# Thread: Decibal Output Question

1. Originally Posted by WhiteRabbit
yes. double cone area, double power, add 3 db. so a second 10 with a second 350 rms would yield a total of 6 dB on the meter..... max

I struggled with this for a long, long time. what it comes down to is that the meter is not a measure of air "pressure" but rather accoustic "energy". Or is it the other way around? Honestly, it doesnt matter.

the point of the matter is that the meter will read 6 dB (max) higher when you double your displacement. which is exactly what you are doing by adding both a subwoofer AND doubling power.

now, the funny thing to keep in mind is that because of this, while the relationship between excursion and power requirements is still linear, its not a 1 to 1 relationship! Thus, to double the excursion (remember double excursion is double the air motion and therefore a 6 dB gain on the meter!) keeping only one subwoofer would require FOUR times the power! Thus we see that the rule of double power for a 3 dB (max!) gain holds true! assuming the cone of the woofer can actually travel that far!

last thing to think about is, why make the distinction between energy and pressure? well, doubling the excursion IS just a 3 dB gain! in pressure (or is it energy? Honestly, it doesnt matter). Thus, we need double the power AND double the cone area to get our 3 dB gain! but the meter will read 6 dB (max!) up.

get it?
Errr.... say what?

Reading this thread at past 3 in the morning, I just about had a fit trying to figure where the heck did everyone get all their wrong numbers... until I remembered that I am not drawing Bode plots here.
so in my world dB = 20 log x which in this case would be 6 (just so you knew why the heck i was confused)

For the audio world, dB = 10 log x, hence 3db = 10 log 2 give or take one hundredth.

p.s. was that your purpose to make it as confusing as possible? or just a natural gift?

2. no confusion, it was written simply to clarify why some say 3 dB and why some say 6 dB. They argue with each other, and they don't realize they are BOTH correct!

But, if you'd like to be less rude about it, try replying during normal day hours, rather than 3 AM. A smiley doesn't compensate for poor behavior.

3. didn't see any rudeness in his response?

maybe I'm just dense though....lol

4. something i dont get here, ive just finished a 10 hour shift so its probably me being stupid

if you have an amp and a sub, and you change the amp for one thats twice the power there will be a 3db increase in SPL right?

here is where i get confused

if you use the original amp and instead of upgrading the amp buy another sub so you have twice the cone area, you guys say you get 3db increase too? how is this possible? 3db is double the power in the air, how do we recieve double the power in the air with an additional sub without extra power? each sub is recieving half the power so would be putting out 3db less, shouldnt this cancel out the fact that there are 2 and give the original SPL?

5. Originally Posted by Nic
if you use the original amp and instead of upgrading the amp buy another sub so you have twice the cone area, you guys say you get 3db increase too?
almost! we have to assume double Sd and the same excursion. if your first subwoofer has an Sd of 100 cm^2 and your amp pushes it 10 mm, and the upgraded subwoofer has an Sd of 200 cm^2 and your same amp pushes it 20 mm, we've not doubled the air displaced, but quadrupled it, and get a 12 dB (max) gain!

so its more than cone area, the bottom line is air displacement.

for our original example, remember the relationship of power to excursion is a 2x linear relationship. Thus, to double excursion we need FOUR times the power. So, you asked what happens when we double cone area (add a second subwoofer) without adding extra power, and get a 3 dB gain? lets do some math

when we double the cone area at constant power, we halve power per subwoofer, just as you said. But we dont achieve one half excursion until we quarter the power to each subwoofer! Thus, when we double cone area without doubling power, we arent cutting air displaced by half *per subwoofer*, its merely a fraction per subwoofer! add them together, and we arent doubling excursion, but its still greater than simply maintaining the same excursion. and that increased excursion assuming all other factors like efficiency run at 100%, you will achieve a 3 dB gain.

6. if power to excursion is a 2x linear relationship dosent that mean that speakers are way more efficient at low power than at high power?

7. What is Sd? i cant remember back to when i did this stuff in physics but my logic goes (assuming an identical speaker and amp added) double the cone area and the same driver displacement, that leads to 2 times the SPL or +3db

on another note if you quadruple the air displacement isnt that a 6db gain? a 3db gain is twice the level below it, so +3db is 2 times +6db is 4 times + 9 db is 8 times etc.

8. Originally Posted by WhiteRabbit
no confusion, it was written simply to clarify why some say 3 dB and why some say 6 dB. They argue with each other, and they don't realize they are BOTH correct!

But, if you'd like to be less rude about it, try replying during normal day hours, rather than 3 AM. A smiley doesn't compensate for poor behavior.
Alright, just as Red noticed, there was no rudeness. There WAS confusion and a single read through your post would easily explain where it comes from. Although the needed facts are there, the explanation is nowhere as concise and precise in terminology as it should be for someone so uptight about supposedly rude comments. In case you wonder why, here is a quote for you.

Originally Posted by WhiteRabbit
what it comes down to is that the meter is not a measure of air "pressure" but rather accoustic "energy". Or is it the other way around?
I hope you enjoy my reply now that I am leaving it during normal day hours. Now Mr. Touchy-feely, flame on about my rudeness.

9. Sd was the piston area for a speaker.

remember nic, that when we measure spl on a mic, doubling cone area OR doubling power results in a 3 dB gain, doing both results in a 6 db max gain! so quadrupling your displacement (four subwoofers rather than one) results in a 12 dB max gian! assuming they each have the same power the original subwoofer did.

as to the first paragraph, remember that doubling your cone area while maintaining your excursion level results in twice the air displacement and thus a 6 dB gain!

10. dosent twice the air displacement correspond to double the W/m^2? and thus a 3db increase?

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