A guide to bat detectors for woodland beginners

Why a bat detector belongs in your woodland kit

Woodland after dark is far from empty. Bats use echolocation to hunt insects, and a detector turns those ultrasonic calls into sounds you can hear. For beginners, the choice often comes down to heterodyne or time expansion. Heterodyne models are affordable and instant. Time expansion models cost more but reveal detail. You do not need the most expensive device to enjoy your first season; you need one that suits how you listen.

Heterodyne detectors: the affordable woodland workhorse

A heterodyne detector tunes to one narrow frequency at a time. You turn a dial, usually marked in kilohertz (kHz), and the detector converts bat calls near that frequency into audible sound through headphones. Set it around 45 kHz for common pipistrelles, or 25 kHz for noctules. The response is immediate, which makes heterodyne perfect for walking a woodland ride and listening as bats pass.

• Best for: real-time listening, woodland walks, learning common species.
• Budget: typically £60–£150 new, less second-hand.
• Limitations: you hear only a narrow slice of sound. Tune to 45 kHz and a bat calling at 55 kHz may vanish.

Practical tip: tune slowly, pause for two or three minutes at each likely spot, and use over-ear headphones. Rustling waterproofs and heavy footsteps will mask more bats than a cheap detector ever will.

Time expansion and full-spectrum: more detail, higher cost

Time expansion detectors record ultrasound and play it back slowed down, often ten times slower. A call lasting five milliseconds becomes fifty milliseconds, so your ear can follow its shape. Full-spectrum detectors record the whole frequency range for later analysis. These are the tools to choose when you want to identify species confidently, study social calls, or compare recordings across nights.

They are not magic. They cost more, use more battery, and often need a computer to review files. For a beginner, a heterodyne detector and a notebook will teach you more in one summer than an expensive recorder you do not yet know how to use.

Recognising common pipistrelle and noctule calls

Start with two species. The common pipistrelle peaks around 45 kHz. On a heterodyne detector its echolocation sounds like dry, rapid slaps or ticks, almost like a small stone skipping on water. The rhythm is fast and regular. The soprano pipistrelle is similar but peaks nearer 55 kHz, with a higher, thinner sound.

The noctule is much lower, around 20–25 kHz. Tune down and you may hear a loud, slow “chip-chop” or “kerr-plop”, with clearer gaps between calls. Noctules often emerge before full darkness and fly high and fast over the canopy. If a call sounds surprisingly deep and carries across the wood, you are probably listening to a noctule.

• Common pipistrelle: 45 kHz, fast wet slaps, fluttering flight around edges and glades.
• Soprano pipistrelle: 55 kHz, higher and thinner, often near water.
• Noctule: 20–25 kHz, loud chip-chop, high fast flight, early emergence.

Do not worry about rare species at first. Learn these three, and you will already be reading the night.

Woodland fieldcraft for better listening

Choose a mild, dry evening with little wind. Bats are most active from sunset to about two hours after, and again before dawn. Woodland rides, glades, pond edges and canopy gaps are reliable. Stand still, use headphones, and keep rustling to a minimum. A red filter on your torch lets you watch flight without disturbing bats as much as white light.

Keep a simple log: time, temperature, habitat, frequency setting, and what the bat was doing. After a few weeks you will notice patterns. Never handle a bat or disturb a roost; all British bats are protected by law. If you find an injured bat, contact a licensed bat worker.

When to upgrade your detector

Give a heterodyne detector a full season. Once you can recognise pipistrelle and noctule without checking the dial every few seconds, you will know what you want next. If you mainly enjoy live listening, stay heterodyne. If you want to record calls, compare nights, or separate soprano from common pipistrelle with confidence, look at time expansion or full-spectrum.

Check frequency range (at least 15–120 kHz), recording options, battery life, and whether the files are easy to review. Second-hand time expansion units can be excellent value. A local bat group may lend equipment or run training nights. Better still, borrow before you buy. The best detector is the one you actually take into the wood.