An audio speaker generates sound waves that are typically characterized as longitudinal waves. Longitudinal waves are a type of mechanical wave in which the displacement of the medium (such as air) is parallel to the direction of propagation of the wave. In the case of an audio speaker, an electric signal is converted into mechanical vibrations by the speaker's diaphragm, which in turn creates changes in air pressure, resulting in the generation of sound waves.
These sound waves consist of compressions and rarefactions. A compression is a region where air particles are compressed, resulting in higher pressure, while a rarefaction is a region where air particles are spread out, resulting in lower pressure. The diaphragm of the speaker rapidly vibrates back and forth, producing these compressions and rarefactions, which propagate through the air as sound waves.
It's important to note that while audio speakers primarily generate longitudinal waves, they can also produce other types of waves, such as transverse waves, depending on the design and construction of the speaker. However, for most conventional audio speakers, longitudinal waves are the primary type of sound wave produced.