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CHAPTER 4
Characteristics
4.3 Frequency analysis of sound
It is said that the human ear senses differently according to the frequency. However, the sound generated from a vibration is
not limited to one frequency, but instead, various frequencies are generated at differing levels. This is expressed by the NC
curve, which is determined according to the difficulty of hearing a conversation.
Even if the sound is a very low level, it is annoying if a specific frequency is emitted very loudly.
These sounds are suppressed to a minimum during product design stages, but, the sound may become very disturbing with
resonance of the ceiling, wall, etc.
Example Continuous frequency analysis
Tolerable noise levels and NC values according to room application
Room application
Broadcasting studio
Music hall
Theatre (approx. 500 seats)
Classroom
Conference room
Apartment
Hotel
Housing (room)
46
Frequency (Hz)
dB
NC value
25
15 - 20
30
20
35
20 - 25
40
25
40
25
40
25 - 30
40
25 - 30
40
25 - 30
NC curve
Min. audible limit
Room application
Cinema
Hospital
Library
Small office
Restaurant
Gymnasium
Large conference room
Factory
Frequency band (Hz)
dB
NC value
40
30
35
30
40
30
45
30 - 35
50
45
55
50
50
45
70
50 or more
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