Audio‑Technica BP3600

Designed specifically for immersive formats, the BP3600 is a novel alternative to more familiar Ambisonic microphones.

It probably hasn’t escaped your attention that immersive audio has become a big thing. From Dolby Atmos and Apple Spatial to binaural radio plays and theatre shows and multi‑speaker exhibitions and ‘experiences’, we’re becoming surrounded by technology that, one way or another, immerses listeners in an extended soundfield. There’s no escape. But the term ‘immersive’ is something of a catch‑all; it potentially covers so much technical ground that it has all but lost any definitive meaning. When, for example, does audio become immersive? One answer might be that it is immersive when the channel count extends beyond stereo. But hang on, binaural audio is stereo and very definitely has the power to immerse a listener in an extended soundfield...

So, the term can describe a wide range of audio technologies and production techniques, but one element of immersive audio that is perhaps a constant across all artistic concepts and delivery technologies is the ‘bed’. You can maybe think of a bed as the wraparound audio canvas on which the action happens. Dolby Atmos, for example, enables beds and objects. Typically, in an Atmos music mix, you might put the string pads in a bed routed statically to multiple output channels, and treat the lead vocal as an object, because of the enhanced degree of dynamic location control that Atmos objects can have. Similarly, in an immersive theatre show or Atmos movie you’ll probably make the background atmospheres a distributed bed and have the actors’ voices routed to discrete object channels. A further characteristic quality that distinguishes beds from objects, especially in the theatrical realm (by which I mean, theatre, movie, TV, audio drama), is that beds are very likely to comprise natural, recorded atmospheres. For a street scene in a movie, for example, you probably need to record a street atmosphere (unless of course there’s an online AI source of artificially generated immersive natural atmospheres that I’ve missed). And it’s those background atmosphere beds that the subject of this review, the Audio‑Technica BP3600, is designed to capture.

Eighth Wonder

The BP3600 from Audio-Technica is an integrated, eight‑capsule microphone designed primarily to capture atmosphere beds for integration in immersive audio productions of pretty much any technical flavour. Now, we’ve reviewed in the magazine over recent years a few multi‑capsule microphones, such as the Soyuz 013 Ambisonic, aimed at encoding 360‑degree soundfields in Ambisonic format. Your first thought might be that the BP3600 inhabits the same territory, but it is in fact significantly different in that, rather than having cardioid microphone capsules in a coincident tetrahedral array, the BP3600 features eight hypercardioid capsules, located at each corner of a virtual 15cm cube.

There are two significant repercussions of this arrangement compared to an Ambisonic array. Firstly, in being spaced apart, the BP3600 capsules embed signal arrival time as well as frequency‑dependent level difference information in the audio they capture. This is analogous to the difference between a crossed coincident cardioid microphone pair and a spaced stereo array. The crossed pair embeds only frequency‑dependent level differences, due to the different orientation of the two microphones, while a spaced pair embeds both level and arrival time differences. Secondly, in order for it to be usefully integrated within a mix, the four (first order) or eight (second order) channels of an Ambisonic microphone require post processing to decode the output matrix. With an Ambisonic mic, you can’t simply route mic outputs to some channels. However, that’s exactly what you can do with the BP3600, and I’ll go on to explain the significance of this in a paragraph or two’s time.

3600 Degrees

The BP3600 arrives in a smart black fabric zip case in which the mic body and eight microphone capsules are packed separately. The capsules have to be attached, with a simple push and turn, to the mic body. The body itself incorporates a 16‑pin LEMO connector for output, and conventionally dimensioned mic stand attachment features. The body also carries colour‑coded bands at the bottom of each mic capsule attachment point that refer to channel numbers. The XLR ends of the supplied 5m connection cable are similarly colour‑coded, so that the assignment of mic capsules to channels is relatively easy to keep track of — and that’s important. The BP3600 case additionally houses eight small optional windshields, and more substantive wind protection is available as an option. A longer (20m) LEMO to XLR connection cable is also available and, it has to be said, I did find the supplied 5m cable a little restrictive. It would of course be possible to increase the reach from mic to interface/recorder by extending the eight XLR ends of the mic cable, but that would fall somewhat into the category of clumsy solutions.

The BP3600’s sensitivity is specified at 8.9mV/Pa, while a self‑noise figure of 23dBA is given (for all the capsules combines). Maximum SPL is 144dB (at 1kHz, for 1% THD). The electret capsules are described as “hypercardioid” but there is no detail provided in the published spec to define just how tight is their directional characteristic. All in all, the BP3600 exudes quality and precision. It’s clearly a serious and professional piece of kit.

