Doppler shift sound calculator
A 440 Hz siren rushing toward a listener at 20 m/s sounds roughly 6% higher, because its crests arrive packed closer together. Enter the pitch and the speed toward or away from you to get the frequency you hear. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Frequency
- 440
- Relative speed
- 20
- Speed of sound
- 343
- Perceived frequency
- 467.2445820433
- Shift
- 27.2445820433
- Shift ratio
- 1.0619195046
When a sound source moves toward you the crests pack closer together and you hear a higher pitch; when it recedes the crests stretch and the pitch falls. That is the classical Doppler effect for sound in a still medium with a stationary listener. Students checking ambulance siren problems, physics teachers building answer keys, game audio designers estimating pitch for moving emitters, and lab instructors verifying ultrasonic rangefinder scenarios all need a fast, deterministic number without opening a textbook every time.
What the Doppler formula for sound returns and which inputs you send
In the classical acoustic model with a stationary observer and a source that moves radially through a still medium, the frequency you hear is f prime equals f times c over c minus v. Here f is the frequency the source emits in its rest frame, c is the speed of sound in the medium, and v is the component of source velocity along the line of sight with the sign convention that positive v means the source is approaching the listener.
Approaching sources raise the perceived pitch; receding sources lower it; zero relative speed leaves the frequency unchanged. Search intent clusters around phrases such as Doppler shift calculator, Doppler effect sound frequency, perceived frequency calculator, siren pitch toward away, and acoustic Doppler formula. People want a numeric answer they can check against a worksheet, embed in a simulation, or feed into a larger pipeline—not only a reminder that the formula exists.
Units stay consistent with whatever basis you chose: hertz with metres per second is the usual classroom pair, but any coherent length-over-time basis for both speeds works as long as you do not mix systems mid-request.
Practical use, limits, pricing, and delivery channels
Use the browser tool when you want an immediate perceived-frequency check while grading a Doppler homework set, verifying a lab microphone run with a moving speaker, sanity-checking game audio pitch for a racing car or drone flyby, or converting a known approach speed into the frequency a fixed sensor will record.
Choose units consistently—if frequency is in hertz and relative speed is in metres per second, leave speed of sound in metres per second as well, whether you keep the default three hundred forty-three or override it for water, helium, or a temperature-adjusted air value you already computed offline. Results are fully deterministic: the same inputs always yield the same perceived frequency and the same direction label.
There is no language model, no external physics service, and no randomness, so the capacity is safe to pin in continuous-integration checks and reproducible lesson materials.
What you can do with it
Homework and lab Doppler drills
Students and graders confirm perceived siren or speaker frequency for a given approach or recede speed, with clear rejection when speed meets or exceeds the speed of sound.
Game audio and sensor pitch estimates
Quick structured numbers for moving emitters, drones, and fixed microphones without standing up a full wave-propagation or spatial-audio stack.
FAQ
What happens if relative speed is at or above the speed of sound?
Magnitudes at or above the speed of sound are rejected because the ideal acoustic Doppler formula is singular for sonic and supersonic approach; failed validation is not charged.
Is positive relative speed toward or away from the listener?
Negative means away (lower pitch). Zero leaves the frequency unchanged and sets direction to stationary.
Is the Doppler shift sound calculator free?
The browser tool runs free on your device.
What speed of sound does the calculator assume?
It uses 343 metres per second, the value for dry air near 20 °C. You can replace it for water, helium or air at another temperature, as long as you already know that speed.
Does it handle a moving listener or wind?
No. It models a source moving straight toward or away from a listener who stands still in still air. Wind, a moving listener and the Doppler shift of light are outside this calculation.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/phys/doppler-sound \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"frequency":440,"relative_speed":20}'const res = await fetch("https://api.kit.forhosting.com/phys/doppler-sound", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"frequency": 440,
"relative_speed": 20
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/phys/doppler-sound",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"frequency": 440,
"relative_speed": 20
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/phys/doppler-sound", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"frequency":440,"relative_speed":20}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"frequency":440,"relative_speed":20}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/phys/doppler-sound", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"frequency": 440,
"relative_speed": 20
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "phys.doppler_sound",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
This capability accepts a positive source frequency and a relative radial speed that is positive toward the observer or negative away, optionally a custom speed of sound that defaults to three hundred forty-three metres per second for dry air near twenty degrees Celsius, rejects speeds at or above the speed of sound with a clear invalid_input error, and returns the perceived frequency plus shift, ratio, and direction as structured JSON.
What the Doppler formula for sound returns and which inputs you send
Delivering frequency, relative_speed, speed_of_sound, perceived_frequency, shift, shift_ratio, direction, formula, and method as JSON makes the result easy to paste into notes, diff in automated tests, or reuse across languages without reimplementing the arithmetic.
How this calculator validates input and applies f′ = f·c/(c−v)
Relative speed must be finite and its absolute value must stay strictly below the speed of sound: at or above the speed of sound the ideal formula hits a singularity for an approaching source and is rejected with a message that names the speed of sound limit. Numeric strings that parse to finite numbers are accepted so forms and CSV-style pipelines can pass values without an extra cast. There is no wind model, no temperature table, no relativistic light Doppler, and no unit conversion; this is ideal still-medium acoustic arithmetic from the two known kinematic quantities. Send a JSON body with required fields frequency and relative_speed. Frequency must be a strictly positive finite number; zero, negative values, empty strings, non-numeric text, NaN, and Infinity all fail immediately with invalid_input before any result is returned, and failed validation is not charged. Optional speed_of_sound defaults to three hundred forty-three when omitted; if you supply it, it must be strictly positive and finite so the medium model stays physical. After validation the engine computes perceived frequency as frequency times speed of sound divided by speed of sound minus relative speed, then reports shift as perceived minus source, shift_ratio as perceived over source, and direction as toward, away, or stationary from the sign of relative speed. Results are rounded to a stable decimal precision so the same input always yields the same JSON across JavaScript engines.
Practical use, limits, pricing, and delivery channels
Each successful request is one billable item at $0.002; failed validation does not consume balance. Wire the endpoint into waves and acoustics drills, contest answer checkers, emergency-vehicle sound demos, or preprocessing steps where you already prefer explicit domain errors—such as rejecting a speed at or above the speed of sound—over opaque black-box widgets that cannot explain why a bad input was refused.
Integration use cases
Deterministic fixtures for physics tests
Pipelines that seed unit tests or interactive acoustics demos can call a pure Doppler oracle instead of reimplementing f·c/(c−v) in every language.
Technical questions
How do I calculate perceived frequency from source frequency and relative speed?
Send JSON with frequency and relative_speed (for example frequency: 440, relative_speed: 20). The response returns perceived_frequency equal to f times c over c minus v, plus shift, shift_ratio, and direction.
Can I change the speed of sound for water or other media?
Yes. Optional speed_of_sound (aliases c, medium_speed, sound_speed) overrides the default 343. Keep units consistent with relative_speed; the engine does not convert units or look up media tables.