You’re sitting in a quiet room and suddenly, a mosquito buzzes past your ear. That high-pitched, irritating whine is a physical manifestation of speed. Specifically, it’s the speed of vibrations. When we talk about sound frequency, we’re basically just counting how many times a second the air molecules around us are being shoved back and forth.
It’s physics. But it’s also how we perceive the entire world.
If the air vibrates slowly, you get the floor-shaking rumble of a passing semi-truck. If it vibrates thousands of times per second, you get the piercing whistle of a teakettle. Most people think of sound as a "thing" that travels, like a bullet. Honestly, it’s more like a wave in a stadium. The people (the air molecules) don’t actually move from one side of the stadium to the other; they just stand up and sit down in sequence. That "standing up and sitting down" happens at a specific rate. That rate is frequency.
What is sound frequency in plain English?
At its most basic level, frequency is the "pitch" of a sound. Scientists measure this in Hertz ($Hz$). One Hertz is simply one cycle per second. If you pluck a guitar string and it moves back and forth 440 times in one second, it’s producing a frequency of 440 Hz. In the music world, we call that a "Concert A."
Everything has a frequency.
Your vocal cords. The diaphragm in your smartphone’s speaker. Even the solid ground during an earthquake. But our ears are picky. Humans generally only hear between 20 Hz and 20,000 Hz ($20 kHz$). Anything lower is infrasound—you might feel it in your chest, but you won't "hear" it. Anything higher is ultrasound. Your dog might freak out at a 25,000 Hz whistle, but to you, it’s dead silence.
The anatomy of a wave
To understand how this works, you have to visualize the pressure. Sound is a longitudinal wave. When an object vibrates, it creates areas of high pressure (compression) and low pressure (rarefaction).
The distance between two peaks of high pressure is the wavelength. There's a direct, inverse relationship here: the higher the frequency, the shorter the wavelength. High-frequency sounds (like a bird chirping) have tiny, tight waves. Low-frequency sounds (like a bass guitar) have long, sweeping waves that can literally wrap around buildings. This is why you can hear the "thump thump" of a car stereo from three blocks away, but you can’t hear the lyrics until the car is right next to you. The low frequencies have enough energy and wavelength to bypass obstacles, while the high frequencies get absorbed or blocked by the first brick wall they hit.
The math behind the noise
While we're keeping it casual, we can't ignore the actual physics that governs this. The speed of sound in air is roughly 343 meters per second (at room temperature). The formula that ties it all together is:
$$v = f \lambda$$
In this equation, $v$ is the velocity of sound, $f$ is the frequency, and $\lambda$ (lambda) is the wavelength. Because the speed of sound ($v$) stays relatively constant in the same air, if you increase the frequency, the wavelength has to get smaller.
It’s a rigid trade-off.
Why does frequency change as you get older?
Here is the depressing reality of human biology: your ears are dying. Well, maybe not "dying," but they are definitely wearing out.
Inside your cochlea—the spiral-shaped part of your inner ear—there are thousands of tiny hair cells called cilia. The cells at the very beginning of the spiral are responsible for picking up high frequencies. Because every sound that enters your ear hits those first, they take the most abuse. Over time, through loud concerts, lawnmowers, or just the sheer friction of existing for 40 years, those high-frequency hair cells get damaged.
They don't grow back.
This is why a teenager can hear a "mosquito" ringtone at 17,500 Hz, but their 50-year-old teacher hears absolutely nothing. It's a phenomenon called presbycusis. It’s also why people with hearing loss often complain that they can "hear" people talking but can't "understand" them. Consonants like s, f, and th are high-frequency sounds. If you lose your high-frequency hearing, you lose the clarity of speech.
Beyond the human ear: Infrasound and Ultrasound
We are living in a narrow slice of reality.
