An alternative methodology for “Immersive Sound” microphone- techniques by the use of Boundary Layer Microphones
“Catching the true essence of an acoustical music performance”
BERT VAN DER WOLF © 2020
NorthStar Recording Services BV | Edison Production Company BV
Molenstraat 13, Haaften, The Netherlands
Over more than a century, musicians, record-producers and -engineers have looked for tools and techniques to capture the true essence of an acoustical musical performance. In Mono (source point), Stereo (2D setup), Quadraphonic sound, or any other elaborate Surround-sound format like 5.1 or 7.1 (2D setup in a horizontal plane) and Immersive Sound (3D with height information), the ultimate goal has been to create an illusion as if “being there” whilst musicians perform their art. Much development was needed, and many obstacles had to be overcome, to eventually get closer to the sensation of experiencing live music by playing a recording through Hi-Fi equipment. There are as many different approaches as there are recordings, some more successful than others, but in general it can be stated, that the results often just offer a limited representation of the musical event that has been coded in the data on the disc. Recorded music, manifesting itself as “frozen time”.
This paper discusses a method to obtain a relative optimum in “realism” through a specific, rather personal recording technique and the use of “alternative tools” that are less common in the general music recording community.
Starting in the recording business some 32 years ago, as a graduate student at the Royal Conservatory of The Hague, I was under the impression that there was an absolute method for making proper recordings. A sort of rulebook one could conform to and thus consistently produce adequate recordings according to this iron standard. Soon I discovered however, that performing well in recording- and postproduction was more complicated.
There was nevertheless a heritage of rules from analog tape-recording and reproduction on vinyl discs, from which we learned several sensible rules for frequency and phase behavior, making sure to deliver decent mono compatibility in stereo recordings. With the introduction of digital audio and the Compact Disc, most of these regulations were abandoned, as the medium allowed for much more “out of the box” experimentation regarding timing and dynamics, i.e. delays, limiters, exciters and use of wide audio frequency-band spectra.
For acoustic recordings, an immense plethora of microphone-techniques and digital reverb engines, using “true” impulse responses of existing halls, came into play. This all to accomplish the dearly desired feat of “transporting the listener back to the original event” from where the recorded music originated. For producer-engineers, like myself, this ultimately translates in time travel and re-living an event from the past, a rather mystical experience. This aspect has always kept the passion for this profession alive and is the main reason why I started recording music in the first place.
To determine what we are looking for, it is imperative to know how real instruments in real acoustics sound. Every instrument is different however, even 2 violins never sound the same and recording venues, i.e. concert halls, churches and studios, are in themselves inherently also musical instruments with unique characteristics. Good musicians go with that. They take what the instruments and hall acoustics offer them and play accordingly “to make things work”. Some spaces inspire more than others, but in general one can produce a good and realistic recording in any environment. Fashion and personal preferences determine ultimately what is considered a good or bad recording, given the fact that the technical quality of equipment and the chosen delivery format is optimal according to the current professional standards. Coding this “mystical experience” in the data through microphone technique into electrical currents and de-coding it with a playback system, so that the original emotion is again apparent, is the ultimate challenge! Many parameters and details are responsible for this effect, predominantly the recognition of the musical content and the delicate timing of the musician’s performance, but certainly also the recognition of acoustical characteristics, creates most of the emotional response. The better the width and depth of the acoustics and the placement of instruments and voices, the more one feels as if “time is being replayed”. Ultimately one should be completely immersed in the soundscape and tonal textures of the original event and forget that the music is played back on HI-FI equipment. This of course is the holy grail for all professionals in the recorded arts!
No matter how elaborate, sophisticated or clever the microphone placement and subsequent techniques are, there always seem to be downsides to the chosen system that prevents the experience to be as thrilling as a live performance. Due to flawed methodology that relies only on trusted measurement and quality of recording tools, the results are often somewhat disappointing. The human ear-and brain apparatus seems to be really clever when it comes to recognizing aberrations in both tonal balance and texture and erroneous warped phantom images in the soundstage, which characterize many recordings in the business. The limited number of speakers and available angles for placing microphones, to somehow mimic the infinite number of reflections triggered in real acoustics, have a hard time convincing the hearing apparatus and possibly deceiving it by not recognizing real from fake. Many recordings sound like a pile of microphones which throw up an incoherent soundscape with all sorts of phase issues and tonal comb-filtering distortions. On top of that, musicians tend to focus greatly on balance and perfection of performance, so making a recording completely natural is not an easy task, certainly not in the business of producing commercial records.
