How do you make speakers louder?

Answer 1

Obviously, increasing the power that is sent to the speakers will increase the loudness, but that very quickly reaches a limit: either the current limit of the voice coil or the maximum excursion (motion) of the cone. Hit either limit and you destroy the speaker.

Beyond that, the only way to increase the loudness is to improve the efficiency by which the kinetic energy (motion) of the speaker is transferred to the air around it. The idea is to design a structure or enclosure which allows the relatively small speaker to effectively put a large volume of air into motion in the space around it.

There are many enclosure designs (ported, reflex) which can improve efficiency at certain frequencies, but at the expense of other frequencies. The one design that is dramatically more efficient than any other is the acoustic horn. A horn is a structure in front of the enclosure that helps match the acoustic impedance at the driver (high) to the acoustic impedance of the room (low). One problem with horns is that they only improve efficiency up to a particular frequency; above that, you have to have another driver connected to a horn with different geometry. The other problem is that, for low frequencies, horns are huge. One clever horn design by Paul Klipsch from the 1950s, which is still in use today, actually uses the walls of the room as the sides of the horn. Under a military contract, one company has built a single horn in a 20-foot shipping container (link attached). Larger horns are only possible as permanent installations (link attached).

The importance of efficiency is demonstrated by the fact that Robert Della Curti's Real Total Horn is capable of producing 110dB with only one watt of input power!