According to IEC 268-5, the frequency response of a loudspeaker is calculated as follows: 1) The frequency response of the loudspeaker is measured along the reference axis using a sine signal. 2) At the maximum sound pressure level, the volume level is averaged at one octave. This mean output level is termed the sensitivity. 3) The frequency response is given as the range of frequencies at which the volume level of the loudspeaker drops 10 dB below the sensitivity level. Dips in the frequency response narrower than 1/9 of an octave can be discounted.