The Good, the Bad and the Ugly Reasons for Using a Shotgun Microphone
By Ray Rayburn
Why would anyone use a shotgun or line microphone? Like anything else there are good and bad reasons.
Those who record sound for film or TV will often use shotgun microphones to pickup dialog on the set. Popular microphones for this application include the modern Sennheiser MKH 8070 or the classic MKH 816 it replaced. Both are long shotgun style microphones. Some sound recordists will use hypercardioid microphones such as the Schoeps MK41 since they are smaller and lighter weight on the end of their microphone booms. A hypercardioid provides in theory 6 dB rejection of background sounds and noise compared with an omni microphone which is excellent. However, it is often desirable to reject unwanted sounds even better than a hypercardioid can. This is where the shotgun microphone comes in.
Shotgun type microphones start with a hypercardioid microphone, in front of which is placed a long slotted interference tube. The interference tube provides a characteristic that narrows the sound pickup angle as frequency increases, thus helping to reject unwanted sounds and noise at higher frequencies. The length of the interference tube determines the frequency above which most of the improvement in directivity happens. The following chart is derived from Figure 6.4 in Eargle’s Microphone Book.
0.8 meters (31.5 inches) – 900 Hz
0.4 meters (15.75 inches) – 1,800 Hz
0.2 meters (7.9 inches) – 3,600 Hz
0.1 meters (3.9 inches) – 7,200 Hz
0.05 meters (2 inches) – 14,400 Hz
Most of the energy in speech is at frequencies ranging from a low of about 100 Hz to a high of about 12,000 Hz. The most critical frequencies for intelligibility are in the 500 Hz to 4,000 Hz range. This chart should help you understand why the most desirable shotgun microphones usually tend to be the longer models. These longer microphones provide additional rejection of noise, echoes, and reverberation starting at lower frequencies. Very short “shotgun” type microphones may look like they should provide extra directionality, but they do so only at the very highest frequencies so their real world benefits are small.
One application where shotgun microphones usually provide little benefit is sound reinforcement. In part this is because those who try shotgun microphones for sound reinforcement are typically using the least expensive models. The Sennheiser MKH 8070 has a list price of $1699.95 and the older MKH 816 was even more expensive. It is difficult to design a shotgun microphone that has good sound quality not only on-axis but off-axis as well. Less expensive shotgun microphones tend to have uneven frequency response outside of the “sweet spot” right in front of the microphone. This uneven off-axis response results in an increased chance of feedback in a sound reinforcement application. As a result many folk who try shotgun microphones for sound reinforcement wind up going back to more conventional microphones after a while.
Shotgun microphones can be valuable tools in specialized applications, most notably when recording dialog under less than ideal acoustic conditions. Like all microphones, they have their limitations, and should only be used where their advantages outweigh the limitations.
Ray A. Rayburn is a Senior Consultant with K2 Audio LLC. He is a member of the AES Standards Working Group on Microphones, and Chair of the Standards sub-committee on Interconnections. He is also a recording engineer with a lifetime interest in microphone use, testing, and design. Ray is also the author of the recently published Eargle’s The Microphone Book, 3e. Visit the book’s companion site!