Since heart sounds are generally of low frequency component and very faint, this method, depending on doctor's knowledge and experience, is subjective and sometimes is unreliable and ineffective.
由于心音的频率较低而且非常微弱，医务人员通过听诊器根据自己的知识和经验对心音做出主观分析判断并不一定可靠。
The very low frequency electromagnetic method (VLF-EM) has such advantages as portable instrument, simple method, economical and high efficiency.
甚低频电磁法(VLF - EM)，具有仪器轻便、方法简单、经济高效等优点。
This analytic method is also suitable for the bands of extremely low frequency (ELF) and very high frequency (VHF).
这种分析方法也适用于极低频到甚高频波段。
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