The discovery that sound travels as a longitudinal wave is credited to the French scientist Augustin-Jean Fresnel (1788-1827). However, it's important to note that the concept of longitudinal waves in general was developed over several centuries by multiple scientists, and it wasn't until the early 19th century that a more complete understanding of sound waves emerged.
In the 17th century, Sir Isaac Newton proposed the idea of sound traveling as a series of compressions and rarefactions, which laid the groundwork for the concept of longitudinal waves. However, it was the work of Augustin-Jean Fresnel and others in the early 19th century that provided experimental evidence and further refined the understanding of sound waves.
Fresnel's contributions to the understanding of sound waves were intertwined with his studies on light and optics. He investigated the behavior of light waves and developed the wave theory of light, which later became one of the cornerstones of modern physics. While working on understanding the properties of waves, Fresnel extended his studies to sound waves as well.
Fresnel, along with the French mathematician François Arago, conducted experiments to demonstrate the nature of sound waves. They observed how sound waves traveled through different mediums, such as air and water, and how they interacted with obstacles and openings. Through these experiments, they were able to demonstrate that sound waves exhibited the characteristics of longitudinal waves, where the particles of the medium oscillated back and forth parallel to the direction of wave propagation.
Their work, along with contributions from other scientists like Thomas Young and Augustin-Louis Cauchy, helped establish the understanding that sound is a mechanical wave that propagates as a series of compressions (regions of higher pressure) and rarefactions (regions of lower pressure) through a medium.
Overall, it was a combination of theoretical and experimental efforts by several scientists, including Fresnel, that led to the discovery and understanding of sound as a longitudinal wave. This understanding has since become an essential aspect of acoustics and wave physics.