A Visual Guide to Stereo Recording

2012/10/10

Stereo Technique Hero Image

I stumbled across a cool website recently while reading about field recording techniques. The website hosts an interactive tool that shows the area of sound captured when recording in stereo.

This “Sengpiel” tool pairs microphone polar patterns with stereo recording methods to show exactly what scope of sound a stereo recording captures. The image above shows an example.

Why would you be interested in this?

Well, it answers valuable questions field recordists have while recording stereo sound effects:

  • What microphone should I choose?
  • Which polar pattern do I need to capture a particular sound clip?
  • How should I arrange my microphones to gather the right depth and breadth of sound?

In today’s post I’ll explain the broad strokes of polar patterns and stereo recording methods. This will give us the background to see how the Sengpiel tool can help choose which combination is best for you.

Stereo Recording

How can a field recordist gather two-channel, stereo recordings?

The quickest way is to use a stereo microphone. Models such as the Rode NT4 or Sanken CSS-5 capture stereo recordings in one tidy package. However, all-in-one microphones like these lock a field recordist into capturing sound effects at a fixed width. Versatility is sacrificed for convenience.

Want more flexibility? Use a pair of very similar or “matching” mono microphones to record two discrete signals. These mono tracks become the left and right channels of a stereo recording.

Of course, that’s a simplified description. Putting this idea into practice is tricky. Precisely how this is done varies greatly. It involves two important aspects: polar patterns and microphone arrangement.

  1. Microphone Polar Patterns
  2. Each microphone is specifically designed to capture a certain breadth of audio. This area is called its polar pattern or directionality. In basic terms, a microphone’s polar pattern describes where it is most sensitive to sound.

    For instance, some microphones pick up audio in a broad arc that surrounds it. Others capture sound strongly in front, but weakly to their side.

    Examples of polar patterns are:

    • Omnidirectional. This polar pattern captures a broad sphere around the microphone.
      Click to enlarge them.

       

    • Cardiod. Named because of its vague resemblance to a heart shape. This polar pattern captures a reasonable spread of audio in front, but rejects sound to the microphone’s side and rear. Hyper-cardiod and super-cardiod patterns are variations.
      Cardiod Polar Pattern

       

    • Bi-directional or figure-of-eight. This microphone type captures sound well in front and behind, creating a polar pattern that looks similar to an “8.”
      Figure-of-8 Polar Pattern

       

    • Shotgun. This pattern captures audio best in a long, narrow lobe to the front. Sounds to its side and rear are rejected.
      Shotgun Polar Pattern

       

  3. Stereo Recording Techniques
  4. The polar pattern of each microphone is only part of the technique. The second half is how the two microphones are arranged.

    There are a number of techniques to do this, including:

    • A-B
    • X-Y coincident
    • Blumlein pair
    • ORTF
    • NOS and DIN
    • Jecklin disk

    These bewildering terms each refer to a specific method used to arrange the two mono microphones.

    For example, some may arrange the microphones at 90˚ to each other. Others may require the two mono microphones be divided by a sound-absorbent baffle. Some demand microphones are spaced precisely 17 cm apart.

    What do these terms mean? Sound on Sound has a meaty two part article explaining microphone techniques.

That outlines how polar patterns and recording techniques influence stereo recording. Let’s seen how the Sengpiel tool helps us visualize our choice.

Visualizing Stereo Recording Methods

What combination of polar pattern and recording technique should you choose?

You’ll have a dozen considerations when choosing the proper stereo recording method.

Common considerations are ease of use, your subject, and mono compatibility. Of course, your ears are the best tools to discover which recordings are rich, deep, and make your subject sound best.

Not comfortable leaving the choice to your ears alone? Want to know which recording technique will capture the scope of sound you need? That’s when the Sengpiel tool helps. When you want a precise, mathematical answer you can use sengpielaudio.com’s stereo visualization tool.

It’s difficult to visualize the effect of pairing a stereo technique with microphone polar patterns. How will your figure-of-eight microphones capture sound when 20 cm apart? How about 30 cm?

That’s when the Sengpiel tool is invaluable. Click the image below or this link to visit the website in a new window.

Tool, Broad View

How to Use the Sengpiel Tool

Select a method with the “microphone system” pulldown on the left. Click a radio button to choose which polar pattern you’ll use in each technique. The result?

The Web page uses the formulas of German sound engineer Eberhard Sengpiel to calculate the stereo image. It displays the spread of sound that will be captured by the arrangement of the microphones in a mauve arc.

You can drag the box at the top of the green line to change the microphone angle. Pull the box at the bottom of this line to modify the spacing between the microphone capsules.

How broad is your source? Will your technique choice capture its full width? Modify the value in “Orchestra angle” or drag the cyan hash mark. Compare this with the purple arc. This will show you the slice of the source your technique will gather.

If you’re using matched mono microphones to capture stereo recordings, the Sengpiel tool will show you exactly what scope of sound your recording method will capture.

Do you know any other tools or resources that help field recordists choose stereo recording techniques? Share them with others in the comments below.





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