In this experiment, we focused on the high-frequency signals but also recorded some sub-frequency ones.

sub-frequency 由前缀 sub-（表示"低于、次于"）和 frequency（"频率"）组成，字面意思是"低于主要频率的"，即"低频"。 frequency 是名词，指"频率"，即单位时间内周期性事件发生的次数。