New recording techniques for solo double bass Cato Langnes NOTAM, Sandakerveien 24 D, Bygg F3, 0473 Oslo catola@notam02.no, www.notam02.no Abstract This paper summarizes techniques utilized in the process of recording solo double bass performances by Håkon Thelin. The aim of the project was a CD production with performances of some of the compositions used by Thelin in his work as an artistic research fellow at the Norwegian Academy of Music. The pieces feature vast dynamic ranges and variations in playing techniques. Micro tonality, harmonics and various bowing techniques are integral parts of the instrument s sound. The challenge was to capture and bring forward the fine and delicate nuances resulting from the playing techniques, while at the same time preserving the free, open resonance of the double bass in the concert hall used for the recordings. These requirements sparked an investigative process that ultimately led to the technical solutions that are presented here. Project background I have been a sound engineer and producer of rock and contemporary music for more than two decades. For a long time I have wanted to use microphone techniques that are common in rock productions for recording classical and contemporary music. During the fall of 2008, composer Anders Vinjar and I experimented with stereo techniques where we compared omnidirectional and cardioid microphones. I was quite skeptical towards omnidirectional microphones, since their lack of directionality compared to cardioid microphones inevitably reduces the source separation between the recording channels. However, the superior depth and correct frequency response resulting from the omnidirectional recordings was a pleasant surprise. After these first experiments I started working with combinations of cardioid and omnidirectional pairs, with very positive results. I used the cardioid pair to focus on salient details in the soundscape, whereas the omnidirectional pair was used to record the timbre surrounding the source. This work led to a more comprehensive investigation of the vast range of possibilities afforded by combinations of different microphones with different characteristics. We began recording the works for Thelin's CD during the winter of 2009. Thelin works with new and unconventional playing techniques, and he wanted to use the recorded medium to convey and reinforce details that are not easily perceived in a concert situation. He has created his own musical language predominantly inspired by folk and contemporary music and by the timbral potential found in the double bass. Thelin's repertoire and technical style requires a close and extremely detailed, but simultaneously natural rendering of the instrument. The recordings that will be discussed here formed part of the documentation for Thelin's research. The investigative aspect permeated the entire process to a considerable degree.
Recording Eight microphones were placed at different distances and angles from the double bass in order to capture all the nuances in Thelin's playing, and the ambience in the room. Fig. 1.: Six out of eight microphones at different angles and distances. Close range single microphones Thelin uses a DPA 4021 attached to the instrument for amplification in concert situations. We attached the microphone to the strings just below the bridge, pointing diagonally up towards the f- hole. This mounting is also used for his concerts (Fig. 2). Fig. 2.: DPA 4021 mounted below the bridge.
In order to achieve a warm character in combination with the good bass response that a large membrane microphone yields in a mix, I interchangeably used an AKG 414 or a Neumann U87, depending on the composed material. This was placed as close to the instrument as possible, at the right side of the bridge when seen from the front. For practical reasons, a space of 30 cm between the bass and this microphone was necessary for Thelin to be able to bow freely (Fig. 10). The purpose of this close- miking technique was to capture the low- amplitude harmonics and Thelin's bow bouncing techniques that produce minimal sound pressure, and therefore cannot be well captured at further distances. This miking technique also added a solid mid- range signal to the mix. Some of Thelin's playing techniques feature much activity on the high end of the fingerboard. Because of this, we decided to place an AKG 461 microphone quite close to the fingerboard to capture these details. It was however not necessary to use this microphone on all of the material (Fig. 3). Fig. 3.: Placement of AKG 461 for close miking of the fingerboard. Close range stereo pair Sound pressure waves radiate from the double bass in highly diverse patterns and across different frequency ranges, and knowledge about these acoustic radiation patterns is crucial in order to select microphone placements that yield an optimal recording of the instrument. The double bass is ultimately a bass instrument; however, in order to obtain depth in timbre, it is important to capture the many rich frequencies from 600 Hz and upward. This is especially beneficial if the bass is used in pop/rock/jazz contexts, where these upper frequencies are crucial for the bass to be heard clearly and distinctly in the mix.
