By Charley Beeman
A single dipole microphone records sound along one axis and rejects sounds coming from the sides and above. This is great when the direction of the user is fixed, however combining two directional microphones in an “XY” configuration can address a wider range of uses. The XY configuration is when we take two directional microphones and point them orthogonally. Using this pair of microphones, the software can identify the direction of arrival (DOA) of a sound, steer a beam in 360°, and record high quality stereo audio with a wide soundstage and precise imaging.
The XY configuration
To arrange two dipole microphones in the XY configuration, align them such that the dipoles are orthogonal to each other as shown below. The microphones should be as close as possible to each other.
This configuration can be integrated into very compact formfactors. Below is an image of our Horizon development kit's XY pair and a gasket that includes this XY configuration with an additional omnidirectional microphone. By adding an omnidirectional microphone, different polar patterns such as cardioids and hypercardioids can be created. For more information check out our article on creating different polar patterns.
The beam pattern of the two overlapping dipoles is shown below in figure 2. With two dipoles, the entire sound field in 360° is captured. In this XY configuration, one Soundskrit microphone is oriented such that it captures sound from the “X” direction, while a second microphone captures sound from the “Y” direction, as on a coordinate grid.
Just like many other physical phenomena, sound waves are a vector that propagate with a specific direction that can be broken down into components that are traveling in the “X” direction, and components that are traveling in the “Y” direction. An example of this is illustrated below.
Direction of Arrival
With two Soundskrit microphones measuring the “X” and “Y” components of the sound wave, the XY configuration can fully characterize the direction a sound wave is propagating from. For example, a sound wave traveling from a 45° angle relative to the microphones will have an equal amount of energy travelling in the “X” direction and the “Y” direction. Both Soundskrit microphones would see signals with equal amplitude. In contrast, if the sound wave were arriving from an angle of 15°, its “Y” component would have an amplitude 3.8 times greater than the amplitude of its “X” component. By comparing the relative amplitudes between the two microphones, the direction from which the sound wave originates can be determined.
Similarly, the signals from the XY configuration can be recombined to output a new dipole pattern pointed along a different direction. This can be used to steer the effective listening direction of the microphones to a desired angle and is done through simple addition of the two microphone signals. For example, the sum of the “X” and “Y” microphone with equal weighting creates a dipole polar pattern oriented along the 45° direction. But weighting the “Y” microphone 3.8 times more than the “X” microphone creates a dipole pointing at 15°. The tool below shows the scale of the "X" and "Y" microphones along side the direction of the beam. The front and rear lobe of a dipole have opposite polarity, so by convention the positive lobe refers to the front. In the tool below you can visualize beam steering as the ratio between them is adjusted.
Stereo sound recordings have a left and a right channel to replicate our natural directional hearing that allows us to tell where sounds are coming from and how far away the source is. The challenge for consumer electronics is that the professional stereo recording methods require large equipment or a lot of space between microphones. To record stereo live, professional audio engineers use several different techniques, all of which require directional microphones. The XY configuration is based on the Blumlein pair recording technique and can scale down profession recording techniques to the size of consumer electronics. To hear a demo of stereo recording using directional MEMS microphones, listen to this video of Soundskrit's own Carly Stalder and Stephane Leahy performing a song. This video compares stereo recording using two omnidirectional microphones and directional MEMS microphones: