Microphones come in all sorts of shapes and sizes. Some are nearly wand-like, built with very long lengths and relatively small diameters. These microphones are typically known as shotgun microphones.
Why Are Some Microphones Long & What Are Interference Tubes? Shotgun microphones are very long but unlike most mic designs their capsules are not near their ends. Rather, shotgun mics are built like pencil mics but with long interference tubes extending from their capsules. The tubes concentrate the polar pattern of the mic to a very narrow acceptance angle.
So in this angle, we’ll talk about the design of shotgun mics and get into the pros, cons, and applications of long microphones and interference tubes.
The Shotgun Microphone
Before we talk about interference tubes, let’s discuss the shotgun microphone.
What is a shotgun microphone? A shotgun mic is highly directional; long and skinny and is often used as a boom mic in film production. The extreme directionality of shotgun mics is not possible in typical capsules, and so long interference tubes are utilized in order to filter out off-axis sound waves before they reach the diaphragm.
Here is a picture of a Sennheiser MZX 8070 long shotgun microphone:
Shotgun microphones are generally condenser mics and are designed with regular small-diaphragm condenser capsules.
To learn more about small-diaphragm condensers, check out my article Large-Diaphragm Vs. Small-Diaphragm Condenser Microphones.
The actual microphone portion itself (the capsule, impedance converter, and inner electronics) typically has the same form factor as a pencil microphone. That is, the mic section is short and the capsule is typically located closer to the output than to the top of the shotgun mic.
In front of the capsule, however, is a long interference tube. This is what gives shotgun microphones their length. These tubes affect the way in which sound waves reach the microphone diaphragm, thus having a huge effect on the polar pattern of the microphone. Shotgun microphones are very directional and have narrow polar patterns.
The MZX 8070 pictured above is actually modular with an MZX 8000 preamplifier and the capsule/interference tube.
Another common shotgun mic is the Sennheiser MHK 416:
Shotgun microphones are commonly used in film as boom mics; in sports as distance mics, and in nature/ambient recordings to capture distant audio in a specific direction.
To learn more about my microphone recommendations, check My New Microphone’s Recommended Microphones And Accessories.
Let’s recap the characteristics of shotgun microphones before getting into detail about interference tubes and the shotgun/lobar polar pattern. Shotgun microphones are typically:
- Small-diaphragm condensers (audio frequency or radio frequency capsules).
- Long and skinny (due to the interference tube).
- Very directional with narrow polar patterns (due to the interference tube).
- Used in film, sports broadcasting, ambient/nature recordings, and other applications.
With that brief introduction, let’s get into the reason why some microphones are so long: the interference tube.
The Interference Tube
What is an interference tube? An interference tube is an extended acoustic labyrinth that yields the lobar polar pattern in shotgun microphones. An interference tube allows on-axis sounds to reach the mic diaphragm unimpeded while causing off-axis sounds to cancel out as they reach the diaphragm with varying amounts of phase shift.
Let’s break this down a bit further with another shotgun mic picture. This time of the Shure VP89L:
With the VP89L and the above-pictured MZX 8070 and MKH 416 shotgun mic, we see that along most of the microphones’ lengths there are slots.
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The interference tube essentially measures from just before the first slot (at the microphone capsule) all the way to the top of the microphone.
So how does an interference tube work?
The top of an interference tube acts as a normal microphone grille, allowing on-axis sound waves to enter the interference tube and travel straight to the diaphragm. In other words, sounds that travel toward the shotgun mic from the direction in which the mic is pointing do so with barely any issues. This makes the microphone sensitive to on-axis sounds.
The slots on the side of the interference tube treat sound waves differently. The sound waves that enter from off-axis angles enter the interference tube and are subjected to reflections within the tube and phase cancellation with other sound waves within the tube.
Sound waves coming from slightly off-axis directions are subject to less deconstructive interference than those entering from the sides and those entering from behind the microphone.
The effectively narrows the polar pattern of the shotgun microphone, which is the next point we’ll touch on.
The Shotgun/Lobar Polar Pattern
The main reason for the interference tube and the shotgun microphone is, of course, the highly directional polar pattern.
Let’s look at the typical polar response graph of a shotgun microphone:
As we can see, the on-axis (0°) response is the most sensitive and the shotgun polar pattern has a very narrow acceptance angle.
This is brought on by the aforementioned interference tube, which has acoustic slots that allow off-axis sounds to enter but subject them to phase cancellation with other off-axis sounds.
It’s important to note here that (as with all microphone polar patterns), the shotgun polar pattern becomes less directional at lower audio frequencies. The longer wavelengths of lower frequencies are less prone to phase cancellation in the interference tube and these waves are also less likely to cancel themselves out in the medium around the microphone.
Looking back at the graph, we see there is also a rear lobe of sensitivity.
To achieve the highly directional pattern, shotgun mics generally use hypercardioid or supercardioid capsules. These hyper/supercardioid capsules have narrower polar patterns than regular cardioid capsules but do exhibit rear lobes of sensitivity.
The interference tube affects the overall polar pattern of the shotgun mic but it is not capable of removing the rear lobe inherent in hypercardioid and supercardioid capsules.
For more information on cardioid, hypercardioid, and supercardioid patterns check out the following My New Microphone articles:
What Is A Cardioid Microphone? (Polar Pattern + Mic Examples).
What Is A Hypercardioid Microphone? (Polar Pattern + Mic Examples).
What Is A Supercardioid Microphone? (Polar Pattern + Mic Examples).
Another interesting part of the shotgun polar pattern response is the presence of side lobes. This is only sometimes the case with shotgun microphones and comes from two parts of the acoustic labyrinth:
- The proximity of the mic capsule to the nearest slot of the interference tube could allow sound waves at 90°/270° to hit the diaphragm with relative ease.
- The rear ports (if there are any) of the capsule could allow sound to enter from the side or slightly to the rear.
For more information on the shotgun/lobar polar pattern, please consider reading my article The Lobar/Shotgun Microphone Polar Pattern (With Mic Examples).
Why does my wireless mic keep cutting out? A wireless microphone “cutting out” is caused by the temporary loss of wireless transmission between the mic transmitters and receiver. This loss may be due to a weak signal at the transmitter or receiver; physical interference between the transmitter and receiver, or interference from other radio waves.
To learn more about wireless microphones, check out my article How Do Wireless Microphones Work?
What is a microphone capsule? A mic capsule is the transducer component of a microphone and is made of a diaphragm, diaphragm housing, and some sort of design that allows for the production of an electrical signal that coincides with diaphragmic movement. The main capsule transducer types are condenser, moving-coil, and ribbon.
For more information on microphone capsules and diaphragm, check out my articles What Is A Microphone Capsule? (Plus Top 3 Most Popular Capsules) and What Is A Microphone Diaphragm?.