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This topic comprises 2 pages: 1 2
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Topic: Mixing room speakers, information about Hornspeakers
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Harold Hallikainen
Jedi Master Film Handler
Posts: 906
From: Denver, CO, USA
Registered: Aug 2009
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posted 12-11-2018 04:38 PM
As mentioned, not everyone agrees with my views on the X-curve. Looking at Michael Karagosian's paper, we see that its copyright date is 2013, a year before the theater testing report was published, so Michael did not have the latest measurement data.
Michael's paper mentions air attenuation. Note that the level of all frequencies decrease by 6 dB every time you double the distance from the source due to the sound being spread over a larger area. Imagine a light bulb at the center of the sphere. If the light bulb emits 1 watt and the sphere has a radius of 1 meter, the "power density" will be 1 W / surface area of sphere = 1 W / 12.6 m^2 = 79.6 mW / m^2. Doubling the radius to 2 m, the surface area becomes 50.3 m^2, so the power density is 19.9 mw / m^2 or 1/4 that we got at 1 m. 1/4 power is a decrease of 6 dB. So, this is not attenuation, it's just spreading the power around. Further, spreading the power around is independent of frequency.
However, since air is viscous, there is attenuation where some of the sound gets converted to heat. The amount of attenuation depends on the distance (how much air is passed through), the air temperature, and the humidity of the air. A nice table showing this is at http://www.kayelaby.npl.co.uk/general_physics/2_4/2_4_1.html . At 10 kHz, attenuation is between 95 and 280 dB/km depending on humidity. If we average the values for 10 kHz, we get 167 dB/km or 0.167 dB/m.
Looking at 1 kHz, the average attenuation is 6.1 dB/km or .006 dB/m.
So, if we go back 30 m, as suggested by Michael, we expect 1 kHz to be attenuated 0.18 dB, and 10 kHz to be attenuated 5 dB, putting 10 kHz about 5 dB below 1 kHz, just as Michael points out.
30 m is pretty far. And, if this is 2/3 of the way back, as specified for the measurement position in SMPTE RP 2096-1, the auditorium would be 45 meters deep (about 148 feet). Big auditorium!
So, while there is indeed some high frequency attenuation by the air, it is probably pretty small in typical auditoriums. Also, I do not think we want to compensate for this loss in equalization. If we were to compensate for it, those sitting in front of the reference position would have excessive highs. Further, we are used to having less highs with distance, so our brains compensate.
I agree with Michael when he says “Notably, neither reflections nor absorption should be addressed with equalization.” We are interested in the frequency response of the “direct sound.”
“Thirty years ago, when attempting to devise a technique that could easily be deployed in cinemas around the world, it was decided that readily available realtime analyzers could be used if one could take into account the effect of room reverberation. Out of this thinking was born the X-Curve sound measurement method.” However, the B-chain report shows that a modern cinema has very little reflection above 1 kHz, so room reflections do not contribute to an RTA reading above 1 kHz. The B-Chain report also reveals this when it finds that short window impulse response measurements (which reject reflections) closely match long window measurements (equivalent to RTA readings which include reflections). Further, near field measurements (very close to the screen) showed the same X-curve. The difference in the far field is likely due to air attenuation (0.167 dB/m). In many theaters, this would be negligible. Therefore, I believe the ideas that X-curve at a distance results in flat at the screen and that it is necessary to attenuate the highs as measured at a distance to compensate for buildup due to reflections (there is very little high frequency reflection) are not correct.
“Users that do not understand the proper application of X-Curve may sometimes treat it as an equalization curve, attempting to equalize the system until an exact replication of the curve as published in ST 202 is achieved, ignoring the response window described by the text in the standard. This generally leads to excessive equalization, and not necessarily the best possible sound.” I believe we do not want to exactly replicate the curve because it would use excessive filters. Fewer filters sound better than exactly matching the frequency response. We want to, instead, equalize the trend instead of each peak and dip in the response (high frequency peaks and dips are probably due to local reflections from seats, the floor, etc.).
“In addition, the X-Curve measurement method is standardized in SMPTE ST 202 and ISO-2969, and methods for equalizing a room, based on first arrival sound are not standardized, causing temptation to discount the use of first arrival methods in cinema. But, in fact, many skilled technicians prefer to use first arrival methods, and proper use of such methods should lead to equivalent results.” First arrival (impulse response with short time windows to reject reflections) are now standardized in SMPTE RP 2096-1. As noted in the B chain study group report, at high frequencies, there is so little reflection in a typical cinema that short window impulse response and RTA measurements are very nearly equal.
“SMPTE established a new Technology Committee, TC-25CSS Cinema Sound Systems. As the name suggests, members of the committee will be evaluating other measurement methods for cinema, based on first arrival of sound. If one or more alternative cinema sound measurement standards should emerge, one should keep in mind that they are an alternative, and not a replacement, for ST 202. X-Curve, the method that takes into account room reflections, and newer methods based on first arrival of sound, both strive for the same thing: a measurement method that allows the cinema technician to best approximate the sound heard in the mixing room.” At this point, we do have the new recommended practice (RP 2096-1). However, we still are using the X-curve as defined in ST 202 because the HF rolloff is not to compensate for room reflections (as revealed by the B chain study group report), but, instead, is (in my view) de-emphasis provided by the screen. I believe those creating content boost the highs so the content sounds good through the screen on the dub stage, thereby also making it sound good in the theater.
Again, not everyone agrees with the way I see it, but I THINK what I’m saying is based on the latest evidence.
Harold
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Marcel Birgelen
Film God
Posts: 3357
From: Maastricht, Limburg, Netherlands
Registered: Feb 2012
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posted 03-04-2019 05:59 PM
The whole idea of the X-Curve, is that it's the response of the room versus a totally flat speaker fed with pink noise. The X-Curve, in the original idea, doesn't include the screen attenuation.
Now, it's indeed striking that the screen attenuation itself shows a similar plot to the X-Curve in many cases. It probably only adds to the confusion and the black art of movie theater tuning. ![[Wink]](wink.gif)
I know that for many in the industry, even questioning the X-Curve has become somewhat of a sacrilege and in general, it has done more good than bad.
But since we're living in 2019, there has been quite some change in the industry since the 1970's, when the idea behind the X-Curve was created.
Since then, the average auditorium has seen quite some changes:
- Rooms have generally become smaller.
- Where mono was the norm, we now have many speakers to consider, in the same room, some behind the screen and some not.
- Speakers and amplifiers have become more powerful.
- Movies have become louder and so has the need for isolation. All in all, rooms have become more "dead" over the years as a result of increased dampening.
- The advent of stadium seating has not only decreased the volume of the average room even more, it also drastically changed the geometry of the average room.
Measurements across the years have shown that the X-Curve seemed to fit the traditional auditorium quite nicely, but different kind of rooms have vastly different characteristics. The ways we build auditoriums have changed over the years, that might have impacted the validity of the X-Curve.
Personally, I think that every room should be considered on its own. Nowadays, we've got the equipment to measure and adjust accordingly. Also, modern auto-calibration and parametric EQ allow us to do stuff we never could about 40 years ago.
In the end though, you can calibrate all you want, the proof is in the listening. That involves humans and is still a very subjective thing.
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