Horses as you may have noticed have two ears that swivel around their heads like little radar.
These two funnel shaped appendages can move in unison or independently. It takes 10 muscles to move them and be moved 180 degrees to locate a sound more accurately.
The direction the ears point help us see what direction they are looking in. This is useful for herd animals when a herd mates body might be blocking a sound or object of interest. There is no difference between the hearing of an adult mare or gelding but older horses often lose some degree of hearing and become unable to hear higher frequency noises, just like people.
They are used to listen for predators so they can make a speedy getaway.
They are also used to display emotion, speak to humans and other horses weather or not we see what they are saying.
One of the most important pieces of information about a horses ears moving is how much he hears. When there is a noise close to the horse suddenly he can move his ears and pinpoint where the noise came from within 25 degrees.
Compared to humans and other predators horses have poor abilities to pinpoint sounds. Humans and dogs have the amazing ability to locate a sound within a degree of where it came from.
Horses may not be able to tell where exactly a noise is coming from but they can hear faint noises up to 4400 meters away and are far better at telling the difference between two noises with similar volume and hear sounds of higher frequency. They are also able to protect their ears from very loud noises by lying them flat. This is seen when two stallions are fighting, not only to display anger and fear but to protect their ears from the loud squeals of their opponent.
Humans prime type of communication is speech. As predators speaking a totally different language than a horse we are unknowingly doing a good thing by talking to our horses. Unlike the signals we send through vision and touch our voice is constant, therefor a more reliable source a horse can use in recognition.Think about it this way: A horse that knows the word WOAH means to stop all actions if trained properly will do so without any pressure on the reins whatsoever when you say that word.
A horse taught to halt with only the reins may require a different degree of pressure every time you pull the reins to stop and get the desired result. It may be because the rider is off balance one time or a completely different rider the next trying to give the same cues. They are just not as consistent as the word coming out of your mouth.
However, just because you use the word WOAH does not mean the horse will respond and stop. It is the tone of voice you use that a horse distinguishes rather than a spoken word. A good example of this is again the WOAH command. In this example the horse in question knows the word WOAH means to park all fours and stand still. Something scares the horse and the owner is yelling frantically WOAHWOAHWOAHWOAHWOAHWOAH!! when normally the horse responds and stops to a long and low "wooooooooahhhhhh" Of course the horse is going to resume having a meltdown over whatever scared it. The difference in the two tones of the same words are like a chicken clucking and a duck quacking.
Try it out. Try using a command the horse knows, like woah and whatever tone you use and substituting it for a word that sounds alike, like slow. The horse responds the same, why? you used the same tone you would for the original command.
I have an interesting scenario with Indigo. A few springs ago we had an incident with a dozen motorcycles driving by and honking when I was riding Indigo down the road. They were right next to us and that was the most scared I have ever seen Indigo to this day. That was also the only time she took off at a full out gallop before I could think about reacting and stopping her; barreling across the neighbors freshly plowed field like a cannonball. Previously to this incident she could have cared less about motorcycles. After this incident my dad could go blasting past us on his motorcycle without even so much as a flick of an ear just to make sure she was desensitized. If another motorcycle went by after him she would lose her marbles. Why? Well just like dogs horses can differentiate the tone and sound of an engine from others. My dad pulls in all the time on his motorcycle and she began to associate the sound of his bike with him and our motorcycle desensitization sessions. Same with the neighbors dirtbike and four wheelers. They hit a certain RPM and the pitch is higher and guess what, Indigo loses her marbles even if she can't see them. They drive by at a lower RPM and shes totally fine. When I am riding her I can hear that pitch I know she is going to react to and she does, right on key...eerr pitch.
So what tones in your commands does your horse respond to? What sounds are really bothersome. Can you see a connection between some things your horse spooks at and the tone/pitch it makes?