Some questions about boxes

Hi folks! I have a few questions for you:
1.) if sealed box is not perfectly airtight, what will happen with Qtc of box?

2.) can someone explain what is happening when I put absorpting material in the box (i.e. fibreglass)? How this affect Qtc of sealed box, and how it affect ripple in vented box? (Qtc gets lower, isn' it?)

3.) in addition to question 2.), when I use CAD programs for calculating box volume, and I get in example 50l for sealed box - is this 50l for empty box or for damped box with fibreglass? And how much fibreglass inside? Similarly for vented box?

Well, thank you for your help!
 

roddyama

diyAudio Moderator Emeritus
2002-01-19 9:25 am
Michigan
Hi Namo,

1.)Greater the leakage, the lower the Qtc.

2.)The absorbing material will dissipate the back wave by converting the energy to heat. This will make the box look bigger to the driver. In a closed box, it will decrease the Qtc. You shouldn't use much filling material on a vented box because you will severely effect the efficiency of the port.

3.)50l = the net volume in the box. The stuffing in a closed box can increase the box compliance by as much as 15%, but that would be the maximum using long fiber wool optimally placed in the box. Fiberglass will be less, and Dacron even less. For a vented box, see #2 above.

Hope this helps;)
Rodd Yamas***a
 
roddyama said:
Hi Namo,
2.)The absorbing material will dissipate the back wave by converting the energy to heat.

Some clarification:
If you stuff a sealed box full of fiberglass-filler you create an iso-thermal box (constant temperature). The added mass holds the box at a constant temperature, thus acting like a 15-18% larger volume of air.
 

roddyama

diyAudio Moderator Emeritus
2002-01-19 9:25 am
Michigan
Hi 'cashman,

Close. Actually, the backwaves from the speaker forms a series of pressures and rarefactions that is the backwave. The air heats on the pressures and that heat is released on the rarefactions. So the process is "adiabatic" (roughly isothermic). When stuffing is introduced into the box the backwave causes the fibers in the stuffing to vibrate against each other. This creates heat, or more accurately uses the energy from the backwave making the volume in the box react as if it were bigger.

Rodd Yamas***a