**SPL VERSUS DISTANCE**

Generally speaking, the sound pressure level in free field such as an anechoic chamber drops by 6dB everytime you double the distance from the source. The actual levels depend on the sensitivity of the source (louspeaker). The formula to calculate the SPL as a function of distance is as follows:

**SPL2 = SPL1 + 20*log(R1) -20*Log(R2)**

where:

R1 = The original reference distance, normally 1 meter

R2 = The desired new distance from the source

SPL1 = The original reference dB level, normally the sensitivity of the speaker

SPL2 = The new dB level at the new distance

The formula to calculate Power as a function of SPL is as follows:

**P2 = P1 * 10^((dB1-dB2)/10)**

Where:

dB1 = The original SPL, normally the sensitivity of the speaker

dB2 = The reduced SPL level as a result of increasing distance

P1 = The original power needed to obtain the sensitivity, normally 1 watt

P2 = The power needed to maintain the original SPL, normally the sensitivity level

The following tables demonstrate the SPL as a function of distance for three hypothetical loudspeakers with sensitivities of 87dB, 90dB, and 95dB. The yellow column shows approximately how much power you will need to maintain the original SPL. The included graphs clearly demonstrate that the relationships between SPL, distance, and power are exponential.

To calculate the sensitivity of a speaker, 1 watt of test signal is send to the speaker in an anechoic chamber and the output is measured at 1 meter. If the power is doubled, the SPL output will increase approximately by 3dB. The relationship between SPL and power is as follows:

**dB2 = dB1 + 10 * Log(P2/P1)**

where:

P1 = 1 Watt

P2 = The value of P1 multiplied by 2, 4, 8, etc.

dB1 = 87, 90, or 95, depending on the speaker's sensitivity

dB2 = The new SPL level as a result of doubling the power

The following tables and graph demonstrate the relationship between SPL and power. As before, this relationship is exponential.

You can find a calculator and additional information at the following site:

http://www.sengpielaudio.com/calculator-distance.htm