What is a Target Curve
and How can I make it Optimum? 

These are the two most frequent questions asked aboutTact room correction systems. We will try to answer them first by defining the target curve concept itself and then by providing some guidelines for optimum target curve selection.

Room correction is based on the target curve concept that was introduced by Tact in the early nineties. Since then it has evolved into one of the most powerful tools for describing desired room acoustic response.

The most common room acoustic description is given by the room frequency response. Room frequency response describes the sound pressure at the listening position over the entire frequency range (from less than 20Hz and sometimes above 20,000 Hz).  Typical room response is shown in Fig. 1

Fig. 1. Typical room response measurement. This measurement was taken by Tact -2.2 XP Dynamic Room Correction preamplifier.

As it can be seen from this sample measurement, room problems can be broken into three bands:

1. Bass region

2. Midrange

3. Treble

Room correction systems should be able to affect the room response throughout the band by covering all three bands. The bass region exhibits most of the abnormalities of room acoustics.  For high performance bass reproduction, it is very important to even out the bass region frequency response. Even though the bass region correction produces most of the audible correction effects, correction of the rest of the band is equally important. Level balancing at all frequencies in the midrange and treble bands results in well-defined and stable imaging.

Definition: Target curve is a user defined desired room frequency response that covers all three bands: bass, midrange and treble.

Fig. 2. Target curve and two channels measurement graph.

Target curve is used to modify room response and at the same time to balance all channels throughout the entire frequency range (20-20,000 Hz). Once the target curve is modified, room correction filters have to be calculated and loaded into the signal processor. WithTact room correction technology, all target curve changes are instantly engaged while the music is playing. In addition to this on-the-fly room correction performance,Tact Dynamic Room Correction technology combines the reference target curve with eight equal loudness compensation curves.

Optimum Target Curve

Does an optimum target curve exist? An optimum target curve is defined by the listening room, the sound system and more importantly by the users themselves. Optimum target curve does exits, however, it is optimum only to a particular setup and to the ears of one or more listeners (if it is possible to find two or more listeners that agree on the same definition of a good sound).

 
There are a few rules to follow when choosing an optimum target curve

1. Before selecting a target curve always look at the room response measurement.

2. Do not extend the target curve bellow the loudspeaker lower cutoff frequency (region 1 in Fig.2 and Fig. 3). This frequency is usually specified by your loudspeaker manufacturer.

3. At frequencies around 1KHz and above, try to follow the room’s natural response. This region controls the brightness of your sound system.

4. For processors that support subwoofers, such asTact-2.2 XP, select proper crossover filters. Crossover filter selection is based on loudspeaker manufacturer specifications. If these specifications are not available, crossover frequency can be determined by examining the subwoofer and main channel measurement graphs.

5. After the initial target curve selection, perform listening tests and modify the target curve to meet your specific listening needs.

Fig. 3. Shows typicalTact-2.2 XP measurement, crossover filters, and target curve selection. This graph shows left channel and left subwoofer measurements only. Right channel measurements are not displayed.

Fig. 3. Left and Left Subwoofer measurement with 160 Hz, 24th order crossover filter.

Note how the target curve in Fig.3 region 1, rolls off together with the subwoofer measurement. Since the subwoofer measurement extends beyond 200 Hz, the crossover frequency is set at 160 Hz with 24th order filters, equivalent to 144 dB per octave. In this example, subwoofer crossover frequency was determined based on a listening test and measurement data rather than manufacturers specifications.

At higher frequencies (treble region) the target curve follows the natural room response with a slight roll off all the way to 20 KHz.

After the initial target curve selection, perform an extensive listening test and make target curve adjustments in all three bands: bass, midrange and treble. Changing crossover frequency and crossover filter slope can also affect the sound of your system.

What should I expect from room correction processors?

 In the past few years, room correction technology has evolved to yet another level. Preamplifiers and power amplifiers that incorporate room correction features should be based on the latest digital signal processing technology. Here is a list of few must-have features for a state of the art room correction processor:

· High frequency resolution – around 1Hz in bass region

· User editable target curve

· Crossover design package with high filter slopes (24th order)

· High frequency resolution measurements – less than 1Hz in the bass region

· On-the-fly target curve adjustments.

· On-the-fly crossover parameter adjustment

· Correction verification capabilities

· Instant switching from correction to bypass. This is very important for AB tests

· Level dependent correction. The target curve shape has to dynamically change with the master level control – equal loudness compensation

· Automatic time alignment and level balancing.

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