[georgia]

“X” CURVE

Pink noise is played through each speaker or channel of a sound sound system. Then, equalization adjustments are made to each channel until the measurement of the pink noiseconforms to curve called the X curve. In the beginning, theatre loudspeakers suffered from both a limited frequency range as well as a poor frequency response. By implementing equalization, you can solve some of these issues. A room the size of a threatre will create acoustic anolomies. When equalized with pink noise for a flat response in a theatre, speakers deliver too much treble. The resulting sound is way too bright and a bit painful. The X curve was an attempt to normalize the sound in a large room.
Taken for an article I had lying around…my compliments to the author….

It was also noted that larger theatres would exhibit a somewhat steeper high frequency roll off, and that smaller theatres would exhibit a slightly reduced roll off of the high frequencies. This finding was officially noted in 1990. Beyond that, there have been few additional guidelines to aid technicians in the interpretation of these measurements and the equalization of cinema systems. Several years ago, the measurement system evolved with the use of four microphones placed around the auditorium to pickup the sound. While some have steadfastly defended this approach, in the final analysis it is no better than a single microphone pickup. Different, yes. But whether one uses a single microphone or four, by including all the reverberation, the resulting measurements are equally unreliable. While some have been critical of the way cinema sound systems are measured and equalized, I think the real disappointment is that as the loudspeakers have evolved, the methods employed to measure their behavior in theatres have not evolved far enough or quickly enough. Ioan Allen’s work of a quarter century ago was important and should not be understated. It represented a valuable component in Dolby’s efforts to introduce Dolby Stereo as well as improve cinema sound. It later became the basis for the SMPTE 202-M as well as the ISO-2969 motion picture audio standards. It also opened the door for many other improvements in all aspects of movie sound and paved the way for the introduction of wideband three-way loudspeakers as well as sound systems with a nine octave response, first introduced to movie theatres by my company in 1979.
In fairness, since the original work on the X curve was done with older theatre speakers having significant frequency response and frequency range limitations, it was impossible to glean further insights into what the shape of the curve might be with full-range highoutput loudspeakers in theatres of different sizes. Such speakers were unavailable at the time. That has now changed and a lot has been learned. Indeed, both Ioan and I have separately presented papers with similar findings on the varying shapes of the X curve.

The Real X Curve also shows that real-time measurements of the frequencies below 100 Hz, are also room dependent. While some theatres will exhibit a slightly rolled off bass region, many will show quite an elevated measurement in these frequencies. From this we see that the practice of automatically and artificially rolling off these lower frequencies, contributes to the lack of bass in many motion picture sound systems.

During the International Theatre Equipment Association technical seminars in 1999, Ioan Allen presented his own findings on the characteristics of real-time analyzer measurements of pink noise in theatres of different sizes. His presentation also included so-called waterfall charts showing how the shape of the pink noise measurement actually evolves as reverberation accumulates over time and results in response curves of varying shapes. The bass build up below 100 Hz is also seen in this graph that he has kindly provided for this article. He pointed out that the X curve itself “is a myth.” That is to say the high frequency roll off seen when measuring pink noise with real-time analyzers does not
indicate a roll off in the frequency response of the sound system. He reminded us that the roll off seen in such measurements is a result of the accumulated reverberation being included in the measurement. Now that the varying shapes of the X curve are more clearly understood, are we now fully prepared to equalize cinema sound systems to perfection? Well, not quite. We have a problem. Before we can properly equalize a sound system with pink noise, we need to know what the shape of the curve should be for the particular theatre we are in, when the response we actually hear with program material is flat. Determining that requires the use of screen speakers with a flat on-axis frequency response. Since most high frequency horns used in cinemas are the constant directivity type, with their own characteristic rolled off high frequency response, finding the correct place for the knee of the curve for a particular room is unlikely. Perhaps less difficult is knowing how the lowest frequencies should measure. The best way to handle the frequencies below 100 Hz is to adopt a what you see is what you get policy and do not equalize.

Another equally frustrating problem is the inability of the pink noise / real-time analyzer approach to accurately convey what is going on in the frequency range from about 100 to 400 Hz. For the sake of simplicity, my own Real X Curve chart does not show how these frequencies can sometimes measure at reduced amplitudes, rather than flat, in good sounding systems. In my experience, however, the actual shape of the frequency response depicted by an analyzer in these frequencies is not consistent from theatre to theatre, even though the sound systems involved may have the same tone. Furthermore, the way speakers behave in these frequencies can be influenced by the room.

How they should measure with pink noise is also room dependent. Sound systems tuned so that the analyzer shows a flat response between 100 and 400 Hz will often sound bloated, boomy or “honky,” while others will sound fine. There seems to be as many solutions to the challenges of tuning motion picture sound systems as there are technicians and authors who choose to write about them. The proof of the success of any technique is in the listening, however, not in the rhetoric. Those really interested in learning what works best merely need to stick their heads in the different rooms, setup different ways and hear for themselves. Fortunately, the differences are very evident, making judgments easy.

A new measurement system is needed. Whenever it arrives, the inventors will surely find themselves standing on the shoulders of Ioan Allen. Until we have a reliable method for measuring what something sounds like, it turns out that his original approach to the equalization of those older theatre speakers of the 1970s, remains the best solution to tuning a sound system. By comparing the sound heard from theatre speakers to a known high quality source, one can hear the difference and make adjustments accordingly. Since there are still no such measurement methods, we will need to rely on our ears for listening.



cheers
geo