How Shotgun Microphones Work: Interference Tubes, Polar Patterns, and Microphone Placement

Shotgun microphones are one of the most recognizable tools used in production sound. Their long, narrow design is commonly seen mounted at the end of a boom pole and positioned just outside of the camera frame.

What makes a shotgun microphone useful for production sound is its directionality. A shotgun microphone is designed to emphasize sound coming from the direction it is pointed while reducing some of the sound arriving from other directions.

However, shotgun microphones are also commonly misunderstood. They do not work like telephoto lenses, and they cannot simply "zoom in" on someone speaking from across a room.

To understand what a shotgun microphone can and cannot do, we need to look at its interference tube, polar pattern, directionality, and most importantly, microphone placement.

What Makes a Shotgun Microphone a Shotgun Microphone?

The term shotgun microphone refers primarily to the microphone's directional design.

The most recognizable feature of a shotgun microphone is the long tube extending in front of the microphone capsule. This is called the interference tube.

The interference tube is what gives a shotgun microphone much of its distinctive directional behavior. Rather than picking up sound evenly from every direction, the microphone is designed to be most sensitive to sound coming from the direction in which it is pointed while reducing a significant amount of sound arriving from the sides.

This makes shotgun microphones particularly useful in production sound, where the goal is often to capture an actor's dialogue while reducing unwanted sound from elsewhere in the environment.

What Is an Interference Tube?

An interference tube is the long, slotted section located in front of a shotgun microphone's capsule. If you look closely at a shotgun microphone, you will usually see a series of openings or slots running along the sides of the tube.

These slots are an important part of how the microphone becomes directional.

Sound coming from directly in front of the microphone travels down the length of the tube toward the microphone capsule. Sound arriving from the sides behaves differently — instead of entering through the front of the tube, off-axis sound can enter through several different slots along its length. Because those slots are located at different distances from the microphone capsule, the sound entering through them has different distances to travel before reaching the capsule.

How the Interference Tube Rejects Sound

Those differences in distance cause off-axis sounds to arrive at the microphone capsule at slightly different times. When similar sound waves arrive slightly out of phase with each other, portions of those waves can partially cancel one another out. This is called phase cancellation.

So instead of allowing all of the sound arriving from the sides to reach the microphone equally, the interference tube causes some of that off-axis sound to cancel itself out. The result is reduced sensitivity to sound arriving from certain directions.

Sound arriving directly from the front does not experience the same degree of cancellation, allowing the microphone to remain more sensitive in the direction it is pointed.

In simplified terms:

Sound from the front → Reaches the capsule more consistently

while:

Sound from the sides → Enters different slots → Travels different distances → Partially cancels

This is what helps create the narrow directional response associated with shotgun microphones.

Shotgun Microphones Do Not "Zoom In" on Sound

One of the biggest misconceptions about shotgun microphones is that they work like telephoto lenses. They do not.

A camera lens can visually magnify something that is far away. A shotgun microphone cannot acoustically magnify a distant voice in the same way.

If someone is standing twenty feet away from the microphone, the microphone is still twenty feet away from that person. Their voice has already traveled through twenty feet of space before reaching the microphone. Along the way, the microphone may also receive:

• Room ambience
• Reflections
• Traffic
• Air conditioning
• Other voices

and other sounds in the environment.

The directional properties of the shotgun microphone can reduce some unwanted sound coming from other directions, but they cannot undo the effects of distance.

Distance Still Matters

The farther a microphone is from the sound source, the more difficult it generally becomes to capture that source cleanly in relation to the surrounding environment. This is why boom operators work so hard to get the microphone close to the actor.

Imagine recording an actor from fifteen feet away. Even if your shotgun microphone is pointed directly at the actor, their voice still has to travel through the room before reaching the microphone.

Now imagine positioning that same microphone approximately two feet from the actor. The actor's voice reaches the microphone much more strongly in relation to many of the other sounds in the environment.

This is one of the most important principles of production sound: directionality helps, but proximity is still extremely important.

Microphone Placement Comes First

Suppose your actor sounds distant. Your first instinct might be to increase the gain. But increasing the gain amplifies everything the microphone is capturing — the actor's voice, but also air conditioning, traffic, room reflections, background conversations, handling noise, and other unwanted sound.

Increasing gain does not move the microphone closer to the actor. Whenever possible, improve the microphone placement first:

• Move the microphone closer to the source
• Aim it properly
• Reduce unnecessary background noise where possible

Then use gain to bring that cleaner signal to an appropriate recording level. A highly directional microphone is useful, but it cannot replace good microphone placement.

