Frequency is the number of times per second that a sound wave repeats itself. In everyday conversation, you’ve likely talked about frequency without realizing it; you just refer to it as pitch, which is the subjective response of human hearing to frequency. For example, a truck engine has a much lower pitch than a bicycle horn, and a dog’s growl has a much lower pitch than a bird’s chirp. Pitch is how we perceive the different frequencies of those sounds.

Alright, let’s get some quick definitions out of the way. The wide range of frequencies is measured in hertz (Hz). The number of hertz represents the number of cycles per second. A cycle is one complete wave of sound. At its most basic level, frequency describes how often something repeats, and with sound waves, those repetitions will determine the pitch of what we hear. Did we just compare “frequency” and “pitch” twice? YES! Because it’s important! A bird’s song, for example, has a frequency between 2,000 and 8,000 Hz. Compare that to a dog’s bark, which measures at about 1,000 Hz. The difference in pitch stems from the oscillation, or the regular back and forth of the sound waves that emanate from those two sources. Sound waves can behave “irregularly”, but let’s stick to the basics. That sort of misbehavior is better suited for a classroom. For most normal hearing people, the audible range in terms of hertz is 20 - 20,000 hz.

A dog bark falls into the category known as mid frequency while a bird song (depending, of course, upon the bird) is often considered high frequency. Mid frequency is comprised of the sounds we perceive most often on a day to day basis and fall into the range of 200-2,000 Hz. Anything below 200 Hz is considered low frequency and anything above 2,000 Hz is high frequency sound.

Anything above 20,000 Hz is referred to as ultrasound. You probably recognize the word ultrasound from its use in the medical industry, but these high frequency waves are not merely a product of modern technology. Dolphins, for example, have one of the highest perception thresholds of any species. Because of the dolphin’s sonar-like echolocation ability, they can detect objects in the water that are way beyond their eyesight. They do this by sending out a high frequency sound (ultrasound), and interpret the echoes to determine what object the sound beam hit and where it is. It’s generally thought that echolocation is the most advanced natural form of auditory perception, at least it was…until the movie Arrival introduced us to aliens who could see the future with sound. TBD on the frequency of those sound waves. We’re still studying them.

But enough about dolphins and aliens. Just like the super low frequency infrasound, ultrasound can affect us even though we can’t hear it. The symptoms of sustained ultrasound are similar to infrasound as well, with some people experiencing nausea, dizziness, headaches, and fatigue.

While these are general averages, hearing ability truly is based on the individual. It’s fairly common for one person to hear a sound that someone else can’t. It’s even more common for someone to be ANNOYED by a sound that another person ignores. For the few who are blessed, or better yet – cursed – with such keen hearing, daily life can be flooded with noises that many people simply don’t notice. Just ask Daredevil – superpowers can be a burden. Yet even if someone else can’t hear a sound, that doesn’t mean it isn’t there. And sounds can still affect us whether we realize it or not. For example, extremely low frequency infrasound (sounds below 20 Hz) have been linked to hearing loss, and in circumstances of sustained acute noise, headaches, dizziness, and fatigue. Check out the video below for example of sound throughout the different frequencies: