新款HDM 27系列头戴式耳机能够为广播评论员和音频工程师提供高质量的声音体验以及佩戴的舒适性。HDM 27采用包耳式封闭耳罩设计，这种超轻型设计不仅可以提供一个大程度的外界噪声衰减，还可以为长时间的工作过程提供舒适的佩戴。柔软的包耳式耳罩确保了佩戴的舒适性。与此相似的HMS 26系列头戴式耳机具有自动适应的分叉式两段头带，因此不会对敏感的头顶部产生压力。对于那些更愿意选择传统方式耳机头带的用户，产品还提供了一个更宽的头带衬垫，从而将两个头带合并为一个头带。
The NoiseGard on/off switch is the switch that allows you to turn the active noise compensation circuitry on or off. It comes with a two colors LED for-power on and low battery-on headsets using a battery pack.
NOTE: If you should turn off the NoiseGard system there is no loss of, or reduction of, the received audio and you still retain the passive capabilities of the headset. You may have to increase the receiver volume to compensate for the increase of ambient noise due to the loss of the NoiseGard feature. The LED is located on the control-unit respective to the battery pack of the headsets. The control unit is included in-line on the models HMEC 45/46 series, HMEC 25/26 series, and HMEC 460 series. An optional battery pack is available for the HMEC 350 series.
It is a common saying that dynamic microphones just have no maximum SPL.
This saying is not technically exactly correct but for every day use use of dynamic microphones you will not find gain limitations. At SPL levels above 160 dB some mechanical problems might occur (for example the voice coil might crash onto the magnet) but these SPL levels are very hard to find in real world.
Here at Sennheiser we use dynamic microphones for distortion measurements on loudspeakers at very high sound pressure levels. At Sennheiser we have measured the SPL and THD inside a large 2" driver/horn combination with a MD 421. The max SPL this combination was able to produce was 150 dB and the THD level recorded by a MD 421 was below 1% (produced by the driver not by the mic)
So as a conclusion we can state the MD 421 has no maximum SPL limitations for practical use. However if using dynamic microphones at high SPL levels a user should be aware that the very high voltage (up to several volts) delivered by the microphone might cause problems when connected to mic preamps.