Sounds can be produced at a desired frequency by different methods. Sirens emit sound by means of an air blast interrupted by a toothed wheel with 44 teeth.
- frequency [ˈfri:kwənsi] n. 频率
- siren [ˈsaiərin] n.汽笛
- emit [iˈmit] v.发出
- blast [blɑ:st] n. 一股(强而突然的气流)
- interrupt [ˌɪntəˈrʌpt] v. 中断
