Fs Thiele Small parameter, playing transducers below it - Home Theater Forum and Systems - HomeTheaterShack.com

Old 12-20-09, 10:37 AM Thread Starter
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Ryan

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Fs Thiele Small parameter, playing transducers below it

Small signal parameters
These values can be determined by measuring the input impedance of the driver, near the resonance frequency, at small input levels for which the mechanical behavior of the driver is effectively linear (ie, proportional to its input). These values are more easily measured than the fundamental ones above.

Fs – Resonance frequency of the driver

Fs = 1 / 2pi * sqrt(Cms * Mms)

Cms - Compliance of the driver's suspension, in metres per newton (the reciprocal of its 'stiffness').

Mms - Mass of the diaphragm, including acoustic load, in kilograms.

----------------------------------------------------------------------------

what actually happens below Fs and why?
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Old 12-20-09, 08:21 PM
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lsiberian

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Re: Fs Thiele Small parameter, playing transducers below it

What actually happens is that distortion rises very rapidly. Distortion is a combination of box/port compression and a drivers ability to cleanly produce the given frequency.

Fs is essentially the lowest frequency a transducer is designed to produce. This is why in a standard ported design you will use the port to reinforce at the frequency just above Fs. Model almost any driver ported in WinISD and you will see it normally suggest this alignment.

We use a port tuning to reinforce the low stuff to extend our subs further, but that doesn't magically make our driver great at producing below Fs. if built proper it simply reduces compression and reinforces the low end.

Now in reference to the maelstrom 21" it has a 16hz Fs. Which is far below anything you'd probably ever encounter in a movie or music track.
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Old 12-20-09, 09:15 PM
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Re: Fs Thiele Small parameter, playing transducers below it

Last edited by soho54; 12-20-09 at 09:42 PM.
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Old 12-20-09, 10:46 PM
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David

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Re: Fs Thiele Small parameter, playing transducers below it

Operating below fs is not at all unusual. Subwoofers using ELF or Linkwitz Transform electronics do so specifically to operate below fs. I have designed many systems designed to operate below fs. In fact, many commercial subwoofer systems I have worked on use drivers with fs in the 150 to 200Hz range specifically because they are designed to operate below fs...

Fs is nothing magical, it is simply the frequency that corresponds to a driver's natural resonant frequency. Voice coil impedance rises dramatically at fs, so it can be difficult for an amplifier to drive past that rise, and the subwoofer system's response rolls off at 12 db/oct below fc, so to maintain a constant output below this point the signal must be electronically boosted and more amplifier power is required. But if the system is designed to operate below fs (or fc) in the first place, the amplifier never has to drive through the impedance peak. Use the proper electronic boost to counter the system roll off and an amplifier with sufficient head room to deliver the very large power demands it will call for, and there is nothing that will cause any more distortion in an ELF system than any other subwoofer system, and extremely low frequency output can be generated.
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Old 12-20-09, 11:55 PM Thread Starter
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Ryan

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Re: Fs Thiele Small parameter, playing transducers below it

so there are or are not driver issues below Fs, be it IB, horn, vented, sealed, or anything else. (obviousely alignments designed below the drivers Fs)
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Old 12-21-09, 08:15 AM
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David

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Re: Fs Thiele Small parameter, playing transducers below it

Quote:
Ryan Anderson wrote: View Post
so there are or are not driver issues below Fs, be it IB, horn, vented, sealed, or anything else. (obviousely alignments designed below the drivers Fs)
I didn't say that. I said there are ways to design a system to operate below fs, not that all systems should be. The driver itself does not generally have any issues operating below fs, just not AT fs.
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Old 12-21-09, 08:35 AM Thread Starter
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Ryan

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Re: Fs Thiele Small parameter, playing transducers below it

Quote:
dyohn wrote: View Post
I didn't say that. I said there are ways to design a system to operate below fs, not that all systems should be. The driver itself does not generally have any issues operating below fs, just not AT fs.
LOL

read that question, not statement, again

Quote:
Ryan Anderson wrote: View Post
so there are or are not driver issues below Fs, ....(obviousely alignments designed below the drivers Fs)
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Old 12-21-09, 08:51 AM
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lsiberian

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Re: Fs Thiele Small parameter, playing transducers below it

Quote:
dyohn wrote: View Post
Operating below fs is not at all unusual. Subwoofers using ELF or Linkwitz Transform electronics do so specifically to operate below fs. I have designed many systems designed to operate below fs. In fact, many commercial subwoofer systems I have worked on use drivers with fs in the 150 to 200Hz range specifically because they are designed to operate below fs...

Fs is nothing magical, it is simply the frequency that corresponds to a driver's natural resonant frequency. Voice coil impedance rises dramatically at fs, so it can be difficult for an amplifier to drive past that rise, and the subwoofer system's response rolls off at 12 db/oct below fc, so to maintain a constant output below this point the signal must be electronically boosted and more amplifier power is required. But if the system is designed to operate below fs (or fc) in the first place, the amplifier never has to drive through the impedance peak. Use the proper electronic boost to counter the system roll off and an amplifier with sufficient head room to deliver the very large power demands it will call for, and there is nothing that will cause any more distortion in an ELF system than any other subwoofer system, and extremely low frequency output can be generated.
Well certainly there are different types of drivers that do well under Fs or even ones designed to do well under Fs. As with most rules they don't apply to every situation and are generally a guideline more than a hard cut off. Still I've not seen any ported driver with a low distortion rating below 15hz. If anyone has one it's time to share.
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Old 12-21-09, 09:48 AM Thread Starter
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Ryan

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Re: Fs Thiele Small parameter, playing transducers below it

Quote:
lsiberian wrote: View Post
I've not seen any ported driver with a low distortion rating below 15hz. If anyone has one it's time to share.
this has absolutely nothing to do with driver Fs! Maybe we need to forget about subwoofers for a sec and cover other drivers 1st, as playing below 15Hz is tough, and not because we don't have any drivers with an Fs of 1Hz or 2Hz
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Old 12-21-09, 09:59 AM
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David

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Re: Fs Thiele Small parameter, playing transducers below it

I'm not talking about traditional bass reflex systems (although designing one with good output at 15Hz is not that big a problem.) I'm talking about ELF (extended low frequency) systems - or what Bag End calls Infra: http://www.bagend.com/bagend/what_is_infra.htm But still, getting a driver to operate below fs is no big deal, it's operating at fs that will cause issues.
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