As known, Bandwidth is the difference between the upper and lower frequencies in a continuous band of frequencies. OK, what means when saying that bandwidth range is 15 ~ 20 KHz ? Does that mean that the bandwidth should be 5KHz ?
Range is the minimum to the maximum value of something. Since bandwidth is the difference between the upper and lower frequencies in a continuous band of frequencies, then the range of bandwidth of 15 to 20 KHz would then mean that the minimum difference (bandwidth) would be 15 KHz and the maximum bandwidth would be 20 KHz. If the saying was instead that the range of occupied frequencies was 15 to 20 KHz then it would then mean the bandwidth was 5 KHz.