Stereophonic Sound with ORTF

slow shutter photo of  D.C. metro subway train
slow shutter photo of D.C. metro subway train

Location sound can present a variety of challenging obstacles to capturing hi-fidelity sound. The infrasound that a fast moving train creates can become a problem for directional mics positioned nearby. Using the techniques described in my previous microphone isolation post allowed me to maximize the available headroom in my recording. Below, listen to a stereo recording I made of the Washington D.C. Metro train. You will hear the subway train arriving from a far left tunnel, after it stops the doors open in the center, then the train rumbles as it fades away into the far right tunnel. The distinct panning or movement of sound in this stereo recording was captured using the ORTF stereo mic technique. Try a pair of headphones or stereo speakers for a fully immersed experience.

Higher quality versions of this recording are available, as well as custom sound designs/effects.

Introducing Moon Professional 3500MP

You may be familiar with Canada’s Sim Audio, they’ve been producing top quality audiophile equipment since the 80’s.  And if my phono preamp is any indication, this new offering in the form of a microphone preamplifier should be interesting. The headroom is very high, 32dBu! No overload indicator or level meters are provided. Take a closer look at the Moon Professional 3500MP:

Moon Professional 3500MP

Moon Professional 3500MP

  • A “purist design” for optimal sonic performance and lowest possible noise floor.
  • AC-coupled transformerless design for increased bandwidth and more accurate sonic reproduction.
  • Wide gain range of 8.0 to 66dB with -3.5dB of variable output trim.
  • Metal film resistors and polypropylene film capacitors.
  • 2 fully independent audio channels, each on separate printed circuit boards.
  • No electrolytic capacitors in the signal path (non-phantom mode).
  • Built-in Power supply using 2 ultra-low noise toroidal transformers (1 for the audio signal circuits; 1 for 48V phantom and control circuitry).
  • Housed in an isolated enclosure constructed from satin coated 14-gauge steel, designed to eliminate all traces of AC artifacts.
  • Swiss-made 24-position gain potentiometers featuring gold-plated contacts and thinsurface-mounted film resistors with 1% tolerances.
  • High-reliability sealed relays and toggle switches.
  • High current output stage to accommodate very long cable runs and capable of driving 600ohm loads.
  • Rigid external chassis with solid aluminum front and side panels and heavy gauge steel on the top, bottom and rear panels for shielding from RF, EMI and external vibrations.
  • Circuit board with pure copper tracings and gold plating that yields low impedance characteristics.
  • Designed to be powered up at all times for optimal performance.
  • Low operating temperature to ensure long life.

The HiFiQC Method For Stereo Microphone Isolation

Schoeps UMS 20 Stereo Bar mounted to Rycote Lyre suspension

ORTF with UMS 20 Stereo Bar mounted to Rycote Lyre suspension

Recording music and sound effects with carefully placed stereo microphones can present some challenges. Since directional microphones are more sensitive to infrasonic noise, special attention should be taken when placing them in various environments. Wind, machines, automobile traffic, and footsteps can all cause rumble or noise to build-up and eventually reach your microphones. The low-frequency nature of these sounds can take up precious headroom in your audio recordings. While wind (blowing on the capsules) can be tamed with various foam enclosures (the W5D is my favorite for Schoeps) don’t forget to isolate mechanical vibrations from reaching your mics too. After trying elastic shockmounts alone, I devised 3-part system to better isolate my ORTF stereo array (also works for MS, XY, and Blumlein) from infrasonic disturbances .

A 3/8 inch mic mount threading adapter allows me to eliminate the 20 mm Schoeps mounting adapter, saving 38 grams of weight

A 3/8 inch mic mount threading adapter allows me to eliminate the 20 mm Schoeps mounting adapter, saving 38 grams of weight

To start, Rycote has a non-elastic suspension made of a super-strong thermoplastic called Hytrel. And unlike elastic, it won’t sag and eventually wear-out (so far, so good). After being impressed with a Rycote Invision suspension for a single Schoeps mic, I decided to see if a solution existed for isolating 2 Schoeps mics arranged on the UMS 20 stereo bar.

After some digging, I came across the Rycote portable recorder suspension and wondered if it might work since the weight of the stereo bar and mics was nearly identical to most portable recorders. As you can see from the picture above, this model easily supports and isolates the Schoeps universal stereo bracket with mics attached.

Carbon Fiber Disc and Sorbothane Hemisphere

Carbon Fiber Disc and Sorbothane Hemisphere

Using a microphone suspension is certainly one method to help prevent vibrations from reaching your mics, but what about infrasonic born vibration from foot traffic, cars nearby, or HVAC compressors. These disturbances can travel up your mic stand and into your microphone.

After experiencing issues from foot steps and traffic rumble, I worked on a solution to isolate the microphone tripod from the ground itself. Both Carbon Fiber and Sorbothane are materials known to dissipate mechanical energy. So, I settled on Sorbothane hemispheres attached to an equal diameter carbon fiber disc.  Sorbothane works best when compressed under a range of weight (depending on size and Duro rating of hemisphere). By choosing a durometer rating of 50 in the 1.5″x.75″ Sorbothane hemisphere, 7-14 pounds of weight on each tripod leg could be isolated.  And after adding some 8 lbs sand bags to my tripod, I was able to get the load heavy enough to make the Sorbothane hemishpere’s truly effective.

Sorbothane being displaced by the weight of microphone stand

Sorbothane being compressed by the weight of microphone stand and sandbags

Lastly, but equally important is making sure your microphone cables do not transmit cable-borne noise to the microphone body. Carefully dressing your mic cables around the tripod is essential to preventing vibrations from reaching your mic. With the help of velcro cable wraps, you can neatly run your mic cables, but don’t forget to pay attention to where and how they are laid on the ground. In some cases you may need to isolate the cable on the ground too.

For those looking for an even smaller stereo mic setup with windjammer possibilities…Check out this ORTF setup with Schoeps CCM mics, Rycote Lyre suspension, and even a Rycote connobox to eliminate cable handling noise!

Happy Recording,

-HiFiQC