Posted May 5, 201015 yr How can one calculate the box rise and how does the shape of the box, etc. affect box rise? I am interested because I want to wire my subs down to .5 ohms, and I hear that it probably wont be an issue because of "box rise."
May 5, 201015 yr Why not?Because it fluctuates every second. Frequency, heat, and box size all make it change to where you can't calculate it on an ongoing basis.
May 5, 201015 yr Why not?Because it fluctuates every second. Frequency, heat, and box size all make it change to where you can't calculate it on an ongoing basis.Okay, thanks man.
May 5, 201015 yr Author Okay, thats what I was wondering.. People were telling me that their box's made a 4x or 3x affect on the impedance, but I know it fluctuates. Think they were talking about on average?
May 5, 201015 yr There is no such thing as "Impedance rise". There is an impedance curve, and this goes up and down, so it rises and falls. Don't get hung up on "rise", because it is only the case half the time. The main things that cause your impedance to change is the different resonances the complete alignment has. You can calculate this using just about any of the modeling programs out there. They take all the things like box size, vent resonance etc into account when predicting impedance. But again, this is only a prediction.... The best way to determain your impedance curve, and more importantly the impednace at whatever frequency or range of frequencies you intend to use, is to measure this when the system is complete. If you are trying to get every last bit out of your amp at the very lowest impedance before it cooks itself to death, it would be nice to know what that impedance actually is.Another little tid bit, the overall impedance will never be below what you measure as DCR with an ohmeter. This is true at any frequency, in any box, at any power level. So if you read say 2.3 ohms DCR with an ohmeter, your system impedance will never dip below this....
May 6, 201015 yr Author There is no such thing as "Impedance rise". There is an impedance curve, and this goes up and down, so it rises and falls. Don't get hung up on "rise", because it is only the case half the time. The main things that cause your impedance to change is the different resonances the complete alignment has. You can calculate this using just about any of the modeling programs out there. They take all the things like box size, vent resonance etc into account when predicting impedance. But again, this is only a prediction.... The best way to determain your impedance curve, and more importantly the impednace at whatever frequency or range of frequencies you intend to use, is to measure this when the system is complete. If you are trying to get every last bit out of your amp at the very lowest impedance before it cooks itself to death, it would be nice to know what that impedance actually is.Another little tid bit, the overall impedance will never be below what you measure as DCR with an ohmeter. This is true at any frequency, in any box, at any power level. So if you read say 2.3 ohms DCR with an ohmeter, your system impedance will never dip below this....I see! So if i wire up the subs to .5 ohms.. it wont go below that.. but I can measure the difference of impedance with a ohm meter playing differnt frequencies to see how much of the time it is close to .5 ohms.. if it doesn't hover around .5 ohms for too many frequencies than it should be fine, especially with the sundown!Thanks! Great post!
May 6, 201015 yr make sure u have the electrical to back that up...even tho u might not be seeing right at .5 ohms, you will still be drawing a shit load more current than if u were running at 1 or 2 ohms.
May 6, 201015 yr I see! So if i wire up the subs to .5 ohms.. it wont go below that.. but I can measure the difference of impedance with a ohm meter playing differnt frequencies to see how much of the time it is close to .5 ohms.. if it doesn't hover around .5 ohms for too many frequencies than it should be fine, especially with the sundown!Thanks! Great post!Kind of on the first half, no on the measuring part.You can't measure the impednace with an ohmeter, only the DCR. To measure impedance the system has to be under power (can't test with an ohmeter with anything else connected to the sub) and you must either due this with an impedance bridge, current vs. voltage (not very accurate using cheap meters), a specific measuing device (Dayton woofer tester for example), etc....Also, you can't ever wire up your subs to .5 DCR. The DCR will always be lower than the average impedance. For instance, if yuo have a sub with a nominal .5 ohm coil, the DCR will be lower, say .4 ohms, even lower maybe. Theoritcally, at some frequencies, you might be right down near DCR, so your .5 ohm sub could be presenting your amp with a .4 ohm or less load. Remember, when manufacturers spec out an impedance for a sub, it is an average.... Edited May 6, 201015 yr by 95Honda
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