SPL questions

Discussion in 'Home Theater Projects' started by Brian-K-Owens, Mar 23, 2004.

1. Brian-K-Owens Stunt Coordinator

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The are theoretical questions. . I know the real world results can vary, but:

I see conflicting information on "double" loudness and power requirements.

Here is what I think to be true:

1 driver with X watts will give a certin SPL

2 drivers with the same X watts (each driver getting x/2 watts) will increase SPL 3 dB due to increased cone area.

2 drivers with 2X watt will increase SPL 6 dB. This tells me "double" loudness is +6 dB, since I have twice as many drivers (with equal output) as I started with.

So why do I see comments that say 10dB is "double" loudness. Did I miss something?

My next question is power requirements for a single driver. I am pretty sure you can prove that it takes 4X power to move a driver cone 2X excursion. In other words, feed a driver a 30 Hz sinewave with 10 watts, and the cone will move twice as far if you increase the power to 40 watts. So would moving the cone to 2X excursion be "double" loudness? once again, I hear the comment that it takes 10X power to double the loudness of a driver. Which is true?

Thanks!
Brian

2. Patrick Sun Moderator Moderator

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You're confusing doubling power and its effect on loudness.

Whatever that you need to do to achieve 10dB will double the loudness from the initial volume level.

Using 2 drivers to achieve 3dB increase in loudness is just that.

Doubling the power sent to one driver results in 3dB loudness. Add another driver with doubled power results in another 3dB, so that's 6dB in loudness increase, but it's not twice as loud.

3. Brian-K-Owens Stunt Coordinator

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That is where I am confused. If I add a second driver and double the power, why is that not double loudness? I am moving two time the air. . .

If I have a driver playing a 30 Hz sine wave with 100 watts driving it, and I add another driver of the same type playing the same 30 Hz sine wave with another 100 watts, I have doubled what I had; I am moving twice as much air. We agree that would be an additional 6 dB, but we are saying it would not be double the loudness?

Same thing with a single driver. If I double the power, I get 3 more dB. If I double the power again (4X power), I will have 6 dB more than the original. This also means the cone is moving 2X the excurison it originally was. Once again, moving 2 times more air, but not double the loudness?

Why? I understand the concept of the dB and the mathmatical formulas, here is even a great explaination of the creation of the dB system:

But logically, we agree that a second driver playing as loud as the first would be 6 dB increase. . . seems like that would be twice as loud!!!

Brian

4. mark alan Supporting Actor

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I did an interesting experiment the other night with my SACD player. It allows you to play continous test tones on any combination of speakers. I calibrated each speaker individually to 75 db (5 speakers plus sub). When I turned on all speakers together at same power levels, the db level increased to 84. It was also significantly louder.

5. Patrick Sun Moderator Moderator

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Not according to the definition.

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