Ported Enclosure Advice

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First post here.

I've built a few kits in the past, but have now decided to start totally from scratch and do everything, all the way through to designing the crossovers.

None of the maths phases me so far, but after running the numbers (using equations from DIYSubwoofer website) for the sub im using, im starting to wonder if i have done something wrong, or am missing something. I think i just didnt expect it to be that big.

Im using a SEAS Prestige L26RFX/P 10" driver. From what i have come up with, the enclosure should be 88 litres (not including port volume) with the port being 12cm in diameter and 15" long.

The port calculations were made from the DIY subwoofer page, minimum diameter came out at 9.6 cm and it said to make it a bit bigger, so i used 12cm, is this ok? The length originally came out 21.6 inches, but after reading around the internet and talking to a couple of well respected builders it seems that it is too long and should be multiplied by 0.7 in order to get a better length. Obviously i will make it longer and play around once the speakers are built.

Some parameters:
Fs: 20Hz
VAS: 171 Litres
QTS: 0.33
QES: 0.39
Xmax: 7mm


Any advice anyone can provide would be greatly appreciated.

Cheers
 
Cheers for the reply's.

@sreten: From what i have read, a port should not be tuned to the exact Fs of the driver as it can cause problems with the port, noise etc. Is this information not accurate? Tuning the port to 20Hz sounds like a fine idea, as i would really like these woofers to perform as low as they can go, but don't want to sacrifice quality in the process.

@Thawach: Is there a benefit to using two ports instead of one? The enclosure is most likely going to be trapezoid shaped, so the positioning of two ports would have to be 'stacked' vertically instead of horizontally. Could this cause problems?
 
Thawach, are those numbers for two ports? The volume is the same as i have been getting, not including the driver, ports and bracing.

The port diameter is the exact same as i have been getting, and reading around it seems making it slightly larger will reduce the chance of port noise. The length however is quite different.
 
JakeDN said:
Cheers for the reply's.

@sreten: From what i have read, a port should not be tuned to the exact Fs of the driver as it can cause problems with the port, noise etc. Is this information not accurate? Tuning the port to 20Hz sounds like a fine idea, as i would really like these woofers to perform as low as they can go, but don't want to sacrifice quality in the process.

@Thawach: Is there a benefit to using two ports instead of one? The enclosure is most likely going to be trapezoid shaped, so the positioning of two ports would have to be 'stacked' vertically instead of horizontally. Could this cause problems?

Hi,

The Fs of a driver does not influence the practical choice of the vent
tuning frequency. The driver has a very low Fs, consequently in a
large vented box handling of subsonics below the the port frequency
will be poor, ideally a subsonic filter is needed for high levels.

Multiple ports do not really help, CSA and length ends up near the same.

Flaring does help = smaller CSA = shorter length.
http://www.subwoofer-builder.com/
Has all the info you likely ever need to know for vented subs.

Dropping the port tuning from ~ 25Hz to ~ 20Hz will improve "quality".
The response droops but transient response and phase delay improves.

:)/sreten.
 
JakeDN said:
First post here.

Any advice anyone can provide would be greatly appreciated.

Cheers

My advice is that you need to be able to choose which alignment will work for you. The software (or a calculator) will then help you implement it. The wrong choice of alignment, for your particular needs, can lead to dissapointment.

If you don't know where you are going, more damped responses (like bessel) are "safer" (less likely to be problematic, though perhaps sub-optimum).

There is a lot written on the subject.

Ken
 
JakeDN i try again with your driver


106.47 litre
fb=23.05 hz
f3=24.55 hz
vent diameter=9.6 cm
vent length=31.15 cm

95 litre
fb=24.14 hz
f3=26.83 hz
vent diameter=9.6cm
vent length=32 cm

110 litre
fb=23 hz
f3=24.9 hz
vent diameter=9.6cm
vent length=30.01 cm


115 litre
fb=22.7 hz
f3=24.4 hz
vent diameter=9.6cm
vent length=29.41cm

here is one port. it is different a little(long) please u consider.
 
