Tunes, Ideas, Tasks

Stereo Microphone Techniques

The three instruments I chose to record as part of this study were Acoustic Guitar, Piano and Clarinet. Below you can see how each technique was used and applied to the instrument.

Guitar

AB – This technique employs time difference to give you the stereo image, sound waves hit the diaphragms of each microphone at different times depending on where it is in relation to the two microphones. This creates phase and time differences which when played back in stereo a human ear can decipher and localise the sounds in the stereo field.
I used the Octava condenser microphones for this technique and used the omni-directional capsules.
I used a distance of 30cm between the microphones to try and simulate an approximate size of a human head as well as think about how wide it needs to be in order to pick up phase differences of low frequencies with long wavelengths. Human localisation stops working at around 150Hz due to the head being to narrow to fit a whole wave shape there not allowing phase/time difference localisation to work. 150hz has a wavelength of around 40cm this is why I chose to make the distance between the microphones 30m which is in-between the average width of a human head (17cm) and the wavelength size of the lowest frequencies we can localise.
The microphones were placed around 60cm away from the guitar with one of microphones aiming at the hole of the guitar and the other at the neck.

NOS – this technique uses both time difference and intensity level differences to pick up the stereo field. Intensity level difference is where sounds are arriving at the microphone diaphragms at different intensities meaning that there will be an imbalance of level over the stereo field placing objects either to the left or right when you listen to them.
I used the Octava condenser microphones with cardioid capsules for this technique.
The NOS set up spaces the microphones 30cm apart and at an angle of 90 degrees to each other. This angle allows for the intensity level stereo image as each mic is facing a different direction meaning sound will hit each mic at different intensities depending on where the sounds source is. Having the microphones set to cardioid is essential as it allows them to have a narrow field in front of them and extenuate the intensity difference.
The microphones were placed at around 45cm away from the guitar. They need to be placed quite close to keep the stereo field realistic and also to avoid a drop in bass frequencies when this set up is placed at further distances from the source.

MS – This technique involves two different microphones. A cardioid Octava set up in the middle of the setup pointing at the sound source and a microphone set on figure of 8 placed perpendicular to the first mic picking up the stereo image – for this mic I used the SE Electronics condenser. The microphones are placed one on top of each other to get them as close together as possible.
For this technique a matrix needs to be applied to them in order to create the desired stereo field. Logic has a plug-in that does this, which I used for the recording. The plug-in places the figure of 8 microphone across the stereo field and adds the mono cardioid microphone in to fill in the middle. There is a stereo field width control on the plugin which I tweaked to create a balanced stereo image as I found it was slightly to wide and I should have put the microphones in a bit closer to make the field tighter, however changing this parameter solved this problem for me. I placed the microphones at about 1 meter from the source with the cardioid mic pointing at the hole of the guitar.

Overall I prefer the AB recording, as it is much warmer than the other techniques, there is a much better bass response, which causes this. The chords especially sound better with this technique. The MS technique sounds a bit thin but is very smooth and would be good if you were planning to add the guitar into a mix of other audio as it sounds very clear and has a narrower frequency band than the other recordings. NOS seemed to be a balance of the AB and the MS where it had more bass and warmth than the MS but was still very thin sounding compared to the AB.

Piano

AB - As with the first AB pair I used the octava condensers and placed them 30cm apart. As a piano has a much wider sound source than a guitar I placed them further away at around 1m to try and incorporate the whole image. Pianos emit different notes from different parts with the lower notes and one end and the higher notes at the other end. This means that placing a microphone set up is important, as you want to include all of the different notes. I placed the microphones slightly towards the left (higher notes – when looking at back of the piano) of the piano as lower frequencies spread out more than higher ones meaning that although it was pointing more to the higher end, lower frequencies were still being picked up clearly.

Blumlein – This setup incorporates two figure of 8 microphones placed on top of each but with the diaphragms at 90 degrees. This creates the stereo image using solely intensity. The microphone was set up with the 90 degree angle between them pointing directly at the middle of the piano at about 1m away. This mic set up is susceptible to spill coming from other sound sources as it picks up from behind as well. This was recorded in an room with no other instruments, this would however gather more early reflections of the sound giving it reverb quality however the room is fairly dead and the effect is subtle.

NOS (with different mic’s) I used two SE electronics microphones for this set up as opposed to using octavas for NOS like on the guitar. I set the mics on cardioid and spaced them 30cm apart and angled the capsules 90 degrees perpendicular to each other. These mics were placed about 45 cm away from the piano. This set up has low bass response so I wanted to bring it in closer but also ensure that the stereo field remains so I placed it quite close to the piano.

Overall I preferred the NOS recording for warmth and depth, it seemed to be a lot nicer and rounder sounding than the other two. The AB mic sounded quite accurate, the sound quality was almost the same as being in the room and hearing the piano directly but it seemed to be slightly thin and not full enough. The blumlein recording was very tinny and the notes were very jangly, although some people may like this piano sound I personally favour the NOS recording.

Clarinet

AB – This set up is used again as a control for this task. I used two octava condenser microphones spaced 17cm apart (average width of a human head) I placed them pointing towards the bottom third of the clarinet about 30cm away. As the sound source is quite narrow I placed the mics closer together as well as to note there aren’t that many very low frequencies coming from it either.

ORTF – This technique uses two cardioid microphones spaced 17cm apart and angled at the slightly wider 110 degrees apart, it is similar to NOS in the view that it uses both intensity and time differences methods of deriving the stereo field but it has a wider spread than the NOS setup. It is quite a similar set up to the human ears, especially if you but a blind between the two to simulate a human head (I did not do this in the recording). As using this set up far away from the sound souce will create a loss of bass I placed it quite close to the clarinet at about 50cm pointing outwards but level with the centre of the clarinet.

DIN – This set up is like the NOS set up except with microphones spaced closer together at 20cm. They are still 90 degrees to each other creating the intensity difference stereo spread as well as the time difference information. I placed this about 1m away from the clarinet to create what sounds like quite a wide stereo spread.

For the clarinet I prefer the AB recording out of the three. It sounds very warm and realistic and sounds a lot smoother than the other two recordings. It sounds very glassy and glistens especially on the higher notes. The ORTF recording sounded similar to the AB but there seemed to be more breathe hiss noise in the sound. The DIN set up recording sounded a lot thinner than the other two and as with the ORTF there was a lot of wind/breath sounds.

Pictures of some of the mic set ups (click to view fullsize)