| 摘要 |
<p>In order to scare away birds, at least two acoustic signals are produced, the frequency of which is periodically modulated within a range of about 30 to 9000 Hz; the modulation period is about 0.01 to 5 seconds. The signals are different from one another in respect of their frequency modulation range, in modulation period and/or time duration. The signals are emitted in a fixed or arbitrary sequence.</p> |
