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Old 8th May 2006, 03:29 AM   #1
ostie01 is offline ostie01  Canada
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Default Where should be fb compare to fs

Hi, where should I put fb compare to the fs woofer. Some prefer to put fb lower than fs ,others prefer very close to fs , even some prefer above fs to protect woofer from clipping.Thanks
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Old 8th May 2006, 10:09 AM   #2
sreten is offline sreten  United Kingdom
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Hi,

don't understand your question, Fbox is always > Fs. (Sealed box).

/sreten.
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Old 8th May 2006, 11:18 AM   #3
Did it Himself
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I think he's asking at what frequency should he tune his vent, in relation to Fs. If this is a correct assumtion, the answer is that it depends on your design goals and the Qts of the drive unit.
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Old 8th May 2006, 01:46 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

If its vent tuning frequency then it is confusing.

It obviously depends on Fs, Qts, Vas and Vbox.

Vas and Vbox determine Fbox and Qtc (for a sealed box).

The port tuning frequency is really in relation to Fbox, not Fs.

Though some classic alignments sort of are, eg. Vbox=Vas.
Then Fbox = 1.4 Fs (= x root 2), and if the port is tuned root2
lower than Fbox then port tuning = Fs.

/sreten.
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Old 9th May 2006, 03:37 AM   #5
Ron E is offline Ron E  United States
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Fb can be above or below Fs. The goal is to give a proper response chape rather than to prevent "woofer clipping".

BTW, woofers don't "clip" per se, but they do overload and reach excursion limits, which can make a nasty sound.

Tuning high will not prevent the woofer from overloading thermally or mechanically. In most cases, tuning higher will increase excursion (woofer movement) below Fb and (depending on the signals sent to the woofer) will actually make it more likely to overload mechanically.

In general woofers tuned for flat response in vented boxes with Qts greater than 0.4 will be tuned below Fs and woofers with Qts less than 0.4 will be tuned above Fs.

Tuning higher than normal will usually give a peak in response and tuning lower than normal will cause a droop. Either is valid if it sounds good to you...
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Old 9th May 2006, 07:52 AM   #6
ostie01 is offline ostie01  Canada
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HI, I can see that my question is a little confusing.But thanks for those replies.My speaker is an Eminence Omega Pro 15 in.Here're the spec.

Nominal Basket Diameter 15" / 381mm
Impedance 8 ohms
RMS Power Rating 800 Watts
Frequency Response 35Hz - 2kHz
Sensitivity (1W/1m) 99dB
Voice Coil Diameter 4" / 101.6mm
Max. Crossover Frequency 2kHz

MOUNTING INFORMATION
Overall Diameter 15.21" / 386.4mm
Bolt Circle Diameter 14.562" / 369.9mm
Baffle Cutout Diameter 14" / 355.5
No. Of Mounting Holes 8
Mounting Hole Diameter 0.275" / 7mm
Overall Depth 6.35" / 161mm
Magnet Weight 109 oz.
Shipping Weight 26 lbs. / 11.8kg.

THIELE-SMALL PARAMETERS
Resonant Frequency (fs) 39 Hz
Impedance (Re) 4.98 ohms
Coil Inductance (Le) 1.48mH
Electromagnetic Q (Qes) .36
Mechanical Q (Qms) 8.57
Total Q (Qts) .35
Compliance Equivalent Vol. (Vas) 196 Liters/6.92cu. ft.
Voice Coil Overhang (Xmax) 4.8mm
Surface Area of Cone (Sd) .08563 m2

Right now, it is mounted in a box that is about 6 cubic feet with fb at
30Hz. The vent is 2 X 15 about 6in long.

My question is what happen if I make the port longer, I know that this will lower fb but what happen to the freq responce. Fs is 39Hz.
Some told me to raise Fb to 40Hz, this mean that I will have to make the port shorter or put somthing inside the box to make the box smaller.thanks for any advices.
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Old 9th May 2006, 08:11 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
in a sealed box speaker, the Fb is the box frequency.

In a vented box, what does Fb represent?
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Old 9th May 2006, 08:56 AM   #8
Did it Himself
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Fb = frequency of box = vent resonance frequency

If you increase Fb to 40Hz it will make the output stronger at the frequency, at the expense of depth and protection against over-excursion damage lower down. It will depend on your application how you need to tune it.

Right now the box volume is about right but tuning could maybe do with bringing up a little, to fit a 'classic' alignment.
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Old 9th May 2006, 11:56 AM   #9
sreten is offline sreten  United Kingdom
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Hi,

your current tuning models as 33 Hz.

Shown is the effect of doubling and thirding the port length.

Generally speaking port tuning should be the lowest frequency
that you expect significant power handling capability, below this
power handling is poor, for really high levels a high pass filter
is a good idea.

40Hz is goood down to bottom E on a Bass, 30Hz good for low
B on a Bass or from Keyboards also gives more realistic kick drum.
Lower than 30hz, say 26Hz is used when the prevention of low
bass overload is preferred to bass extension and loudness.

/sreten.
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