Fig. 4.: Illustration from Jürgen Meyer & Uwe Hansen (2005). Acoustics and the Performance of Music. Berlin: Springer. The late double bass professor at the Norwegian Academy of Music, Knut Güttler produced the CD. His extensive knowledge of how sound radiates from a double bass was hugely important during the recording sessions. On his recommendation, I alternated between placing a pair of DPA 4015 wide- cardioid or Neumann KM 130 omnidirectional microphones pointing toward each f- hole at 45- degree angles. In regards to placement, a compromise was reached where the need for Thelin to move the bow and hands freely was balanced against the need for as close of a recording range as possible. I ended up with the microphones placed at a distance of approximately 50 cm from the f- holes with a 70 cm separation between the two microphones (Fig. 4). This gave such good results that the stereo pair was used as the main stereo track when mixing. The technique works very well also for cello recording. The dynamic range a close microphone picks up is larger than what you get when you set up microphones far from the source. Be careful with the recording levels on extremely close mikes ask the musician to play through the loudest part and leave 6 db headroom when you adjust the levels. It is most often necessary to use compression on close- miked tracks and often to adjust automation or use clip gain adjustments to avoid distortion through the compressors.
Fig. 5.: Placement of a DPA 4015 stereo pair and 4021 (on the instrument). Spaced stereo pair Throughout my career I have accumulated broad experience in recording chamber music ensembles for documentation purposes and for radio broadcasting. In these situations, I have often used a spaced pair of cardioids placed as close to the ensemble as possible, without allowing any instrument to become more dominating than the rest due to its proximity to the microphones. This placement technique requires careful monitoring through headphones in order to find the ultimate balance between the musicians and the recording space. Usually, I prefer capturing as much of the direct signal as possible without too much reverb from the room. I used this technique on the Thelin recordings as well. In Lindemansalen (small orchestra concert hall) at the Norwegian Academy of Music, we ended up positioning a spaced par of cardioids 2-3 meters in front of Thelin, pointing outwards (A- B technique) at an angle of approximately 90 degrees. These microphones were placed quite high so they had to be pointed down towards the instrument, thus avoiding the capture of reflections from the floor (Fig. 6). For microphones placed at these positions I have experimented with both omnidirectional and cardioid pairs in A- B formation (as opposed to near- coincident X- Y where the angle is inverted and the microphone heads are brought together). The omnidirectional microphones often yield a fuller timbre with plenty of depth. And they pick up more ambience from the room. Cardioids tend to capture details such as bowing and valve clicking more accurately.
Fig. 6.: Placement of AB stereo pair, top left in picture. Ambience/room microphones Recordings of acoustic instruments are more interesting if you also capture room acoustics. To a certain extent, this is also inevitable. The sessions with Thelin were recorded in Lindemansalen and Levinsalen (small chamber concert hall) at the Norwegian Academy of Music, and both halls have superb acoustics. In order to capture the timbre of the room the way it is typically experienced by an audience, we used a pair of omnidirectional microphones placed about 10 meters in front of the bass, with an equivalent distance between the two microphones. The long distance to the instrument required quite a lot of preamp gain, and the recording contained a great deal of noise from the room in addition to unwanted sub- bass from about 50 Hz and below, probably from the ventilation system. In the mix, therefore, these channels were filtered through a 50 Hz low cut before used. Reverb in the final mix is based mainly on these ambience channels. Double MS A double MS microphone set- up was not used at all in Thelin's project, but this technique has yielded good results in many other projects, and therefore deserves mention. If one wishes to further develop recordings of room acoustics in the mixing process, the double MS technique provides the possibility of experimenting with the stereo width through the decoding process and a subsequent balancing of the resulting channels. For MS recording one uses a forward- pointing cardioid microphone (mid) together with a figure- 8 microphone (side) that captures the right and left side of the room. The microphones are recorded to separate tracks and are easy decoded to a stereo signal. You can import your tracks to any DAW,
duplicate the figure - 8 track and invert one of the duplicated tracks. Move the two figure- 8 tracks to a stereo track (side) and mix it with the cardioid track (mid) and you get a nice stereo signal. Double MS recording includes an additional backward pointing microphone that enhances the perception of depth in the room. These signals can be decoded into 4 or 5 channels if the goal is a 5.1 mix. The decoding process is well described in the Schoeps Surround Brochure, which can be downloaded here: http://www.schoeps.de/en/products/categories/doublems Phase issues The recordings were all done with Sound Devices 788T recorders and imported to Pro Tools for editing and mixing. When using multiple microphones on one source, playing it back later will introduce phasing problems because the source sounds will reach the microphones at different times (Fig 7). Before editing, we could clearly hear comb- filtering artifacts between the microphones that were relatively close together, and very distinct echo effects between the close- range and ambience tracks. Fig. 7.: Edit window in Pro Tools after import of files, pre phase- correction. Notice that the signals have different starting points.
