Passive Crossover Design

Crossovers cannot be calculated. The heart of the speaker system is the crossover (unless you are using a single full range driver – although in reality these do not exist – most drivers can only cover about 3 to 4 octaves without generating considerable levels of distortion). Crossover design is critical to the loudspeaker’s performance – it has many jobs to do...

1. Pass the correct band of frequencies to the appropriate driver

2. Prevent the wrong frequencies from getting to drivers not designed to handle them (in particular stop low frequencies getting to the tweeter)

3. Obtain an acoustic response which is smooth and flat* across the audio spectrum

4. Ensure the load impedance presented to an amplifier is within design limits – i.e. a speaker with 8-ohms nominal impedance should not drop below 80% of this value (6.4 ohms) anywhere in the frequency spectrum.

5. Adhere to acoustic design parameters at the crossover points to ensure good transient performance – this means each of the low-pass, band-pass and high-pass filters must have the correct: Roll-off slope; Corner frequency; Summing point.

Note: * A perfectly flat response doesn’t guarantee a good sounding speaker – in some cases a response with a downward slope towards the HF end, or the so called ‘BBC dip’ in the mid range, may produce the best overall sound for a given loudspeaker -room combination.

The word ‘acoustic’ is underlined above - this is one reason why crossovers cannot be calculated – a filter can be designed electrically to be, for example, 12dB per octave with a Q of 0.5 but when measured acoustically can be something entirely different. It is true that electrical filters can be calculated and their behaviour is very predictable - this is the basis for the many calculators that can be found in books and on the internet. These formulae only work for a resistive load, i.e. loads that do not vary with frequency – a loudspeaker is nothing like a resistor which means all these calculators are fundamentally flawed. The best they can do is to provide you a starting point which will ensure you won’t damage your tweeter. To correctly design a crossover you need the following information: -

1. Impedance (magnitude & phase) data by frequency for each (actual*) driver mounted in the intended cabinet.

2. The distance between the intended listening position and the front of the cabinet

3. Anechoic frequency (magnitude & phase) data for each (actual*) driver mounted in the intended cabinet

4. Non-linear distortion (Energy-Time-Curves) data for each (actual*) driver mounted in the intended cabinet

Note * It is always best to use the actual drivers because there can be performance differences due to production spreads etc. However drivers from highly regarded manufactures such as Scanspeak, SEAS, Volt, etc are likely to be very similar for a given manufacturing period.

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