The cat's second most important sense is its hearing, and with thirty muscles working each external ear, as compared with six in man, it can turn its eats precisely to locate sound. This ear-turning is done far quicker by a cat than by a dog.
The outer ear is more than just a funnel for collecting sound waves and channelling them down to the ear drum. Its shape is not as simple as the round Victorian ear trumpet, but is irregular and asymmetrical. This shape, combined with the eat movements, produces variations in the quality of received sound that allow the cat to localize its source with precision. A cat has the ability to discriminate between two sounds separated by an angle of five degrees with an accuracy of around seventy five per cent.
At high frequencies, the cat's hearing (and the dog's) is far more acute than ours. A cat can hear sounds up to two octaves higher than the highest note we can hear, and that is half an octave higher than the best a dog can do! In the high-frequency range, where one may expect to find the high-pitch noises produced by small prey animals, the cat exhibits particular sensitivity. It has great powers of discrimination between notes in this range, being able to distinguish one-fifth to one-tenth of a tone in difference between two notes. The large echo chambers in the skull play an essential part in magnifying sounds for the purpose of analysis by the feline ear and brain.
Most cats learn to recognize, without any training, words uttered by the human voice. They will respond to their name, a call to dine, and so on, but their vocabulary never grows as large as that which can be learned by dogs.
As in human beings, age brings its toll upon the hearing of cats. Their sensitivity to high notes reduces quite quickly with the passing of the years, often beginning to decline as early as three years of age and usually showing marked loss by the time that the cat reaches four-and-a-half.
Senility and diseases of some kind may result in a cat becoming completely deaf. Ear infections and blockage with wax generally respond well to prompt treatment by the veterinarian. White cats, particularly ones with blue eyes, have a tendency to deafness induced by a rogue gene in their make-up that causes shrivelling of the inner ear structure. This type of deafness is not amenable to therapy. In general, cats cope extremely well with deafness when it occurs.