Acoustic Glossary


 

Sound Level Calculations - a simpler modern way


Sound Power is measured in Watts but sound power levels are expressed in decibels (dBs).

Sound Pressure is measured in Pascals but sound pressure levels are expressed in dBs.

The reason for using decibels is to compress the immense range of watts we can hear, 0.000000000001 watts up to 100 watts or more, into manageable numbers. Similarly the range of sound pressure levels we can hear is 0.00002 Pascals up to 200 Pascals. Converting these levels into dBs results in a range of values 0 dB up 140 dB, much easier to 'handle'.

However sound level calculations involve logarithms which many people are not comfortable with, so we describe a simpler modern way.

Our sound power example is typical of all sound power quantities calculations.
Our sound pressure example is typical of all root power quantities (sound field) calculations.

First the basics


Sound Power Level (Lw) = 10·log (w/wo) dB, where w is the sound power in watts and wo is the reference sound power level of 10-12 watts ≡ 0 dB **

Sound Power Level is a sound energy quantity and uses the 10·log factor so, as a rule of thumb:

3 dB = a factor of 2 in sound energy (+3dB is double the sound power and 3dB less is half the power)

To see a table of typical sound power levels and other 'rules of thumb', click here

** When you divide watts by the reference watts, for example, the result is a simple ratio (no units).


Sound Pressure Level (SPL) = 20·log (p/po) dB, where p is the sound pressure in pascals and po is the reference sound pressure of 0.00002 pascals ≡ 0 dB in air = the threshold of hearing at 1KHz

Sound Pressure Level is a sound field quantity and uses the 20 log factor so, as a rule of thumb:

6 dB = a factor of 2 in sound pressure i.e. doubling or halving the sound pressure

To see a table of typical sound pressure levels and 'rules of thumb' click here

To understand the 10·log sound power vs the 20·log sound pressure formulae see our root power quantity entry. See also the IEC decibel definition

Sound Level Calculations
Acoustic engineers often use spreadsheets to add, subtract and average acoustic levels, like the following tables. If you don't have access to this software or the experience to programme spreadsheet columns you can simply ask Google to convert the dB levels back into sound level ratios, add them up and then ask Google to convert the 'new' total sound level ratio into decibels again.
Adding Energy Quantities - sound power example ..
10·log (w/wo)w/wow/wo totals10·log (w/wo) totals
60 dB 1,000,000    
60 dB 1,000,000 2,000,00063.01 dB
70 dB 10,000,000 12,000,000 70.79 dB
61 dB 1,258,925 13,258,92571.23 dB
 

Column 1 lists the dB levels we want to add up, note the column headings

In column 2 we removed the 10·log decibel conversion by simply typing 10^(60/10) into Google, to convert both the 60 dB levels and up comes the answer 1,000,000

Then enter 10^(70/10) into the search engine to convert the 70dB level and finally 10^(61/10) to convert the 61dB level

The column 3 levels are the arithmetic running totals of the w/wo (sound power quantities).

Finally in Column 4 we have re-converted the individual totals back to decibels using Google again, this time entering 10*log (2,000,000) for row 2, the addition of (60 dB + 60 dB). Next enter 10*log (12,000,000) into the search engine for the row 3 total and finally enter 10*log (13,258,925) = 71.23 dB, the sum of the four sound power dB levels.

It also follows that the average sound power level, of the four dB levels is 10*log (13,258,925/4) = 65.20 dB

The above example also confirms the sound energy rule of thumb that 3 dB is a factor of 2, double or half and 10 dB = a factor of 10.

Adding Field Quantities, a sound pressure example ..
20·log (p/po)p/pop/po totals 20·log (p/po) totals
60 dB     1,000    
60 dB1,000 2,00066.02 dB
70 dB3,162 5,162 74.26 dB
61 dB1,122 6,28475.96 dB
 

Sound pressure level calculations are the same as sound power level calculations detailed above, except the 20 log(p/po) factor applies, instead of the 10 log(w/wo) ratios.

For example to convert the 60 dB sound pressures to the p/po levels type 10^(60/20) into Google and the answer is 1,000. Repeat this procedure for the 70 and 61 dB levels.

Then to convert the total p/po levels to decibels type 20*log (6,284) into Google and the sum of the four sound pressure dB levels is 75.96 dB.

Similarly the average sound pressure level = 20*log (6,284/4) = 63.92 dB

The above example also confirms the sound pressure rule of thumb that 6 dB is a factor of 2, double or half and 10 dB = a factor of 3.

Our power quantity and the root power quantity entries explain the reason for the 10 and 20 dB factors.

We use Google in our examples, only because it is currently the most popular search engine.
 
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