fredk
The perceptibility of loudness, and other differences caused by one being louder than the other, is an interesting test. By simply having the only difference in your double blind listen test between position A and B a difference in dB you can judge the effect of this phenomenon in isolation. When you are 2 dB louder you will get comments referring to the difference in SPL along with other comments about the sound difference which will be favourable for the slightly louder one. As you get down to 1 dB the comments in regards to SPL will start disappearing leaving just the odd comment about minute advantages for the slightly louder one. Once you are down to a ½ dB difference the comments about differences of any nature will disappear. If you can keep your test products within a ¼ dB of each other then you know this variable is out of your results.
For setting up the levels we use IEC 268-5 weighted noise. It is rolled off below 40 Hz and above 5 kHz. You do want a good meter for setting this up too. Though the SPL meter only needs to read a difference between the two products and not any sort of absolute SPL, it does need to be linear and broadband. We use our B&K SPL meter as the benchmark.
