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Old 6th October 2003, 08:31 PM   #1
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Default Could be a really stupid question!

I've been looking at the Fostex full range drivers and wondered if it would be possible to fit one in a sealed box?

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I'm particularly interested in the FX 200 (8") because it seems to have a pretty flat response.

If it would work what size box would i need ? deep bass is not important to me but speed is.

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Matt
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Old 7th October 2003, 12:07 AM   #2
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Default Re: Could be a really stupid question!

Quote:
Originally posted by fatmarley
I've been looking at the Fostex full range drivers and wondered if it would be possible to fit one in a sealed box?

Link

I'm particularly interested in the FX 200 (8") because it seems to have a pretty flat response.

If it would work what size box would i need ? deep bass is not important to me but speed is.

Cheers
Matt
All I know is what I've read in books over the last couple of months, but it would appear that the published parameters are in the ballpark for a sealed enclosure. Two books I've read say you want a ratio Fs/Qes of about 50. 38/.48 = 71.2, which is sort of about 50. The box would be on the largish side. I don't have the books here at the office to figure just how large.

Fostex lists recommended ported boxes: http://www.fostex.co.jp/int/pages/pr...llrange2.html#
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Old 7th October 2003, 01:33 AM   #3
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Using some measured specs I get a max flat alignment (Qtc = 0.707) in 55.75L, F3/Fb = 52.63Hz with no stuffing. To stay linear though, max power handling below ~186Hz falls off at 12dB/octave, so by the time it gets to Fb it can only handle ~0.3W.

WRT 'speed', for a critically damped alignment (Qtc = 0.5), 343.97L is required, F3 = 57.83Hz/Fb = 37.23Hz.

GM
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Old 7th October 2003, 07:24 AM   #4
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Quote:
Originally posted by GM
Using some measured specs I get a max flat alignment (Qtc = 0.707) in 55.75L, F3/Fb = 52.63Hz with no stuffing. To stay linear though, max power handling below ~186Hz falls off at 12dB/octave, so by the time it gets to Fb it can only handle ~0.3W.

WRT 'speed', for a critically damped alignment (Qtc = 0.5), 343.97L is required, F3 = 57.83Hz/Fb = 37.23Hz.

GM
Could you explain this to me because i don't understand?

Does this mean i would need a huge box (343.97L)?

How would it effect the sound if i use a 55.75L box?
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Old 7th October 2003, 02:06 PM   #5
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Could you explain this to me because i don't understand?

Does this mean i would need a huge box (343.97L)?

How would it effect the sound if i use a 55.75L box?

====

Transient response is a function of the system's Qt where 0.5 = critically damped, or perfect transient response. Above this point it's underdamped ('overshoot', or 'rings'), getting progressively worse with increasing Qt (hump in response), ergo below this point it gets progressively more overdamped (inability to complete the signal before decaying).

Yes, if perfect transient response is a primary performance goal.

It will have less LF extension, more efficiency before roll off begins, so will sound louder, more tonally balanced, and will sound a little more resonant due to the slight 'ringing'. The room and the speaker's placement normally affects it to some degree, so going for the largest practical cab size and then experimenting with distance from a wall or corner to boost it back up is what many folks do.

GM
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Old 7th October 2003, 03:28 PM   #6
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Quote:
Originally posted by GM
To stay linear though, max power handling below ~186Hz falls off at 12dB/octave, so by the time it gets to Fb it can only handle ~0.3W.

GM
Fatmarley:

To briefly explain GM's comment on "linear": to move bass requires large movements of the driver back and forth. Any driver has a "linear range", where the cone moves back and forth with the signal, ( a low distortion condition), and then enters into a "nonlinear range", where the cone does not quite follow the signal, ( a high distortion condition). A typical driver will be "linear", for instance, at a movement of 4mm. However, if the same driver needs to move 6 mm, it will start playing in a "nonlinear" fashion. That is, it will produce high distortion.

A quick glance at these Fostexes seems to show that they have very little capacity for movement back and forth, in either linear or nonlinear fashion. That is, they have small "excursions".

Two more things.

A) A sealed box gives less distorted bass than a ported system. However, it should be pointed out that this distortion is in the bass range, where it is not as hearable as it is in the midrange. Different sealed box setups have varying degrees of distortion, ("ringing", "overshoot",etc), but almost all of them are lower than the same pehenomena in ported boxes.

B) Ported boxes, for various reasons too involved for this post, can produce more bass for less cone movement than sealed boxes. For the same maximum cone movement, a speaker in a ported box can produce four times the output, with the same cone movement, that it could in a sealed box.

Yes, you read that right. Up to four times the output for the same cone movement. But the output will have somewhat higher distortion.

I believe that the reason that Fostex recommends ported boxes for it's drivers is that since the Fostexes have very limited cone movement compared to other speakers, it is imperative that they get the most bass output for the limited cone movement they have. That means a ported box, (or Transmission Line), and not a sealed box.

A speaker in a Transmission Line will typically have twice the bass output, with the same cone movement, that the same speaker will have in a sealed box. Not as good, output-wise, as a ported box, but still better than the sealed box in this regard. Some people prefer the sound of a Transmission Line to a ported box.
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Old 8th October 2003, 04:20 AM   #7
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Just looked up the Xmax for the FX200 specifically. The Xmax is 1.0 mm.

A normal 8" speaker would likely have an Xmax of 4.0 mm or so. So the Fostex has an Xmax that is one quarter of a normal Xmax. The Fostex 8 inch has the air moving capability of typical 4 inch speaker.

With such a small Xmax, I would say that a sealed box is not appropriate if you want to get any appreciable bass out of this speaker.
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Old 8th October 2003, 07:01 AM   #8
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Thank's Kelticwizard, GM and everyone else,

I will use the ported box recomended by Fostex.

Cheers
Matt
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Old 8th October 2003, 07:42 PM   #9
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I have found a pair of boxes which have an internal volume of 33 litres. would these work with a port? if so, what diameter and length would i need?

does anyone know how it would affect the sound having a slightly bigger box than the recommended 25 litres?
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Old 8th October 2003, 08:33 PM   #10
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Both sizes are way too small for this driver, max flat = ~117.43L, so the 33L will be less 'boomy' than the 25L, with slightly less power handling as the tradeoff. A 5.08cm dia x 13.2cm long vent should tune it down to around Fs. Tuning it up to ~40hz with a 7.8cm long vent brings the 'boom'/power handling back up to Fostex standards.

GM
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