Why does the sum of filters not always match the overall reported level?

The reason is due to the fact that what you are calculating when you sum the values of the 1/1 and 1/3 Octave bands is not an overall level. This number is simply the summed level of the 1/1 or 1/3 octave bands. This is best characterized in the plot below:

This means that when adding up the 1/1 and 1/3 octave bands, the frequencies at or around the cutoff frequencies of the filters will have a slight error. This is why these numbers can not be used to accurately calculate the overall sound pressure level. Even more error is seen when performing a Weighting after Filtering, as seen below.

When applying weighting curves via postprocessing, the attenuation at the cutoffs of the Octave Filters in addition to another factor can cause even greater error. As an example, applying the attenuations of an A weight filter by hand will not likely yield the same results as running the raw data through a filter, and calculating the overall level. The plot below shows the error on these calculations:

So if a frequency is close to the cutoff, the attenuation applied by simply subtracting it from the 1/1 or 1/3 octave band may be off by as much as 15 dB off. This creates an even larger difference between the Overall Level and the 1/1 and 1/3 Octave Sum.

Why does the sum of filters not always match the overall reported level?

The reason is due to the fact that what you are calculating when you sum the values of the 1/1 and 1/3 Octave bands is not an overall level. This number is simply the summed level of the 1/1 or 1/3 octave bands. This is best characterized in the plot below:

This means that when adding up the 1/1 and 1/3 octave bands, the frequencies at or around the cutoff frequencies of the filters will have a slight error. This is why these numbers can not be used to accurately calculate the overall sound pressure level. Even more error is seen when performing a Weighting after Filtering, as seen below.

When applying weighting curves via postprocessing, the attenuation at the cutoffs of the Octave Filters in addition to another factor can cause even greater error. As an example, applying the attenuations of an A weight filter by hand will not likely yield the same results as running the raw data through a filter, and calculating the overall level. The plot below shows the error on these calculations:

So if a frequency is close to the cutoff, the attenuation applied by simply subtracting it from the 1/1 or 1/3 octave band may be off by as much as 15 dB off. This creates an even larger difference between the Overall Level and the 1/1 and 1/3 Octave Sum.

Why does the sum of filters not always match the overall reported level?

The reason is due to the fact that what you are calculating when you sum the values of the 1/1 and 1/3 Octave bands is not an overall level. This number is simply the summed level of the 1/1 or 1/3 octave bands. This is best characterized in the plot below:

This means that when adding up the 1/1 and 1/3 octave bands, the frequencies at or around the cutoff frequencies of the filters will have a slight error. This is why these numbers can not be used to accurately calculate the overall sound pressure level. Even more error is seen when performing a Weighting after Filtering, as seen below.

When applying weighting curves via postprocessing, the attenuation at the cutoffs of the Octave Filters in addition to another factor can cause even greater error. As an example, applying the attenuations of an A weight filter by hand will not likely yield the same results as running the raw data through a filter, and calculating the overall level. The plot below shows the error on these calculations:

So if a frequency is close to the cutoff, the attenuation applied by simply subtracting it from the 1/1 or 1/3 octave band may be off by as much as 15 dB off. This creates an even larger difference between the Overall Level and the 1/1 and 1/3 Octave Sum.

Why does the sum of filters not always match the overall reported level?

The reason is due to the fact that what you are calculating when you sum the values of the 1/1 and 1/3 Octave bands is not an overall level. This number is simply the summed level of the 1/1 or 1/3 octave bands. This is best characterized in the plot below:

This means that when adding up the 1/1 and 1/3 octave bands, the frequencies at or around the cutoff frequencies of the filters will have a slight error. This is why these numbers can not be used to accurately calculate the overall sound pressure level. Even more error is seen when performing a Weighting after Filtering, as seen below.

When applying weighting curves via postprocessing, the attenuation at the cutoffs of the Octave Filters in addition to another factor can cause even greater error. As an example, applying the attenuations of an A weight filter by hand will not likely yield the same results as running the raw data through a filter, and calculating the overall level. The plot below shows the error on these calculations:

So if a frequency is close to the cutoff, the attenuation applied by simply subtracting it from the 1/1 or 1/3 octave band may be off by as much as 15 dB off. This creates an even larger difference between the Overall Level and the 1/1 and 1/3 Octave Sum.