Re: slight change of topic but...
Hey man, AFAIC it's your thread! (I hope Cal isn't watching.. sorry Cal
)
Foam is definitely a good idea!
The foam won't affect your port as long as it is well clear of the back end of it. Foam will increase the effecive box volume slightly which can reduce tuning frequency to a less extent. It can also reduce Q slightly. None of these are a worry if the volume taken up by the foam is small compared to the box volume. It might be an idea to put more of the foam on the back wall between the driver and port where direct reflections would otherwise occur.
I often put a timber brace across the inside of the cabinet between the port and the driver such that any mid frequency waves bouncing from the back wall will be broken up by partial reflection off the brace.
I also put sculptured foam on most internal surfaces to speed up the decay of standing waves.
As far as the effects of bass response are concerned, the most extreme case I can recall was when I filled about 60% of a speakers volume with large chunks of foam. The effective box volume increased about 25% the Fb dropped about 12% and on top of these changes the net response was as if the driver had reduced its Qts by around 6%.
I'd doubt that you would go that overboard with your foam!
bremen nacht said:
Hey man, AFAIC it's your thread! (I hope Cal isn't watching.. sorry Cal
)Foam is definitely a good idea!
The foam won't affect your port as long as it is well clear of the back end of it. Foam will increase the effecive box volume slightly which can reduce tuning frequency to a less extent. It can also reduce Q slightly. None of these are a worry if the volume taken up by the foam is small compared to the box volume. It might be an idea to put more of the foam on the back wall between the driver and port where direct reflections would otherwise occur.
I often put a timber brace across the inside of the cabinet between the port and the driver such that any mid frequency waves bouncing from the back wall will be broken up by partial reflection off the brace.
I also put sculptured foam on most internal surfaces to speed up the decay of standing waves.
As far as the effects of bass response are concerned, the most extreme case I can recall was when I filled about 60% of a speakers volume with large chunks of foam. The effective box volume increased about 25% the Fb dropped about 12% and on top of these changes the net response was as if the driver had reduced its Qts by around 6%.
I'd doubt that you would go that overboard with your foam!
Re: Re: slight change of topic but...
Yes, as my cabinet is narrow and deep I can put a double thickness of foam ("eggbox" style stuff, sold as acoustic treatment panels for studios) on the back wall without getting close to the pipe.
I've arranged two "shelves" full of 100mm holes as braces and standing-wave breakers. I've also placed the foam 25mm off the walls.
Interesting figures! How did you measure the effective volume? I don't plan using that much foam but I'd like to be able to put a figure on how much effect the foam is having.
I took a day off work to work on the boxes today. It seems to be taking forever, and I seem to be spending more on blades and abrasives (MDF
) than I am on beer...looking good though, and I might just be ready to put a driver in a box and make some measurements tomorrow.
Cheers!
Yes, as my cabinet is narrow and deep I can put a double thickness of foam ("eggbox" style stuff, sold as acoustic treatment panels for studios) on the back wall without getting close to the pipe.
I've arranged two "shelves" full of 100mm holes as braces and standing-wave breakers. I've also placed the foam 25mm off the walls.
Interesting figures! How did you measure the effective volume? I don't plan using that much foam but I'd like to be able to put a figure on how much effect the foam is having.
I took a day off work to work on the boxes today. It seems to be taking forever, and I seem to be spending more on blades and abrasives (MDF
) than I am on beer...looking good though, and I might just be ready to put a driver in a box and make some measurements tomorrow.Cheers!
Measuring the effects of damping material
Your un-foamed box volume can be calculated by multiplying the dimensions as I'm sure you know.
To calculate the rest you need to be able to measure AC impedance and frequency rather accurately. Can you do this?
If you need instructions to measure Fcb and Qcb let me know.
To determine the change in effective box volume with foam you need to Block the port so that the enclosure is as air tight as possible then determine the system resonance frequency (Fcb) without and with the foam.
call them Fcb and Fcbx (x stands for with foam
)
effective volume relative to real volume is given by
Vb change = (Fcb / Fcbx)^2 where ^2 means squared
a result of 1.25 is obviously a 25% increase
The change in box tuning frequency (Fb) is proportional to the change in Fcb..
