When it comes to recording conversation between a group of people – such as a focus group – you need a powerful microphone that can record every participant’s voice clearly and not look too intimidating in the process. If setting up multiple lectern microphones is not an option, a boundary microphone is a great choice.

The boundary microphone – also known as PZM or Pressure Zone Microphone – is an unassuming little quadrangular or circular mic that can be placed at the center of a table or fitted on the ceiling if required.

How it Works

Let’s try to understand this without getting too technical.

Basically when a speaker talks into a microphone their voice not only enters the the microphone but also reflects off of any hard, flat surface around them.

If the reflected sound reaches the microphone a few seconds after the original sound, it causes a double signal and therefore distortion of sound.

A boundary microphone takes this phenomenon of reflected sound and turns it into an advantage.

The microphone is placed on a hard reflective surface (like a table) with it’s diaphragm very close and parallel to the surface.

This reduces the time reflected sound takes to reach the microphone, thus reducing distortion and actually improving signal strength (since the sound pressure is doubled).

Placement

Because of their omnidirectional nature ( i.e. the ability to receives sounds from all directions), boundary microphones can capture voices of participants all around the room.

They should be placed on a flat surface like a table, wall or ceiling otherwise they are not very effective. Since they thrive on reflected sound, placing them in corners considerably improves the recording quality.

A single microphone should be enough to cover a small group. But if you plan to use multiple mics, be sure to follow the 3:1 distance rule i.e. the distance between the microphones should be three times the distance between each microphone and the source of sound. [Source]

Pros

• A single microphone can be used for a group.
• It’s small and unobstrusive.
• Produces great sound quality.

Cons

• When placed on a table, the boundary microphone can easily catch sounds off of the table – like rustling paper or thumping on the table.

Best Boundary Microphone

There are a lot of great boundary microphones out there from Shure, Crown, Audio Technica etc.

Here are some to consider:

MXL AC404 USB Conference Microphone

Shure Beta 91A Kick Drum Microphone

Audio-Technica PRO 44 Cardioid Condenser Boundary Microphone

Read: 4 Ways to Improve Focus Group Recordings