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With @amirm's experiment with the 5128 coming to a close, I thought it might be useful to take a moment to think about what makes headphone measurements useful to us and what we're looking for in them - particularly since Amir will be faced with the cost/benefit analysis of choosing a headphone test fixture if he does end up going that route.
I tend to separate headphone measurements into two overarching categories, each with its own sub categories:
The first is measurements which correspond to audible performance for end users, the "consumer utility" measurements. Frequency response in relation to a subjectively pertinent target response curve is the most obvious of these, as per Sean Olive's excellent work, it corresponds most significantly to subjective sound quality, but it's not the only one, and it's not entirely monodimensional. Sensitivity (I prefer to express this in dBSPL/V, personally), maximum linear output level, noise isolation from outside sound sources, leakage level outside the cups for a given level at the eardrum, and in the fairly niche cases where it comes up audible levels of distortion at conventional listening volumes are all significant to the subjective experience of using headphones.
Additionally, linear frequency response, as Amir's experiences in mounting headphones show, isn't just one curve - it's a spread of behavior under varying circumstances, and in my opinion there's value to capturing some of that breadth. A "typical"/"average" frequency response plot, showing reasonably charitable coupling and an average of multiple positions, is useful for comparing frequency response to a target curve. A plot of average and worst-case deviation from this average by frequency with positioning can be useful for showing the range of on-head behavior that can occur even on a single ear. Measurements with deliberately compromised seal - glasses, wigs, gaps of an intentional size under the pads, etc - can differentiate headphone tolerance of suboptimal wear, something which definitely occurs in real usage.
The second is measurements which can provide some technical or engineering interest, but do not directly have significance to audible behavior in most cases, a.k.a. "nerd bait". Examples here would include the electrical impedance of the headphone, most analysis in the time domain, nonlinear behavior at ludicrous extremes of output level (yes, I've played a 20hz sine wave at 120dBSPL before, no, I won't apologize), behavior beyond the audible band (I may be the only one, but I always got a chuckle out of Rin Choi forcing headphones to play 2hz sine waves), acoustic and electrical crosstalk, atypical measures of nonlinearity (coherence, swept IMD, etc), etc.
For my purposes, as a complete nerd, pretty much all properly conducted measurements have at least some interesting component - I don't buy that many headphones anymore, and I spent a lot of time tinkering with headphone design, so anything that lets you look inside another design is interesting to me. Sadly for me, Amir doesn't run this site for my sole benefit, so I thought I'd drop this here to ask: what do you want from headphone measurements? What do you go in looking for? What's the focus you're looking for from headphone analysis? Perhaps this might be helpful to Amir in deciding both what he's looking for from test equipment, and whether this headphone measurement malarky is worth his time at all!
I tend to separate headphone measurements into two overarching categories, each with its own sub categories:
The first is measurements which correspond to audible performance for end users, the "consumer utility" measurements. Frequency response in relation to a subjectively pertinent target response curve is the most obvious of these, as per Sean Olive's excellent work, it corresponds most significantly to subjective sound quality, but it's not the only one, and it's not entirely monodimensional. Sensitivity (I prefer to express this in dBSPL/V, personally), maximum linear output level, noise isolation from outside sound sources, leakage level outside the cups for a given level at the eardrum, and in the fairly niche cases where it comes up audible levels of distortion at conventional listening volumes are all significant to the subjective experience of using headphones.
Additionally, linear frequency response, as Amir's experiences in mounting headphones show, isn't just one curve - it's a spread of behavior under varying circumstances, and in my opinion there's value to capturing some of that breadth. A "typical"/"average" frequency response plot, showing reasonably charitable coupling and an average of multiple positions, is useful for comparing frequency response to a target curve. A plot of average and worst-case deviation from this average by frequency with positioning can be useful for showing the range of on-head behavior that can occur even on a single ear. Measurements with deliberately compromised seal - glasses, wigs, gaps of an intentional size under the pads, etc - can differentiate headphone tolerance of suboptimal wear, something which definitely occurs in real usage.
The second is measurements which can provide some technical or engineering interest, but do not directly have significance to audible behavior in most cases, a.k.a. "nerd bait". Examples here would include the electrical impedance of the headphone, most analysis in the time domain, nonlinear behavior at ludicrous extremes of output level (yes, I've played a 20hz sine wave at 120dBSPL before, no, I won't apologize), behavior beyond the audible band (I may be the only one, but I always got a chuckle out of Rin Choi forcing headphones to play 2hz sine waves), acoustic and electrical crosstalk, atypical measures of nonlinearity (coherence, swept IMD, etc), etc.
For my purposes, as a complete nerd, pretty much all properly conducted measurements have at least some interesting component - I don't buy that many headphones anymore, and I spent a lot of time tinkering with headphone design, so anything that lets you look inside another design is interesting to me. Sadly for me, Amir doesn't run this site for my sole benefit, so I thought I'd drop this here to ask: what do you want from headphone measurements? What do you go in looking for? What's the focus you're looking for from headphone analysis? Perhaps this might be helpful to Amir in deciding both what he's looking for from test equipment, and whether this headphone measurement malarky is worth his time at all!