The 4Hz Myth: Understanding Earbud Frequency Response (and What You Can Hear)

When you’re shopping for audio gear, you’re buried in numbers. And one of the most prominent, and most misleading, is Frequency Response.

You’ll see a specification like “4 Hz – 24,000 Hz,” as listed on products like the Sony MDR-XB55AP earbuds. It looks impressive. But let’s start with the hard truth.

You cannot hear 4Hz. You also cannot hear 24,000Hz.

So, are manufacturers lying? No, but they are using engineering specifications as marketing, and it’s confusing. Let’s break down what those numbers really mean.

The Human Limit: What You Can Actually Hear

First, let’s establish our baseline. A healthy, young human ear has an audible range of approximately 20 Hz to 20,000 Hz (or 20 kHz). * 20 Hz: This is the lowest, deepest rumble you can perceive as a musical tone. Think of the lowest note on a massive pipe organ. * 20,000 Hz: This is the absolute highest-pitched sizzle, like a dog whistle. This upper limit drops significantly as we age.

Anything below 20Hz is called Infrasound (sub-bass). Anything above 20,000Hz is Ultrasound.

So, if we can only hear down to 20Hz, is that 4Hz number an outright lie? Not exactly. It’s just confusing ‘hearing’ with ‘feeling’.

The 4Hz Myth: The Difference Between ‘Hearing’ and ‘Feeling’

Because you can feel it. What manufacturers are advertising here is the driver’s physical ability to move that slowly. This is the realm of psychoacoustics—the study of how we perceive sound.

A 4Hz wave isn’t a “note”; it’s a pressure pulse. When you’re in a movie theater and an explosion shakes your seat, you are feeling sub-bass frequencies. You perceive this infrasound as a physical rumble or vibration, not as a sound.

When an earbud like the Sony MDR-XB55AP claims a 4Hz response, it’s not saying you’ll hear a 4Hz note. It’s signaling that its 12mm dynamic driver and “Powered Bass Duct++” technology are engineered to be “loose” and powerful enough to move air slowly, creating that tactile, rumbling sensation that bass-lovers crave.

The 24,000Hz Myth: The ‘Headroom’

What about the high end? If we can’t hear past 20,000Hz (and most adults can’t hear past 16,000Hz), why is 24,000Hz (or 24kHz) on the box?

This number signifies driver headroom. It means the driver can vibrate that quickly without immediately distorting or “rolling off” (fading).

Think of it this way: if a driver can hit 24kHz, it means it can reproduce the frequencies you can hear (like 15kHz) with less effort and more accuracy. It’s a sign of a capable, fast-responding driver. It is not, however, the same as “Hi-Res Audio.” The official “Hi-Res” logo (from the Japan Audio Society) typically requires a driver to be capable of 40,000Hz, a standard this particular model does not meet, as confirmed in user questions.

The Most Important Spec You’re Not Seeing

Here is the real takeaway: The Frequency Response Range (e.g., 4-24kHz) is mostly a marketing number.

The specification that actually matters is the Frequency Response Curve. * Range: “My car’s speedometer goes to 150 mph.” (Tells you the potential). * Curve: “My car accelerates quickly to 60 mph but struggles up hills.” (Tells you the performance).

The “Curve” (or “Sound Signature”) tells you how loud the earbuds play at each frequency. An earbud tuned for “Extra Bass” will have a curve with a huge hump in the low-end. A “Neutral” earbud will have a flatter curve.

So, when you see “4Hz - 24,000Hz,” don’t take it literally. Interpret it as engineering shorthand: “This driver is built to be a powerful bass ‘woofer’ with a fast ‘tweeter’.” It’s a sign of capability, not a promise of what you’ll hear.