A boundary microphone (PZM or pressure zone microphone) has a mic capsule that is placed flush against or as close as possible to a boundary (wall, floor, ceiling, baffle, etc.). The best boundary mics are flat and offer an omnidirectional pick up pattern that really picks up a hemisphere of sound around it.
The idea behind boundary microphones is that by place their mic capsule against a surface, a phase coherent mic signal will capture a truer sound of an acoustic space. Since there will be much fewer sound reflections entering the microphone (particularly from the rear), the mic will reproduce a cleaner signal of the sound in the acoustic environment.
Boundary mics are often applied as piano microphones, on hockey boards, and in studio lives rooms to capture room sound.
So what are the best boundary microphones on the market? Here are my top 2 recommendations:
- Crown PZM 30D: The Crown PZM 30D (link to check the price on Amazon) is an industry standard boundary microphone. Whether we’re miking a room for individual instruments, full orchestras, or speech, the PZM 30D is a top recommendation. The 30D features two selectable frequency responses that will suit just about any boundary mic position. This microphone is built with durability in mind and sounds amazing, getting it my top recommendation for a boundary microphone.
- Audix ADX60: The Audix ADX60 (link to check the price on Amazon) is another top performing boundary microphone. This microphone is rugged and sounds great in practically any situation that would call for a boundary microphone.
Before talking about each of the above mics in greater details, let’s list out some points that make a great boundary microphone.
“Best” is a dangerous word. There is really no such thing as a “best microphone” for any situation. The microphone(s) listed in my Recommended Microphones And Accessories” page are simply my recommendations. These recommendations are based on my own experience and are mindful of budget. It would be easy to suggest an ELA M 251 or U47 for most scenarios. However, these tube mics are very expensive, putting them out of a hobbyist’s price range and making it difficult for professionals to make their money back on the gear.
Another important note is that the microphone or equipment you choose is not the most important part of recording audio. In fact, there are many factors that are arguably more important than the choice of microphone. These include:
- Performer (whether a musician, speaker, or otherwise)
- Microphone technique/placement
- Number of microphones used
- Natural sound of the room
- Content (whether that’s the song, discussion, or otherwise)
- Signal chain (including mic cable, preamplifier, console, and/or interface/computer)
With that being said, some microphones and gear suit some instruments better than others, prompting this series of articles under “Recommended Microphones And Accessories.”
What Makes A Great Boundary Microphone?
- Hemispherical pickup pattern: A great boundary microphone has a hemispherical polar pattern. This is important since PZM mics sit against surfaces and do not benefit from being sensitive to sound reflections and vibrations at the surface they’re placed against.
- Flat shape: Flat boundary mics benefit from having capsules closer to the surface and from being less of a tripping hazard when set on the floor.
- Durability: Choose a durable boundary mic, especially when taking them on the road and when miking the floors of a space.
- Extended frequency response: Select a PZM mic with a flat and extended frequency response for a natural capture of sound waves in an acoustic space.
Now let’s talk about the two recommended microphones in the light of the above criteria.
The Crown PZM 30D
The Crown PZM 30D is an industry standard electret condenser boundary microphone. Its rugged and flat design make it a durable mic that sounds excellent on any surface.
Crown Audio is featured in My New Microphone’s Top Best Power Amplifier Brands In The World.
The Crown PZM 30D has a balanced XLR output, low self-noise, and a flat frequency response across its hemispherical polar pattern (when set against a wall).
Pickup Pattern Of The Crown PZM 30D
The Crown PZM 30D has an excellent hemispherical polar response. Really, the mic is fairly omnidirectional. However, in proper application against a boundary surface, the 30D exhibits a phase coherent hemispherical polar response that is consistent in picking up sound from every direction.
When positioned flush against a surface, the capsule of the PZM 30D will capture the direct sound waves and some early reflections from other surfaces but not the reflections from the flush surface. This yields a stronger mic signal with less phase cancellation and colouration.
For more information on the hemispherical/boundary microphone polar pattern, check out my article The Hemispherical Boundary Microphone/PZM Polar Pattern.
Dimensions Of The Crown PZM 30D
The Crown PZM 30D is a beautifully crafted boundary microphone with the following dimensions:
- 3″ (7.62 cm) wide
- 2.5″ (6.35 cm) long
- .375″ (0.95 cm) tall
Here are a couple of diagrams from the Crown PZM 30D data sheet:
The flat shape of the PZM 30D allows it to act with extreme efficiency as a boundary mic. There is no space for sound to reflect of the boundary surface back into the mic capsule.
The PZM 30D is also easily mountable, whether that be on a wall, ceiling, floor, or other baffle/surface.
Durability Of The Crown PZM 30D
The detachable XLR connection makes the Crown PZM 30D more durable than its cabled counterparts.
The outer casing is also quite durable, though I would suggest exercising caution when positioning Crown PZM 30Ds on the floor, especially in high-traffic areas.
Frequency Response Of The Crown PZM 30D
The frequency response of the Crown PZM 30D is given as 20 Hz – 20,000 Hz. Here is the boundary microphone’s frequency response graph:
As we see, the response of the 30D is quite flat. This is another factor that makes the mic sound so natural.
There is a “rising” frequency response switchable option that adds slight brilliance and air to the mic’s capture as needed.
The low-frequency response depends on size of surface. Specifically, the low-end frequency response begins to shelve down at and below the frequency F, where F= 750/D and D= the shortest boundary dimension in feet. At approximately F=188/D, the output of the microphone is down 6 dB.
Note that the above frequency response graph is tested at an “infinite surface.” These types of surfaces include walls, floors, etc.
For more information on microphone frequency response, check out my article Complete Guide To Microphone Frequency Response (With Mic Examples).
The Audix ADX60
The Audio ADX60 is a sensitive pre-polarized condenser boundary microphone. Its rugged build offers excellent durability and its hemispherical pickup pattern is very consistent. This PZM mic sounds very natural and is designed for use on the road, at home, and in the studio.
Audix is featured in My New Microphone’s Top Best Microphone Brands You’ve Likely Never Heard Of.
Pickup Pattern Of The Audix ADX60
Unlike many PZM microphones which have an omnidirectional capsule, the Audix ADX60 is actually a cardioid microphone. Here are its polar pattern graphs:
When positioned flush against a surface, the ADK60 exhibits a pickup pattern very close to a hemisphere. This allows for a natural pickup of sound waves travelling from any direction toward the boundary against which the ADX60 is placed.
Dimensions Of The Audix ADX60
The ADX60 has a rugged but sleek design. Its dimensions are as follows:
- 65 mm (2.56″) wide
- 80 mm (3.15″) long
- 21 mm (0.83″) tall
Here is a couple of diagram from the Audix ADX60 specifications sheet:
As we can imagine, this flat design is a big factor that makes the ADX60 an excellent boundary mic. The capsule will sit practically flush against the surface and capture phase coherent signals from the acoustic space without any rear reflections.
Durability Of The Audix ADX60
The Audix is designed to be durable. Its shape makes it difficult to snap and it can bear some weight when used as a floor mic (althought I would highly recommend keeping the ADX60 away from foot traffic areas.
The detachable mini XLR connection adds to the durability and longevity of this microphone. As mentioned before, a mic with a built-in cable is likely to experience problems before a mic without a built-in cable.
Frequency Response Of The Audix ADX60
The frequency response of the Audix ADX60 is given as 50 Hz – 18,000 Hz. Here is the frequency response graph of the boundary mic:
We see that the ADX60’s response is fairly flat but also fairly coloured.
The presence/high-end sensitivity boost picks up the brighter details and characteristics of a room and the sound happening with that room. This can be of great benefit in music recordings and in broadcast applications.
The low-end response of the ADX60 appears to be weak on the graph and is only rated down to 50 Hz. However, in listening to this microphone, we hear a natural enough low-end and mid-frequency response that makes this a practical boundary microphone for most practical applications.
So there are my top two recommended boundary microphones and my reasons for recommending them. Whether you’re recording drums or a full band in a live room, a sporting event in a stadium, an orchestra in a concert hall, or a meeting in a teleconferencing room, these two mics should be on the top of your considerations list.
- Crown PZM 30D: Best boundary microphone.
- Audix ADX60: Best durable boundary microphone.
For all the My New Microphone mic/gear recommendations, please check out my page Recommended Microphones And Accessories.