Lavalier Transmitter

The Wireless Audio System

The lavalier transmitter is the heart of a wireless microphone system, a technology that gives filmmakers immense freedom in how they capture dialogue. While often referred to simply as the ‘mic pack’, the transmitter is one of three critical components that work in unison to liberate an actor from a cabled connection:

1. The Lavalier Microphone (‘Lav’): A very small microphone capsule designed to be attached to clothing or hidden on an actor’s body. It captures the sound directly.
2. The Bodypack Transmitter: The lavalier microphone’s thin cable plugs into this small, battery-powered box. The transmitter’s job is to take the weak audio signal from the microphone, amplify it, and then broadcast it wirelessly as a radio signal.
3. The Receiver: This unit, often mounted on the sound mixer’s audio bag or on the camera, is tuned to the same frequency as the transmitter. It catches the radio signal, converts it back into an audio signal, and feeds it into the audio recorder or camera.

Together, this system allows an actor to move freely through a scene—to walk, run, or get into a car—without being physically tethered to the sound department.

How the Transmitter Works

The transmitter is a miniature radio station. Its primary function is to perform frequency modulation (FM), encoding the audio signal onto a specific radio frequency, typically in the UHF (Ultra High Frequency) band. Professional transmitters offer several key features that are essential for high-quality results:

• Frequency Agility: The ability to change the broadcast frequency is crucial. The RF spectrum is crowded with television broadcasts, emergency services, and other wireless devices. A sound mixer must be able to scan for and select a clear, open frequency to avoid interference, which can manifest as static, hissing, or complete signal dropouts.
• Adjustable Input Gain: The lavalier microphone outputs a very low-level signal. The transmitter’s preamp boosts this signal. The input gain must be set carefully to be strong enough to provide a clean signal, but not so strong that it ‘clips’ or distorts on the loudest sounds (like a shout).
• Power and Antenna: They are powered by batteries (either AA or rechargeable packs) and have a short, flexible antenna to broadcast the signal.

The Art of Hiding the Mic and Pack

Placing the transmitter and its associated microphone is a specialized skill, often handled by the production sound mixer or a sound utility person. The goal is twofold: make the equipment completely invisible to the camera, and achieve the best possible sound quality.

• Hiding the Transmitter: The pack is often placed in a pocket, but for tighter costumes, it may be hidden in the small of the actor’s back with a neoprene belt, strapped to an ankle, or sewn into the wardrobe itself. Comfort and preventing the pack from moving or creating a bulge are key considerations.
• Hiding the Microphone: This is even more challenging. The lav mic must be positioned to pick up the actor’s voice clearly while avoiding the dreaded sound of ‘clothing rustle’. Common techniques include taping the mic under a shirt collar, placing it in the space between buttons on a shirt (the placket), or using a variety of specialized clips and sticky mounts to attach it directly to skin or fabric. The cable must also be carefully taped down to prevent it from rubbing and creating noise.