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Can someone please tell me whether the total resonance of a speaker system or the Qtc, will change if dampening material is added to a sealed chamber with specifically the righ t amount of volume to achieve the desired Qtc?
I'm trying to create a speaker with a Qtc of .707, and the woofer/midbass diver needs .273 cubic feet to achieve this. I always read that I should stuff the chamber with Acousta-Stuf, but that in doing this, the chamber is effectively increased. How does a perfectionist like me deal with this? PLEASE HELP!!!
Follow Ups:
Yoav's approach is probably the best overall. Trying to calculate your way into the answer probably is an exercise in frustration (or futility)But I do have something to add, and that you need to keep in mind. A small, sealed, and filled enclosure is properly called an acoustic suspension enclosure, and the filling is not there primarily for Q control or reflection control. It's there for thermodynamic reasons. A closed volume of air if compressed or expanded will not only change pressure, it will heat or cool. (adiabatic) This because no heat transfer is taking place. So the compression obeys a power law relationship and is not linear. Therefore, it introduces harmonic distortion. In Vilchur's Acoustic Suspension design he used the stuffing to transfer heat into and out of the air during the compression and expansion at an audio frequency rate. Since the mass of the filler, and it's thermal capacity is many thimes that of the air, the temperature of the air remains nearly constant.(isothermal)If the temperature remains constant, then the pressure/volume relationship is linear. This reduces the pressure rise (making the box look bigger), but it also reduces the harmonic distortion.
So if you leave out the filling so that your math works more precisely, you will lose the major benefit of the acoustic suspension principal. Further fiberglass has a lot more thermal capacity and better thermal properties than polyester fiber or Acousti Stuff. I have used 6" unbacked home insulation succesfully. JUst be sure you place the sheets edge on to the woofer. The surfaces of the insulation is pretty dense and closed, and will impede expansion and compression. Edge on the glass fiber is porous to the pressure waves.
Vilchur's original experiments used very fine brass wool (like 000 steel wool),but the production speakers used glass fiber. There is a measureable difference in harmonic distortion between glass and brass fiber, but supposedly the difference was inaudible. So the production AR's used glass since they din't need to worry about electrical insulation problems that way.
The fact that the filler is a good absorber, and prevents internal reflections at higher frequencies is just a happy side effect of it's primary purpose. The approach I take is to completely fill the box, and shift the Q by changing the packing density.
Typically, you can expect a 10-12% decrease in Q, and a 6% decrease in the in box resonance from the basic calculations. (that's from a paper by P. J. Snyder)
Hope this helps. I'm really an acoustic suspension buff, and I'd like to see better understanding of its non-obvious, but rather elegant principals. Hope you don't mind a thermo lession.
Wow- thats about all I can say to you. Your level of enthusiasm inspires me. And don't worry about being verbose in your thermo lesson; I found it very interesting.
I was aware of the idea of an acoustic suspension, but I was unaware that it was linked to the thermal aspect of the speaker. A few questions remain though...
You said that I should't leave the damening material out of my, by now, very involved equations, less I neglect the acoustic suspension benefits. I don't understand where I am susposed to account for it though. You said that all you do is fill the box completely, changing the Q by changng the density. Did you design the box 6% smaller and with a Q 10-12% higher? I do still intend to use dampening material, so wouldn't experimenting with the right amounts to find the right Q still take care of the thermo aspects, as long as there is dampening material present?
Dear grssplst,Damping material has a significant effect on enclosure Qtc and Fc. This is indeed because it effectively increases the enclosure volume.
Do not be concerned with achieving 'magic numbers' such as 0.707. Even though these numbers have a mathematical significance, in practice enclosure losses, room placement etc. have a great influence on Fs and the Qtc. This means that the only way of achieving good response is experimentation:
If the bass sounds overdamped ('dead'), remove some damping material.
If the bass sounds underdamped ('boomy'), add more damping material.You will be surprised at how easy it is to achieve excellent results this way. I have worked this way with all of my designs, and after measuring the results I found out that the response was exactly of the desired Qtc.
Sincerely,
Yoav Gonczarowski
YG Acoustics
If the Qtc. of the enclosure is right, then you're ok. What is the enclosure Q now? If it's higher than you like, add more stuffing. That makes the enclosure appear larger acoustically, lowering the Q. If the Q is too low, either remove some stuffing or add solid mass to decrease the size of the enclosure (acoustically-speaking), and raise the Q.
I haven't built the enclosure yet, jus done some calculations to find out what I need to build. Hypothetically, if I build an enclosure with .273 of volume, I shoul achieve a Qtc. value of .707. So if I can do this, should I even worry about dampening material? because I read somewhere that it is good to use some, so that sound waves aren't reflected back into the cone of the driver. Thanks for your help!
To design your enclosure, did you use factory supplied parameters or did you measure the Thiele/Small parameters for yourself? Factory figures can be off by 50%, so unless you measure your own drivers, you can't be sure how accurate the parameters are. Either way, the actual Q of the enclosure needs to be measured and adjusted if necessary, to achieve the desired results. Your calculated figures may be close enough, but there's no way to be sure unless you measure.You should use damping material like fiberglass, AcoustaStuff, or any of several other products available to control standing waxes and/or adjust the Q of a closed box woofer or mid range.
By the way, your enclosure volume is .273 of what? Liters? Cubic feet?
No, I didn't measure myself. I only wish I had enough surplus cash to be buying cool measuring devices. I am aware that the listed paramaters could well be way off, so I am going to go with the approach of building it, and then testing how much dampening material sounds best. By the way, it is cubic feet, and the driver is a 6.5" mid-bass driver about mid way on the performance scale I would say. Thanks for all your help, it is much appreciated!
Well, I sure can't tell you the formula, but Danny Richie of GR Research turned me on to Black Hole 5.He said that this was the best dampening product there is. I used it and it sure did wonders for my Paradox 1 speakers. I just covered all the exposed inside surfaces there were.
You'll have to get a formula from someone that makes Black Hole 5.
Cut-Throat
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