You might have seen one of these unusually long microphones. You might have also wondered why it has a strange shape. Well, this microphone is called “Interference Tube Microphones.” they are also referred to as “shotgun microphones.”
The shotgun microphones are highly directional. You need to point it at the sound source if you are going to use it for recording. This type of microphone can achieve high concentration on the source of the sound.
How Does Shotgun Microphone Work?
It can have a tight pickup of sound when the source of sound is in front of it. But if you move it away from the sound source, its pickup of sounds worsens. Like a shotgun, you should aim it at the sound source. Hence, it is referred to as the “shotgun microphone.”
Moreover, the shape of this microphone appears like a shotgun barrel. And like the shotgun that you need to aim at your target, you should aim this mic at the target sound source.
The polar response of a typical shotgun microphone is easy to imagine. The direction, for example, where the shotgun mic is pointed, is at the 0° reference point. At this point, the sound pickup of this microphone is at its highest. At this point, too, this microphone is at its gain. Its highest gain, of course, is at -11dB.
As you turn the microphone away from the reference point, its gain starts to decrease. At 180° reference point, its back is facing the sound source. At this point, the gain of this mic is almost nil or at its lowest. At this point likewise, its gain is at -28dB. You’ll see the radical diminishing of its gain as it moves away from the sound source.
Sound Pickup of the Shotgun Microphones
The interference tube was conceived to have an unimpeded movement of the sound. The on-axis sound passes unimpeded down the tube’s length to the diaphragm capsule of the mic. Yet, the on-axis sound needs to take a circuitous path on the side slots. The multiple slots vary in distances from the diaphragm.
The polar pattern of this mic appears like a spider that has been squashed. It exhibits multiple narrow peaks and nulls of sensitivity at different degrees and angles for different sound frequencies. This is due to the effect of its design (interference tube principle). Hence, the off-axis sources of sound appear to be innately colored. The coloration also varies according to the position of the off-axis sound source.
Thus, the off-axis sound reaches the diaphragm in varying phases. These varying intervals of arrivals at the diaphragm allow for partial cancellation of off-axis sound. Hence, we call this mic—the interference tube. If you look at the graphical representation of the polar pattern of this mic, you’ll be amazed. This is because the polar pattern narrows at the front compared to other mics.
Distinctive Polar Patterns of Shotgun Microphones
Shotgun microphones can be classified according to three types of polar patterns. These classifications include the supercardioids, hypercardioids, and ultracardioids.
The supercardioids present a narrow range of polar patterns. This mic exhibits sensitivity in the diaphragm’s front. Thus, it rejects off-axis sounds, although it exhibits very minimal rear pickup. If you are the operator of this mic who positions yourself behind this mic, you don’t need to create noise at the back of this mic.
The primary sensitivity of the hypercardioid microphones is in the mic’s front part. Yet, the hypercardioid mic differs from the cardioid mic because its least sensitive points are at 150-160 degrees and 200-210 degrees while the cardioid has it behind the microphone.
The hypercardioids, of course, is similar to the supercardioid mics because of its narrow pickup pattern as well as its extended pickup at the rear. This mic is said to be more directional than cardioid mics. This is because it has less sensitivity on its sides. If you want more isolation of the sound sources, you should use this mic.
This mic carries the narrowest pickup pattern outside of those omnidirectional mics. You should not use this on a boom pole because it is highly sensitive to the movement of the subject. This is because it is so directional.
The Pros and Cons of Using Shotgun Microphones
The shotgun microphone comes with many advantages. The obvious advantages of this mic include the following:
- It directly focuses on the source of the sound.
- It is almost oblivious to the noises present in the environment.
- It picks up only the sound that you want to pick up.
- You can use it to record the sound in its front directly.
- It records sound that is in a fixed position in front of it.
The shotgun mic also comes with many disadvantages. These disadvantages include the following:
- If you move it away from the sound source, its pickup of sounds worsens.
- It picks up sound at its rear, and thus those who are positioned at the back should be silent.
- Its gain starts to decrease as the sound source moves away from the reference point.
When to Use the Shotgun Microphones?
You will often see a shotgun mic being used for speeches or talks in conferences, lectures, and meetings. Its use free the speaker from wearing a wired microphone or from holding a mic in his/her hand. With the shotgun mic at hand, the speaker can simply stand in front of the mic and talk even at a distance. He can do it as long as he is talking right in shotgun mic’s front. With this mic at hand, the speaker can continue talking without the need to handle this heavy mic.
Hence, it is ideal for use when distant speaking is needed. It is also suitable for use in formal conferences and meetings wherein the speaker needs to stay in one place while he/she explains or delivers a speech or talk. Moreover, it is perfect for use if you want to record only the sound source right in front of the microphone.
Shotgun mics function at its best when there is a significant difference between the unwanted off-axis sound and the on-axis sound that you would like the mic to pick up. Moreover, it perfectly works when the sound source doesn’t move.
You should not use it in rooms that carry high echoes. You should not use it in small rooms likewise. In such rooms, the off-and on-axis would sound almost the same. Plus, it will not function well if there are strong off-axis sounds that circle the mic.
It is also not advisable to move the mic while using it. This cancels the directionality of this mic. This is because its directionality is limited when the off-axis sounds are attenuated. Additionally, off-axis sounds usually become distractingly colored.
You will often see some shotgun mics with a shorter length. This shortening, however, of shotgun mics is just for stylish purposes and for convenience. This shortening, however, is a disadvantage because short shotguns only function with higher frequencies. Thus, it becomes irrelevant to use shortened shotgun mics if you intend to record low and mild frequencies. So, if you intend to record these frequencies, you better go for longer shotgun mics.
Which Shotgun Mics Should You Use?
When choosing which shotgun mics to use, you should first consider several factors. You can use the longer shotgun if you intend to record distant sound with noises along the sides. This is because longer shotgun mics are sensitive to positions of the sound source. Longer shotgun mics also need to be fixed. The operator of this mic, likewise, should be very skilled.
The medium-length shotgun, however, functions well in many situations for voice and booming pickup. So, it is also nice to use in different situations.
The short-length shotgun mics, on the other hand, exhibits less directionality as compared to the longer shotgun mics. Yet, they come in handy if the space between the sound source and the mic needs to be shortened. It still rejects off-axis high frequencies.
You should also consider the quality and the degree of rejection when choosing the length of the shotgun mics. Yet, if you are looking for the best shotgun to use, you should look for shotguns with high sensitivity and low self-noise. It should also have a uniform and consistent off-axis rejection and natural on-axis sound.
There is a common misconception among users of mics about the shotgun microphone. This misconception pertains to the idea that the shotgun mic magically grabs the source sound. Yet, the reality is that the shotgun mic just rejects much of the off-axis sounds. These sounds include excessive room sound and even the sound coming from the busy streets.
Shure and Rode shotgun mics are the two major brands that pioneer in this kind of mic. The majority of shotgun mics, of course, can reasonably capture on-axis audio. Yet, if you are looking for the best shotgun mics, you should look for those that have low self-noise. Your choice should also have high sensitivity relative to the on-axis sound and should be capable of uniform off-axis rejection. If you would look for these types of shotgun mics, no doubt you could zero in on the best shotgun mics. You should always remember that the degree of rejection is as important as the quality of rejection.