ADDING AND AVERAGING SOUND PRESSURE LEVELS
Topics covered here are:
Adding individual sounds
Averaging sounds
Still need help -Formulas for Excel?
Sound levels are in decibels. This is a logarithmic measure, so dB cannot be added directly. This is common sense because one sound of 100 dB and another of 90 dB cannot make 190 dB.
Sound levels can be added or averaged under certain circumstances......
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but these cannot be
added or averaged together: |
| a) | Using the definition of a decibel, a sound level must first be converted to sound pressure squared. So divide the dB by 10 and use this result to exponential 10. i.e. sound pressure squared is 10 to the power of dB/10. |
| b) | Calculate the sound pressure squared for each sound individually. |
| c) | Add all the sound pressures squared and take the logarithm to the base 10. Then multiply the result by 10 to obtain the decibel level of the sounds occurring together. |
| Note: | The above calculation is for independent sounds. If there are two sounds in phase, such as when listening to a stereo sound, and/or the microphone is within 1/2 a wavelength of a reflecting surface then this calculation will need some modification not included here. For further help, e-mail Grozier. |
| a) | Leq sounds can be averaged easily by averaging the sound pressures squared. See above. |
| b) | For example, the questioned is asked. “You have all this 1-minute Leq data for the Beach Boys from your Grozier system, what is the 7-minute Leq data for the concert? (Crazy question but what the…) Sure right-away. |
| c) | For each of the seven minute periods calculate the sound pressures squared and add them up. Divide by 7 to get the 7-minute sound pressure squared. |
| d) | Take log 10 of the result and multiply by 10 to get the 7-minute average sound pressure level. |
| e) | The example below shows how to get the 7 min average sound pressure level (Leq min). The average is in the lowest right cell. |
Still need help?
Try using this spreadsheet
For further information e-mail Grozier.
