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Old 25th November 2011, 07:04 PM   #161
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Quote:
Originally Posted by bear View Post
Afaik, the cone movement does not change between equivalent cone structures based on a variance in motor construction, meaning the SPL is solely based on cone motion be it provided by a 1,000 watt signal or a 1 watt signal, the difference being in the conversion of energy in the motor.

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This is correct. Sound compression and rarefaction is purely a mechanical phenomenon.
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Old 25th November 2011, 08:00 PM   #162
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Originally Posted by Overkill Audio View Post
Hi Tom,

I see you are busy on other threads but it would be great to hear your thoughts on the attached doc.
I have huge respect for you and your loudspeaker designs...Your opinion counts to me and so many others on this forum and the doc raises some important issues that will really help DIY'ers choose the right drivers and how best to use them... it will for me anyway!!

I wrote the doc a few years ago and never had it " reviewed " by an independant expert...please, please have a read...Feel free to blow it out the water or disagree or whatever, I will not be offended!!

Thanks
Derek.
It is an interesting topic, suggest starting a thread for this discussion?

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Old 25th November 2011, 08:11 PM   #163
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Hi
I would agree with much / most of what you say, the differences being mostly a different way of seeing how the driver properties tie together.

How a loudspeaker driver works can be explained in great detail but rarely does one hear discussion of how sound works and what a driver really does. I like to say so far as size and time, sound is something like a set of Russian dolls, the traditionally smallest one (20KHz) only 5/8 inch tall while the largest at 20Hz is 1000 times large. You see this volume relationship with woofers, if you want the same sensitivity but an octave lower corner F, it requires you cube the volume not double it.
If you lower the corner frequency an octave but keep the size the same, the sensitivity goes down 9dB.

A lower frequency is larger in wavelength and naturally takes a longer time too.
For that reason, when looking at an event covering a wide bandwidth, time is often not a good frame of reference as the natural period for any event is directly related to frequency.
For example, some foam about subwoofer group delay without considering that any response feature always occupies twice the time if it happens an octave lower, that is to say a large GD is part of a low frequency response corner. In fact the phase response is locked into the amplitude response, any change in one causes the change in the other.
Conversely, a better way to view these things would scale the Russian dolls to the same size so that only deviations from the design are shown, like Cumulative burst decay or in wavelets. Here what would be ime is now related to the wavelength or period (like phase) for that particular frequency.
You thoughts about pistonic motion are mostly right, nothing in infinitely stiff so nothing is absolutely ridged. Conversely, when operating as a piston radiator, when a small fraction of a wavelength across, then the small amount of flex is averaged out as total displacement. In a high pressure design like some at work, the flexure of the cone is visible as an un-accounted for compliance between the horn and driver.

Cone breakup (and any other sharp resonance driven by the system) is really bad! Breakup magnitude relative to that below is sort of a radiator badness indicator.
The drivers motor is the primary distortion generator, the nonlinearity between the signal Voltage and motion. If one had a breakup that was say 20dB tall at 3KHz, then one might be tempted to think, ĎI can cross that out at 1500 with a 96dB /oct crossover and all is well. Yes, when you measure the response the effect of the breakup is not visible, cool!.

Do a distortion tracking measurement and you see that there is a large mound of 3rd harmonic at 1KHz, a similar mound of 4th harmonic at 750Hz and at every submultiple of the mound at 3K, that same resonant gain is applied to the existing motor nonlinearity and that harmonic and raise about 100X or 20dB.

When a woofer is acoustically small even what it does may not be obvious.
For example, what is known for sure is that if you put a microphone or pressure transducer inside a sealed box enclosure with a woofer mounted, one seeís that the internal pressure is directly related to the driver excursion at any frequency (where the enclosure is small compared to the wavelength).
In the range where one is in the box roll off (-12dB/oct) one finds that the driver excursion does not increase as the frequency falls, -12 dB per octave is a constant displacement slope.

Yet, if one monitors the internal microphone, one seeís flat response and a square wave in equals a square wave pressure what gives????
If you wanted such a woofer to produce a flat response externally, one needs to have the excursion increase by four times each time you go down an octave. This is to offset the changing radiation resistance which has a radiator size term relative to the wavelength look into radiator. With that requirement, what does the excursion look like required to produce a square wave now?
This is a constant acceleration response. Here it is the moving mass and the current to force transduction, in series with itís Rdc makes it an acceleration device. That is why / how one can add mass to a given driverís motor and NOT have an effect on itís attack, speed of response step response or whatever measure of time one would apply. It is already controlled by mass, adding more only lowers the main resonance F and the efficiency etc. Conversely, the reason why large massive woofers do not respond quickly is because they have a larger motor which has more series inductance and this is what rolls the response off / slows its rise time.
Like the Russian dolls again, drivers can be suited to a frequency range, a big slow subwoofer driver maybe perfect doing that job even being slow because everything coming out of the low pass crossover IS slow.
You are concerned with an issues near to my heart, also the Manger is a driver I have had some time to play with and measure. In time response, it is the best driver I have ever measured, it occupies one point in time, over a very broad band and radiates as a simple source. AS you suggest it REALLY needs to be crossover over in the lower mid as itís harmonic distortion skyrockets even at 1w @ 200Hz.

Understand what it does, it radiates a very simple portion of a sphere (read my other post about disappearing), it does that by producing a disturbance at the center which radiates outward at the speed of sound in air and then is absorbed. From that standpoint, it is conceptually similar to a Quad ESL-63 which constructs a portion of a spherical radaiton with sequential rings of ess radiators.
Our Synergy horns produce this also but by producing the high frequencies at the apex of a conical horn and progressively adding the mid and lower frequencies in time with the wavefront as it progresses forward (compensating for the order they emerge form the crossover) radiating a spherical segment that appears to have been from one source at the apex.

One thing that is required to act like a single source like the manger is the drivers acoustic phase must be around zero degrees over a broad band and the Manger does that and will reproduce a nice square wave. A woofer with flat response canít do that until way above the low corner because of itís acoustic phase.
Anyway, some random thoughts, time for turkey Phase II.
Best,
Tom
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Old 26th November 2011, 12:45 PM   #164
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Default Great post, thanks!

Hi Tom,

Thanks so much Tom there is lots of great info in your reply, I will take a day to digest it and hopefully come back with some worthwhile ideas to explore further.

Thanks Bear, I will start a new thread for this and please contribute if you have time, your opinon also is a good'un!

Cheers
Derek.
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