Tyco 579-769 6dB/division, 87dB, Equation 2-10. Directivity Factor Q for a Conical Source

Speaker Basics, Continued

Speaker Dispersion		The figure below includes a typical polar plot graph and the interpretation of the dispersion angle.
Angle and “Q”
(continued)				Simplex 4902-9721
				Ceiling Mount Speaker
				Polar Plot - 2kHz 0º
				6dB/division
Note:	See Figure 2-5 on
	the following page
	for a more detailed
	view of a Speaker
	Polar Plot.				Ceiling
				7
				5º
					off axi
			87dB			s
						87dB
				Dispersion Angle
				Sensitivity = 93dB @10 Feet, 1 W
				Figure 2-4. Speaker Polar Plot Interpretation
		The “Coverage Angle” is defined as the angle where the speaker SPL drops 6 dB from the
		on-axis SPL. For the speaker above, the coverage angle is 150 degrees.

Another common representation of speaker directivity is “Directivity Factor” or “Q.” For speakers having a conical coverage pattern (typical of single driver speakers used in fire alarm applications), “Q” is determined by:

Q =	2
	⎛	θ ⎞
	1− cos ⎜	2	⎟
	⎝		⎠

Equation 2-10. Directivity Factor “Q” for a Conical Source

For the speaker above, the coverage angle (θ) is 150 degrees at 2 kHz, resulting in a “Q” of 2.7.

Continued on next page

2-11