Enclosure Volume

[sonu]

I have made a good enclosure for my sub, having a good tight low bass, as the thiele parameters were not known i made it in assumption to have a volume on 85 liters for 12" driver.

I am using a active Linkwitz crossover (200Hz).
The response is very good at moderate volume (ie at 60W), am using a 100 W RMS Amp. but when i increase further there is rattling sound from the woofer. Is that because of improper volume / port. Port used of 5" dia with 5" length.

I also want to know what effect would a Sub /Mids / Highs have if one uses Active crossover with a Passive crossover, which is already installed in the enclosure.

Regards,
Sonu.

[Cro maniac]

but when i increase further there is rattling sound from the woofer

The rattling is probably caused because coil reaches the magnet in moments when you put too much power to speaker. The same thing happened to my friend when his 12" subwoofer was driven in very large box with (120 liters) with weak amplifier which couldn't control the speaker very well.

There are few possibilities to solve this problem !

1. using smaller box
2. using smaller port (with less diameter)
3. using amplifier with larger damping factor
4. stuffing the box and (or) port, so speaker will have to work much harder to reach the bottom.

I suppose the last solution is most acceptable to you.

[kelticwizard]

The port you have in your 85 liter box should tune it to 44 Hz.

That's not bad, but depending on the kind of music you listen to, and whether or not you ever use a turntable, you mighty be getting some low frequency tones that are well below the box tuning and can drive the speaker into very large excursions. This might cause rattling.

Check to see if your rattling is accompanied by the speaker cone seeming to move very, very far.

[sonu]

Thanks,

Visually it seems that it is moving to its extremes when it starts to rattle, but my problem is that my woofers wattage is 150W, but still it's cone is moving to its extreme only @ about 60W, does an improper design would lead to this, OR damage my Sub.

Is there any perfect approch to measure the thiele parameters of the Woofer i.e VAS, Qtc, Fs etc. for calculating a perfect enclosure.

Also can I check the SPL.

[Vivek]

There are several methods. Go to www.diysubwoofers.org and do some research. You will get the method to measure the parameters and also design guidelines.
All the best.

Vivek

[Nappylady]

Sonu,

Something people often misunderstand about speakers is power ratings. The speaker might be able to handle 150 watts before the voice coil falls apart, but with the wrong enclosure, it might only handle 5 watts before it reaches maximum excursion and starts distorting terribly. Most woofers are designed to have the suspension keep the speaker from over-extending itself, but I have used some cheap ones in the past that will slam the voice coil against the back plate before the suspension is able to stop it.

The above comments are completely correct.

Open baffle speakers are the worst in terms of driver control. There's nothing to stop the speaker from just flopping around. Vented boxes are the next worst; playing sounds below the tuning frequency of the port leads to very high excursions. Small sealed boxes lead to the best control of the driver, assuming that the walls of the enclosure are strong enough and the stuffing is done properly.

Good luck in controlling your subwoofer!!

[kelticwizard]

Originally posted by Nappylady
Sonu,

Vented boxes are the next worst; playing sounds below the tuning frequency of the port leads to very high excursions.

Absolutely correct. There are two basic alternatives with your vented box.

A) Lower the tuning frequency so that the low notes of whatever music you are playing is included above the tuning frequency. Don't laugh-most records don't have that much of a deep bass content. Extending the length of the port tube is all you need to do here.

B) Put in an electronic filter to cut the signal beneath the tuning frequency. I don't know how handy you are with electronics, or what kind of filters your amp has.

It should be noted that many people tune their box volumes about half an octave below the normal tuning frequency on purpose, not so much because they want to limit excursion, but because they want to splice into the "room gain", (the tendency of many rooms to reproduce low frequencies at a higher volume than high or middle frequencies). This is called an "underdamped" setup, but all it amounts to tuning the box to about half on octave or less underneath the normal tuning. It yields a sound that gives slightly less bass in some bass notes, but more bass in the deep bass notes. And as an added advantage, it helps to cut down on cone excursion in the deep, loud bass notes.

To tune your box to 30 Hz, instead of the 44 Hz that it is presently tuned to, increase your 5" port to 15.5 inches. If you have to, just use an elbow so the pipe can fit into the box. Make sure that the internal opening of the pipe is not blocked by stuffing and has room to breathe.

[sreten]

Just a couple of comments on KC's post :

He is of course referring to "overdamped" bass alignments.

A simpler way of trying it is lining the port to reduce diameter to ~ 3".
If you like the result then optimally dimension the port.

However how well this works does depend on driver parameters,
but I will point out for an unknown driver the lower the port
tuning the better the likely result, overdamped alignments are
far more forgiving of innapropriate parameters.

Lets take an inexpensive 12" driver at random the BSB-12 :

http://www.bkelec.com/

Fs= 27Hz, Qts = 0.32 Vas = 260litres and use your Box.

No forward planning on my part, sealed loading is good as is
reflexed at 32 Hz, here 44Hz is not a good tuning frequency.

Note that port tuning is critical, flying blind you should experiment
with different port tuning frequencies until you hit a good result.

sreten.

[kelticwizard]

Some 12 inchers might sound OK with a 3" diameter port, but considering the size of your box, (85 liters), I would suggest you go to a 4" diameter port just to make sure.

To tune an 85 liter box with a 4" diameter port:
To 44 Hz: 2 3/4 inches
To 30 Hz: 9 3/8 inches

And you can tune anywhere in between, if you like.

It is my experience that if you tune by ear, as you have done here, you likely will end up with a tuning somewhat higher than optimum. That is because the higher the tuning, the more the bass notes above the tuning frequency get emphasized, (at the expense of the real low bass notes found below the tuning frequency). Many people find a little extra "oomph" gained by tuning higher sounds good.

I would definitely tune to at least 30 Hz, and listen to it for awhile. It might not have the bass punch gotten by tuning higher, but over the course of time you might find it more accurate that way. In fact, by experimentation, you might find yourself deciding to tune below 30 Hz.

[sonu]

I searched the website for measuring thiele para, and found it on ESP Site.
After measuring the parameters i found the following.

VAS - 75.19l
Fs - 54Hz
Qts - 0.870
Qms - 5.750
Qes - 1.020

Are the driver parameters OK

Now I simulated this on WINISD but could not come to a conclusion of Vb & ports needed, please suggest me the best ported design I could do with this driver.

Now one more thing I wanna know as how to select a best driver i.e. what parametrs i should look for when selecting. so that i can improve my driver on that basis, what all affects the change in VAS, Fs, Q etc.

Regards.