ROAR!

Cats of the genus Panthera – the tiger (P. tigris, Tiger - Spy in the Jungle [2008]), lion (P. leo, Animal Nation - Lions), leopard (P. pardus, Wildlife Special - Leopard [1997]), and jaguar (P. onca, Big Cats) - are distinguished by their ability to roar; the snow leopard (P. uncia) is the only exception. In 1834, it was reported that in the big cats the hyoid apparatus is different to their smaller cousins. The hyoid is a small bone in the neck of many mammals (including humans). In panthers it is suspended from the skull by an elastic ligament, rather than a rigid series of bones. This is in contrast to other cat species. It was believed for many years that this allows the big cats to elongate their throat (expanding the supra-laryngeal pharynx) creating the deep and extremely loud roar.

However, this anatomical anomaly is probably not what makes roaring possible. It was only in 1986, 152 years after Owen’s original observation, that Hast reported that the larynx of the panthers is clearly different to that of the cats that do not roar.

The vocal folds sit on top of the larynx and shape the sounds made by air rushing past from the lungs. The vocal folds of the big cats are especially large and thick, with an extra pad of fibrous tissue. The snow leopard (U. uncia, pictured below, Natural World - Snow Leopard), which has an incompletely ossified hyoid apparatus but does not have these specialised vocal folds, cannot roar. Recently, the snow leopard has been reclassified (to Uncia uncia), partly because of this fact, even though genetic studies still place it firmly within the Panthera genus. 

A large mass has a naturally low resonant frequency. Several of the muscles of the larynx are also larger, which further increases the power and range. The expansion of the throat may help as well, creating an expanding acoustic pipe, like a trumpet