HD650 technical measurements And how single number metrics provide extremely limited information Measurement setup Klippel QC analyzer Furman SP-20A headphone amp ACO Pacific 7052SYS microphone Brüel & Kjaer 4231 mic calibrator Mr BlockHead™ flat plate coupler density similar to average human head 1.08 g/cc width consistent with average human head 15.25 cm Mr BlockHead is mounted on a high mass lighting stand to minimize externally induced vibrations. View of ACO Pacific 7052SYS in Mr BlockHead flat plat coupler Alert assistant is nearby ready to help. HD650 is specified to have 0.05% THD+N at 1 KHz with 1Vrms stimulus Historically a low distortion 1 KHz sine generator would have stimulated the electro-acoustic transducer using a low distortion amplifier. Then a steep 1 KHz notch filter removes the stimulus and the measured level of what remains is compared to the stimulus level. That ratio results in THD+N. Now consider the capability of a modern acoustic analyzer. Klippel generator is adjusted to 1Vrms (circled in red.) Low distortion amplifier is set for 0 dB gain to follow the generator settings displayed. Headphones carefully positioned on flat plate coupler. Sound level observed is 100.6 dB SPL which is consistent with published sensitivity. THD+N at 1 KHz is about 0.05% THD+N (the red dot on the graph.) Looking at this graphic limited to the single number data point offers minimal information. SINAD at 1 KHz provides similarly very limited information about the characteristics of an amplifier. A single dot on a graph. Expanding the view to include the entire analyzer 1 Vrms sweep from 20 Hz to 20 KHz vastly more information is observed Upper left graph: Frequency response, rub+buzz Upper right graph: THD+N, H2 (offset by -4% for visual clarity) and H3 (offset by -2% for visual clarity) Lower graph: Impedance Why would we want to limit our view of an electro-acoustic transducer or amplifier to a single number representing an infinitesimal aspect of performance? Single number metrics used for audio component performance rankings are absurd when one realizes how much information is missing. Notes about the analyzer graphs 1) frequency response is not compensated in any way. Data can be exported such that others may use their favorite compensation methods. 2) rub+buzz is a measure of all that remains when fundamental and harmonics are removed. It is sort of an acoustic crap factor. 3) impedance measurements are often noisy and unreliable below 0 dBu in normal environments outside an anechoic chamber or very well controlled sound space. Every electro-acoustic transducer can transform electricity to sound (speaker) and sound to electricity (microphone.) They are usually optimized for one or the other but still can function in reverse. To avoid outside influences impedance is typically measured with a sine sweep of between 0 and +10 dBu. It is a balancing act between outside influence and analyzer input level of which there are more considerations than will be discussed here. 4) rub+buzz below 40 dB SPL is inconsequential Note the rub+buzz bump at 6 KHz corresponding to a small THD+N and H2 bump at 6 KHz. Watch these points in the following graphs.