If you’ve looked at high-end headphones or speakers, you’ve probably noticed numbers on the spec sheet that read something like “20Hz-20KHz.” What do these numbers mean?
For any device that uses a standard speaker driver, the Hz-KHz value is the range of audible sound vibrations that the speaker can produce. It’s generally labeled as the “frequency response” and expressed in hertz, with kilohertz being a thousand hertz. So the typical frequency response for headphones, twenty hertz to twenty thousand hertz, is quite full indeed. More expensive models can go even higher and lower; the $700 Sony set in the image above has a range of 4Hz-100KHz.
To understand how all this works, you need to know a little about the physics of sound. Sound travels in waves. The distance between the crests (topmost points) of one wave and the next is called the wavelength. Waves with higher frequencies come closer together, and so have shorter wavelengths. Waves with lower frequencies come further apart, and so have longer wavelengths. Hertz are units used to measure frequency. One hertz is defined as one cycle per second. So, a frequency measured at 20 Hz is traveling at 20 cycles (or waves) per second.
Most speakers and headphones convert electrical signals into sound by using a magnetic field to move a flexible diaphragm back and forth very quickly. Those vibrations create the sound waves that travel to our ears. How quickly those vibrations happen affect the wavelength of the sound waves, and our ears hear those different frequencies as sounds in different ranges.
The vibrations that create a sound at 20 hertz are very low—a bass rumble—and have a long wavelength. It’s vibrating the driver twenty times every second. Most music and audio plays in the range of about 80 hertz to 15,0000 hertz. At 15,000 hertz the pitch is a high whine due to its extremely short wavelength, like a smoke detector alarm.
We also have different styles of speakers—woofers, mid-range, and tweeters—that are specialized to play sound at specific wavelengths.
You can easily put your speakers or headphones to the test. The video above runs through the spectrum of sound from 20Hz all the way up to 20KHz. Note that at the bottom and top of the range your speakers may not be able to reproduce the sound, especially if they’ have smaller drivers—like the one on your cell phone’s primary body speaker. You can test it out with different headphones and speakers to see which ones can reproduce the widest range of sound.
The better the range for your speakers or headphones, the wider the spectrum of audible sound they can reproduce. Some can go higher and lower than the standard 20Hz-20KHz range, like 16Hz-22KHz. But is that important?
Unless your hearing is exceptionally good, not really. You see, the range of human hearing is about 20Hz-20KHz. But that’s an ideal range, covering almost all of the population. Most infants will be able to hear that full range, and a few people might be able to hear frequencies a bit higher or lower. But just like eyesight, your ability to hear deteriorates as you get older, especially for high-frequency tones. If you’re over 25 or so, you probably can’t hear above 18,000 hertz—less, if your hearing has been damaged by exposure to extremely loud sounds.
So you can test your speakers all day long, but you won’t be able to tell the difference between a set with a 20Hz-20KHz range and one with a 16Hz-22KHz range if your ears are physically incapable of hearing the highest and lowest frequencies. It’s still an interesting statistic, and a set of headphones or speakers with a higher frequency response range will generally be higher-quality than one with a lower range, made with better components and engineering and producing more accurate, rich tones for music and videos. Think of it as you think of horsepower for a car’s engine: an important specification and something people generally like to know, even if they’re unlikely ever to use all 300 horsepower on the highway.
Note that this range concerns the tone or pitch of the sound you hear—not the volume of the sound, which is expressed in decibels (dB). Another way of expressing the ability to output volume is in electrical watts per driver or total watts for all drivers combined. That’s not very precise regarding sound, but it’s a decent shorthand for knowing how powerful the speakers are.