Tyco 579-769 specifications SPL = Sensitivity + 20 log, Equation 2-9. On-Axis SPL Calculation

Speaker Basics, Continued

Sensitivity

Speaker Dispersion Angle and “Q”

The amount of sound that a speaker can be expected to produce is found in the speaker’s sensitivity rating provided in the manufacturer’s literature. “Sensitivity” is the amount of sound (SPL) produced by the speaker with a known signal frequency, power level and distance from the speaker. For fire alarm listed speakers approved under UL Standard 1480, the sensitivity is rated at 1 W of power and 10 feet (3 meters) from the speaker. By knowing the speaker’s sensitivity, you can determine the on-axis SPL (SPL measurements taken directly in-line with the speaker) at any distance from the speaker using the following equation:

Where:			⎡Dr ⎤
• SPL = Sound Pressure Level		SPL = Sensitivity + 20 log
• D = Distance from the speaker			⎢	⎥
• Dr = The reference distance			⎣	D ⎦
• Sensitivity = The SPL at the
	Equation 2-9. On-Axis SPL Calculation
reference distance.

Simplex speakers have two sensitivity ratings listed on their respective data sheets, a reverberant chamber test as required by UL Standard 1480 and an anechoic rating as defined by ULC-S541. The reverberant chamber specification is derived from a test where the speaker’s sound is emitted in a chamber specifically designed to reflect all of the sound so that a total sound power measurement can be made. Correlating the speaker’s reverberant chamber sensitivity rating with real-world acoustics has proven to be difficult. Typically, the anechoic rating at 1 kHz is more representative of real world performance.

The speaker sensitivity rating, while useful for comparing speaker models, tends to oversimplify the true response of a speaker. Speakers “beam” sounds analogous to the way a flashlight produces light: the beam of sound is loudest directly in-line with the device and becomes quieter the farther the listener moves away from the center. This beaming effect is also dependent on the frequency of the signal.

The beaming effect is referred to as the directivity or “polar response” of the speaker, and is occasionally provided by manufacturers in the form of “polar plots.” For typical fire alarm speakers the beam is very wide for low frequencies (low directivity) and becomes more focused for higher frequencies. When determining coverage area, it is common practice to use the directivity information at 2 kHz: a critical band for intelligibility. Fire alarm speakers produce the highest output in the 800Hz to 4 kHz frequency range.

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