Audio Engineering

Measurement 59

Example
We have a 100 W amplifi er but plan to use a 12-dB margin for “ head room. ” How many
watts will our program level be?

1. Above 100 W fi nd  50 dBm.


2. Subtract 12 dB from 50 dBm to obtain  38 dBm. Just below  38 dBm fi nd
   approximately 6 W.

Example
A 100-W amplifi er has 64 dB of gain. What input level in dBm will drive it to full power?

1. 
   

   Above 100 W read  50 dBm.

   
   


2. 
   

    50 dBm  64-dB gain   14 dBm.

   
   

Example
A loudspeaker has a sensitivity of LP  99 dB at 4 ft with a 1-W input. How many watts
are needed to have an LP of 115 at 4 ft?

1. 
   

   115 LP – 99 LP   16 dB.

   
   


2. 
   

   A t  16 on the 1-W scale read 39.8 W.

   
   

2.15.2 The 20 Log x Chart

Refer to the chart in Figure 2.10. A 2:1 voltage, distance, or sound pressure change is
found by locating 2 on the ratio or D scale and looking directly above to 6 dB.

0 5 10 15 20 25 30 35 40 45 50 55 60

1 2 4 6 8 10 20 40 50 80 100 200 400 600 1000

0 − 10 − 20 − 30 − 40

1.0 0.6 0.4 0.2 0.1 0.06 0.04 0.02 0.01

ΔD (dB)

Ratio or D (feet)

Ratio or D (feet)

ΔD (dB)

Figure 2.10 : The 20log 10 x chart.