IEC61305-5规定了一个“Maximum sound pressure level limited by distortion”,对应频率为有效范围的下限(the lower limit of the effective frequency range),此频率的总谐波失真,二次和三次谐波全部小于等于10%。
同时,IEC61305-5规定的“有效频率范围”为:“Frequency range as the range over which the response does not vary by more than 10 dB”,很多时候这个频率范围可能会覆盖到谐振频率以下。