So I'm gonna buy a new bass and probably a new amp..

The bass will be a 5 stringed and so I've heard that a 15 inch amp sounds better than 12 inch amp when you use a 5 stringed, is this true?
If so, why? What's the difference between a 12 and a 15?

What should I get?
So how am I supposed to take anything you say seriously,
when you've swapped free will for faith, hope and pre-destiny?
12" and 15" simply refer to the speaker size.

The larger the size, the more air the speaker pushes, the louder it seems. In order words, directly comparing the 12 to the 15, you will feel the 15 is louder. (and if louder is a criteria for being better, than yes, 15 would be better)

Due to the way the human ear hears certain frequencies, in order to "hear" bass at comparative levels to things like guitars/drums/vocals/etc... you will typically need a lot of wattage behind your amp and big/lots of speakers.

However, to note... the number of strings on your bass really shouldn't matter. The larger speaker pushes more, therefore seems better. That's it really. Though a 5-string produces lower frequencies than a 4-string... a 12 inch or 15 inch should both be equally capable of producing those frequencies.

This is all from a completely technical stand-point. As always, I'd recommend you try it out personally to see what you like better. What sounds "good" or "better" can be fairly subjective.
Actually, relate it to the bass in a car.
I don't know all the little bits about frequencies and acoustics, but.
You get a deeper sound out of a larger speaker. Like mentioned above, it also carries a little better. When you see two gangbangers rolling down the street in their caddies, you'll hear the person with 2 15" before the person with 4 12". Deep bass has a long requency, which is why you can hear it from farther away. It takes less energy to propel the wave the same distance on a low freqency (the 15").
Simple terms - the 15" will carry more, so if your playing on your low strings, that would probably be the way to go. I don't know why it would matter with a 5 string though, because you aren't really going any lower...
...invest and get a big-ass amp with both. Then your set.
This is a clever, witty signature. Rofl at my glory.
The 15" will be able to produce a more... bigger sound and is capable of giving a lot of headroom and a better bass response than 12". 15" speakers are better for bass no matter if its 4 or a 5 stringer.
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Old custom made 1x12" cab x2

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