Mid-Side recording and Microphone Sensitivity

Small technical backgrounder this time.

When recording stereo it is essential to have equal gain settings for the microphones at the risk of the stereo image shifting to either left or right. More gain on the right mic channel will shift the image to the right and the other way around. Equal gain settings are especially important in XY, ORTF or any other spaced or coincident microphone positioning.

With mid-side recording the gain setting for the mid and side channels allow you control the stereo spread. More gain on the side channel compared to mid will widen the stereo image. More gain on the mid channel will narrow the image. Ultimately, no gain on the side channel will leave you with a centered mono signal from the mid mic.

Reading gain or trim pot settings is easy enough but you usually end up with differences in signal strength if you don’t take into account microphone sensitivity even with an equal gain setting on your recorder or mixer. Unless you have a matched pair, microphones have different sensitivity characteristics.

A typical MS setup is the Sennheiser MKH60/ MKH30 microphone pair.

The MKH60 has a sensitivity of 40mV/Pa, the MKH30 has 25mV/Pa. With equal gain you will end up with a much stronger mid signal and consequently a narrow stereo image.

The calculation to compensate for sensitivity is as follows:

±dB = 20 x log(MmV/SmV)

where

MmV = sensitivity of the mid mic in mV/Pa
SmV = sensitivity of the side mic in mV/Pa

In our example we need 4.1dB more gain on the side channel compared to the mid channel to get equal signal strength on both of the recorded channels.

This is easy to realize on mixers such as the Sound Devices 302. With MS stereo linking set trim pots allow you to set gain for the mid and side channels separately. The input 2 fader controls the overall gain for the pair.

North India Field Recording on SoundCloud

Just posted an excerpt of the field recordings I did in North India earlier this year. Using SoundCloud for the first time for this.

Recorded with a Sennheiser MKH8020/ MKH30 MS stereo microphone configuration to a Sound Devices 702 recorder. MS converted to AB stereo at the recorder stage.

SoundCloud uploads are transcoded to 128 kbps mp3 format for streaming. That’s still twice as good as MySpace.

Plug-ins for the field recordist

As anyone familiar with processing audio on a computer will testify there are hundreds of plug-ins available for any application. Searching the KVR site for an EQ for OS X based applications gives you a choice of 141 different plug-ins. For the Windows platform you will find 226. And that’s just for EQ.

Without any exception these plug-ins were engineered with music in mind and that makes life for the field recordist slightly more difficult. While the Abbey Road TG12413 compressor/ limiter plug-in is a brilliant piece of software that does wonders on a drum kit, few field recordists will find a use for it. Vintage style EQ plug-ins can bring excessive coloration to a recording. Many plug-in developers will advertise the ‘warmth’ their processors will bring to your recorded material but that does not interest me, and I suspect most field recordists,  at all. What I am looking for is audio processors that keep the recorded material intact. No added warmth required, not looking for increased presence or transients adjustment to obtain better attack.

My search for the ideal field recording plug-in pack is an on-going thing but here is the result so-far:

EQ: Waves Q10 Paragraphic equalizer, DDMF LP10 Mastering Equalizer

Dynamics: Flux Pure Compressor II, PSP Xenon Limiter

Channel strip: Waves Audio Track

Audio restoration: iZotope RX

Analysis: RNDigital Labs Inspector XL

‘What We Left Behind’ on iTunes

UK based band A Dancing Beggar are releasing their second album called ‘What We Left Behind’ on iTunes today. The album features samples of recordings of mine. A big thanks for that. I can only hope the recordings helped in the inspiration process.

Go check these guys out:

http://www.myspace.com/adancingbeggar

Soundwalkers film release

Soundwalkers, a film by Lisbon, Portugal based video director and sound researcher Raquel Castro after having been available on VIMEO has seen its release on DVD format.

The documentary film captures the importance of sound in the environments we live in and the way people interact with these sounds. Above all it is a film about hearing, listening, perception and reaction and it does a brilliant job at it.

Raquel used some of my field recording work in the film. I met Raquel at Chris Watson’s Sound Recording course hosted by Wildeye in Norfolk, 2007. A few scenes of that course are in the film. I like the shot where the microphones are facing the sea. Waves in the background they stand as devices out to try and get a glimpse of the sounds that live in that particular place by the sea at that particular time.

Check it out on VIMEO here: http://vimeo.com/1737899