Locating Technologies
Ultrasound for real-time location: how it works, range and accuracy
Ultrasound location systems use sound, not radio, above the upper limit of human hearing, typically in the 20–40 kHz range. Sound travels vastly slower than radio waves, roughly 343 metres per second in air compared with the near speed of light for RF signals. That huge difference in propagation speed is what makes ultrasound useful for location: it allows precise distance measurement using timing hardware far simpler and cheaper than the nanosecond-scale timing that RF-based Time of Flight measurement, described in our measurement basics guide, generally requires.
Key specifications
Property | Typical value |
|---|---|
Frequency | Typically 20–40 kHz (ultrasonic, above human hearing) |
Range | Usually under 10 m, strictly line of sight or within a single enclosed space |
Accuracy | Can achieve centimetre-level accuracy under good conditions |
Measurement methods | Time of Flight, often combined with an RF signal for synchronisation |
Update rate | Limited by the slow propagation speed of sound; typically a few updates per second at most |
Tag power profile | Active tags require a battery; ultrasound transmission is generally more power-hungry than low-power RF beaconing |
How it works
Because sound travels so much slower than radio, roughly a million times slower, the time it takes an ultrasonic pulse to cross a typical room is measured in milliseconds rather than nanoseconds. That's a duration ordinary, low-cost electronics can time accurately, which is precisely why ultrasound can achieve fine-grained distance accuracy without the specialised wideband hardware that RF Time of Flight systems, such as UWB, need to time nanosecond-scale intervals precisely.
Most ultrasound location systems combine sound with a radio signal to solve a practical problem: something needs to mark the exact moment the ultrasonic pulse was sent, and radio, travelling near instantaneously by comparison, is well suited to that role. A typical approach sends an RF signal and an ultrasonic pulse simultaneously; the receiver treats the RF signal's near-instant arrival as the starting reference point, then measures the delay until the much slower ultrasonic pulse arrives. That delay, multiplied by the speed of sound, gives a precise distance, the same Time of Flight principle described in our measurement basics guide, just applied to a much slower wave.
Ultrasound shares a key limitation with infrared: it doesn't pass through walls or most solid objects, and its range is short. Sound also degrades with air temperature and humidity, both of which affect the speed of sound slightly, a factor that precise systems need to account for. Because of these constraints, ultrasound is generally deployed within single rooms or enclosed areas rather than across an open floor plan.
Measurement methods available
- Time of Flight (typically RF-synchronised) – an RF signal marks the transmission moment, and the delay until the ultrasonic pulse arrives is converted into distance
Advantages
- Can achieve centimetre-level accuracy using comparatively simple, low-cost timing hardware, thanks to sound's slow propagation speed
- Doesn't share spectrum with RF-based systems, avoiding interference from Wi-Fi, Bluetooth or other 2.4 GHz traffic
- Naturally contained within a room, similar to infrared, which can be useful where unambiguous room-level boundaries matter
Limitations
- Short range and strictly line of sight, or at least within the same enclosed space, since sound doesn't pass through walls
- Slower update rates than RF-based systems, due to the physical propagation speed of sound
- Accuracy affected by air temperature and humidity, which change the speed of sound
- Background noise, echoes and physical obstructions between tag and receiver can all interfere with reliable detection
Typical applications
Ultrasound is typically used where high, room-contained precision matters more than range or update speed: fine-grained asset or equipment location within a single room, laboratory and cleanroom tracking, and specialised research or industrial applications where centimetre accuracy is required within a defined, enclosed space.
Frequently asked questions
Why is ultrasound more accurate than some RF systems, despite being simpler technology? Sound travels far slower than radio, so timing hardware doesn't need the extreme nanosecond-level precision that RF Time of Flight measurement requires, making fine-grained distance accuracy achievable with relatively simple electronics.
Does temperature affect ultrasound accuracy? Yes. The speed of sound in air changes with temperature and, to a lesser extent, humidity, so precise ultrasound systems typically need to account for or compensate for these environmental factors.
Can ultrasound track objects across multiple rooms? Not directly. Because ultrasound doesn't pass through walls, it's generally deployed within single enclosed spaces, and multi-room coverage requires either receivers in every room or pairing with an RF technology for broader tracking.