I had a conversation with a salesman the other day that got me thinking about how much power is really needed for a home theater vs what a salesman might want to sell me.. Mine HT is 500 square feet and the math I did surprised me, so I thought I better run this past the people that really know this kind of thing.

A little googling found that peak commercial theater sound levels are around 95 decibel, but that is really loud. Noise of 90 decibels feels loud to the ears. Lawmakers in California are trying to pass legislation to restrict commercial theaters to a maximum of 85 db. So 95 db gives plenty of head room.

I'm going to focus on the subwoofer since I know I'm going to use two 12" ultimax subs. Maybe four later, but starting with two. These subs are rated at 86.7 dB for 2.83V/1m. I am going to make an assumption that equates to 86.7 db / watt since that is a standard rating, but I'm lost on how someone gets from 2.83 volts with a two ohm speaker to one watt. Sure P = EI, but I'm just going to push the "I believe" button until corrected.

The primary seating position is ~4 meters from the subs (and the L-C-R speakers). So to reach 95 db at 4 meters I would need to double that output twice at the source. I know there is some benefit from reflection, but I'll negate for calculating max power. So 95 db + 6db + 6db = 107 db. Doubling power increases out put 3 db, so from 1 watt I would need to double power 4 times to 8 watts.

Well that can't be right so lets through in 3 db of head room. Now we are at 16 watts. Ok another 3 db of head room = 32 watts.

Clearly I must be doing something wrong or there is a big misconception out there about how much power is "really" needed. A 1000 watt amp would generate 118 db. Or more reasonably a 250 watt amp could generate 111 db. Still way too much.

Did I go horribly wrong somewhere or miss some serious consideration?

A little googling found that peak commercial theater sound levels are around 95 decibel, but that is really loud. Noise of 90 decibels feels loud to the ears. Lawmakers in California are trying to pass legislation to restrict commercial theaters to a maximum of 85 db. So 95 db gives plenty of head room.

I'm going to focus on the subwoofer since I know I'm going to use two 12" ultimax subs. Maybe four later, but starting with two. These subs are rated at 86.7 dB for 2.83V/1m. I am going to make an assumption that equates to 86.7 db / watt since that is a standard rating, but I'm lost on how someone gets from 2.83 volts with a two ohm speaker to one watt. Sure P = EI, but I'm just going to push the "I believe" button until corrected.

The primary seating position is ~4 meters from the subs (and the L-C-R speakers). So to reach 95 db at 4 meters I would need to double that output twice at the source. I know there is some benefit from reflection, but I'll negate for calculating max power. So 95 db + 6db + 6db = 107 db. Doubling power increases out put 3 db, so from 1 watt I would need to double power 4 times to 8 watts.

Well that can't be right so lets through in 3 db of head room. Now we are at 16 watts. Ok another 3 db of head room = 32 watts.

Clearly I must be doing something wrong or there is a big misconception out there about how much power is "really" needed. A 1000 watt amp would generate 118 db. Or more reasonably a 250 watt amp could generate 111 db. Still way too much.

Did I go horribly wrong somewhere or miss some serious consideration?

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