Sound absorption coefficients for frequencies higher than 8kHz and lower than 80Hz? I am building a matlab simulation that applies room compensation to a 3D audio rendering system, and I'm searching for the frequency dependent sound absorption coefficients of various materials in order to extract the reflection coefficients.

However all the tables I found provide absorption coefficients for different octave frequency bands that have center frequency from 125Hz to 4kHz or at most 8kHz, but in my case I need to consider frequencies up to 20kHz.

My question is why are tables for sound absorption coefficients limited to such frequency range and how should I consider the coefficients for higher frequencies(i.e. higher than 10kHz) and also for lower ones(i.e. lower than 80 Hz)? 

I found the tables in the annex section of the book "Auralization" by Dr. Michael Vorländer, which uses the ISO 354:2003 standard in order to perform the measurements.

The only resource where I found tables for frequency values higher than 8kHz  was this paper: https://www.degruyter.com/downloadpdf/j/aoa.2013.38.issue-2/aoa-2013-0020/aoa-2013-0020.pdf where it is proposed an alternative method w.r.t. the previously mentioned standard in order to take the measurements from 50Hz to 50kHz, however these are provided just for very few materials.
The main reason why the tables don't have it, is that it's hard to measure. The measurement technique in ISO 354:2003 relies on measuring the difference in reverberation times in a reverberation rooms with and with/out a material sample.  At higher frequencies, the reverb time is dominated by air absorption and and the sound field becomes less and less diffuse which violates the basic assumption behind the measurement. It's not uncommon to measure absorption coefficients larger than 1. That's not good !

I wouldn't mingle air absorption and wall absorption. They are different effects and should be modelled separately. They are basically additive. At 20 kHz air absorption is around 50-ish dB/100m. The early reflections in a room have a travel distance of only a few meters so there is some attenuation, but there is still plenty of energy left. 

Keep in mind that wall absorption measurement is NOT done at 8 kHz but it's averaged over the 8 kHz octave, so the data covers frequencies up to 11.5 kHz. If you need to go hire, you can often simply extrapolate to 16 kHz by looking at the difference between the 4 kHz and 8 kHz value. 

Easiest is to extrapolate in dB for the reflection coefficient. Example: 4 kHz = 0.6, 8 kHz = 0.8. Reflection coefficients are 0.4 and 0.2 or -4dB and -7dB respectively. So it's 3 dB drop of reflected energy per octave. Extrapolation yields -10 dB for 16 kHz, which is a reflection coefficient of 0.1 or an absorption of 0.9.

Something like this
$$\alpha_{16} = 1-10^{0.1 \cdot (2 \cdot 10 \cdot \log_{10}(1-\alpha_8)-10 \cdot \log_{10}(1-\alpha_4))}$$