You measure the Qtc of the system. ( look up measuring TS parameters on Google). Lots of info on the Net on how to do this. To know the effect of the damping material, compare before and after damping , the values of Qtc. Qtc will fall with increasing amount of damping material used. 'Ideally' you want it to be around 0.707 though practically it might not be what you require for your installation. Increasing Qtc will start to give a rising bass response before eventually falling of again( with decreasing frequency).
Remember that too much filling also reduces cabinet volume by the finite volume of the absorbent and so beyond a point it will start to raise the Qtc. But to raise Qtc all you need to do is reduce cabinet volume by say placing a brick inside or a bag of sand etc.
Adding damping material will decrease Qtc ! That will mean bass will roll off faster around the 'in box' resonant frequency. May not be a bad thing if you find the balance between box positioning ( typically near a wall or corner) and the Qtc. Close wall or corner will raise the low frequency response significantly. If the Qtc is too high it can cause 'boomy' bass. You could listen and evaluate or better than that is to make an acoustic measurement .
Without tools, hearing could be quite sufficient. Start your tests with no absorbent.
Using absorbent to alter Qtc is completely different from "damping cabinet walls". Cabnet walls can be stiffened with braces and struts glued to them.This will raise the first 'cabinet resonance' to beyond the operating range of the sub.You can also glue bitumen felt to the walls which will weigh down the walls and also damp them due to the lossy bitumen content.For small enclosures like a 15 inch cube , this isn't difficult to manage , unless your walls are thinner than 19 mm MDF. There are numerous types of stiffening methods shown on the Net. Google is your best friend.
About "does it alter sound by 'much' "! Some like rising bass which could boom and others prefer a flatter bass response till it rolls off.......with 'extended' bass. While it doesn't directly affect the sounds coming from the mid and upper range, the combination will sound different with different levels of bass or different bass extension. Qtc affects the sound far more than box ( wall) damping effects !
Check out the effect of using a sub near a wall in a graph that I posted on this forum.That was for the LAB12 driver. If the box Qtc is high then a near wall placement might end up having a bass peak causing boomy bass if the Qtc is too high.
measuring Qtc - i dont have any instruments et al..
except the guideline of Qtc 0.707 for critically damped , Qtc > 0.707 for over damped and Qtc < 0.707 for underdamped response.
I did read a lot of this stuff you mentioned on the net... but that remains on paper.
I'm lazy
regards
Due to this precise reason i dont prefer a sealed enclosure and went for TL designs 6 years ago. in a TL you dont need to damp for cabinet but need them only for the enclosure.
@mpw,
Yesterday we discussed a lot about the points you have mentioned in the post. I did a quick calculation after our discussion and below are the results as per my calculation,
- Before you stick the absorbant material to the cabinet wall weigh them.
I have considered a damping coefficient of 1.6 pounds/cu.ft based on your Fs, Vas and Qtc.
For an internal volume of 43.5 liters the following is the weight required,
Critical Damping - Qtc - 0.71 - 1111 grams
Under Damping - Qtc - 0.604 - 1333 grams
Over Damping - Qtc - 0.85 - 890 grams.
The polyester cotton should be very low density type and should allow passage of air and should be loosely packed in the whole of the cabinet.
If your absorbant material weights say 500 grams you will need to subtract this value from the above to keep the net weight the same. You can safely begin with the above values and then adjust based on subjective listening. My suggestion will be to start with critical damping and then under damp and overdamp after listening tests.
Cheers,
haribhai,
Only 1 reason ( of damping ) cannot be the reason to junk the sealed concept altogether.
a. By absorbant material - you mean the cabiner bitumen and felt and foam right ??
b. Thanks for the inputs - they do form a decent starting point.
c. is there a formula to calculate the damping co-efficient of 1.6 pounds per cu ft ? what would happen if the figure is 1.4 or say 1.8 pounds / cu ft ??
d. The nett internal volume is 38 liters and not 43.5 as we have to subtract the driver volume projecting into the cabinet.
regards
mpw
Not scientific, but it has worked for me - start with zero filler, listen to how the bass sounds. If it's boomy, add stuffing just about enough to fill up the internal cavity without compressing the filler much (loosely filled). Listen again. Add more if bass weight and texture isn't right yet. When you think it's just right, don't stop but fill even more and listen, and see if you like it even more. If not start removing some filling. Blah, blah, and blah. Do this over multiple sittings so that ear fatigue doesn't cloud your judgement. You will arrive at what's best sounding to your room and ears. It's just like using a pot to fine tune some values in a circuit.
Jls001,
you made a subwoofer too right ?
a. what cabinet damping did you use ? if at all.
b. By filler - you mean the white fluffy material for the cabinet right ?
c. Where do you get that in Mumbai ?
can you put up some pics of the innards of your subwoofer just before sealing it ?
or email me please.
d. I am apprehensive that i may not get more than 2-3 chances to put / remove filler material as i think threading on MDF is like walking on this ice..
e. Is there a way to drill and tap on MDF such that i can remove the driver and fill / remove filler material.... at will ?.... well... almost..
regards
mpw
