# Versatile Voltage, Power, and Decibel NomogramsAugust 1962 Electronics World

 August 1962 Electronics World   Table of Contents  Wax nostalgic about and learn from the history of early electronics. See articles from Electronics World, published May 1959 - December 1971. All copyrights hereby acknowledged.

August 1962 was a good month for lovers of nomographs and infographics. Electronics World magazine published three articles feeding the mania, including this one, Sound | Audio | Music Infographic, and Coil-Winding Charts. All make good printed references to keep on-hand. One nomograph converts the ratio of two power levels (in watts, milliwatts, etc.) to equivalent decibels of gain. It can also be used to find the unknown power level if one is known and the gain in dB is known. The other nomograph facilitates graphically calculating voltage or power based on the source resistance (assumed to have only real components; i.e., no complex values). If you are not sure how to keep units constant for voltage, power, and resistance (milli, micro, etc.), calculate using the base units and then convert afterward.

Versatile Voltage, Power, and Decibel Nomograms

By Jim Kyle

Chart 1 - Voltage to power conversion nomograph.

Two useful charts that enable the audio technician to find amplifier gains and losses even when voltage measurements are taken across different impedances.

Calculations of power levels and decibel ratios from voltage readings often lead to confusion for both experienced technicians and beginners, since the conventional formula for determining decibel ratio from voltage readings assumes that each reading is taken at the same impedance level.

Many charts, tables, and graphs have been published to aid in solving such problems. However, the charts shown here offer features not to be found in such previous aids. With them, power corresponding to any voltage reading can be determined if resistance is known, voltage can be determined if power is known, and the gain or loss in decibels of any equipment can be determined if input and output voltages and resistance can be measured.

Chart 2 - Power (watts) ratio to decibels conversion nomograph.

Chart 1 is used for voltage-power-resistance calculations. Chart 2 converts power levels directly to decibels gain or loss.

The voltage and resistance scales of Chart 1 bear two sets of graduations, labeled E1 and R1 and E2 and R2 respectively. Scales bearing the same suffix number are used together.

For example, suppose an amplifier is under test. A 10-volt signal applied to the 500-ohm input produces an output measured at 5 volts across 8 ohms.

First, determine input power from Chart 1. The line connecting 10 volts (E2 scale) and 500 ohms (R2 scale) passes through 0.2 watt. Output power is next. This time, the E1 and R1 scales of Chart 1 are used, yielding an answer of 3.1 watts.

Now we turn to Chart 2. Connecting the 3.1-watt output (PI scale) and the 0.2-watt input (P2 scale) gives a total amplifier gain of just under 12 decibels.

Nomographs / Nomograms Available on RF Cafe:

Posted June 21, 2021