RCF HDL 38-AS OWNER'S MANUAL

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LOW FREQUENCY PRESET USING REAR PANEL ROTARY KNOB

The available presets in the rear panel of the loudspeaker, named as “Local” in the software, are only four of the nine

available in RDNet. The numbers are proportional, in terms of gain, to the reduction applied to the low frequencies of all

the clusters.

HIGH FREQUENCY PRESET

The sound propagation, in particular the high frequencies (1.5 KHz and up), depends essentially on the conditions of the

air in which it travels. We can generally afﬁrm that air absorbs high frequencies and the amount of absorption depends

on temperature, humidity and the distance that the sound should would carry.

The decibel decrease is well modelled by a mathematical formula that combines the three parameters (temperature,

humidity and distance) giving a proﬁle of the absorption in function of the frequency.

In the case of a loudspeaker array the goal is the audience coverage

with the best possible uniformity, obtainable only by compensating

for the absorption introduced by air. It is easy to understand that

every cabinet should be compensated differently from the other array

cabinets because the compensation should consider the distance at

which the cabinet is aiming: the cabinet at the top of the cluster will

have a bigger compensation than the one below, which in turn will

compensate more than the one below that, etc. The compensation

should be translated in terms of decibels that should be progressively

added with the increase of high frequencies.

It is important to note that the formula gives an absorption that

exponentially increases with the increase of the frequency: in

particular conditions the compensation requests a gain too high for

the ampliﬁer.

Consider as example the following conditions: 20°C of

temperature, 30% of relative humidity and 70m of distance to

cover. In these conditions the necessary compensation, from 10

KHz and up, starts from 25 dB up to a maximum of 42 dB at 20

KHz (Figure 5). The headroom of the system cannot permit such

high gains.

Considering everything described, 15 compensation levels were

selected in order to approximate at best the inﬁnite number of

compensation curves derived from the mathematical formula.

A low pass ﬁlter is progressively introduced with the increase of the compensation gain: the system does not need to

reproduce frequencies that could hardly reach the desired distance and that could lead to a waste of useful energy.

The picture below (Figure 6) shows the behavior of the 15 ﬁlters. These ﬁlters are designed as very small FIR (ﬁnite

impulse response) ﬁlters in order to preserve the phase coherence of the system.

The RCF Easy Shape Designer algorithm calculates the curve that best ﬁts the one that would be seen in the real world.

Considering it is an approximation, the generated ﬁlters set should be validated with measurements or listening and

eventually changed in order to reach the desired listening experience.

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RCF HDL 38-AS ACTIVE FLYABLE SUBWOOFER MODULE