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#### Fish Chris 2

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I have a feeling that this is going to bring up such crazy, science, that I'm just too stupid to follow it... But I've often wondered why air movement doesn't always equate to loudness ? Or maybe it does ? Lets talk about this. Hopefully somebody smarter than me will know how to dumb this subject down to where I can actually understand it.

Here's what got me to thinking about this (yet again). I was figuring the Vd (volume displacement for my two 15"s, and it only works out to about 21,600 cubic centimeters. They only have an X-max of 12.6 mm's. So then I wanted to compare that to one of my new 18"s (although I will have two, I will be mounting them isobarically, so that I will only have the cone area of 1 18") However, my 18"s have a much longer X-max, of 26.5 mm's. So the Vd of one of my 18"s (or both, in my case) is 30,700 cubic centimeters ! Nearly 30% more Vd than my "two" 15"s ! Now that would be awesome if the 18"s were that much louder than the 15"s used to be, in a standard tuned ported enclosure... because they were freaking LOUD ! But I just don't think it really works this way. Somebody please teach me about Decibles, vs. X-max (Vd)

But another thought... I was giving the 15"s 965 wts RMS total, for the two of them. While I'll be giving the 18"s 2500 wts RMS.... So that should make a little difference too, huh ?

#### gijoe

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One reason is that it doesn't scale linearly.

In the same way that you need twice the power for 3dB more output, doublling displacement only gives you 3dB more. If you have 500 watts and send it all to a single woofer then you can get 3dB more adding a second woofer and sharing the power. If you also double the power so that you now have 500 watts for each woofer, then you get a 6dB increase over a single woofer off of 500 watts.

The decibel is not the only unit of measurement used to describe sound. Phons are another unit, but the decibel scale makes more sense with regard to human's perception of sound.

#### Fish Chris 2

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I kind of get that part of it. But you know to be honest. if my new build were only "as loud" as those 15"s used to be, but played flatter, and dug down deeper, with authority, and still sounded tight and musical in the 50-80hz range, as well, Id be pretty darn happy with that

#### gijoe

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I kind of get that part of it. But you know to be honest. if my new build were only "as loud" as those 15"s used to be, but played flatter, and dug down deeper, with authority, and still sounded tight and musical in the 50-80hz range, as well, Id be pretty darn happy with that
This comes down to the box, proper crossover between the sub and midbass, time alignment, and a bit of EQ helps. You have the displacement to do it, the rest depends on the box, and how you tune.

#### Fish Chris 2

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This comes down to the box, proper crossover between the sub and midbass, time alignment, and a bit of EQ helps. You have the displacement to do it, the rest depends on the box, and how you tune.
Well, depending on how accurate WinIsd Pro is (because I know I can build a box as close to specs as anybody) it says that weighting my PR's with 150 gm's each, will produce a tuning of 30.5 hz.... So I guess we are going to find out. So long as I can do my rideshare where I want to this weekend, I'll be ordering that last piece of equipment (the other 18" sub... well, I still need a good router too) next Monday.....

BTW, time alignment and EQ in my truck is definitely not good, and won't ever be without a DSP. Honestly, it would be an absolute necessity, in an all new build, in a new vehicle... but I don't really see that being added in my old truck....

#### VegasStereo

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One reason is that it doesn't scale linearly.

In the same way that you need twice the power for 3dB more output, doublling displacement only gives you 3dB more. If you have 500 watts and send it all to a single woofer then you can get 3dB more adding a second woofer and sharing the power. If you also double the power so that you now have 500 watts for each woofer, then you get a 6dB increase over a single woofer off of 500 watts.

The decibel is not the only unit of measurement used to describe sound. Phons are another unit, but the decibel scale makes more sense with regard to human's perception of sound.
I was just going to say, Decibels vs Phons and then I scrolled down and saw it in your post. Lol

#### VegasStereo

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One reason is that it doesn't scale linearly.

In the same way that you need twice the power for 3dB more output, doublling displacement only gives you 3dB more. If you have 500 watts and send it all to a single woofer then you can get 3dB more adding a second woofer and sharing the power. If you also double the power so that you now have 500 watts for each woofer, then you get a 6dB increase over a single woofer off of 500 watts.

The decibel is not the only unit of measurement used to describe sound. Phons are another unit, but the decibel scale makes more sense with regard to human's perception of sound.
[email protected] Human hearing is not linear, its logarithmic.

#### Fish Chris 2

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Decibels and Phons.... I hear that lol

#### ckirocz28

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I have a feeling that this is going to bring up such crazy, science, that I'm just too stupid to follow it... But I've often wondered why air movement doesn't always equate to loudness ? Or maybe it does ? Lets talk about this. Hopefully somebody smarter than me will know how to dumb this subject down to where I can actually understand it.