The Proper Channels

Perhaps the most striking aspect of the BP3600 is the ease with which it can be used to record an atmospheric bed that can then be easily integrated within an Atmos session (other immersive audio delivery technologies are, of course, available). It’s so easy because the outputs of the eight BP3600 capsules can be simply routed directly to four listener‑plane channels and four height channels. For example, in a 5.1.4 arrangement, the mic channels would be routed as follows:

• Channel 1 = Front Left
• Channel 2 = Front Right
• Channel 3 = Rear Surround Left
• Channel 4 = Rear Surround Right
• Channel 5 = Front Height Left
• Channel 6 = Front Height Right
• Channel 7 = Rear Height Left
• Channel 8 = Rear Height Right

For The Birds

To get some experience with recording a BP3600 bed, and to demonstrate my deep commitment to Sound On Sound, I got up very early (5:00am) one morning just after the beginning of British Summer Time and set the microphone up on a stand in the woods behind our studio. This is the point where I felt that the standard 5m cable of the BP3600 could usefully be longer, because I ideally wanted to position the microphone further into the woods and more distant from the wall of the studio than I was able. However, the plan was to capture the dawn chorus in full‑fat BP3600 immersive audio. If I was lucky, I thought, I might even get the sound of a badger or fox moseying past. The dawn chorus is particularly fine in my neck of the woods on the south coast of the UK: not only do a wide variety of common garden and woodland bird species celebrate surviving the night with a song, but the distant cry of seagulls on the coast over the hill is also mixed in. It makes for a pretty evocative audio atmosphere.

The BP3600 on duty as a dawn chorus mic.

I recorded the eight BP3600 channels via my Audient ORIA interface with an ADAT‑connected Audient SP8 eight‑channel mic preamp (kindly loaned by Audient for this review), directly into a Pro Tools session. I routed the mic channels directly to the front, rear surround and four height speakers of my Dynaudio BM5‑based Atmos monitoring. The results were spectacularly good. Sitting in the primary listening position with eyes closed, I would have needed just a gentle breeze (and maybe the odour of decaying organic matter) to be fully convinced I was in the woods. I’ve recorded dawn choruses before, and they generally sound great, but never have I felt so effectively transported to the spot where the recording was made. It also worked very effectively over headphones via a Dolby Atmos binaural re‑render.

Having raised the prospect of folding down the BP3600’s eight spaced mics to stereo, there’s a mild health warning to be aware of. As with any spaced array (as opposed to coincident), there’s always the possibility that combining their output will result in some comb filtering as signals that arrive at the capsules out of phase will cancel. I wasn’t specifically aware of any such issues with my dawn chorus recording (perhaps its inherently ambient, diffuse nature made any comb filtering benign), and I think the type of material most likely to be captured by a BP3600 will make spaced‑mic comb filtering less likely to be a problem, but the possibility of comb filtering from downmixed BP3600 output is something to be aware of.

Bed Time

Leaving the romance of the dawn chorus (now regularly playing on a loop in the studio for relaxation) to one side for a moment, there’s obviously things to say about using the BP3600 in immersive audio workflows. Firstly, readers familiar with Dolby Atmos will be aware that standard Atmos beds are configured with a 7.1.2‑channel layout. This means there’s no direct equivalence between the orientation of the eight BP3600 capsules and all Atmos bed channels. In particular, a default Atmos bed only employs two height speakers. However, this is not really an issue once you get into building a mix session, because there’s at least a couple of workarounds. The most obvious is that the left and right upward‑facing BP3600 capsules could be summed to create just two height channels (although this of course would be at the cost of front‑to‑back information in the height plane and may also result in comb‑filtering issues). Alternatively, it is of course possible to mute one pair of upward‑facing mics. A second, and probably preferable, workaround would be to create a custom Atmos bed from discrete objects. This ‘object bed’ could comprise all eight mic channels, or perhaps just the four height channels. Alternative immersive audio formats, such as Sony 360 Reality, will of course make their own demands on routing and workflow from a BP3600, but the beauty of it is that configuring such things is made so much easier because there is none of the matrix decoding required with Ambisonic microphones. Add a spatial/immersive workflow plug‑in such as New Audio Technology’s Spatial Audio Designer to the mix and you have the ability very quickly to create bed material from the BP3600 for pretty much any immersive audio format.

The results were spectacularly good... Never have I felt so effectively transported to the spot where the recording was made.

Finally, while the BP3600 is clearly aimed at the very specific niche of creating immersive bed material, there’s no reason at all why it shouldn’t be put to other more radical creative uses also. It is, after all, an array of eight very high‑performance Audio‑Technica microphones. I didn’t have the opportunity to experiment with such things while the BP3600 was with me, but I’d love, for example, to have tried putting one among a string quartet playing in the round in a lovely reverberant environment, or maybe even recording a drum kit from just above the drummer’s head. Beyond recording beds, the possibilities for immersive audio creativity with the BP3600 expand the more I think about it. The Audio‑Technica BP3600 isn’t an inexpensive proposition (although thinking about it in terms of price per microphone perhaps makes it seem more affordable), but it fulfils its intended role extremely effectively and at the same time potentially offers some really intriguing creative recording possibilities.