Fig. 8.: Post phase- correction. Once phase was in place it started to sound nice, issues with comb filter effects and echo was eliminated. The first thing that needed to be done was to position the files into phase. I used the DPA 4021 microphone that had been mounted on the bass as the main reference, it being closest to the source (Fig 8, the purple- colored track), and brought all the other tracks in line with the attacks on this track. In regards to the stereo pair, a great deal of what makes the difference between the right and the left channels is time- of- arrival between the two microphones. This is especially true for the AB- setup used here, as opposed to the XY- method, where difference in amplitude is the most defining factor. In any of these cases, one should take care not to correct phase within the pair, but rather as a rule of thumb treat the pair as one when dragging the tracks into phase with the rest. However, phase issues do occur in stereo recordings, potentially generating comb- filtering effects. For the ambience microphones, which were spaced approximately 10 meters apart, there was a significant difference in the time- of- arrival of the signal. To deal with this issue, I made some adjustments between the left and the right side and made sure that the most salient features in the attacks were in phase with each other. Editing and mixing Mixing was done together with Håkon Thelin. With the number of microphone placements and recording techniques at our disposal, we were quite free to shape the sound. We had a considerable degree of control between the relatively dry close- range recordings of the instruments and the room ambience, and we could quite simply extract details in the mix by compressing the close- range tracks and thereby bringing details to the forefront, without affecting the overall mix.
The omni/wide- cardioid close- mike pair was used as the main sound source in the mix. The pair provided good representation of dynamics, with a full and rich stereo image. Frequencies below 60 Hz were filtered out of the tracks recorded with the 4021 microphone, and the tracks were subjected to heavy compression in order to bring forward micro details in the playing as clearly as possible. The compressor was used basically as a limiter (short attack and release, 1/10 to 1/20 ratio, and the threshold was lowered to a level just above when the compressor started distorting). In some of the mixes a duplicate of the large diaphragm microphones track was compressed the same way. When I raised the level of these tracks in the mix it still sounded dynamic and open, and all the delicate details in the playing came through clearly in the mix. The middle of the stereo image sounded solid and full when the large diaphragm microphones were combined with the close- microphone pair. Some very soft parts were still almost inaudible in the mix, and this was fixed with automation of volume on the close- microphone tracks, bringing also the extremely silent parts to an audible level (Fig. 9). Fig. 9.: Amplitude automation in Pro Tools. Fig. 10.: DPA 4021 mounted on the instrument, Neumann U87 (large diaphragm microphone) at approximately 30 cm distance, DPA 4015 pair pointed at f- holes.
