difference between 3rd-Octave spectrum of sine sweep vs. its Impulse Response

Hi there,

I've been using MATLAB's octave band functions (latest attempt has used poctave()) in order to look at the 3rd-Octave spectrum of some sound pressure data (in Pascals). I'll summarise the process here, and then pose my question after:

1. The source signal was an exponential sine sweep that was recorded in various receiver locations.
2. I then calculate the Impulse Response (IR) with the inverse sweep and each recording (as per standard methods).
3. Then I use MATLAB's poctave() function to calculate the 3rd-Octave spectrum of the IR
4. I then decided to use the poctave() function to analyse a chopped recording of the sweep to compare it's 3rd-Octave spectrum with the calculated IR of the same sweep recording. Expecting the spectrum to very closely match.

Following step 4 however, the results were not what I expected. I'm hoping someone can explain to me what is going on!!

Below is a plot displaying the 3rd-Octave spectrum of the recorded sweep and the 3rd-Octave spectrum of the IR derived from the same recorded sweep:

Whilst the data has a similar trend. The level in the IR appears greater than that of the sweep recording. I calculated the difference between the two and the result is an almost linear increase the higher in frequency:

This 2nd graph shows the difference in level between the spectrum of the IR compared to the spectrum of the recording (i.e. IR minus recording)

My Question:

Why is this happening?

(I use exactly the same parameters when using poctave() on both sets of data)

I have a feeling perhaps it has something to do with amount of data?!

Any help greatly received! Thank you!