Customers are calling building-biologist Oliver Zenkel whenever they hear very quiet sounds. The hum is imperceptible with normal hearing. Particularly disturbing is the monotonous and recurring frequency. The person concerned enters a feeling of helplessness. Mr. Zenkel has adjusted to the soft tones and plugged a headset to the sound level meter as technical reinforcement. How the search for the hum can succeed, describe the following examples.
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