Fig. 11.: Bussing and compression in Pro Tools. Bussing and compression in Pro Tools There was a clear contrast in the tracks with the DPA 4021 and the AKG 414/Neumann U87s. This is to some extent due to the difference in microphone characteristics. Both close- miking setups provided good capture of the fine details in Thelin's playing, and worked well with the extreme variation in dynamics. Due to the longer distance from the instrument, the large diaphragm microphones U87s and 414s had slightly more room ambience than the 4021. The instrument- mounted microphone gave richer bowing sounds, and stronger low frequencies in attacks where the bow changed direction. It is however difficult to establish a general rule for what provided the best solution, and we ended up using both tracks in the mix. (Fig. 10) A combination of the close microphones provided good results. In the mix, all close- miked tracks were sent to the same bus and compressed, while simultaneously being routed directly to the master bus without compression - a technique known as parallel compression (Fig. 11). Recordings will often sound dull if not placed in a room. Without placement in a room, the recording provides a representation of how the instrument sounds from the perspective of the musician, which is not necessarily the more interesting perspective for listener. In the same manner that the close- miking provided a good representation of the musician's perspective, the microphone position 2-3 meters in front of the instrument was useful for providing a natural sense of the room in the mix; an audience's perspective. I have with good results used a double MS rig for the same purpose. If you have enough microphones and tracks available it is a good idea to do both. Both techniques provide good representation of a sound source in room and the MS recording may be decoded to both stereo and surround. In situations lacking ambience microphones, or where the ambience recordings are too noisy to be used in the mix, I have created artificial room ambience by feeding the signal from such stereo- or double MS- setups into the artificial reverb, rather than using the signal from the
close microphones. This has reduced the degree of quick transients and sharp, squeaking sounds in the reverberation, and has made it sound more soft and dense. The ambience microphones were exclusively used for adding natural reverb to the recordings. Heavy bowing and hard pizzicatos sound poorly with added reverb, while softer bowing will benefit from more ambience. When balancing these concerns, a lot of amplitude automation was used for the ambience track. We reduced the reverb in the sections where Thelin would play louder, and turned it up in the softer parts (Fig. 12). In order to achieve a homogenous reverb, a lot of precision work with automation was required at the transition points between soft and loud amplitudes. Fig. 12.: Automation of volume on surround tracks in Pro Tools. The master bus was subjected to EQ and compression. The EQ lifted some areas of the frequency spectrum that did not sound energetic enough in the mix, and the compressor was set to reduce the loudest peaks with approximately 2 db. Summary The experiences from making recordings with Håkon Thelin have been of use in almost every recording session I have done since that time. If time and resources allow, I now use close- miking to capture smaller details, stereo pairs for capturing the ensemble as a whole, and stereo or double MS for the acoustics of the room. If the pieces have been previously recorded, I listen to them for reference, and discuss them with the musicians and composer to achieve an understanding of what they are looking for and would like to emphasize. I spend a lot of time analyzing the recording situation in order to decide where to position microphones to capture the small details of the music, how to achieve the best possible separation of instruments, and how to best describe the room where the music is performed. Music is almost always complex, even with only one performer. Instruments sound different depending on how close you are to the sound source. This is not only related to the frequency characteristics of the room, but also due to how lower frequencies travel further through the room than higher frequencies, due to their higher energy levels. When you are located far away from the sound source, you will not hear the higher frequencies at the same level as the musician does, since they will be absorbed before reaching the listener. The same principle applies for the acoustics of the room the reverb time for lower frequencies is longer than for higher frequencies, so they will linger for a longer
time in the room. Certain details in the sound of instruments will drown when they are played together with other instruments, or emerge far away from the listener/microphone. That makes close miking essential for recording them. It is important to pay attention to phase and the sounds travel time to the microphones when multiple microphones are used to capture the same source, and the microphones are placed far apart. Regarding music for solo instrument, as this paper describes, or small ensembles, phase issues are fairly easy to correct, compared to more complex recordings with huge ensembles. The question at every recording session is how many microphones are required, and how they should be positioned to give the desired balance between the instrument and the existing acoustics to the listener. More information about Thelin s research project can be found here: http://haakonthelin.com/multiphonics/