Pitch and frequency have a lot to do with perceived loudness. Sound pressure levels are recorded across the full spectrum of audible sound and it is the peak that is reported. As an example let's say the D&D had a peak of 124dB @ 100Hz and at 1kHz it was only 120dB. Then lets say that the Jardines peak of 123dB was at 1kHz and only 120 dB at 100Hz. Now lets factor in the possibility that the testers hearing is not very good down low. It's possible that his hearing at 100Hz is attenuated by 6dB, but is perfect at 1kHz. Therefore in his hearing range the Jardine will indeed sound louder. All this is saying is that perceived hearing is very subjective and measure SPLs are only truelly accurate for those will level hearing across the audible frequency range.
What this ultimately means is the the only way to tell for sure which muffler is the loudest or quietest to you is to hear them all and preferably at the same time or have your hearing tested and find out where you hear the best then campare that to the audio waveforms (something that we will never see)of the various mufflers.
I realize this doesn't really help you decide which pipe to buy but my only suggestion would be to stick with stock untill you've had a chance to hear the others on the same bike as you own.
