from baffle edges
 

nasubi

Member
2016-05-01 4:06 am
Japan
Thanks xrk971
I'm sorry for late reply.

I'm going to equalize its frequency responce to be flat as possible including the problem of diffraction. 
Also, I'm concerned about another problem in time domain.
Complex waves caused by diffraction would mix up with waves directly radiated from the driver spoil the quality of sound.
Ofcourse it should be avoided by adding a rectangular baffle around the driver as you said.
But I think the baffle would still reflect these waves at the edge of it.
In addition, because of long distance between the edge of the battle and the driver, it would cause more delayed waves than the speaker which has no baffle, which cannot be correct.

In short, adding the baffle would reduce diffraction and cause more delayed waves which cannot be correct, while no baffle would cause severe diffraction which can be equalized and less deleyed waves.
I think it is difficult to choose which one is better. I suppose there is nothing to do but try it.

>If I am understanding you right, you want to put a driver on an open ended pipe as a resonator?
That's right. ;)
I'm using URL below as reference.
TimeDomain Theory :: TimeDomain :: Bigston Audio
Yoshii9??????????#timedomain : ???????

Thanks again
nasubi
 
You can't EQ out diffraction as it affects the time and frequency domain differently for different wavelengths. It might work for one angle but will not be consistent over all listening angles and at different distances. Also, the effect of diffraction here is substantial 10dB peak to valley fluctuation. Get a smooth frequency domain and your time domain will be smooth as they are related via Fourier transform.
 
I just use commercially available drivers so the light Mms and high Bl value are just whatever comes with the drivers. I know it will have more authority for high Bl but that is more important for woofers. The low Mms gives better articulation but at the expense of potential cone breakup.

Compare the Mms and Bl to other popular commercial drivers and see if its worth it? Look at some highly regarded drivers like ScanSpeak 10F/8424 or Faital Pro 3FE22 or Visaton B80.
 
A minor point: all widebanders produce the majority of their BW through resonance of the cone etc. For a given driver type, size & motor power a low moving mass does imply higher efficiency. It may sacrifice some response linearity as it will probably possess less self-damping, so you're back to the 'name your poison' in terms of what you want / are trying to do. The 83 is fairly well balanced in this regard, notwitstanding an issue at about 2.2KHz which most listeners seem to feel is benign. Where it falls short in some regards is its high F0. Couple that to Fostex's standard short-throw motor design, and it's not a driver that thanks you for actively EQing the LF end, unless used in large numbers of course.

Re the baffle, if I'm reading the intention correctly, it's to minimise the baffle area to the extent that it's essentially the same size as the driver itself, which can produce interesting results, providing you address any potential issues with early reflectons coming back through the cone.
 
You can't EQ out diffraction as it affects the time and frequency domain differently for different wavelengths. It might work for one angle but will not be consistent over all listening angles and at different distances. Also, the effect of diffraction here is substantial 10dB peak to valley fluctuation. Get a smooth frequency domain and your time domain will be smooth as they are related via Fourier transform.

Exactly.
I thought was not enough.I'll have second thoughts about it. Thank you for letting me know about it. ;)
 
I just use commercially available drivers so the light Mms and high Bl value are just whatever comes with the drivers. I know it will have more authority for high Bl but that is more important for woofers. The low Mms gives better articulation but at the expense of potential cone breakup.

Compare the Mms and Bl to other popular commercial drivers and see if its worth it? Look at some highly regarded drivers like ScanSpeak 10F/8424 or Faital Pro 3FE22 or Visaton B80.

I know there are many well-balanced drivers in terms of both frequency and time domain.
But I want to challenge the limit even throw away most of the LF output.
At least, the better transient response drivers have, the larger its SPL at the beginning of waves.

Indeed, cone breakup is one of most important problems should be avoided.
That's the reason why I chose the 8cm full-range driver.
A cone whose diameter is more than 10cm causes cone breakup severely and has high directivity at high frequency.
And avoiding cone breakup leads to increase the cone weight, which do good to LF output but do harm to transient response.
So I found 8cm is a reasonable compromise but I also seek a better solution like "Earthworks Sigma 6.2".
 
FWIW I see your project quite reasonable.

1) the original datasheet shows cone resonance 165 Hz , flat response (@88dB/W efficiency which is not bad at all) down to that frequency and just 8dB down @ 100Hz ... not bad for a 3"speaker !!!!

2) I would expect easy cone breakup in an 1.5g cone ... if it were 6" or 8" because it would be printing paper thin, ... but on a 3" unit? Meant for full range, not just a woofer? .... sounds right to me; doubly so because it's curvilinear/exponential, which is structurally way more rigid than a straight cone one.

3) Fostex suggests 2 reflex cabinets, one straight tuned to 90Hz and the other with one chamber at 60Hz, pretty impressive for such a small speaker.

I bet it will sound very good as a full range speaker, with obvious lack of power at low frequencies.

As of the aluminum tube tuned to 86 Hz, it's almost the same as one of the preferred tunings, so no big deal.

Yes, a straight metal tube will have higher Q than a wooden tuned box , and if left alone *might* be too much and unnatural, but you can add as much acoustical damping material as you wish ... worst case fill the tube with it ;)
In principle not much different to a transmission line loaded cabinet, save that here it's straight.

Testing your idea is easy, just have the tube cut, mount the speaker, and *you* tell us :)

Good luck and don't overthink it.
 
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nasubi

Member
2016-05-01 4:06 am
Japan
A minor point: all widebanders produce the majority of their BW through resonance of the cone etc. For a given driver type, size & motor power a low moving mass does imply higher efficiency. It may sacrifice some response linearity as it will probably possess less self-damping, so you're back to the 'name your poison' in terms of what you want / are trying to do. The 83 is fairly well balanced in this regard, notwitstanding an issue at about 2.2KHz which most listeners seem to feel is benign. Where it falls short in some regards is its high F0. Couple that to Fostex's standard short-throw motor design, and it's not a driver that thanks you for actively EQing the LF end, unless used in large numbers of course.

Re the baffle, if I'm reading the intention correctly, it's to minimise the baffle area to the extent that it's essentially the same size as the driver itself, which can produce interesting results, providing you address any potential issues with early reflectons coming back through the cone.


I'm sorry for late reply.

That's right. I'll consider that deeply.

Sorry I don't know whether I'm understand you correctly, but I think you said "if I'm reading the intention correctly, it's to minimise the baffle area to the extent that it's essentially the same size as the driver itself, which can produce interesting results. And if I'm reading the intention correctly, you address any potential issues with early reflectons coming back through the cone."
In my understanding, your understanding is correct, but I don't get what 'interesting results' indicates. Can you please build on that?