Education and theoretical studies on microphone techniques are available in abundance. All have their own merits and deliver plausible solutions for the dilemmas faced during recordings of acoustic performances. There are however also distinctive schools of preference and temperament when it comes to how to create the ultimate auditive experience. Personally, I have generalized the audio community into two distinct groups: “the sound-people and the temporal sensitive-people.” The first seem to be predominantly debating tonal colors and balance, the latter focus on localization and 3-dimensional projection and layering of the soundstage. The same division can be found amongst musicians and laymen.
“It sounds dull or too bright, or I want more bass, and could it sound louder please” is what the “sound-people” tend to emphasize on. The “temporal sensitive-people” however, respond very strongly to phase coherence and an undistorted impulse response in the projection and share descriptions like: “the loudspeakers have vanished, and the soundstage is multi-layered and deep”. The thing is that all these parameters are equally important, but the two groups do not easily agree on the methods to use, i.e. the two types do come up with rather different microphone arrangements. The first often propose a wide range of microphone choices, often looking for specific characteristics for different situations and ultimately solving problems “in the mix”. They don’t hesitate to alter reality to personal liking and create an alternative soundstage that, at best, adequately translates the intended emotions by the musicians. You find them mostly in POP-music. The latter however tend to stick to a few pairs of optimal omnidirectional microphones, often with small membranes and painstakingly move them around the instruments and voices to find the best trade-off for ultimate transparency and balance. Acoustics are very important for them and imperative for the transmission of musical communication. No surprise this goes for acoustical recording specialists, like me, in acoustical Jazz and Classical music. A big difference between these two types of engineer’s preferences, is the way of “mixing”, i.e. blending sound sources. Either by electrical currents through wires and mixing boards, or alternatively by air molecules colliding in the recording space. There is no way to interchange the two principles as the results are totally different in texture and projection. This has led to a lot of misunderstandings as to how create a convincing balance and soundstage in recordings which results in very different recording traditions.
In the quest for finding the ultimate recording process, I have gone through most of the known microphone techniques and matrices available and have always been at the forefront of the analogue and digital technological development. Several intense collaborations with hardware manufacturers have resulted in fantastic electronics and highly transparent recording equipment that inherently should be able to create the most convincing recording possible. All of this however never completely delivered the desired results, especially in regard to recordings in 5.1 Surround Sound. Most techniques show a reasonably consistent blend and tonal balance, but often the slightest random dynamic impulse destroys the transcendent illusion of reality and immediately trigger the awareness of listening to a recording that fails to convince. Instruments creeping in and popping out from the loudspeaker drivers, warped phantom images and distorted acoustical attributes, but most of all a vague representation of the acoustical characteristics of the original recording venue. This problem gave me the incentive to look for microphones that were better suited for “coding” the holographic attributes of a recording space into an electrical signal that will indeed be “de-coded” by the ear/brain apparatus as true characteristics for that particular space and everything that sounds in it. It led me to Boundary Layer microphones. (BLM) (fig.1) This type of microphone transmits indirect sounds that reach its membrane via wall, floor and ceiling reflections with a high degree of fidelity, and these are the particular sounds that convey to the listener important information on the size and nature of the recording venue. Furthermore, a microphone mounted in a “boundary surface” is in a sound pressure maximum all the time, which delivers a sound texture that is far less colored by early reflections from the floor or walls that typically color the sound of microphones on a stand. (comb-filtering) In the BLM, sound waves are in phase, because the direct- and reflected sound path arrives at the same time at the microphone’s membrane, hence there is almost no such interference. Also transients are optimally preserved and the free-field and diffuse-field frequency response of the microphone are identical, therefore the listener has the impression, more than with conventional microphones, of actually being in the recording room, and can identify remote sources quite well, as would a person actually in the room. It translates in a true “holographic imprint” of the room, which our hearing decodes accordingly as a true 3-dimensional space. What makes a boundary layer microphone also rather practical is that its positioning is much less critical. All sounds in the recording room are optimally stored in the “code”, almost no matter in which boundary field the BLM is placed. From the first experiments it became clear that I found a promising route to follow. The first challenge was to obtain BLMs with a quality matching my whole recording chain, only finding those to be non-existent on the market. That’s why I worked together with the Dutch manufacturer Rens Heijnis, from Sonodore, to construct a BLM model that met the technical requirements we had become accustomed to. Using BLM’s for the rear channels in 5.1 surround sound recordings immediately delivered results with a far more natural acoustic imprint in the reproduced sound and a virtually seamless integration with the front channels. Most striking was the realization that the sound pressure level (SPL) of the rear channels could be presented much higher than with traditional omni-directional microphones being the source, surprisingly without destroying the frontal soundstage. As if the envelope of acoustics simply folds around the direct sound of the instruments and voices in a frontal presentation. It seemed that the ear/brain decodes the texture of the BLM source rather as “a wall” and therefore dismisses the sound as non-directional, hence not disturbing but enhancing the frontal localization of phantom images in the soundstage. From there on it was an inspiring journey for discovering the optimal arrangement of multiple BLMs which could be implemented in stereo-mixes, 5.1 surround sound productions and immersive formats like 9.1 AURO-3D (fig. 2). Eventually I came up with the High-Quality Musical Mastering principle (HQMM), whereby a matrix of 4 to 8 BLMs is implemented to create a full immersive soundscape around the phantom images in the frontal loudspeakers without pulling those in the rear loudspeakers, although presented with a substantial SPL(!)
