How do boom mics really work?

In video production, there are many different types of microphones, each with its own specific uses. One of the most popular types of microphones used on set is boom mics. Operated by boom mic operators, boom mics are essentially shotgun microphones attached to boom poles and used to capture audio at a distance. But how do they work? Let use delve into how these types of microphones work, as well as where they came from and how to set them up correctly.

History of boom mics

While boom mics today are high-tech equipment for professional video productions, they come from humble beginnings. The first boom mic was nothing more than a microphone attached to a fishing rod. It was on the set of “The Wild Party” (1929) to allow the film’s lead, Clara Bow, to move freely on set. She was reportedly struggling to deliver her performance because she had to stand still so the microphone on set could capture her audio. Director Dorothy Arzner had his technicians attach a microphone to the finish rod, and thus, the boom mic was born. Though Arzner created the first boom mic prototype, it wasn’t patented until a year later. It was officially patented by a Fox Film Corporation sound engineer, Edmund H Hansen.

Over the decades, microphones have evolved significantly. Modern versions can be highly directional, allowing sound recordists to capture audio from a farther range while ignoring unwanted ambient sounds. The directionality provided by the likes of hypercardioid and shotgun mics has allowed for far more flexibility when it comes to capturing good audio from a distance. 

Understanding the boom mic

A boom mic setup typically consists of a microphone mounted on a boom pole, equipped with a shock mount to prevent handling noise. It’s also often covered with a windscreen to reduce noise interference. Boom mics are the go-to audio solution in scenarios where lavalier mics could disrupt a scene’s authenticity, such as an interview or film.

The science behind boom mics lies in their ability to focus on a sound source from a distance, making them perfect for film sets where the mic needs to be invisible and not intrude on the scene. The microphones used are usually shotgun mics, known for their directional pickup patterns that can handle capturing sound from a distance. But this isn’t always the case. Boom mics are typically equipped with microphones that have hypercardioid or lobar pickup patterns.

Pickup patterns: Hypercardioid vs. lobar

Hypercardioid mics have a pickup pattern that is tight and focused directly in front of the microphone and have a small lobe of sensitivity to the rear. They’re generally best suited for indoor environments, especially ones with low ceilings and untreated walls that are prone to echoes and sound reflections. Stationary setups like interviews and dialogue recordings would benefit most from the use of a hypercardioid microphone.

Microphone polar patterns
Illustration of different microphone pickup patterns

Lobar pickup patterns are found in shotgun microphones, which are highly directional. These mics are designed to record audio at longer distances due to their interference tube design. This design means they don’t reject reverberations and reflections as well as hypercardioid mics, so they’re best to use in outdoor environments. Also, boom mic operators can take advantage of the mic’s increased directionality in an outdoor setting, allowing them to stand further away from a subject while still capturing clear audio. Check out this article by Shure if you wanna learn more about how shotgun mics work.

Setting up a boom mic

Step 1: Attaching the microphone to a boom pole

Once you have a microphone (check out our microphone buyer’s guide for boom mic suggestions) and a compatible boom pole, secure the mic onto the boom pole using a shock mount. Shock mounts are crucial for reducing handling noise caused by vibrations and movements from the boom operator. 

Next add any microphone accessories you may need. If you’re filming outdoors, we recommend a dead cat or blimp.

Then, attach the XLR cable’s female end into the microphone. Typically, even the most basic and affordable booms come with an internal XLR cable. This means you don’t have to run the cable around the pole. But if yours doesn’t, go ahead and wrap it around the boom pole snugly. Pro tip: Use velcro straps so that the cord stays secure during operation. Connect the other end to your audio recording device. If your boom pole has an internal cable, it’s output may be at the bottom. In that case, use another XLR cable to complete the connection.

Step 2: Positioning

Work with the subject and camera operator to find the edge of the camera’s frame for the shot. The boom operator should then practice moving the boom pole to find the best position for capturing audio while staying out of this frame. Once you’ve got that down, position the mic as close as you can to the subject just outside of that frame — either above or below.

Step 3: Testing

Start monitoring the sound coming from the mic using headphones. Have the subject stand and speak similarly to how they will in the scene. Then adjust input levels on the recorder until it’s at an appropriate volume with minimal background noise. Once the voice is loud and clear, also make sure the subject isn’t distorting at their loudest points.

Step 4: Final checks

Before it’s time to start rolling, make sure to:

  • Double-check all connections and settings. 
  • Ensure that the boom pole’s handling noise is minimized during any movements you’ll have to perform — such as turning the pole’s direction between two subjects engaging in dialogue.
  • Check that there are no obstructions, such as cords, that will trip you while recording the scene. If this is a scene that involves moving subjects, make sure you know the start and end points so you can keep up. 
  • Confirm that the recording device has enough storage and battery power for the session.

Step 5: Action

Continuously monitor the audio through headphones while recording. Be ready to make minor adjustments to the boom pole’s position or the recording levels if the sound dynamics change during the shoot.

More boom mic tips

Here are a few more tips we have for you if you’re operating a boom mic:

  • Use a relaxed grip and keep your movements smooth to avoid creating handling noise.
  • Be aware of potential noise sources around the set. If you can, try to control them (for example, turning off a noisy air conditioning unit). If you can’t, try your best to position the mic in a way that they’re not as noticeable — your audio engineer will appreciate it.
  • Use specialized windshields, like blimps, when recording outdoors to drastically reduce wind noise.
  • In echo-prone areas, position the mic closer to the sound source or use absorptive materials around the set to dampen the echo. In these cases, a hypercardioid microphone may be most appropriate.
  • Keep extra batteries and SD cards on-hand.

There you have it

Boom mics are important tools in professional filmmaking that are the best microphone choice in many instances. And now that you know how boom mics work, take your knowledge to the set and experiment with different techniques. Also, consider learning more about audio recording for free here.

Kyle Alsberry
Kyle Alsberry
Kyle Alsberry is a multimedia producer and audiovisual technician at California State University, Chico and is Videomaker's associate editor.

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