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On-Axis/Off-Axis
If you have ever tried to wade through a spec sheet for a microphone or loudspeaker, you have seen the words on-axis and off-axis. However, for many, the terms may be very mysterious. But, if you are going to take full advantage of the features of your microphone or loudspeaker, you would be well advised to understand these basic terms. Let's explore the term axis. Webster defines axis as "a main line of direction." Axis could also be defined as "one of the reference lines of a coordinate system." Putting those definitions together, "on-axis" would describe the primary direction from the microphone or loudspeaker. The on-axis direction would also become the reference for other measurements. Figure 1 
On-axis, for most microphones, is a line in the same direction as the long dimension of the microphone (Figure 1). If the microphone has a directional pattern, it will be the most sensitive on-axis. In other words, a talker will be picked up the best if the microphone is positioned so the talker's mouth is on-axis to the microphone (Figure 2). | Figure 2 | | 
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| The Polar Chart in a specification sheet will show the directional characteristics of the microphone or loudspeaker. The Polar Chart will define the on-axis direction and then show the response of the microphone or loudspeaker at other directions in relation to the on-axis response (the other directions are called the off-axis response). Figure 3
 The Polar Chart in Figure 3 is a 1-Dimensional graph that depicts a 3-Dimensional phenomena (one "slice" of the pick up pattern). It represents the level a microphone will pick up at the same distance in a 360° sphere. The Polar Chart is a good tool to use as an indicator of how well the microphone or loudspeaker will perform for a particular application. by Ron Huisinga © 1996 Internet Sound Institute (www.soundinstitute.com). This article is for personal use only. Any commercial reproduction is not permitted without permission. To obtain permission, contact ISI at hopi@soundinstitute.com
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