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Old 24th August 2008, 10:26 PM   #1
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Question Speaker Specs....Fs

The Fs is the resonant frequency of a driver ( I assume in free air).
In the real world the driver will be mounted in some enclosure (and will be tightly secured to a baffle); does this "real world" mounting change the Fs of the "driver & baffle assembly" to a new frequency? If true...then why is the free air resonance frequency of a driver considered important?
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Old 24th August 2008, 10:39 PM   #2
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It raises Fs. Fs is important because if it's already high putting it in a box is going to make it higher, so you know where you are starting from by knowing Fs.

You seem keen to learn so I would recommend you invest in the loudspeaker design cookbook by Vance Dickason, or any of the books by David Weems.
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Old 24th August 2008, 11:04 PM   #3
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Yes, the box and driver interact; to get a feel for this you could download WINISD and try changing parameters to see how they interact
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 25th August 2008, 02:28 AM   #4
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Quote:
Originally posted by PeteMcK
Yes, the box and driver interact; to get a feel for this you could download WINISD and try changing parameters to see how they interact
..Agreed.
That's how I learned all the basics of loudspeaker drivers, enclosures, T-S parameters, etc.
It's helped a ton, and I use that knowledge all the time.
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Old 25th August 2008, 01:32 PM   #5
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Default Fs behavior

I shall take the advice ...do some research. The fact that the Fs INCREASES when a driver becomes solidly mounted into a baffle....totally baffles me. It seems counter-intuitive. If I have a small object (a 6" driver) that has a free air (self) resonance of XHz....and I "weld" it onto a baffle that is 2Ft square.....It seems to me the resonance of the larger "assembly" will now be much LOWER than that of the free air unmounted driver alone. Am I missing something? (Yes..must be....so I will begin some additional reading per your suggestions.) THANKS
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Old 25th August 2008, 01:35 PM   #6
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No. On a baffle like you say Fs will barely be changed, because it's fully open at the back. Also the baffle is not moving, just the cone. ALl the baffle is doing is stopping waves from the back of the speaker reaching round the front and cancelling the sound.

When you put the speaker in a box Fs rises because the pressure the cone has to work against becomes much stiffer.
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Old 25th August 2008, 03:24 PM   #7
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Default Fs Sealed Driver

Thanks...What happens if the driver has a sealed back ? Is the back pressure (any more) constant; (or) independent of the box ? The original driver in my TL speakers was a Radford (actually made by Goodman) That midrange featured a sealed back. It looked almost like a second skin behind the cone....stiff Aluminum...very shallow. The Morel EM1308 is also a closed back device. I chose it because I had to mount it in the same chamber as the woofer.
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Old 25th August 2008, 04:19 PM   #8
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If it's got a sealed back then anything to do with boxes is pointless. Once you get up past 200Hz or so the box size becomes pretty much irrelevant anyway as the compliance of the speaker is much stiffer than the air.
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Old 25th August 2008, 05:35 PM   #9
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Default Sealed Drivers

Thanks again for your technical insight. I didn't know of many quality midrange drivers that featured sealed backs. The Fs of the Morel EM1308 (a sealed back unit) is 320Hz and I'm crossing around 500Hz I know this is close and tighter than the 2X Fs rule of thumb....but I don't have much alternatives to choose from. XO for my mids is 2nd order 12dB per octave bandpass. I don't have any specs on the Radford BD25 woofer.
Many thanks to all who have responded so far. Your info has been a real education for me as I continue to refurb my 33 year old speakers.
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