When a loudspeaker projects sound, the sound waves do primarily propagate in the direction of the cone movement, but they also spread out in all directions. This spreading of sound in three dimensions is known as spherical spreading or spherical divergence.
The reason for spherical spreading of sound waves is based on the wave nature of sound. Sound waves are longitudinal waves, which means they propagate through compressions and rarefactions of the medium (usually air molecules). When a loudspeaker vibrates its cone, it creates compressions and rarefactions in the air adjacent to the cone, generating sound waves.
Initially, the sound waves do travel primarily in the direction of the cone movement, forming a cylindrical wavefront. However, as the waves propagate further away from the source, they start to encounter neighboring air molecules in all directions. These air molecules act as secondary sources of sound, creating their own spherical wavefronts.
Due to this spherical wavefront formation from multiple secondary sources, the sound energy spreads out equally in all directions around the loudspeaker. This spreading occurs because sound waves obey the principle of Huygens' wavelets, which states that each point on a wavefront can be considered as a source of secondary spherical waves.
As a result, the sound wavefront expands as it moves away from the loudspeaker, forming an expanding spherical shell. This means that the sound radiates in a three-dimensional pattern, allowing it to reach listeners from various angles and directions.
It's important to note that the intensity of sound decreases as the distance from the source increases, following the inverse square law. This means that the sound energy spreads out over a larger area as it propagates, resulting in a decrease in sound intensity with increasing distance from the loudspeaker.