Using a Shotgun Microphone on a Boom Pole

On a film set, a shotgun microphone is commonly mounted in a shock mount at the end of a boom pole. The boom pole allows the operator to position the microphone close to the actor while remaining outside of the camera frame.

A typical arrangement might be:

Boom Pole → Shock Mount → Shotgun Microphone

The microphone may be positioned above the actor and pointed toward their mouth or chest. Depending on the shot and environment, it may also be positioned from below or from another angle. The exact position may change, but the goal remains the same: get the microphone as close to the actor as possible while keeping it out of the shot.

The directional properties of the shotgun microphone then help reduce some of the sound coming from areas away from where the microphone is aimed.

What Is a Polar Pattern?

To understand exactly how directional a microphone is, we can look at its polar pattern.

A polar pattern is a visual representation of how sensitive a microphone is to sound arriving from different directions. Imagine looking straight down at a microphone from above — the microphone is in the center of a circle, and sound can approach it from the front, the sides, the rear, and every direction in between.

The microphone's polar pattern tells us how sensitive it is to sound arriving from each of those directions. Some microphones pick up sound fairly evenly from all around them. Others are much more sensitive to sound coming from the front. Shotgun microphones fall into the second category.

The Lobar Polar Pattern

Shotgun microphones are often described as having a lobar polar pattern. A lobar pattern is highly directional — the microphone has its greatest sensitivity toward the front, creating a relatively narrow area in which sound is picked up most strongly. Sound arriving from the sides is reduced significantly.

However, this does not mean the microphone rejects everything outside of that narrow front area. A lobar pattern can also contain smaller areas of sensitivity toward the rear or sides of the microphone.

This is important because a shotgun microphone does not create an invisible wall around everything except the person it is pointed toward. It simply has different levels of sensitivity depending on the direction from which a sound arrives.

Polar Patterns Are Not Perfect Walls

It is useful to think of a polar pattern as a map of sensitivity rather than a map of what the microphone can and cannot hear.

If a sound is located directly in front of the microphone, the microphone may be very sensitive to it. If another sound is located off to the side, the microphone may be less sensitive to it. But that does not necessarily mean the microphone will not hear that sound at all — if the unwanted sound is loud enough, it can still be recorded.

For example, pointing a shotgun microphone away from a loud air conditioner does not guarantee that the air conditioner will disappear from the recording. The polar pattern helps reduce unwanted sound. It does not eliminate the environment around the microphone.

Shotgun Microphones Change Directionality Depending on Frequency

Another important thing to understand is that a shotgun microphone does not have one perfectly consistent polar pattern at every frequency. Its directional behavior changes depending on the frequency of the sound.

The interference tube is generally more effective at rejecting mid- and high-frequency sounds than very low-frequency sounds. At lower frequencies, a shotgun microphone may behave more like a supercardioid or hypercardioid microphone. As frequencies increase, the interference tube becomes more effective and the microphone's response becomes more strongly lobar.

This means a polar pattern diagram for a shotgun microphone may show several different lines representing different frequencies. The microphone might reject a high-frequency sound very effectively from one direction while still picking up more low-frequency energy from that same direction.

Why Lower Frequencies Are Harder to Reject

Lower-frequency sound waves have much longer wavelengths than high-frequency sound waves. Because the interference tube relies on differences in the distance sound travels before reaching the capsule, those differences have less effect on very long wavelengths.

This is one reason a shotgun microphone can still pick up low-frequency environmental sounds even when carefully aimed. You might successfully reduce some higher-frequency sound coming from the side while still hearing low-frequency rumble from:

• HVAC systems
• Traffic
• Machinery

or other environmental sources. Directionality helps, but no microphone completely removes the environment in which it is being used.

Aiming a Shotgun Microphone

Because a shotgun microphone is directional, aiming matters. The front of the microphone should be pointed toward the sound you want to capture. Whenever possible, unwanted sounds should fall into areas where the microphone is less sensitive.

For example, imagine you are recording an actor and there is an unwanted noise source somewhere nearby. You want:

Actor → Most sensitive part of the polar pattern

and, when possible:

Noise source → Less sensitive part of the polar pattern

This allows you to use the microphone's directionality to your advantage. A good boom operator is therefore doing more than simply getting the microphone close — they are also thinking about where the microphone is pointed and what exists around the actor.

On-Axis and Off-Axis Sound

You may hear the terms on-axis and off-axis when discussing directional microphones.

On-axis sound comes from the direction the microphone is primarily pointed. For a shotgun microphone, this generally means sound arriving from directly in front. Off-axis sound arrives from other directions.

