11SUPER-X PRO CX3400 User Manual

3.4Setting the crossover frequencies

The use of extremely high-grade potentiometers made it unnecessary to install fixed-frequency plug-in modules. Thus, you have a wide range of setting options available that even more expensive crossover networks hardly give you.

The CX3400 works in two specific frequency ranges: 44 though 930 Hz and 440 Hz through 9.3 kHz. The Linkwitz-Riley filters employed in the SUPER-X PRO feature a slope of 24 dB/octave. High-grade components such as 1%-tolerance metal-film resistors ensure a perfect phase and amplitude response at all crossover frequencies.

Please consult the manufacturer’s specifications of the various speaker components to set the crossover frequencies. When polar plots of specific speakers or horns are available, use them too. Don’t set the crossover frequencies around peaks or drop-outs in the frequency response, but try to find a range that is largely linear. When folded woofer horns are used, you also need to take the horn length into account (see chapter 3.5 “Runtime Correction”).

◊Never operate speaker/horn drivers below the frequency range specified by the manufacturer!

3.5 Runtime correction

3.5.1 Background

The ideal transducer would be a point source of sound, i.e. a speaker of infinitely small dimension, which could still reproduce the entire frequency spectrum. Unfortunately, such a sound source is impossible in reality, so that we have

to accept some compromises.

If the drivers in a multi-way system (i.e. the diaphragm set in motion by the voice coil, but not e.g. the opening of a horn) are not exactly aligned on a vertical axis, the varying distances between sound source and listener result in phase errors and cancellations (also called “comb filter effect”). In particular, in the high- frequency range it is imperative, due to the shorter wavelengths, that the drivers be positioned one above the other, not side by side. The old-fashioned horizontal rows of radiators follow this principle: while the speaker power is summed up in the horizontal plane, the signals cancel each other on a vertical axis. Thus, unwanted reflections from the ceiling can be reduced.

Consequently, a speaker stack whose systems radiate towards the same area should have all speakers arranged in a vertical line. And even if the front sides of all systems are perfectly aligned, runtime differences still occur due to the different speaker designs (horns, bass reflex cabinet, etc.).

The BEHRINGER SUPER-X PRO allows you to delay the Low bands by up to

2 milliseconds. In this way, you can virtually push back a specific speaker cabinet by as much as 68.6 cm (which is quite useful, for example, when you place

a constant-directivity horn (CD) on top of a closed speaker cabinet).

Runtime correction is not the same as phase correction. Speaker systems that have the same run times are also in phase (unless, the polarity of one speaker is reversed). However, the opposite is not true.

3.5.2 Basics of electronic runtime correction

It is important to know how the dimensions of time and space are connected with each other, e.g. by using a tape measure and a pocket calculator.

Example: a delay of 2 ms corresponds to a distance of 68.6 cm; when you measure an offset of 30 cm you can calculate the necessary delay as follows:

2 ms x 30 / 68.6 = 0.87 ms.

If it is impossible to measure the offset with an accuracy of at least 1 cm, you can perform the runtime correction with the help of a measuring microphone and tunable sine generator, using the SUPER-X PRO’s feature of variably adjustable crossover frequencies. More on this below.

The speed of sound is 343 m/s or 34.3 cm/ms approximately (hence, 2 ms of delay correspond to a virtual speaker offset of 68.6 cm). Frequency is measured in oscillations per second (1/s); the unit of measurement is Hertz (Hz).

For example, when you adjust a crossover frequency of 3 kHz between the horn and midrange systems, the wavelength λ is calculated as follows: λ = c / f

(c = speed of sound; f = frequency). So, the wavelength at 3 kHz is:

34, 3 cm/ms

=

34300 cm/ms

= 11, 43 cm

3000 1/s

3000 1/s

 

 

With a virtual distance of 68.6 cm, the control range of the potentiometer will provide at least six positions that ensure phase coincidence. Perhaps none of these positions will correct the runtime differences completely, for example, if the offset between the drivers is greater than 68.6 cm.

Is that important? It is, because only a system whose runtime differences have been corrected will be capable of:

1.reproducing pulse peaks correctly.

2.maintaining phase coincidence when the signal frequency moves away from the crossover frequency.

3.5.3Runtime correction in a P.A. system using the SUPER-X PRO midrange/high midrange/tweeter range

Before you perform the following steps to correct both runtime and phase,

you should measure the offset between the drivers in cm and move the cabinets (or delay their bands), until you think they are aligned correctly. This will save you a lot of time later on. Now, do the fine adjustment as follows:

Using an analyzer

•Use pink noise as your sound source and connect the measuring microphone to the measuring input of the analyzer.

•Adjust the bands below/above the crossover frequency so that each one alone produces a 0 dB reading at the crossover frequency; mute the remaining bands. If both bands together produce a +3 dB reading, they can be considered in phase.

•Now, raise the crossover frequency by a factor of 1.5 and perform the same steps as above. Here, too, the analyzer should read +3 dB.

•Finally, check your setting by raising the crossover frequency by a factor of 1.4.

•Here, too, you can temporarily reverse the phase of one of the bands and check the sound for cancellations. If no runtime correction is achieved

•Check whether the distance between the two drivers is or could be greater than 68.6 cm. If so, try to correct it by moving the speaker cabinets.

•If this still doesn’t solve the problem, one of the bands could be reversed in polarity. Experiment with the front-panel PHASE INVERT button.

Subwoofer/Woofer/Midrange

It is often claimed that phase or runtime correction below 150 Hz is unnecessary, because the sound waves feature a spherical dispersion pattern at such low frequencies. We disagree with this view.

Modern systems often use bass reflex cabinets for their woofer or subwoofer systems. Consequently, when stacking the cabinets, the drivers are usually aligned along the vertical axis of the speaker front, or can at least be aligned using the available control range of the SUPER-X PRO. Here, runtime correction follows the same principle as in midrange/high-midrange/tweeter systems.