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8.2 - DIRECTIVITY

8.2.8 - HIGHLY DIRECTIONAL MICROPHONE SYSTEMS

The Rifle Mic

The rifle mic is a combination of a normal 1st order directivity microphone mounted in an interference tube. The interference tube is not normally sold as an accessory for an existing microphone, but rather as the complete unit (microphone + tube). The reason is that some small modifications may be needed to the mechanical construction of the microphone capsule to improve frequency response characteristics. A certain amount of frequency response equalisation is sometimes necessary which it is preferable to design into the circuitry of the microphone preamplifier. The normal 1st order directivity microphone is mounted at the extremity of a relatively long tube which has entry ports at regular intervals along the tube - the tube is usually about the same diameter as the microphone body. This tube can be anything from about 30cm in length, up to a couple of metres. The short rifle mic with a length of only about 20cm exists (the interference tube is about 12cm in length), but really it can only be considered as a small improvement on the standard hyper/supercardioid directivity pattern. This interference tube technique has also been applied to measuring low frequency variations of atmospheric pressure (microbaroms) believed to be produced by storms at sea ^[12], the tube in this case being some 600 metres in length!

The function of the tube is to sum the individual sound coming from each entry port along the interference tube. Sound arriving on the axis of the tube, will be conveyed to the microphone capsule through the tube without any interference. Whereas for sound coming from off-axis, each entry port will be slightly in advance of the wave progressing down the tube and the sum of the individual ports will therefore be attenuated with respect to the direct sound level.

First of all some basics - a plane wave which passes through a small entry port (hole or slot) in a rigid screen will be transformed into a spherical wave front radiating from the hole, as shown in Figure 55.


Figure 55 - A Plane Wave passing through a Small Orifice becomes a Spherical Wave

If the plane wave now passes through a number of holes, as shown in Figure 56, then we will see spherical wave fronts radiating from each hole, to form a complex spherical wave pattern. At a certain distance from the holes, this complex pattern will combine to produce a roughly plane wave front.


Figure 56 - a plane wave passing through a set of small orifices
becomes a complex pattern of a spherical waves

Within an interference tube the individual spherical wave fronts are canalised by the tube, and propagate along the tube in both directions, as shown in Figures 57 and 58 - the rest of the original sound wave will pass round the tube and continue on its way. We could also add to Figure 57 the complex pattern of reflected waves within the tube, but this would make the diagram almost incomprehensible!


Figure 57 - plan view of spherical wave propagation
canalised within the interference tube


Figure 58 - Section View within the Interference Tube

In the interference tube of a rifle mic, we are obviously only interested in the propagation of the individual wave fronts towards the microphone capsule. When the sound comes on axis of the rifle mic the sound inside the interference tube is unaffected, and the sound wave reaches the capsule at all frequencies with approximately the same amplitude as in free field conditions. The sound wave advances down the inside of the tube at the same rate as the sound wave around the outside of the tube as shown in Figure 59.


Figure 59 - Sound Wave On-axis of a Rifle Mic

When sound arrives off-axis, the interference tube will enter into action. As each element of the sound wave passes by a hole or slot in the tube, a spherical wave front will be generated inside the tube. However as the path followed by the wave inside the tube is out-of-phase with the external sound wave, each spherical wave front will be itself slightly out-of-phase with the preceding wave as shown in Figure 60.


Figure 60 - Sound Wave Off-axis of a Rifle Mic
The addition of each of these out-of-phase spherical wave fronts will generate a signal at the microphone capsule which will be considerably attenuated with respect to the on- axis response. In the blow-up in Figure 61 it is possible to see the difference in phase between the sound wave in the tube and the sound wave outside the interference tube


Figure 61 - Details of Out-of-Phase Propagation within Interference Tube
As spherical wave front generated at each slot is slightly out-of-phase with respect to the next, there is a gradual decrease in amplitude of the signal, the signal arriving at the capsule will be considerably attenuated. The diagram should therefore show a gradual decrease in amplitude as the wave advances down the tube as shown in Figure 62.


Figure 62 - Sound Arriving at 30° to Rifle Mic Axis
Gradually Attenuated towards the Microphone Capsule
However this simplified explanation is not quite correct, as the sound waves are in fact travelling along the tube. So the summing process is a dynamic process and takes place as each part of the sound wave passes a slot in the tube and generates a progressing spherical wave-front within the tube. The best teaching demonstration of this process that I have seen, uses small waves generated on the surface of a small water bath, with mechanical obstacles to simulate the reflection and/or diffraction. Maybe a future web-site video or animated computer graphics would fit the bill. For the time being we only have our imagination. However a vector diagram may help to get a clearer idea of the process - each spherical wave front with its respective shift in phase (generated at each slot) is represented as a vector – the sum of the vectors representing the signal arriving at the diaphragm. Figure 63 shows the addition of each vector representing the spherical wave propagation inside a tube with 12 individual slots. The vector AB represents the amplitude (and phase) of the signal arriving at the diaphragm for a series of incident angles (0°, 5°, 10°, 15°, 20° and 25°) – we would expect complete extinction of the signal at about 30°. We can see that the vector AB decreases gradually as the incident angle increases. This diagram represents the summation process within the interference tube for a relatively high frequency – each slot is producing about 5° shift in phase.


Figure 63 - vector diagram showing summing in the high frequency range
for different incident angles

At a lower frequency there will be less phase shift at each slot (about 2° per slot as in Figure 64) - the attenuation will be more gradual and therefore the extinction point will be at a greater angle. These two theoretical diagrams (Figures 63 & 64) apply to a 12 slot interference tube. In practice the number of slots will be considerably more, and the spacing in between slots will be only a few millimetres. The amplitude of each vector will therefore diminish as will the phase shift, and the vector diagram will tend towards the arc of a circle, however the overall profile of the curve of vectors will be practically the same.


Figure 64 - vector diagram showing summing in the medium frequency range
for different incident angles
The directivity index will depend very much on frequency. In the bass frequencies the rifle mic will be almost omnidirectional, as frequency content increases it will become more and more directional, and at high frequencies it will have a very tight pick-up front lobe. Sensitivity on axis will be the same as the integrated microphone in free field conditions. Remembering that this type of microphone is mainly used to pick-up distance sound sources, it is unsuitable for conditions where there is a high level of ambient sound in the lower frequency range. It has however the advantage of being reasonably small (~50cm) and therefore visually unobtrusive - often mounted directly on the camera for news gathering. However the low sensitivity, omnidirectional response at low frequencies coupled with the tight front lobe at high frequencies, mean that it is certainly not the miracle solution to sound pick-up at a distance. Figure 65 shows the polar diagrams of four commercially available rifle microphones.


Figure 65 - four commercially available rifle microphones
© polar diagrams by courtesy of Audio Technica, AKG and Beyer

The interference tube is about 30cm long for the Audio Technica 4071a, 36cm long for the Audio Technica 815b, 32cm long for the AKG 69-ULS and 36cm long for the Beyer MC 837.

[12] 1959 - "Noise reducing line microphone for frequencies below 1c/s' by F.B.Daniels, Journal of the Acoustical Society of America, Vol 22 no 5