52 Chapter 4 For example, an inverse­Chebyshev filter has notches in the stopband. Notch-type crossovers contain either inverse­Chebyshev filters, or notch filters as such, like the Bainter filter. These filters incorporate zeros as well as poles, so they are also non-all-pole crossovers. Notch crossovers are dealt with in Chapter 5. 4.2 Symmetric and Asymmetric Crossovers Symmetric crossovers have filters with the same slope at the crossover point. For example, a two-way second-order crossover has 12 dB/octave slopes for both HF and LF filters. Asymmetric crossovers have differing slopes at the crossover point; the HF filter might have a 18 dB/octave slope while the LF filter has a 12 dB/octave slope. Asymmetric crossovers do not in general sum to flat unless a rolloff in the response of one of the drivers is being used to make the slopes equal in terms of acoustic output. Subtractive crossovers in their straightforward (non-delayed) form always give asymmetric slopes for orders greater than one because the output derived by subtraction always has a 6 dB/octave slope, whatever the slope of the filter. A slope of 6 dB/octave is usually inadequate unless especially capable wide-range drive units are employed. Adding a time delay in the path going to the subtraction stage can give equal slopes, but the delay needs to be very precise for this to extend over an adequate frequency range, and this appears to be a major problem with the concept. This problem is explored in Chapter 6 on subtractive crossovers. 4.3 All-Pass and Constant-Power Crossovers Crossovers are classified as either "All-Pass Crossovers" (APC) or "Constant-Power crossovers" (CPC) in accordance with the way that the outputs recombine. An All-Pass Crossover has filter outputs that sum in the air in front of the loudspeaker to create a sound pressure level (SPL) with a flat amplitude/frequency response. The human ear is only sensitive to the pressure changes at the ear hole, and has no way to integrate the acoustic power bouncing around a room. If the filter outputs are summed electrically instead of acoustically, as a test of proper recombination, then the filter outputs should sum to a flat voltage response, voltage being equivalent to SPL. This is sometimes called the amplitude response. Summation is by vector addition (i.e., phase must be taken into account) of the highpass output V HP with the lowpass output V LP : V SUM = V HP + V LP (4.1) APC crossovers are the usual choice because they give the loudspeaker a flat on-axis frequency response, and this is usually where the person who pays for the audio system sits; however, this certainly does not guarantee a flat off-axis response. The phrase