The basis of this recording principle is a realistic and holographic 3-dimensional representation of the musical instruments, voices and recording venue, according to traditional concert practice. For most older music this means a frontal representation of the musical performance, but such that width and depth of the ensemble and acoustic characteristics of the hall do resemble “real life” as much as possible. Some older compositions, and many contemporary works do specifically ask for placement of musical instruments and voices over the full 360° sound scape, and in these cases the recording is as realistic as possible, within the limits of the 5.1 Surround Sound and Immersive Sound AURO 3-D standards. This requires a very innovative use of all loudspeakers and the use of completely matched, full frequency range drivers for all discrete channels.
In fig. 3 is a typical set-up described where multiple BLMs are implemented to capture the total of acoustical information in a room during a musical performance. To enhance, but also modify the impression of size, tonal texture and reverberance, the BLMs are placed on baffles that can be placed anywhere in the room. These panels create new boundary fields which again contain the “holographic” DNA of the whole room, yet with greater fidelity and less comb-filtering. Placing them strategically gives the flexibility in the post-production process to adapt this imprint optimally to the frontal directional presentation of what is recorded with traditional microphone techniques. These techniques are relatively standard with a main system, more or less elaborate in a matrix of omni-directional microphones, complemented by subtle use of spot-microphones. Half the amount of BLMs are responsible for the rear channels from the audience’s perspective and half are situated on the sides of the soundstage, to mimic sidewalls, all picking up reflections from all angles. The sensitivity on-axis of the BLMs is slightly dimmed as the polar response resembles a “donut shape”, so this again enhances the feature of relative independence from the frontal imaging in the recording. This combination delivers tremendous flexibility in stretching the soundscape outside and above, of mainly the acoustics, but also phantom images exceeding the width of the main stereo pair of speakers.
Implementing high quality Boundary Layer Microphones, in acoustical recordings, delivers an exceptional high fidelity and accurate representation of the natural acoustics in recording rooms. In traditional Surround Sound and Immersive Sound recordings, the characteristics of these microphones deliver an astounding holographic soundstage over the 360° monitor range. Placement at recording is far less critical than conventional microphones and the texture of the signal proves to be highly flexible and effective during mixing. In my personal experience the use of BLMs offer the best solution in any multi-channel recording for Surround Sound (2D) and Immersive Sound (3D) productions.
One might think that adding more channels and more loudspeakers automatically leads to a more natural sound reproduction. It is however proven by many experiments, that there is a tradeoff between adding channels, thereby delivering more directional information, and introducing harmonic and temporal distortion through increasingly complex phase issues. Furthermore, there are commercial formats to conform to and the true craft of the recording engineer is to obtain the highest fidelity and holographic realism within the constraints of these pre-set formats.
I do like the 9.1 AURO 3D Immersive Sound format, because it offers an efficient number of channels for my High-Quality Musical Mastering principle (HQMM), allowing the use of multiple boundary layer microphones. The format is optimally suited for creating the ultimate illusion of “being there”. Furthermore, it offers a very good backward compatibility with the traditional audio formats like 5.1 Surround sound, which is less the case with for example the 22.2 standard format.
05-04-2020 Haaften, The Netherlands
- Microphones “Methods of Operation & Type Examples” Gerhart Boré / Stephan Peus
Fig.1 Sonodore BLM-21 and PS 402 Power Supply
Fig.2 Speaker arrangement for 5.1 Surround Sound and Immersive Sound 9.1 AURO-3D
Fig. 3 Typical 5.1/9.1 set-up with BLM-21 microphones.
 The AURO 3D format was introduced by Wilfried van Baelen at the 2006 AES conventions in Paris and San Francisco and in 2010 at the AES Spatial Convention in Tokyo, where he coined "Immersive Sound" to be used as the new generic term for "Surround Sound with Height".
 High Quality Musical Mastering with the use of 8 boundary layer microphones for a true holographic sound representation of the recording room.
Photo: Brendon Heinst