I just got around to reading this thread because it was published when I was out of commission, and I didn't notice it until new posts started up again at the end of July. I assumed the content would be the usual dreary exchanges of subjective opinions, but I was happy to see the thread was actually spawned by one of Shady's excellent articles. This is a subject that has always fascinated me. Why are seemingly significant grill diffraction effects generally not audible, but peaks and dips caused by resonances in the drivers themselves or poor crossover design are readily audible, particularly on pink noise or white noise?
Shady ventures an explanation for the reduced audibility of diffraction effect, noting that diffraction peaks and dips in the reflected sound will differ in frequency depending on the angle of the reflected sound to the speaker. Although not explicitly stated, I assume the theory is that the various differences in the location of the diffraction peaks and dips will tend to cancel each other out, producing a smoother room response than would be suggested by an on-axis anechoic measurement. As Shady notes, an implication of this phenomenon is that diffraction effects should be more audible when listening close to the speaker, since reflected sound will play less of a role in what you hear than when you are in the usual listening position.
All of this is certainly interesting, and I hadn't thought thought of it before. To test the theory out (albeit not rigorously),I played some white noise from my FM tuner through the exact same BMR speaker that Shady used for his Audioholics review and as an illustration of grill effects earlier in his posted article. I've attached my with-and-without-grill measurements to save you the trouble of pulling up the article again. My measurements track Shady's very closely except for small differences in our mic calibrations in the highs. I think we can all agree that if the severe dip and peak in the with-grill measurement also showed up with the grill off, you could probably hear the irregularity and Shady wouldn't have awarded 5 stars to the BMR.
In any event, I enlisted a friend to remove and replace the BMR grill while I listened to white noise about 9 feet from the speaker. ( I kept my eyes closed, but I could hear when the grill was removed and replaced). To my ears, the sound was identical either way. My friend and I switched places, and he couldn't hear any difference either. Then we repeated the experiment listening very close to the speaker, about 8 inches away. The only difference I could hear was when the solid frame of the grill passed in front of the speaker. I'm sure you can fault the design of the experiment, but it was pretty convincing to me, and you can easily replicate the test on your own speakers. I have to conclude that the relative inaudibility of diffraction effects remains something of a mystery.
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