Tweaker's Asylum

Posted by Wrenchhead on January 04, 2002 at 03:53:08

In Reply to: Re: AC cord swap for powered subwoofer posted by groman on January 03, 2002 at 20:40:40:

You assumption of 94 db SPL, or even 90-95 SPL is way off. Much more like 85 db or like that. With that, your power requirements go way up. And with tranisients, you need even more. Plus, I have a very big room (16 feet x 40 feet x 9feet), so to keep sound levels high in the room, you need more power still. Also, you should note, that the SPL efficiency ratings at a one meter distance. I don't listen to my speaker one meter away. More like 4 0r 5 meters away, and sometimes further away. So for every 3 db's in lower sensitivitiy you need to double the power. Everytime you double the distance from the speaker you loose 6 db's. So at 4 meters, to produce 85 db'd you need more like 4 watts. To produce 100 db's at 4 meters, you already then need 128 watts. With tranisients, you can easily double that. Strange it is with my full range speakers with an 86 db SPL, that I can bring my 200 watt amp to clipping without making the music unbearable loud - it's a big room. You need to look at you calulations again, and consider distance from the speaker, subwoofer efficiencies (which are low), and room size. You will then come to a very different conclusion. Further your cable gauge calculation is based on the output of the amp. However, the cable doesn't carry the output power. You need to base you AWG cable on how much power the amp takes IN. Since amp's create heat, and nothing is 100% efficient, we can very safely assume that it will take in more power than it sends to the speaker. This increase your AWG requirement again. Fruther more, how LONG your cable is also a factor on what gauge it needs to be. The long you need a cable to be for a given current, the thick the gauge needs to be. I think you need to sharpen your pencil and reconsider your analysis based on reality, not a one meter test condition.