Microphones for Investigation — PRN Equipment Guide

## Beyond the built-in microphone The internal microphones on digital voice recorders and cameras are adequate for general documentation but have significant limitations: they pick up handling noise from the device itself, they are omnidirectional and cannot differentiate between sources, and their frequency response is typically optimised for the human speech range (roughly 100 Hz–8 kHz). For more rigorous audio work, external microphones expand both capability and specificity. ## Directional microphones A directional (cardioid or hypercardioid) microphone has a polar pattern that accepts sound primarily from in front and rejects sound from behind and the sides. In a noisy investigation environment, this allows you to aim at a specific area of interest and reduce pickup from investigators elsewhere in the room. **Supercardioid and hypercardioid patterns** have a narrower front lobe and are useful for focusing on a point source at distance, though they also have a small rear lobe of sensitivity — ensure investigators are not positioned directly behind the microphone. **Shotgun microphones** use an interference tube to produce a very narrow directional pattern. They are useful for focusing at distance but are sensitive to wind noise and handling noise; they require a good shock mount and, outdoors, a wind shield (deadcat). ## Omnidirectional microphones Omnidirectional microphones pick up sound equally from all directions. They are useful for capturing the ambient acoustic character of a room and for stereo or multi-channel arrays where spatial information matters. They have a flatter frequency response than most directional designs and are less prone to proximity effect (bass boost when very close to a source). A stereo pair of matched omnidirectional microphones spaced appropriately (A-B stereo technique) captures a wide, natural soundfield and is valuable for full-room ambient recording during unattended monitoring. ## Contact microphones (geophones) Contact microphones (also called geophones, accelerometers, or piezo contact mics) are attached directly to a surface and detect vibration transmitted through that surface rather than airborne sound. They are useful for: - Detecting structural vibration (footsteps on a floor above, pipe knocking, settling timbers). - Separating surface-transmitted sound from airborne sound — if a knock is heard on a recording but the contact mic on the wall does not register a corresponding vibration, the knock may be airborne only, narrowing its likely source. They are most useful as part of a multi-sensor array, not in isolation. ## Practical guidance - Use a shock mount for any external microphone on a stand or held hand-held — handling and floor-coupled vibration are major sources of low-frequency noise. - Record a 30-second silence reference at the start of each session with all investigators still: this captures the room's ambient noise floor and any equipment self-noise. - Match your microphone impedance to your recorder's input. Most consumer recorders expect a high-impedance input; professional low-impedance (balanced XLR) microphones need a preamp or an interface. - Phantom power (48V) is required by condenser microphones and is not available on all consumer recorders. Check compatibility before purchase.