Sound frequency ranges

Not every vibration reaches your ears. An elephant's rumbling call, a bat's high-pitched shriek, and the ordinary hum of a conversation are all the same kind of thing — a longitudinal mechanical wave moving through air at the same speed — yet only one of the three is something a human can hear. The dividing line is frequency: how many times per second the wave vibrates. Sound scientists sort every vibration into three named bands purely by counting that number, and this simple act of counting turns out to explain why elephants can "talk" across kilometres, why bats can navigate in total darkness, and why doctors can peer inside the human body without a single cut. This article fixes the three bands, their boundary numbers, and the two easily-confused speed-based cousins — subsonic and supersonic — that trip up almost every Class 9 student the first time they meet them.

The core insight

Sound is classified purely by its frequency in hertz into infrasonic (below 20 Hz), sonic or audible (20 Hz to 20,000 Hz), and ultrasonic (above 20,000 Hz), and these frequency-based names must never be confused with the speed-based terms subsonic and supersonic.

The core idea

Every sound is a longitudinal mechanical wave, and every such wave obeys the same relation linking its speed, frequency, and wavelength: $V = f\lambda$ . What changes from one sound to the next is simply the value of $f$ , the frequency — the number of complete vibrations the source makes in one second, measured in hertz (Hz). The human ear happens to respond only to a limited window of that possible range, and it is this biological limit, not any deep physical boundary, that gives the three bands their names.

Infrasonic sound — frequency below 20 Hz. The vibrations are too slow, too few per second, for the human ear to register them as sound at all. Natural sources include the low rumble that precedes an earthquake and the deep calls elephants and whales use to communicate across many kilometres, since low-frequency waves lose less energy over long distances.
Sonic (audible) sound — frequency between 20 Hz and 20,000 Hz (20 kHz). This is the window a healthy human ear can detect, and it covers ordinary speech, music, and the everyday sounds of the environment.
Ultrasonic sound — frequency above 20,000 Hz. Too fast for a human ear, but well within range for bats, dogs, and dolphins, which use it for echolocation or simply because their hearing extends higher than ours. Elementary human applications include cleaning delicate jewellery and instruments, detecting hidden cracks inside metal castings, and medical imaging — named here only as applications, without their internal mechanism, since that belongs to later study.

Two sound-alike terms describe something entirely different: how fast an object travels, not how fast a source vibrates. Subsonic means an object's speed is less than the speed of sound; supersonic means it is greater. A supersonic aircraft is simply flying faster than sound travels through air — it is not automatically emitting ultrasonic waves, and an infrasonic call from an elephant says nothing about how fast the elephant is moving. Frequency-words describe vibration rate; speed-words describe motion through a medium. Keeping this pairing straight is the single most exam-relevant skill in this topic.

Figure

A frequency number line from zero to beyond twenty thousand hertz marked with the infrasonic sonic and ultrasonic bands and their boundary values of twenty hertz and twenty thousand hertz

Derivation AHow wavelength shrinks as frequency climbs across the three bands

The goal is to show, using nothing beyond the wave relation $V = f\lambda$ already known from the earlier part of this chapter, that at a fixed speed of sound the wavelength of a wave falls as its frequency rises, so that ultrasonic waves are physically shorter-waved than sonic ones, which are in turn shorter-waved than infrasonic ones — a consequence that follows purely from rearranging one familiar equation, with no new physics needed.

Step 1. Start from the universal wave relation for sound travelling through a given medium: $V = f\lambda$ , where $V$ is the wave speed, $f$ is the frequency, and $\lambda$ is the wavelength. In a fixed medium such as air at a given temperature, $V$ is effectively constant, commonly taken as $340\ \text{m/s}$ .

Step 2. Rearrange for wavelength: $\lambda = \dfrac{V}{f}$ . Since $V$ is held fixed, $\lambda$ and $f$ are inversely related — as one goes up, the other must come down to keep their product equal to $V$ .

Step 3. Compare the three bands at the same speed. An infrasonic wave at $f = 10\ \text{Hz}$ gives $\lambda = 340/10 = 34\ \text{m}$ , a wave many metres long. A mid-band audible wave at $f = 340\ \text{Hz}$ gives $\lambda = 340/340 = 1\ \text{m}$ . An ultrasonic wave at $f = 50{,}000\ \text{Hz}$ gives $\lambda = 340/50{,}000 = 0.0068\ \text{m}$ , under a centimetre.

