Dogs have sensitive hearing - everyone knows that. Take a silent dog whistle as an example. Blow on an ultrasonic whistle and we humans hear nothing - and there's the potential for looking rather silly if the dog doesn't respond. However, the well-trained dog who has learned what the signal means will turn for their master and high tail it back to their side.
This is an example of dogs hearing ultrasonic frequencies, in other words, those frequencies that are high-pitched - too high for the human ear to pick up. If you want to get technical, an ultrasonic wave is one measuring 20,000 Hz and above. This is a cinch for dogs, since their upper hearing limit goes all the way to 60,000 Hz.
But what about at the other end of the scale to 'infrasonic' waves? These are sound waves below 20Hz that are too low-frequency for the human ear to detect. Can dogs hear infrasonic sound waves? Actually, probably not, but they may be able to 'feel' them.
Signs a Dog Hears Infrasonic Sound Waves
Dogs have the ability to hear sounds with frequencies ranging from around 40 Hz to 60,000 Hz. This means dogs are less sensitive to low frequency (or bass) noises than people. Dogs are therefore not likely to hear infrasonic sounds, however, they may 'feel' them.
This is because infrasonic sounds are often low and rumbly, such as a deep bass in a rock song or the low rumble of an erupting volcano. It's therefore entirely possible that a dog may detect vibration if an infrasonic sound has a high amplitude wave (the equivalent of being very loud.)
When listening or trying to localize a sound, the dog may stop and listen, cocking their head to one side. Their ears will change position, flicking this way and that to better concentrate on the sound waves. The dog stands in an alert posture, legs straight and head held high, perhaps moving the head from side to side as if scanning the air.
The dog is most likely to feel vibration through their whiskers, or through their paws if the infrasonic noise vibrates the ground. Again, the dog will stand to attention, concentrating on the vibration. They may appear alarmed or unnerved, and depending on the dog's character,3 may slink away or run to a safe place where they feel secure.
A History of Infrasonic Sounds and Hearing
In the modern day, we know that sound travels as a wave in the air. Sound has properties depending on its frequency (the length of the wave and how many repetitions there are in a second) and amplitude (the height of each sound wave determines the volume.)
It was the ancient Greek mathematician Pythagoras who first made a connection between a vibrating string and how the length of the string altered the tone of the note. This discovery was called "String theory" and was still being added to and refined into the 17th and 18th centuries.
The mid-1600s were a fruitful time for the theory of sound, with French scientist Marin Mersenne investigating the speed at which sound traveled through the air, and Robert Boyle working out that sound can't travel through a vacuum.
The latter point is pertinent because sound causes vibrations of molecules in the air. Without the presence of those molecules (such as in a vacuum), there is no sound. Infrasonic sound is still a sound wave, albeit one that is too low for dogs to hear. However, by its very nature as a sound wave, it causes vibration which can be detected as rumbling in the air or ground.
Indeed, infrasonic sound is still bound by the law of physics. The amplitude (height) of the sound wave governs its volume. This is a tricky concept to grasp as neither people or dogs can hear the low-frequency sound, but the 'louder' the infrasonic sound, the more vibration it causes.
The Science of Infrasonic Sounds
It's not often that human senses are better at detecting sound than a dog, but this is arguably the case with infrasonic sounds. This is due to the range of human hearing, extending from a low 20 Hz to a high 20,000 Hz. Contrast this with the much wider range of canine hearing, ranging from 40Hz to 60,000Hz.
In practical terms, this means dogs miss out on some of the lower frequency noises which we can hear, such as the lowest 'C' note on a piano.
However, infrasonic frequencies are below 20 Hz, so both dogs and people miss out on hearing an audible sound. But we may both feel the 'noise' as vibration. This is because low-frequency sound waves can cause physical vibration in the environment. Think of the low booming bass at a rock concert that you feel through the soles of your shoes.
Another example is the rumble of an exploding volcano where the infrasonic sounds are both low-frequency but high amplitude (amplitude is a measure of volume, so if these were audible sounds, they would be uncomfortably loud). When near an exploding volcano, people report feeling strong vibrations in the air, which are the result of those infrasonic waves.
Training a Dog to Hear Infrasonic Sounds
If you want to train an animal to hear infrasonic sound, then it's best to start with an elephant, rather than a dog. Elephant hearing extends to lower frequencies than any other mammal and goes as low as 14 - 16 Hz. Indeed, the elephant uses these low-frequency noises to communicate over great distances. A loud, low call emitted by one elephant can be heard by another over a mile away.
Okay, but let's say we don't have an elephant but we do own a dog. In theory, it's possible to train a dog to detect and react to infrasonic noises, such as those of a volcano before it erupts. This could act as an early warning for people living close to active volcanoes, to help then know when an eruption is imminent.
Much like any training, reward-based methods work best. The dog wouldn't hear the noise but would sense the vibration. Thus, you would need some sort of device that emitted a noise of the same frequency as an erupting volcano. Play that 'noise' at sufficient volume to cause vibration. When the dog shows signs of having felt the vibration, reward the dog with an enthusiastic "Yes", and give a treat.
Once the dog is regularly acknowledging the low-frequency sound, you change the focus to have the dog perform a specific action when they sense the vibration. This might be you getting the dog to bark so that they raise the alarm. To do this you would play the infrasonic noise and when the dog reacted, you then make a noise that would trigger the dog to bark. This could be an action as simple as standing with your back to a door and knocking on it with your fist.
When the dog barks, say "Yes" and reward them. The idea is that the dog links the low rumbly vibration with a requirement to bark.
By Pippa Elliott
Published: 05/18/2018, edited: 04/06/2020