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The loudspeaker may use up

Acoustic impedance, intensity and power is an introduction in terms of physics and the wave equation. However, the power is the pressure times the flow, so the power can be the same at both ends.

Third, the temperature of water rises less under a given imposed pressure than does that of air. This is only true if the wavelength of the sound is much greater than the size of the enclosure, and that explains the limited frequency range of this technique. However, other periodic effects in its sound tend to complicate perception of the Doppler effect here. The blue curves show the direct and reflected waves at successive time instances indicated by the red dotted lines. Roman period theatres had semi-circular orchestra and taller and more elaborate stage building.

The loudspeaker may use up electrical power, but it doesn't transmit any sound power. It's worth mentioning that, even if the walls were perfectly reflecting, the reverberation time would be finite, because some energy is lost in transmission.

Because it works best for high frequencies, it has important effects for the loudness and timbre of brass instruments. However, when two instruments play the same note, it is always louder, and never softer. However, even in that extreme condition with two identical sine wave sources, cancellation is restricted to a small fraction of the space.

And they often have vibrato. Along with the forward propagating wavefronts, backscattered and reflected waves from the seating rows are produced from Lokki et al.

It's worth mentioning

Even when the conditions are such that two harmonics cancel, other harmonics may not. Yes, the horn works in both cases and I can understand why you say that it amplifies in both directions. Given that today the stage building does not exist, the first reflection arrives very shortly from the orchestra ground. These two effects do not cancel out because the time taken for diffusion of heat or chemical components is proportional to the square of the distance. Although it is small, this non-adiabatic non-heat conserving process is responsible for the loss of energy of sound in a gas.