Recitaiton room; Teaching room; Class-room; Lecture room

classroom; lecture room; recitation room; teaching room

Classroom;Teaching room;Recitation room;Lecture room

Lecture room; Classroom; Teaching room; Recitation room

Teaching room; Classroom; Recitation room; Lecture room

trap-to-trap

One class elementary school; One room school; One teacher school; Single room school

one class school; one room school; one teacher school; single room school

One room school; One class elementary school; One teacher school; Single room school

One teacher school; One class elementary school; One room school; Single room school

room in room

room in room structure

Single room school; One class elementary school; One room school; One teacher school

’trap’effect

3rd harmonic trap

50 Hz wave trap

50 Hz-trap filter

absorption trap

absorption wave-trap