The source level of an acoustic source compares the intensity emitted by the acoustic source to a reference source. This of course, also enables direct comparison of acoustic sources with each other i.e. which one is the most powerful? The reference source is an omni directional source (DI=0dB) with an acoustic output power of 1W taken at the reference distance r=1m from the acoustic center. In terms of acoustic intensity the reference source has an acoustic intensity I0:
This is where the (to some well-known) reference level 170.8dB re 1μPa @ 1m derives from and it should be noted that the “dB re 1μPa @1m” should be understood as “the intensity level relative to the intensity of a plane wave with an rms-pressure of 1μPa taken at the reference distance 1m from the source”. Most acoustic sources have an acoustic power output different from 1W and they are not always omni-directional. To find the source level of such a more generic source we simply add (in dB) the directivity and the ratio of power output relative to 1W:
In this formula, and in many similar, it is always understood that Pa is relative to unity with the proper unit assigned i.e. 1W. So “Pa” is really an abbreviation for “Acoustic output power relative to 1W” just like “DI” is short for directivity relative to an omni-directional source. The transmit response to voltage, TRV, is defined in such a way that the source level can be calculated from:
The TRV value is, however, often measured at low power and since the electric-to-acoustic efficiency can drop significantly with increased power levels it is often best to used the TRV relation with caution. It should be emphasized that the number and term source level refers to an acoustic source, not to the level of a particular sound, and that a source level is merely a practical definition.
The source level of a transmitter can be estimated (ignoring attenuation) by measuring the output voltage of a hydrophone submerged in the vicinity of the transmitting transducer, see the sketch below.