Friday, December 15, 2017

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I measured the HD 4.50 BTNCs using a G.R.A.S. Model 43AG ear/cheek simulator, a Clio 10 FW audio analyzer, a laptop computer running TrueRTA software with an M-Audio MobilePre USB audio interface, and a Musical Fidelity V-CAN amp. I used the Model 43AG’s original KB0065 simulated pinna for most measurements, as well as the new KB5000 pinna for certain measurements, as noted. For Bluetooth-sourced measurements, I used a Sony HWS-BTA2W Bluetooth transmitter to send signals from the Clio 10 FW to the headphones. These are “flat” measurements; no diffuse-field or free-field compensation curve was employed.

Frequency response

The HD 4.50 BTNCs’ frequency response, taken with a Bluetooth signal with noise canceling (NC) on, may look a little weird due to its two prominent peaks centered at roughly 50Hz and 2.2kHz. Actually, it’s not far from what’s considered a standard “flat” headphone response: a broad bass peak, a midrange dip, a prominent response peak around 2.5kHz, and a lesser peak around 6kHz.

Frequency response

This chart shows the HD 4.50 BTNCs’ measured right-channel frequency response measured with the old KB0065 pinna (which I’ve used for years) and G.R.A.S.’s new KB5000 pinna, which I’ll be switching to because it more accurately reflects the structure and pliability of the human ear. I include this mostly for future reference rather than as something you should draw conclusions from; I intend to show both measurements in every review for at least the next year before I begin to use only the new pinna.

Frequency response

This chart shows the right-channel frequency response of the HD 4.50 BTNCs measured with Bluetooth and NC on, with Bluetooth on and NC off, and with a wired connection. Obviously, switching NC off substantially changes the sound of these headphones. Although I don’t show it in the chart here, I also measured the response of a wired connection, adding 70 ohms output impedance to the V-CAN amp’s 5-ohm native output impedance, but saw no notable change in response.

Frequency response

This chart shows the HD 4.50 BTNCs’ measured right-channel frequency response compared with those of three other NC headphones: the AKG N60 NC Wireless, the Bose QuietComfort 35, and the PSB M4U 2 (the last generally considered to rank among the best-sounding NC headphones). The HD 4.50 BTNCs’ response is similar to that of the AKG.

Waterfall

The spectral-decay (waterfall) chart shows a few very narrow, low-magnitude resonances between 1.5 and 2.1kHz; these are not likely to be audible.

THD

The total harmonic distortion (THD) of the HD 4.50 BTNCs, measured with a wired connection because the Clio 10 FW’s sine sweeps can’t accommodate Bluetooth’s latency, is negligible at 90dBA. At 100dBA, it’s about average for dynamic over-ear headphones; it rises to 3% at 60Hz, and spikes from 3% at 30Hz to 13% at 20Hz -- but note that 100dBA is an extremely loud listening level, and that few music recordings have significant content below 30Hz.

Isolation

In this chart, the external noise level is 85dB SPL; the numbers below that indicate the degree of attenuation of outside sounds. (I recently switched to measuring at a level of 85dB instead of 75dB; this doesn’t change the way the isolation curves look, but a level of 85dB allows me to get better measurements of NC headphones, which demand a lower noise floor.) In this measurement, the isolation of the HD 4.50 BTNCs looks somewhat below average for NC headphones, which surprises me because my subjective tests showed it to be better, and this measurement usually corresponds closely with subjective impressions.

Impedance

The HD 4.50 BTNCs’ impedance magnitude and phase in wired mode are nearly flat, averaging about 23 ohms and with negligible phase shift.

The sensitivity of the HD 4.50 BTNCs in wired mode, measured between 300Hz and 3kHz with a 1mW signal and calculated for the specified 18 ohms impedance, is 100.9dB. This means that if you have to use a wired connection, the Sennheisers will still play plenty loud.

. . . Brent Butterworth
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