58 Chapter 2
Example
Subtract 81 dB from 90 dB; the difference is 9 dB. The 9-dB vertical line intersects the
curved line at 0.6 dB on the vertical scale. Thus the unknown level is 90 dB – 0.6 dB, or
89.4 dB.
2.14 Combining Voltage ..................................................................................................
To combine voltages, use the following equation:
EEEEEaaT 12 22 2 12 [cos( 1 2 )] (2.34)where ET is the total sound pressure, current, or voltage; E 1 is the sound pressure, current,
or voltage of the fi rst signal; E 2 is the sound pressure, current, or voltage of the second
signal;a 1 is the phase angle of signal one; and a 2 is the phase angle of signal two.
2.15 Using the Log Charts ...............................................................................................
2.15.1 The 10 Log x Chart
There are two scales on the top of the 10 log 10 x chart in Figure 2.9. One is in dB above
and below a 1-W reference level and the other is in dBm (reference 0.001 W). Power
ratios may be read directly from the 1-W dB scale.
Example
How many decibels is a 25:1 power ratio?
- Look up 25 on the power–watts scale.
- Read 14 dB directly above the 25.
1000 400 200 100 60 40 20 10 6 4 2 1 0.6 0.4 0.2 0.1 0.04 0.02 0.01 0.004 0.002 0.00130 20 10 0 10 20 3060 50 40 30 20 10 0
Decibels above and below a one-watt reference leveldBmPower (watts)
Figure 2.9 : The 10log 10 x chart.