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Crossover design and listening room conditions

Peter StrassackerDr. Peter Strassacker
involved in loudspeaker design since 1977
Books on material and field research
speakers like Lagrange 98, Pascal,

How to design a crossover
Peter Strassacker
interviewed by Philip Zukowsky (9/2007).
 
 

Philipp:
The main purpose of a crossover is to distribute those frequencies to a driver that can be reproduced well by this unit. Additionally, it's important that all drivers are in phase in the region of their crossovers. Nevertheless, there are speakers that have a unbalanced sound even though the frequency response is flat. What's the reason for this?

Peter:
One problem is surely the energy distribution in a room, depending on frequencies.Frequenzweiche

Let me give you an example. Imagine, you have designed a crossover for a loudspeaker with a total frequency response that is totally flat on axis.

The individual frequency responses should look like pictured on the left.
Frequenzweiche Even when we look at the sound radiation coming from the speakers, the frequency response on axis is still flat. When we look at angles, however, we notice that the level decreases with increasing angle, depending on the frequency (more precise: depending on quotient of wave length and diameter of the driver λ/D).

Since the decline in volume with increasing angle depends on the diameter of the driver, we obtain a volume pattern as pictured on the left. If a smaller driver takes over at higher frequencies, the volume decline from this frequency upwards will be smaller.

Philipp:
But, where is the problem? If the loudspeaker is pointed towards the listener, all frequencies will be absorbed by the human ear at the same value.

Peter:
That's correct only in an anechoic chamber or free air. Our listening rooms have, however, walls all around that are reflecting the sound radiating from the speakers. Depending on how the room is dampened, a large portion of the sound doesn't reach the ear directly but as reflections from the walls.

Philipp:
That would mean that in our example the frequencies around 300 Hz and 3000 Hz are not reflected as well as the other since these frequencies are radiated to the sides with less intensity.

Peter:
Correct, an experienced human ear would miss some information.

Philipp:
What's the best way to design a crossover?

Peter:
An unorthodox method that initially leads to excellent results is to measure the loudspeakers at an angle of 15 degrees and to optimise the crossover in such a way that the frequency response below 15 degrees is flat.

At frequencies where the volume depends heavily on the angle, the volume below 30 degrees will be too low, while the volume below 0 degrees will be too high. These will be offset mostly.

If you require the direct radiation to have the same volume without depending on the frequency, then the speakers should not be pointed towards the listener, but rather parallel to the side walls.

Philipp:
I understand, below 15 degrees I'll have a perfectly flat frequency response for the direct radiation and otherwise a mostly balanced energy density over all frequencies in the listening room.

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