Step 4. Read the result physically. Moving from the infrasonic band through the audible band into the ultrasonic band, frequency rises by orders of magnitude while wavelength shrinks by the same factor, since their product must always equal the fixed speed $V$ . This is exactly why ultrasonic waves, with their short wavelengths, can resolve fine detail (such as a small crack in metal) that a long infrasonic wave never could — a fact used, without derivation, in the applications named above.

Worked example

A bat emits a call of frequency 50,000 Hz that travels through air at 340 m/s. Classify this sound by frequency, and find its wavelength.

Step 1. Compare the given frequency to the two boundary values. $f = 50{,}000\ \text{Hz}$ , and since $50{,}000\ \text{Hz} > 20{,}000\ \text{Hz}$ , this frequency lies above the audible range, so the sound is ultrasonic.

Step 2. Apply $V = f\lambda$ , rearranged as $\lambda = V/f$ , using the given speed and frequency: $\lambda = \dfrac{340}{50{,}000} = 0.0068\ \text{m}$ .

Step 3. Convert to a more readable unit: $0.0068\ \text{m} = 6.8\ \text{mm}$ .

The bat's ultrasonic call is classified as ultrasonic, with a wavelength of 6.8 mm — far shorter than an ordinary audible wave travelling at the same speed, exactly as the derivation above predicts for a higher frequency.

Competitive stretch

A source vibrates at 12 Hz. State its classification, and predict (without calculating) whether its wavelength in air would be longer or shorter than the bat's 6.8 mm call. Since $12\ \text{Hz} < 20\ \text{Hz}$ , the sound is infrasonic; because frequency and wavelength are inversely related at a fixed speed, this much lower frequency must correspond to a much longer wavelength than the bat's call — indeed, $\lambda = 340/12 \approx 28.3\ \text{m}$ , thousands of times longer.

Common traps

Confusing infrasonic (a frequency below 20 Hz) with subsonic (an object's speed below the speed of sound) — one is a vibration rate, the other is how fast something moves.
Confusing ultrasonic (a frequency above 20 kHz) with supersonic (an object's speed above the speed of sound) for the identical reason.
Treating the 20 Hz to 20,000 Hz audible range as a strict boundary for every individual — it is the standard textbook range for a healthy young ear, and Class 9 exams expect these exact numbers unless a question states otherwise.
Thinking ultrasonic sound means "very loud" sound — frequency (how fast a source vibrates) and loudness (intensity) are unrelated properties; a whisper-quiet source can still be ultrasonic.

Ask the tutor: Still mixing up ultrasonic with supersonic, or infrasonic with subsonic — ask the AI tutor for a quick memory trick and a couple of extra classification questions.

Is 20 Hz to 20,000 Hz exactly what every human can hear

Not exactly — it is the standard range quoted for a healthy young ear, and the numbers you should use and remember for Class 9 exams. In practice, the upper limit of 20,000 Hz tends to drop as people age, and individual hearing varies slightly from person to person. Unless a question tells you otherwise, always use 20 Hz and 20,000 Hz as the audible boundaries.

Is a supersonic jet also making ultrasonic sound

Not necessarily, and this is one of the most common mix-ups in this topic. Supersonic describes how fast the jet itself is travelling compared with the speed of sound in air. Ultrasonic describes the frequency of a sound wave, above 20,000 Hz. A supersonic jet can produce ordinary audible engine noise, or an ultrasonic squeal, or both — its speed of travel and the frequency of the sound it emits are two completely independent things.

Why can bats and elephants hear frequencies humans cannot

Because the range of frequencies an ear can detect depends on the biology of that particular ear, not on any universal law of physics. A bat's ear is built to detect very high frequencies, letting it use ultrasonic echoes to navigate and hunt in the dark. An elephant's ear and vocal anatomy are tuned toward very low, infrasonic frequencies, useful for communicating over long distances since low-frequency waves travel farther with less loss of energy. The 20 Hz to 20,000 Hz window is simply where the human ear happens to be most sensitive.

Sound frequency ranges

The core idea

Worked example

Competitive stretch

Common traps

Before this

Where this leads