Most people know that a dog's hearing is more sensitive than humans', but they may not know that the nature of the sounds dogs hear is also much different.
Dogs hear much higher tones. Even the most sensitive people can't hear above 20,000 cycles per second, while some dogs can hear above 45,000. The "silent whistles" sometimes used to signal dogs produce sound at around 25,000 cycles per second-too high for humans to hear, but not for dogs.
Smaller dogs hear higher tones than bigger dogs. It has to do with the size of their ears. Small ears are more sensitive to high-pitched tones because high sounds are amplified. But dogs with big, square, mastiff-type heads (including Saint Bernards) can hear subsonic tones. These are very-low-frequency sounds-far too low for humans to hear.
Thus Saint Bernards are able to hear the faint low-frequency sounds made by people trapped under snow by avalanches, which dogs with smaller heads cannot sense at all. Some can even hear the very low sounds made when snow starts to move over ice or rock. This is why some Saint Bernards have been able to warn of an impending avalanche minutes before it is detectable by people.