Fb change = Fcbx / Fcb
Note that the change in Vb af Fb are linked and if you bring Vb back down by reducing actual box volume that Fb will move back to where it should have been also.
The change in box volume and tuning is a definite effective change that I always compensate for! If you want to go heavy on the foam you can kill back some of that box volume increase by putting more bracing in!
To estimate the change in effective Qts, determine the closed box system Q (Qcb) without and with foam.
Call them Qcb and Qcbx
New effective Qts -> Qtsx =Qts x (Qcbx/Qcb)x(Fcb/Fcbx)
Its worth noting that this is not a true change in Qts but actually realy a measurement of the resistivity of the foam interfereing with the box volume. This may not interfere with the drivers Qts as far as the port is concerned. It is a better idea to minimize this affect than to attempt compensation for it. The effect is reduced if there is no foam close to the drivers' frame and the foam does not interfere with the flow of air around the box.
Your un-foamed box volume can be calculated by multiplying the dimensions as I'm sure you know.
To calculate the rest you need to be able to measure AC impedance and frequency rather accurately. Can you do this?
If you need instructions to measure Fcb and Qcb let me know.
To determine the change in effective box volume with foam you need to Block the port so that the enclosure is as air tight as possible then determine the system resonance frequency (Fcb) without and with the foam.
call them Fcb and Fcbx (x stands for with foam
)effective volume relative to real volume is given by
Vb change = (Fcb / Fcbx)^2 where ^2 means squared
a result of 1.25 is obviously a 25% increase
The change in box tuning frequency (Fb) is proportional to the change in Fcb..
Fb change = Fcbx / Fcb
Note that the change in Vb af Fb are linked and if you bring Vb back down by reducing actual box volume that Fb will move back to where it should have been also.
The change in box volume and tuning is a definite effective change that I always compensate for! If you want to go heavy on the foam you can kill back some of that box volume increase by putting more bracing in!
To estimate the change in effective Qts, determine the closed box system Q (Qcb) without and with foam.
Call them Qcb and Qcbx
New effective Qts -> Qtsx =Qts x (Qcbx/Qcb)x(Fcb/Fcbx)
Its worth noting that this is not a true change in Qts but actually realy a measurement of the resistivity of the foam interfereing with the box volume. This may not interfere with the drivers Qts as far as the port is concerned. It is a better idea to minimize this affect than to attempt compensation for it. The effect is reduced if there is no foam close to the drivers' frame and the foam does not interfere with the flow of air around the box.
Thanks, Filgor, very useful stuff.
I can measure f of resonances by measuring driver impedance, don't know how to measure Q.
I read somewhere that signals below the resonant frequency of the port will kill the driver. Should I apply a high-pass filter to remove these signals, or is it sufficient to keep an eye on stupid amounts of cone excursion when listening loud?
I can measure f of resonances by measuring driver impedance, don't know how to measure Q.
I read somewhere that signals below the resonant frequency of the port will kill the driver. Should I apply a high-pass filter to remove these signals, or is it sufficient to keep an eye on stupid amounts of cone excursion when listening loud?
bremen nacht said:
Measuring Qcb is the same as measuring driver Qts except that you have it mounted in your box..
Heres a recipe for measuring Qts
http://sound.westhost.com/tsp.htm
bremen nacht said:
Frequencies more than half an octave below FB can kill the driver at very high levels by stressing the suspention and banging the coil on the back plate. More importantly they can cause horrible distortion at much lower power levels because they can drive the coil to its "x-max". Basically you will hear a problem long before you destroy your driver.
I often put in a 12dB/octave Butterworth highpass at about 2/3 of the tuning frequency to reduce distortion.
Its worth noting that most recordings dont have a lot of power below 40Hz so if your tuning frequency is below 40Hz you probably dont need the high pass. Most good amps have a high pass built in just below 20Hz to remove "rumble" from LP turntables. Also I'm guessing your 12 inch drivers need a low Fb anyway.
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