No reflections coming from walls and ceiling may be included. The result
of which is that the microphone should be placed quite close to the loudspeaker
(approx. 70 to 140 cm).
The phase response is dependent on how the drivers are fitted and arranged.
That's usually quite simple: the preferred source of radiation is located
in-between tweeter and midrange. To be precise, the line with constant phase
difference is actually a hyperbola, having its apex between the drivers. In
the simplest case the hyperbola turns into a straight line that runs in-between
the centres of tweeter and bass driver.
If the sound pressure level of the entire loudspeaker is measured on this
straight line (or hyperbola), then the phase difference is recorded correctly.
It's a bit more complicated when several sound sources have to be
considered (pictured below)
We are actually interested in the total sound pressure level of the loudspeaker.
Now the problem arises that - depending on the distance to the loudspeaker
- the phase difference between tweeter and mid / bass driver changes according
to the distance.
The further away the listener is located from the loudspeaker the smaller
the phase difference, that depends on the difference of x - a.
The sound path difference is:
x-a = square root (a2 + h2) - a
If h = 29 cm (we are exaggerating a bit), then the phase changes as follows:
The table shows clearly that developing the speaker with a microphone distance
of 70 cm is absolute nonsense when later the average listening distance is
5 metres. In later applications the phase difference between tweeter and bass
driver would be 122 degrees - 13 degrees = 109 degrees different to the design
in the lab. We therefore recommend the following procedure when building a
multiple driver loudspeaker:
First, we just measure two neighbouring drivers in order to calculate a filter
for them. The distance to both has to kept the same, therefore, the phase
doesn't change with increasing distance.
That's how the crossover for just one transition. The second transition between
midrange and bass driver is measured with the microphone positioned between
midrange and bass driver.
Later, when the loudspeaker is finished the total frequency response has
to be checked from a considerable distance. This can only be done covering
a small frequency range since the time window has to be kept small to exclude