Here's what got me to thinking about this (yet again). I was figuring the Vd (volume displacement for my two 15"s, and it only works out to about 21,600 cubic centimeters. They only have an X-max of 12.6 mm's. So then I wanted to compare that to one of my new 18"s (although I will have two, I will be mounting them isobarically, so that I will only have the cone area of 1 18") However, my 18"s have a much longer X-max, of 26.5 mm's. So the Vd of one of my 18"s (or both, in my case) is 30,700 cubic centimeters ! Nearly 30% more Vd than my "two" 15"s ! Now that would be awesome if the 18"s were that much louder than the 15"s used to be, in a standard tuned ported enclosure... because they were freaking LOUD ! But I just don't think it really works this way. Somebody please teach me about Decibles, vs. X-max (Vd)

But another thought... I was giving the 15"s 965 wts RMS total, for the two of them. While I'll be giving the 18"s 2500 wts RMS.... So that should make a little difference too, huh ?
You could get closer to a linear comparison between the different subs by making your vehicle airtight and completely rigid, like spl competitors do. But, like most of us, you drive your car, so that sort of treatment is impractical. Leaky, flexible cars make poor subwoofer enclosures.

#### Fish Chris 2

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I should have asked..... Could it have something to do also with this ? So, can you shove a smaller amount of air around, with more energy, vs. shoving a greater amount of air around, with less energy ? OR, does it always just take more energy to shove a greater amount of air around ?

#### Fish Chris 2

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You could get closer to a linear comparison between the different subs by making your vehicle airtight and completely rigid, like spl competitors do. But, like most of us, you drive your car, so that sort of treatment is impractical. Leaky, flexible cars make poor subwoofer enclosures.
Right. Our vehicles are kind of a crappy environment to try to make really nice sound... But if you spend a lot of time in your vehicle, and enjoy nice sound, what are you gonna do ?

#### dgage

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I have been pretty big into home theater, especially home theater subwoofers. In general, a pair of 15” drivers with double the power almost equals the output of a single 18“ of comparable capability. And a pair of 18s with double the power almost equal the output and depth of a single 24” sub. My testing of this was with SI HST-15s, HST-18s, and HS-24s in sealed enclosures.

Now regarding air, I don’t know the physics of it but larger drivers pressurize the air better. As an example, let’s use a 6” and 8” midbass with the same output level (decibels). A 6” driver has less cone area so to get the same volume output, it will have to move the cone more, say 3mm. For the 8” midbass, it doesn’t need to move the cone as far, maybe 2mm. So technically the 8” is moving faster because it doesn’t need to move as far for a given output level. This is why larger midbass drivers can give more chest thump. I liken it to a snapping towel, it charges the air better.

#### Fish Chris 2

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Right on. TY. Those 18" and especially 24" home setups sound fun 🙂
If I had a place big enough, and some extra \$\$\$, and a place where neighbors were not a consideration, I'd LOVE to play with some BIG subwoofers, and design some enclosures where size was no object. 🙂 Ya know something rediculous.... Like a 1/4 wave folded horn for a few 24"s 🙂 .....made out of concrete 😀 lol

#### ckirocz28

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Right. Our vehicles are kind of a crappy environment to try to make really nice sound... But if you spend a lot of time in your vehicle, and enjoy nice sound, what are you gonna do ?
Buy bigger subwoofers, of course!

#### Fish Chris 2

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#### syc0path

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But another thought... I was giving the 15"s 965 wts RMS total, for the two of them. While I'll be giving the 18"s 2500 wts RMS.... So that should make a little difference too, huh ?
Power is just a way to drive the subs to xmax. So assuming those power ratings are enough to drive the 15s and the 18 to full xmax, then u have already taken power into account.

So, can you shove a smaller amount of air around, with more energy, vs. shoving a greater amount of air around, with less energy ? OR, does it always just take more energy to shove a greater amount of air around ?
A smaller sub will shove a smaller amount of air around, but that can still get loud IF it's in a relatively small space that is constructed in such a way that the sound doesn't easily escape. That's what cabin gain is. An 8" sub in a car will generally get louder than a 12" sub in the middle of a field.

#### JMikeK

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Speakers don't shove columns of air around (edit: the air directly inside a port is an exception, but you can't listen to that). If that were true, you'd only be able to hear the bass if you were sitting directly in front of the sub in a 8" or 18" circle or whatever, and if you moved just outside that circle, you'd lose the bass. Speakers vibrate the air around them in a mostly spherical shape. There are many ways this can be untrue (naturally or altered intentionally) to make the sound more direction, but none are applicable in the sub bass octaves inside a car.

#### gijoe

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Speakers don't shove columns of air around (edit: the air directly inside a port is an exception, but you can't listen to that). If that were true, you'd only be able to hear the bass if you were sitting directly in front of the sub in a 8" or 18" circle or whatever, and if you moved just outside that circle, you'd lose the bass. Speakers vibrate the air around them in a mostly spherical shape. There are many ways this can be untrue (naturally or altered intentionally) to make the sound more direction, but none are applicable in the sub bass octaves inside a car.
Correct, every point on the wave front can be represented as the source of a new wave. that's how diffraction works.

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