Ideally, the actor's dialogue is on-axis while unwanted sounds are positioned as far off-axis as practical. This is another reason simply holding a boom microphone somewhere above an actor is not enough — the microphone needs to be aimed correctly. If the microphone is pointed away from the actor's mouth, the actor's voice itself can become increasingly off-axis, and you may then lose some of the benefit of using a directional microphone in the first place.

The Microphone Is Listening to an Area, Not a Single Point

Another helpful way to think about a shotgun microphone is that it listens to an area, not one microscopic point in space.

Pointing the microphone at an actor does not mean everything else disappears. The microphone still exists inside a real acoustic environment. Sound reflects from walls. Background noises travel through the space. Other people speak. Air conditioning runs. Traffic passes outside.

The goal is therefore not to create a recording containing nothing except the actor's voice. The goal is to position and aim the microphone so the actor's voice is captured as strongly and cleanly as possible compared with everything else around them.

Expensive Shotgun Microphones Cannot Fix Bad Placement

It is easy to assume that buying a more expensive microphone will automatically produce dramatically better production sound. Microphone quality certainly matters. But an expensive shotgun microphone positioned poorly can still produce a worse recording than a less expensive microphone positioned correctly.

Imagine an expensive microphone 10 feet away compared with a good microphone 2 feet away. In many situations, microphone placement will have a larger effect on the usefulness of the recording than the difference between the prices of those two microphones.

This is why learning boom technique and microphone placement is such an important part of production sound. Equipment helps you capture good sound. It does not replace good technique.

Shotgun Microphones and Our Signal Flow

We can also place the shotgun microphone into the signal-flow road we have been building throughout these guides.

The road begins acoustically:

Actor → Sound Waves → Shotgun Microphone

At the microphone, those sound waves become an electrical signal:

Shotgun Microphone → Audio Signal

That audio can then continue down the road:

Shotgun Microphone → XLR Cable → Mixer/Recorder

If the microphone requires phantom power, power simultaneously travels in the opposite direction:

Mixer/Recorder → +48V → XLR Cable → Shotgun Microphone

Once the microphone is properly positioned, powered, and connected, the recorder can amplify and record its signal.

But remember that the recorder only receives what the microphone gives it. If the microphone begins the road with poorly captured sound, everything farther down the road begins with that same problem.

Placement Before Processing

This brings us back to one of the most important ideas in production sound: get the sound right as early in the signal path as possible.

If the microphone is too far away, moving it closer is usually better than simply adding more gain. If the microphone is aimed incorrectly, aim it correctly. If an unwanted sound is directly inside the microphone's strongest pickup area, reposition the microphone when possible.

Gain, equalization, noise reduction, and post-production tools can all be useful later. But the cleaner the original microphone signal is, the less you need to rely on those tools. The beginning of the signal road matters.

What Shotgun Microphones Are Good At

A shotgun microphone is useful when you need a directional microphone that can be positioned near a sound source while reducing some unwanted off-axis sound. In production sound, this makes it particularly useful as a boom microphone for dialogue.

Its design allows a boom operator to:

• Aim toward the actor
• Reduce some sound coming from the sides
• Keep the microphone outside of the camera frame
• Move the microphone as actors move through a scene

The interference tube gives the microphone directionality, while the boom pole gives the operator the ability to put that directionality in the right place. The two work together.

What Shotgun Microphones Cannot Do

A shotgun microphone cannot:

• Zoom in on someone from across a room
• Completely remove all background noise
• Ignore every sound coming from the sides
• Fix poor microphone placement
• Make a noisy location silent
• Replace proper boom technique

Its directionality is a tool. Like every tool in production sound, it works best when you understand both its strengths and its limitations.

Conclusion

Shotgun microphones are highly directional microphones that use an interference tube to reduce some sound arriving from off-axis directions.

Sound arriving from the sides enters through different openings along the interference tube and travels different distances before reaching the microphone capsule. These timing differences cause some of the off-axis sound to partially cancel itself out. This contributes to the microphone's characteristic lobar polar pattern, which makes it particularly sensitive toward the direction it is pointed while reducing sensitivity to sound arriving from other directions.

However, shotgun microphones do not zoom in on distant sound. Distance still matters. Microphone placement still matters. Aiming still matters. And the microphone will still pick up some of the environment around it.

The best results come from combining the microphone's directionality with good technique: get the microphone close, point it toward the sound you want, position unwanted sounds off-axis whenever possible, then set the rest of your signal path appropriately.

Once you understand what the interference tube and polar pattern are actually doing, a shotgun microphone becomes much easier to use effectively. Instead of relying on it to magically isolate dialogue, you can use its directional characteristics deliberately to capture a cleaner signal at the very beginning of the recording process.