Palm-sized highly directional microphone system
This page introduces a new microphone system that offers any desired polar patterns without using either of a interference tube or microphone array with numerous microphones.
The proposed system consists of only 4 omnidirectional microphones and software.
Directionality of a microphone is usually described by a polar pattern. An omnidirectional microphone, for instance, has the polar pattern shown on the top left of Fig. 1. The figure-of-eight on the top right is another typical polar pattern of a microphone. By combining the omnidirectional and figure-of-eight, you can easily get the cardioid pattern that is shown on the bottom left of the figure.
Fig.1 Polar patterns of microphone
By using a microphone with a cardioid pattern, you can suppress sounds from the rear. However, if you want to pick up sounds comming only from a desired direction, the cardioid microphone is not good enough and you will need much more highly directional polar patterns such as pattern 1 and pattern 2 in Fig. 2.
Fig.2 Highly directional polar patterns
To generate the patterns shown in Fig. 2, you can use an interference tube or a microphone array (Fig. 3).
Fig.3 Interference tube (Left) and microphone array (Right)
If the target direction is known in advance, the interference tube can be used. To deal with low frequency sounds, however, the interference tube should be very long.
When you want to control the target direction after the recording, you should use an adaptive beamformer that requires a microphone array. To achieve a highly directional polar pattern, the adaptive beamformer requires numerous microphones.
The physical property of the sound wave at the observation point can be measured by a Miniature Head Simulator system and given mathematically. If the spatial property of the sound at the point is known perfectly, any disired polar pattern can be created programmatically.
Since the miniature head simulator consists of only 4 omnidirectional microphones (see Fig. 4), the highly directional polar pattern can be achieved by a very small system.
Fig.4 Tetrahedral microphone system
Consequently, the palm-sized highly directional microphone system offers any desired polar pattern programmatically without using either of a long interference tube or microphone array with number of microphones.
When using an interference tube, you can control the aiming direction only during the recording. Once the recording is made, you cannot change the aiming direction afterwards.
In the proposed system, directionality is created programmatically
during the reproduction. You can, therefore, change the target direction
whenever you like while you are listening to the sound.
Sounds in the following demo clips were recorded with the palm-sized highly directional microphone system. Please use stereo headphones to listen to the sounds.
By clicking one of the thumbnails above, you can start the demo.
On the demo screen, a panoramic image will be displayed below which, you will see a button that says 'OMNIDIRECTIONAL.'
Once the image is loaded and the instruction saying 'Click to start' or 'Tap to start' appears, you can start playing sound by clicking (tapping) on the image.
While the sound is reproduced, you can toggle the polar patterns by clicking (tapping) the button below the image.
There are 4 patterns, that are 'Omnidirectional,' 'Cardioid,' 'Narrow' and 'Narrower.' Hear the difference of the patterns.
The figure below shows polar patterns of the 'Ominidirectional,' 'Cardioid,'
'Narrow' and 'Narrower' directionalities.
Fig.5 Polar patterns available in the demo clips
You can control the target direction by panning the image. If you are using a mobile device, panning can be controlled by the rotary motion of the device.
It requires FFT for analyzing spatial property of the sounds and inverse FFT for reproducing the sounds. They need several tens to several hundreds milliseconds.
What can be reproduced is the sound at the observation point. If interference of sound waves is occurring at the point, the reproduced sound is affected by the interference.