Boundary Microphone

Microphones / Miking

Published March 2024

We follow sE Electronics on the highway to the pressure zone.

Boundary microphones, or pressure zone mics (PZMs), are often overlooked in the studio. Typically associated with live sound, where their high SPL tolerance can be valuable, boundary mics can be placed inside, or fixed to the side of instruments, and so reach places that other mics can’t. Although they’re not things I use on every session, I’ve found some great applications for boundary mics in my studio — on piano in particular — and I was keen to see what this new release from well‑known mic company sE Electronics could bring to my recordings.

Overview

A boundary microphone is designed to be positioned flush, or very close, to a flat surface — usually a floor, wall or ceiling. The close proximity of the capsule to a hard surface delivers two important benefits: reduced comb filtering (because the microphone only picks up what hits the surface, rather than any reflections coming off it), and increased sensitivity (because boundaries are where sound waves reach their maximum pressure).

The BL8 is phantom‑powered and uses the same small‑diaphragm capacitor capsules as the company’s sE8 and sE8 Omni models, which means it can accept either cardioid or omnidirectional capsules. I was given both options with the review mic; swapping them out was reasonably quick and painless using the included miniature screwdriver.

The BL8 is a weighty, reassuringly solid and sleek‑looking microphone, and it ships with a nice red leather case to keep it dust‑ and scratch‑free when not in use. As boundary mics are typically placed or mounted on a flat surface, the BL8 caters for either scenario with both a non‑slip rubber base and well‑thought‑out holes for attaching the mic to a wall or ceiling within a room, or for screwing to the lid of a piano, the side of a cajon, and so on. Another very popular placement for boundary mics is inside bass drums, and sE are keen to point out that the BL8 is compatible with the popular Kelly Shu Flatz microphone mount system, which allows a boundary mic to be suspended and vibration‑isolated within a kick drum.

The base of the BL8 hosts the mic’s pad, filter and EQ switches.In another nod to this popular application, sE have included Classic and Modern EQ options. Activated by a small switch on the base of the mic, these are aimed at sculpting the sound of kick drums, respectively offering a mildly or more aggressively ‘scooped’ tonality compared with the default sound. Lastly, there are switches for engaging a high‑pass filter at either 80Hz or 160Hz, and the option to pad the signal by 10 or 20 dB.

In Use

My very first experiments with home recording involved a cheap boundary mic mounted on the ceiling above my drum kit in our terraced shared house (I feel bad for my neighbours in hindsight!). So it seemed fitting to start my review with drums, and before I looked at putting the BL8 inside the kick drum I experimented with a few different positions around my drum kit to see how the mic would fare in my studio’s live room. I found myself liking it placed on the floor about three feet back from the kick drum, where it worked great as a kind of hybrid kick/room mic that proved very usable in a mix.

Many potential users will be looking at a mic like this with bass drums in mind, and I’m happy to report that the BL8 does not disappoint. I left the mic in its ‘neutral’ EQ position and positioned it inside the shell about six inches away from the beater head of the drum, on top of the soft cushion that lives inside my 22‑inch Rogers kick. This gave me a plain but very focused capture of the kick drum that responded very well indeed to EQ. I then experimented with the mic’s onboard EQ, discovering that the Classic mode adds a nice boost at around 60Hz, combined with a cut around 300‑400 Hz. To my ears, the Modern setting is just a more pronounced version of the same curve, but both settings worked very well and were nicely tuned in to my idea of a good kick drum sound!

Boundary mics can be very effective for recording pianos. My piano technician friend once showed me his technique for using a pair of the cheap Realistic PZM mics commonly found on eBay — taped to the inside of the removable lower panel on an upright piano — and I’ve had a pair permanently in situ since. I was keen then to hear how a more ‘hi‑fi’ boundary mic option would sound in this setting, and whilst it would have been nice to try a pair to cover the full range of the piano, the BL8 did a great job as a mono option. Tonally, I didn’t perceive a huge difference compared to my cheaper in‑house options, but I liked the fact that the mic always felt comfortable and not prone to any distortion if the player started hammering the keys.

More than once I preferred the boundary mic option over my usual Neumann FET U47...

Another use I found for the BL8 was recording an acoustic guitar, with the mic placed on a small table next to the player. It did a nice job of capturing a bright, clear guitar sound that provided a different perspective to my typical mic setup. The BL8 also sounded superb placed on the floor just in front of a loud bass cabinet, where it produced everything you would want for a loud rumbly bass sound with a clear and full low end. More than once I preferred the boundary mic option over my usual Neumann U47 FET positioned close to the speaker.

Summing Up

The BL8 is a rugged, stylish boundary mic that seems very competitively priced compared to the other high‑quality options available. Capable of handling very high sound pressure levels with ease, the BL8 proved a very handy addition to my studio’s mic options during the review period. The ability to just quickly put a mic like this on the floor in front of a bass cab or drum kit — and get good results — was great, and I often found myself preferring the BL8 over mics costing 10 times the price.

I was also reminded of just how good this type of mic is for getting usable room and ambient sounds out of less‑than‑stellar spaces. If you’re recording drums in a small room, for example, you can open up some very creative options by placing a boundary mic on the floor, wall or ceiling.

Summing up, then, if you’re on the lookout for a dedicated boundary mic, this seems a very good choice. Whether you’re after a solid performer inside a bass drum, or you’re interested in experimenting with what this type of mic can offer elsewhere in the studio, the BL8 comes highly recommended.

Omni Or Cardioid?

I made a point of trying both of the available capsules for the review, and not surprisingly, the main difference was that the cardioid capsule was more directional! Either capsule would work great for most of the applications I tried it on, although you would probably be better off with the more focused cardioid pattern if eyeing it up as a dedicated kick‑drum mic. If you’re looking for a more flexible option, or want to be able to capture conversations with multiple voices, or ensemble performances, the omnidirectional capsule would be the better bet.