Every model will be different, but the important thing is that the boundaries represent the frequencies within which the signal will stay above a particular thr

Every model will be different, but the important thing is that the boundaries represent the frequencies within which the signal will stay above a particular threshold of amplitude. A good spec sheet will tell you that threshold, and I've seen things like -3, -6, or -10 dB. It will still pass audio outside of the range but at an undisclosed attenuation.