The perceived loudness of a sound is governed primarily by the amplitude of the sound wave, not by its frequency, wavelength, or speed. This is because the intensity (energy transported per unit area per unit time) of a wave is proportional to the square of its amplitude:

Intensity ∝ Amplitude²

A wave of larger amplitude carries proportionally more energy to the listener's ear, and the human ear (and the associated psychoacoustic sensation of loudness) responds to this energy flux — so a larger-amplitude wave is perceived as louder, even without any change in the actual acoustic power emitted at the source.

For an acoustic engineer designing a concert hall who wants the audience to perceive louder sound without pumping more electrical/acoustic energy into the speakers, the goal is effectively to increase the amplitude of the sound wave as it reaches each listener's ear, using the geometry and materials of the hall itself rather than the source. Typical architectural-acoustics techniques that accomplish this include:

• Reflective surfaces (hard walls, ceilings, and reflector panels) positioned to redirect sound waves that would otherwise be lost, concentrating and reinforcing them toward the audience — constructive superposition of direct and reflected waves increases the local amplitude.
• Controlled reverberation so that reflected sound arrives shortly after the direct sound, reinforcing it rather than causing echo or muddiness.
• Resonant design elements that reinforce (amplify) particular frequency ranges important for speech or music clarity.

All of these strategies work by manipulating amplitude at the listener's location, which is why amplitude is the correct characteristic to focus on.

The other wave properties do not control loudness: frequency determines the pitch of a sound (how "high" or "low" it sounds), not how loud it is; wavelength is simply related to frequency and the speed of sound (wavelength = speed/frequency) and governs things like diffraction and interference patterns rather than perceived volume; and the speed of sound is essentially fixed by the properties of the medium (air, at a given temperature and pressure) and is not something a hall's interior design can meaningfully change to make the source sound louder.