Elephants use infrasound (below 20 Hz) to communicate over miles of savanna. These sounds are so low and have such long wavelengths that they can travel through the earth and the air for huge distances without losing steam. On the flip side, bats and dolphins use ultrasound (above 20,000 Hz) for echolocation. They need those tiny wavelengths because they act like high-resolution sonar. A long wavelength would just "wash over" a small moth, but a high-frequency, short-wavelength sound will bounce right off it, giving the bat a clear picture of its dinner.
In the medical world, we use these high frequencies for ultrasounds to look at babies in the womb or check for gallstones. Because the frequency is so high (in the millions of Hertz), the waves reflect off tiny internal structures, creating a visual map.
The "Ghost in the Machine" (19 Hz)
There’s a famous paper by Vic Tandy titled "The Ghost in the Machine." He discovered that a specific low frequency—roughly 18.9 Hz—can trigger feelings of unease, anxiety, and even hallucinations in humans. This is just below the threshold of hearing.
Because the human eyeball has a resonant frequency of about 18 Hz, sounds at this level can cause your vision to vibrate slightly, creating "grey blobs" in your peripheral vision. People think they’re seeing ghosts, but really, a nearby exhaust fan is just vibrating their eyeballs.
Sound frequency in music and technology
When you're messing with an equalizer (EQ) on your Spotify or in your car, you're playing with frequency bands.
- Sub-bass (20 Hz - 60 Hz): This is the stuff you feel in your seat. It’s cinematic and heavy.
- Mids (250 Hz - 2 kHz): This is where the human voice lives. If you boost this too much, everything sounds like a "honky" telephone call.
- Highs (6 kHz - 20 kHz): This is the "air" or "shimmer." It’s the crispness of a cymbal hit.
Engineers at companies like Bose or Sennheiser spend billions of dollars trying to manipulate how these frequencies reach your eardrum. They use digital signal processing (DSP) to flatten out the "peaks" where a speaker might vibrate too much or "mask" certain frequencies to make the audio feel more natural.
The common misconceptions
People often confuse frequency with amplitude. They aren't the same.
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Amplitude is the "height" of the wave—that’s volume. Frequency is the "width" of the wave—that’s pitch. You can have a high-frequency sound (a whistle) at a very low amplitude (quiet), or a low-frequency sound (a bass drum) at a very high amplitude (loud).
Another big one: the "Brown Note." There is a long-standing urban legend that a specific low-frequency sound can make humans lose control of their bowels. While it makes for a funny South Park episode, NASA and various acoustic researchers have debunked it. While high-intensity infrasound can make you feel sick or vibrate your internal organs uncomfortably, there’s no "magic frequency" for a gastrointestinal disaster.
How to use this knowledge
Understanding what is sound frequency isn't just for physics nerds. It has real-world applications for your health and productivity.
If you're working in a noisy office, "white noise" is actually a combination of all audible frequencies played at once. It works because it "masks" the sudden spikes in frequency (like a coworker’s high-pitched laugh) that would otherwise distract your brain.
Next Steps for Better Hearing and Audio:
- Test your upper limit. Use a (safe) online tone generator with headphones to see where your hearing cuts off. If you’re under 30 and can’t hear 15,000 Hz, you might be blasting your earbuds too loud.
- Check your environment. If you feel unexplained "haunted" vibes or constant headaches in a room, check for low-frequency vibrations from HVAC units or heavy machinery nearby.
- Optimize your EQ. If you struggle to hear dialogue in movies, don't just turn up the volume. Boost the 2 kHz to 5 kHz range. That’s where speech clarity lives.
- Protect the "Highs." When you go to a concert, wear earplugs. You aren't just protecting "sound" in general; you’re specifically protecting those fragile cilia that allow you to hear the shimmer of music and the nuances of human speech.
Frequency defines the texture of our lives. From the "Infrasonic" rumbles of the deep ocean to the "Ultrasonic" pings of a medical scanner, the vibrations of the world are constantly shaping how we move through it. Treat your ears with some respect—they're the only frequency decoders you've got.