Chapter 5: Using Speaker Specifications; Determining Maximum Output: Sensitivity And Power Handling - Toa BS-1030 User Manual

TOA Electronics Speaker Guide

Chapter 5: Using Speaker Specifications

Determining Maximum Output: Sensitivity and Power
Handling
A thorough system design must establish the maximum SPL required from each speaker at a
given listening position. In general, a speaker should be able to produce a sustained long-
term average level 15–25 dB higher than the background noise in its area. If the noise level
is less than 45 dB SPL, the speaker should be able to produce a long-term average level of
70 dB SPL in the listening area, with undistorted peaks 10–20 dB higher. As noted on page 18,
a speaker's rated sensitivity is the on-axis loudness (dB SPL) measured at a specific distance
that results from applying a specific amount of power (i.e., 1 W @ 1 m). The sensitivity may
be used to calculate loudness at other distances and power levels. Three specifications are re-
quired to calculate the maximum SPL capability of a speaker in its environment:
• The speaker/transformer's maximum continuous power rating, or the available
amplifier power;
• The speaker's sensitivity rating (dB SPL @ 1 m on-axis with 1 W input);
• The distance between the listener and the speaker.
Using these three specifications, the maximum on-axis output can be calculated (the formulas
for decibels gained with power and decibels lost with distance are presented in Chapter 4:
Audio Basics). Since the formulas use the log function and require a scientific calculator,
simplified charts (Figure 5-1 and Figure 5-2) are included here for convenience.
Example: A paging horn in an outdoor area needs to reach an average level of 90 dB SPL at
80 ft from the horn. A 30 W model is selected with a sensitivity of 112 dB, 1 W @
1 m. To allow for short-term transients, 6 dB of headroom is added to the average
level requirement, yielding a target level of 96 dB SPL.
Question: How much power is needed to reach the target level?
The rated sensitivity is 112 dB SPL, with 1 W @ 1 m. Use the chart for level change with
distance (Figure 5-1) to see how much the level is reduced at 80 ft compared to the ref-
erence distance of 1 m (answer: 27.7 dB, or about 28 dB). This tells us that 1 W sensitiv-
ity at 80 ft is 112 – 28 = 84 dB SPL. This is 12 dB less than the target level of 96 dB. Use
the chart for level change with power (Figure 5-2) to find the power required to increase
the level 12 dB (answer: about 16 W).
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