Room Tuning In the Box

Jun 1, 2008 12:00 PM, By Bob Hodas

CONSIDERING DIGITAL EQUALIZERS FOR STUDIO MONITORS

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PUTTING THE TEST TO THE TEST

I measured one auto-equalization system in two different rooms to illustrate its effects on room response. A client of mine decided to replace his aging analog equalizers with one of these digital self-tuning systems and asked me to measure the results to make sure they maintained a good response after the changeover. One room was a medium-sized space with a freestanding 5.1 system used for remixing DVD soundtracks. The other was a small quality-control room with a smaller 5.1 system mounted into the walls. This particular automatic system uses equalization throughout the audible frequency range. There is no set upper limit on the frequencies that the equalizer will adjust, and the user can make adjustments once the auto-EQ has done its job.

Figure 1: High-frequency detail

Figure 1: High-frequency detail

The tuning for the room in Fig. 1 on page 34 was performed by one of the developers of the auto-equalization system. The developer's concept is to take an average of the frequency response from a fairly large section around the console. One microphone was used to sample noise; the mic was moved live over a fairly large area throughout the mix position while pink noise was played through the speakers. The tuning for the room in Fig. 2 (also on page 34) was done by the studio engineer, along with a representative of the company that developed the auto-EQ system. For this system, the same basic measurement technique was used, but the engineer kept the microphone restricted to a much smaller space around the mixing console. The room in Fig. 2 is about half the size of the room in Fig. 1.

Let me briefly explain the charts that you are seeing. The bottom window represents the curve of the room prior to the application of equalization, with the inverted equalizer curve overlaid. The top window represents the equalized room. The blue trace in the top and bottom windows represents the frequency response. This is viewed in 48th-octave resolution. The red trace at the top of each window represents coherence and, in this instance, is an indication of the amount of reflections mixing in with the direct signal. The green line in the bottom window is the inverse curve of the equalization that the system applied. (So if the trace is above the zero line, it is actually a cut; if it is below the zero line, it is a boost.) The inverse display simply allows you to see how well the equalization fits into the room curve. I placed my mic at the center mix position as most engineers will reference to this position when listening to a finished mix.

Figure 1: Low-frequency detail

Figure 1: Low-frequency detail

Remember that these are high-resolution shots of the room (48th-octave resolution). While I consider ±6 dB to be a quite good response, the response curves would never look smooth like you might see using 1/3-octave display. The first thing that struck me about Fig. 1 was the 6dB boost that the system applied in the high frequencies. The tweeter has a natural roll-off above 16 kHz. Although, as you can see, the system does a very nice job of taking the rolled-off tweeter and making it flat out to 20 kHz, boosting this area to get a flat frequency response is going to stress the tweeter a great deal, and most likely will result in excess noise in the system, as well as distortion. Moving lower in the frequency response, you can see that quite a few adjustments were made in the midrange. These are minor adjustments in areas where there are first-order reflections and I really don't see how they are necessary. In the case of these reflections, as you move across the mixing console, peaks will become dips and dips become peaks, so the adjustments are not meaningful. Of particular concern to me are the boosts at 300 Hz and around 600 Hz where I measure excess energy to begin with, and a cut around 800 Hz where I measure a hole due to a first-order cancellation. In the low frequencies, the system does a nice job of not trying to fill in the large holes such as those around 100 Hz and deals with a couple of high-Q resonances quite well. What disturbs me in the very low end, though, is that the system applies a shelving boost of the frequencies below 50 Hz. This would put undue stress on the woofer and remove headroom from the speaker system.

Figure 2: High-frequency detail

Figure 2: High-frequency detail

Looking at Fig. 2, we see the system's same tendency in the high frequencies to boost beyond the design capacity of the tweeter. It is also obvious that a roll-off has been inserted above 4 kHz. We must take into account that this room is being used for quality control for DVD movie soundtrack releases. In this situation, one would want the room to have a fairly flat frequency response, not a personality curve. The HF roll-off was applied by the engineer after the automatic EQ had been performed to “sweeten” the sound of the system. (Perhaps boosting the tweeter beyond its capacity made the system a bit harsh?) This sweetening will result in the engineer missing high-frequency problems, such as vocal sibilance, that need to be addressed in the post-production process. Looking lower on the chart, many small adjustments were also made to the midrange — again, something I feel is unnecessary. The system appears to do a pretty good job in the area between 60- and 500 Hz. But as in the other room, I'm amazed to see a 12dB shelf from 60 Hz on down. This results in a 10dB boost at 30 Hz — something that is completely unacceptable in a room where accuracy is the goal.

I am not saying that all of the auto-alignment systems available will perform as strangely as the examples I used; I've only looked at a few. But the point I'm trying to make is that completely trusting these systems, without any backup measurements, may not be prudent at this point in time.

Figure 2: Low-frequency detail

Figure 2: Low-frequency detail

To date, I have yet to see an automatic tuning equalization system that can do as good a job as a well-trained professional. However, I am sure that these systems will continue to become more and more sophisticated. A system that gives the user extensive capabilities to override and tweak the auto-EQ adjustments could prove to be quite desirable. For the time being, my recommendation would be to find someone to help you properly set up your speaker system and address the acoustical issues. Then if you want to use an auto-equalization system, have someone perform some measurements alongside, just to make sure you don't get yourself into too much trouble.


Acoustic consultant Bob Hodas has tuned more than 1,000 rooms across the world. Visit his Website at www.bobhodas.com.

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