how to make a louder speaker
A horn can increase the loudness remarkably. It does it in 2 ways:


better acoustical matching between the air and the speaker cone increases the power of the soundwave
the directivity concentrates the acoustic power to smaller sector. The in-beam dB levels can be much higher than the average or off-beam levels at the same distance.


The horn is that effective only if it's designed for quite narrow frequency range. A single frequency horn can be much more effective than one designed for example reproducing speech understandably.