I measured the HD 630VBs 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 headphone amplifier. In most cases, I used the clamping mechanism on the Model 43AG to ensure a good seal of the earpads against the simulator’s fake rubber earlobe. This is a “flat” measurement; no diffuse-field or free-field compensation curve was employed. Except as noted, measurements were made with the Variable Bass (VB) control in the middle setting.
The HD 630VBs, measured here with the VB dial set to its midpoint, have an idiosyncratic frequency response. Obviously, there’s a large dip between 150 and 600Hz. The peak in the 2-3kHz range, a prominent feature of most headphones’ measured response, is unusually broad, spanning the range from about 1.5 to 4kHz, and there’s a strong peak centered at 8kHz. This is the best match I was able to achieve between the left and right channels; it’s possible the inclusion of the VB control on the right earpiece has some effect on the acoustics of that earpiece.
This chart shows how the VB control affects the HD 630VBs’ response. The control’s operation is not linear; the magnitude and bandwidth of its effect are much greater between the middle and Max positions than between the middle and Min positions. Sennheiser specifies the control’s range as +/-5dB at 50Hz, which is accurate. At 20Hz, the adjustment range is +/-8.8dB. The control has no significant effect above about 800Hz.
Adding 70 ohms output impedance to the V-Can’s 5 ohms to simulate the effects of using a typical low-quality headphone amp suggests that the design of the source device will have considerable effect on the HD 630VBs’ sound. The response, measured with the VB control in the middle setting, is considerably different with the two source impedances. Unsurprisingly, however, the difference is in the range where the VB control is effective; when users adjust the bass control to their taste, they will likely dial out any effect of the source impedance on the headphones’ response.
This chart shows that the HD 630VBs’ voicing is considerably different from that of two well-regarded, closed-back, over-ear models, NAD’s Viso HP50 and Sennheiser’s own Momentum. The HD 630VBs’ output is much lower between 150 and 400Hz, and significantly greater between 3 and 5kHz.
The HD 630VBs’ spectral decay (waterfall) plot shows three prominent resonances, all at a very high Q and a low level of about -40dB; I suspect that these are inaudible. There is a low-Q resonance band between 2 and 4kHz, but the resonance is at only -20dB, and dies out in less than 4ms; this resonance is a result of a peak in the headphones’ response, so it can be considered a part of the HD 630VBs’ sound rather than an audible flaw. The resonance below 1kHz is visible in this measurement with almost every headphone I test; the HD 630VBs’ resonance is lower than average.
The total harmonic distortion (THD) of the HD 630VBs is low, maxing out at 2% even at the extremely high output level of 100dBA. This measurement was taken with the VB control at its midpoint setting.
In this chart, the external noise level is 75dB SPL; numbers below that indicate the degree of attenuation of outside sounds. The HD 630VBs are a little above average for passive closed-back headphones, with 6-10dB of additional noise reduction between 200 and 400Hz compared to the NAD Viso HP50s, closed-back headphones with isolation measurements that are typical for their type. This is especially useful because it’s within the “jet-engine band,” the most prominent noise in a typical airliner cabin.
With the VB control at its middle position, the HD 630VBs’ impedance runs from 100 ohms in the bass to 24 ohms in the midrange. The phase shift is modest, maxing out at +32 degrees at 20kHz.
The VB control has a large effect on the HD 630VBs’ impedance at low frequencies. With the control set to Min, the impedance measures 242 ohms at 20Hz; at Max, it’s 30 ohms at 20Hz; and at the midpoint setting it’s 100 ohms.
The sensitivity of the HD 630VBs, measured between 300Hz and 3kHz with a 1mW signal calculated for the rated 23 ohms impedance, is 100.6dB. This means that a smartphone or tablet should be able to drive the HD 630VBs to adequate volume levels.
. . . Brent Butterworth