According to Ravi Sir, Our Senior Faculty and Head of Department (Audio Technology), many sound engineers are still confused about the basic concept of Sound Level Unit and still, many professionals get in touch with him to get it clarified.
The above mention situation has raised thoughts in the mind of Ravi Sir to discuss the topic and he expressed the feeling that information provided below will be an eye-opener for many working in the field especially for live sound professionals.
Talking a little bit about the history of Sound Level Units,
About a century ago, the phone company in USA decided that the power level on the audio cable should be 1 milliwatt instead of 1 watt. The phone lines are always 600 ohms impudence in analog wiring with the power of 1 milliwatt.
It is very difficult to measure the wattage (power) on an audio cable, but it is very easy to measure voltage. According to ohm’s law the voltage corresponding to 1 milliwatt against 600ohm is 0.775 volt.
dB is the power ratio, when the dB is calculated in terms of 1 milliwatt = 0dBm = 0.775 volts (m denotes milliwatt).
Considering the distortion element and noise, the signal level is increased to 4dBm from 0dBm that is an addition of +4dBm.
The corresponding voltage of +4dBm is 1.22 volt.
In United States of America, the National Association of Broadcasters (NAB) took the voltage form of phone company (instead of 1 milliwatt) standard and called it as 0 dBu (u stands for unterminated – an open circuit with no impedance)
The modern equipment standard is +4dBu which means 1.228 volt as measured.
International Electro-Technical Commission (IEC) picks up the simple reference as 1 volt. 1 volt is specified as 0dBv (decibels referred to a volt)
Professional equipment has a nominal level of 1.228 volt (+4dBu). In Hi-Fi equipment (camera, recorder etc) the signal level is 0.316 volt (approximate of 1/4 of +4dBu). This level is represented as -10dBV.
dBm and dBu are functionally same in old and modern equipment.
1) Should not plug higher level signal (+4dbBu) to a low-level signal input (0.316 volt / -10dBV), and the result will be a distortion due to overload.
2) To avoid overload, attenuation is required to the high-level signal.
Points to remember for live sound professionals
- 0dbm = 0.775 volt
- +4dbu = 1.228 volt
- -40dBV = 0.316 volt
Hope you have found the above-mentioned tips useful.
Do share your perspective about the tips shared above as a comment below.
and as always Happy Sound Engineering !!!