I am planning to build a sound system utilizing Art Welter's Keystone subwoofer design, outfitted with one of the suggested B&C 18inch drivers. Question I had was whether to go with 4ohm or 8ohm driver variants. 4Ohm seemed preferable because it would allow for greater amplifier headroom for a given amplifier, but then I came across a thread somewhere that suggested something to the effect of: "All else equal, a higher impedance driver/amplifier combination will sound tighter." I interpreted this as: Running from the same amplifier, 100db from an 8ohm driver would tend to sound tighter than 100db from an otherwise-identical 4ohm driver.
This led me down a rabbit hole, where I uncovered the following: this thread here partially answered my question with respect to physical construction of drivers. Summarizing posts #8 and #11: "lower impedance means not only thicker wire and less turns, but more moving mass in the voice coil - therefore, voice coil movement becomes "sluggish", compared to different , lighter coils, high frequency response suffers, same with "sharpness", transient attack". I also found another thread that I can't re-find that suggested the following, focusing on the amplifier & current: With total output power being equal, the higher voltage into the 8ohm driver equates to the amplifier having a stronger 'grip' on the driver. The hose analogy was invoked - for the same amount of water, the 8ohm load sees a higher pressure and therefore should be more responsive. Ok, got it - higher impedance has sound quality advantages.
The downside then would becurrent draw & amplifier strain/headroom, since an 8ohm system will draw more current than a 4ohm system run at the same power/volume and also utilize more of an amplifier's maximum output power. (Edited to correct backwards math.)
I'm willing to believe that the sound quality difference is meaningful, and I will prioritize SQ to the extent possible, but I don't have an unlimited amplifier budget for amps to provide ample headroom for the 8ohm load.
So - which way to go? I'm sure some of you pro audio techs have wrestled with this before - is the extra ~3db of headroom (assuming roughly double the power at 4 ohm is available from a given amp model vs 8 ohm) worth the tradeoff?
This led me down a rabbit hole, where I uncovered the following: this thread here partially answered my question with respect to physical construction of drivers. Summarizing posts #8 and #11: "lower impedance means not only thicker wire and less turns, but more moving mass in the voice coil - therefore, voice coil movement becomes "sluggish", compared to different , lighter coils, high frequency response suffers, same with "sharpness", transient attack". I also found another thread that I can't re-find that suggested the following, focusing on the amplifier & current: With total output power being equal, the higher voltage into the 8ohm driver equates to the amplifier having a stronger 'grip' on the driver. The hose analogy was invoked - for the same amount of water, the 8ohm load sees a higher pressure and therefore should be more responsive. Ok, got it - higher impedance has sound quality advantages.
The downside then would be
I'm willing to believe that the sound quality difference is meaningful, and I will prioritize SQ to the extent possible, but I don't have an unlimited amplifier budget for amps to provide ample headroom for the 8ohm load.
So - which way to go? I'm sure some of you pro audio techs have wrestled with this before - is the extra ~3db of headroom (assuming roughly double the power at 4 ohm is available from a given amp model vs 8 ohm) worth the tradeoff?
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No you got that backwords, with any given amplifier a 4ohm driver draws more current than an 8ohm and that lower impedance load stresses the amp more. And If the 4ohm driver has the same true 1w sensitivity as the 8ohm version it will produce more SPL but you have to look real closely at the specs for both drivers... sometimes it's not obvious if that is the case or not. For the record 1w at 4ohms requires 2v while it takes 2.83v at 8ohms.
Now all that said, a 4ohm load on a single channel of a high quality amp isn't considered to be excessively stressful.. in fact it's considered by some to be the sweet spot. However If the amp has to be bridged to deliver enough power that will have a noticable impact on sound quality so this use should be reserved for emergencies, so if building a system from scratch try to avoid setting it up this way.
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Thanks @conanski ... math is not my strong suit. Appears you are correct on that point and I multiplied where I should have divided, or something. (For simpletons like me: 1W @ 4Ohm = 2.0V = 0.5Amps, whereas 1W @ 8Ohm = 2.83V = 0.35Amps.) Also understood that "all else is equal" is not a reality, and specs are different even within the same model. Edited the first post since the other questions still stand.
You say 4ohm is the sweet spot - why? Granted that modern amplifiers can handle a 4ohm load, what makes that more appealing than an 8ohm load if both SQ considerations and overall current draw favor 8 ohms? The only variable that's left is maximum output watts & SPL, and by extension, cost per watt. Is that right? (again, assuming all else truly is equal.)
Good callout on (not) bridging amps, I have seen that several times in my googling and it's worth noting in this discussion.
You say 4ohm is the sweet spot - why? Granted that modern amplifiers can handle a 4ohm load, what makes that more appealing than an 8ohm load if both SQ considerations and overall current draw favor 8 ohms? The only variable that's left is maximum output watts & SPL, and by extension, cost per watt. Is that right? (again, assuming all else truly is equal.)
Good callout on (not) bridging amps, I have seen that several times in my googling and it's worth noting in this discussion.
A 4ohm load strikes a balance between power delivered and driver control and will result in maximum SPL with minimal SQ impact driving two 8ohm drivers in parallel, and that is because you get a free 3dB increase in sensitivity when doubling the driver radiating area.
Many people get hung up on the idea of "fully powering" subs in particular but that is not always the most efficient use of the available power. For example if you were to power 2 modern drivers to 3000w each or power 4 of them to 1500w each.. all loaded in the exact same enclosure type. The 4-box set will produce upwards of 6dB more SPL because of the increase in sensitivity and because the drivers are operating with less power compression... which is loss of output with voice coil heating.. something that always occurs with any system that has been run at this level for any amount of time. Of course if you don't have room for 4 boxes then the point is moot but I offer it here as it may be relevant.
Many people get hung up on the idea of "fully powering" subs in particular but that is not always the most efficient use of the available power. For example if you were to power 2 modern drivers to 3000w each or power 4 of them to 1500w each.. all loaded in the exact same enclosure type. The 4-box set will produce upwards of 6dB more SPL because of the increase in sensitivity and because the drivers are operating with less power compression... which is loss of output with voice coil heating.. something that always occurs with any system that has been run at this level for any amount of time. Of course if you don't have room for 4 boxes then the point is moot but I offer it here as it may be relevant.
Right, and again that points to SQ upside for 8ohm, and cost being the downside (plus space which is a good point).
Seems like 8Ohm cabinets scales better and is the clear choice if I go beyond my initial 2 cabinets. If sticking with 2, or aiming for absolute maximum SPL, then perhaps it favors the 4's.
Thanks!
Seems like 8Ohm cabinets scales better and is the clear choice if I go beyond my initial 2 cabinets. If sticking with 2, or aiming for absolute maximum SPL, then perhaps it favors the 4's.
Thanks!
i just want to weigh in about one factor not fully discussed, the amp that's powering the sub's.
long before i would worry about driver impedance (or in this case load impedance) just what have i got to power subs with? from there it gets clearer what can be achieved in terms of number boxes vis a vis what there load impedances are.
the other factor is driver "sensitivity" and that one easily become a factor in sound quality with respect to clean headroom of the system or sub in this case.
long before i would worry about driver impedance (or in this case load impedance) just what have i got to power subs with? from there it gets clearer what can be achieved in terms of number boxes vis a vis what there load impedances are.
the other factor is driver "sensitivity" and that one easily become a factor in sound quality with respect to clean headroom of the system or sub in this case.