JakeDN said:
sreten, would you mind explaining the advantages/caveats of Bessel and Butterworth Q?


Hi,

Butterworth is maximally flat frequency response.
Bessel is maximally flat group delay.

Most decent alignments are somewhere between the two, though
for subwoofers higher Q than Butterworth and active EQ can give
you a more compact cabinet, as will a Linkwitz transform.

There are no simple answers to the above. But a good rule of thumb
is detuning the Butterworth 4th order port frequency by 0.7 to 0.8 .

:)/sreten.
 
sreten said:



Hi,

Butterworth is maximally flat frequency response.
Bessel is maximally flat group delay.

Most decent alignments are somewhere between the two, though
for subwoofers higher Q than Butterworth and active EQ can give
you a more compact cabinet, as will a Linkwitz transform.

There are no simple answers to the above. But a good rule of thumb
is detuning the Butterworth 4th order port frequency by 0.7 to 0.8 .

:)/sreten.

Ahh, would this be where the 0.7 would come in to the port length calculations i was advised to make? Or am i missing something?

[Edit] A quick question i forgot to mention. If i tune the port to 20Hz with the box at ~25Hz is room gain potentially going to be a problem? I want tight deep bass, nothing boomy.
 
JakeDN said:


Ahh, would this be where the 0.7 would come in to the port length calculations i was advised to make? Or am i missing something?

[Edit] A quick question i forgot to mention. If i tune the port to 20Hz with the box at ~25Hz is room gain potentially going to be a problem? I want tight deep bass, nothing boomy.


Hi,

It might be but to detune by ~ 0.7 you need to ~ double port length.

Room gain is always potentially more of a problem with the standard
Butterworth maximally flat tuning then the detuned alignment. TBH
if you are concerned about room gain and bass quality, not quantity
sealed box loading of the Seas driver is the more obvious choice.

:)/sreten.
 
JakeDN said:


Ahh, would this be where the 0.7 would come in to the port length calculations i was advised to make? Or am i missing something?

[Edit] A quick question i forgot to mention. If i tune the port to 20Hz with the box at ~25Hz is room gain potentially going to be a problem? I want tight deep bass, nothing boomy.

On your first point:-

No, the 0.7 is an estimate of the end effect (the difference between the physical length of the tube used and the number needed in the formula to estimate the mass of air that is moving). It depends a bit on the shape of both ends of the port, on the location on the baffle, distance to nearby surfaces and so on.


On the other point:

That is what I was hinting at above. It is hard to choose the best alignment without lots of information about your room. The idealised target alignments only behave as advertised in open space. If you are worried about a resonance (room mode or speaker resonance) some choices are:-

a) aim for slightly low Q (bessel rather than butterworth, if you want to use the names), or just plot the response and guess what might work well in your room (if you know the worst room modes). That way you are more likely to end up with dips and less severe peaks rather than a very bad peak.

b) use EQ dedicated to your speaker and room.
(For example, many of the effects are minimum phase and so can be corrected with a parametric equalizer.)

c) make the port removable and try various lengths before finally fixing it. (Test tones are useful for that.)

Unfortunately it seems there is no universal solution.

Good luck though!

Ken
 
Cheers for all the help guys. I've got a while to think things over and decide on a final plan of action, this is going to be a summer project.

The port is going to be removable, im currently looking into the best way to affix it so it doesn't leak. I've got the right router bits so im going to make the port from scratch.
 
...minimum diameter came out at 9.6 cm and it said to make it a bit bigger, so i used 12cm, is this ok?...

Ok if you choose a dia.12 cm port and want to tune your 88L box to 20 Hz, then the port length is about 76 cm, but the maximum power you can put in the woofer is very limited and a port with dimensions: 7 cm dia, 25 cm in length, will result in a vent air flow less than 9 m/s at maximum input power that should be restricted at 20 Hz with a 12 dB HP filter.

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