Conclusions and Recommendations
According to the study “Measurement of the sound-absorption coefficient on egg cartons using the Tone Burst Method”; the egg carton absorbs sound very well above 2 KHz. This means to say that egg cartons is not completely worthless as absorption materials. You can use egg cartons as:
a.) A cheap sound absorber in front of your nearfield monitors. This will do a great job in absorbing mid to high frequencies (starting at 2000Hz and beyond) which travels in line of sight. Bear in mind that this is not a perfect solution and based on the research, egg cartons are poor diffusers so you cannot use them for diffusion purposes. See screenshot below: