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Topic: The X-curve for theatres

Antonio Marcheselli
Phenomenal Film Handler

Posts: 1260
From: Florence, Italy
Registered: Mar 2000

posted 09-18-2003 10:58 AM

Ray,

Are you talking about the reflections that comes out of phase in a larger room instead of a smaller one?

Bye
A

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Ray Derrick
Master Film Handler

Posts: 310
From: Sydney, Australia
Registered: Sep 2001

posted 09-18-2003 08:58 PM

Antonio, as the size of a room increases the amount of reverberation and colouration due to reflections usually increases. Reflections and reverberation have reduced high frequency energy because high frequencies are absorbed by most surfaces more easily.

If you stand in a large cinema, your ear discriminates between the direct sound from the speakers and the reflected sound and reverberation, at least in the upper part of the frequency spectrum. A microphone placed in this same space cannot discriminate between direct sound and reflected sound, it can only average all of the sound that it receives and send this to an analyser. So in a larger room the reflected sound and reverberation form a greater percentage of the sound that a microphone picks up and hence the average will be deficient in high frequency energy, even though the direct sound may be flat and sound flat to a listener.

So the X-curve essentially compensates for the inability of the microphone to duplicate what the the human ear (or rather human brain) does by nature.

Now this is a slightly over-simplified explanation but it will give you some idea of why the X-curve is necessary.

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Manny Knowles
"What are these things and WHY are they BLUE???"

Posts: 4247
From: Bloomington, IN, USA
Registered: Feb 2002

posted 09-18-2003 09:11 PM

What happens to this theory when the room isn't that reflective because it has really good wall treatment?

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 09-18-2003 09:38 PM

I usually choose the slope based on the RT60 calculation

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Ray Derrick
Master Film Handler

Posts: 310
From: Sydney, Australia
Registered: Sep 2001

posted 09-18-2003 09:53 PM

Firstly if you read the literature you will find there are alternate theories about all of this, including more recent ideas that there are psychoacoustic factors involved, such as that a person expects a large room to sound duller than a small room. I think the jury is still out on this idea despite some interesting research.

What is clear is that the process is complex and to my mind no one theory provides a totally satisfactory answer. However the explanation I gave above is the oldest theory and to my mind is still the easiest to grasp.

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William Hooper
Phenomenal Film Handler

Posts: 1879
From: Mobile, AL USA
Registered: Jun 99

posted 09-19-2003 12:47 AM

quote:
What happens to this theory when the room isn't that reflective because it has really good wall treatment?

The ear still expects the same thing. As Ray said:

quote:
If you stand in a large cinema, your ear discriminates between the direct sound from the speakers and the reflected sound and reverberation, at least in the upper part of the frequency spectrum. A microphone placed in this same space cannot discriminate between direct sound and reflected sound,

An illustration is if you've gone to a concert in a large auditorium, then listened to a bootleg tape made at that concert with a mike in the auditorium near where you were. It sounds very inferior to the concert you heard, because part of your perception when listening live was discriminating to minimize the banging & booming reflections. The X-curve, as I understand Ray to be describing it, mimicks the way that perception works by attenuating the high end (what would be the banging part).

It would seem that in larger rooms where the highs would decay faster you could push more of the highs, but this is still counter to what you hear in that environment. Listening to a show in a big arena is more open & with fewer highs than if you listened to it in a small, club-like space. The small one is more bangy & immediate, the big room is big & with fewer highs. That characteristic is what the X-curve mimicks.

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Ray Derrick
Master Film Handler

Posts: 310
From: Sydney, Australia
Registered: Sep 2001

posted 09-19-2003 01:39 AM

In answer to Manny's question, I doubt if it would make much difference at all unless you were comparing a heavily treated room to an excessively live one. In my experience the speakers in "live" cinemas do tend to sound a little brighter than "dead" ones when equalised to the same X-curve, which is what you might expect if the theory is correct. But this impression may be caused by other factors such as increased hf content in the reverb of live rooms.

Many years ago when single mic analysers were the norm, I did some interesting experiments with microphone types when equalising studio audio control rooms and later, cinemas. I found that I got better results at hf end of the spectrum when using a cardiod pattern mic pointing towards the sound source, than with an omni pattern mic. Of course the omni was still better for equalising the mid and low end. This lead me to conclude that it was because the cardiod mic was not being influenced so much by reflections and reverb. Whether or not that was a valid conclusion was largely immaterial, the results spoke for themselves.

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 09-20-2003 09:02 PM

I have done several very large rooms that had ajustable accoustics via flown in resonators and damping materials
The bottom line was the reaction to the film sixing staff present to the playback of their source material was the more reverberant the room the less High frequencies they wanted

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Ray Derrick
Master Film Handler

Posts: 310
From: Sydney, Australia
Registered: Sep 2001

posted 09-22-2003 11:00 PM

So Gordy, I guess your experience is further evidence that the simplest (and in this case the oldest) theory is the right one. Somebody mention Arcom's Razor?

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Edward Jurich
Master Film Handler

Posts: 305
From: Las Vegas USA
Registered: Jul 2003

posted 09-24-2003 09:36 AM

I recall that when magnetic sound came out in the 50's, the audio had a much brighter high end than optical. In fact, one of the reasons for doing magnetic was for "high fidelity" sound. And this was in large ( 1500 to 3000 seat ) theaters in Chicago. Seems like in this case any curve was thrown out the window, the high end was quite bright. I think those old studies were made with live concert music in mind and not necessarly movie sound. Seems like you'd want to try and make your sound as good as possible under any conditions. I know that when I went to those magenetic presentations in the 50's, the sound sounded outstanding compared to normal mono optical of the time.

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 09-24-2003 09:55 AM

"I supplied dailies for a Disney movie where the production manager insisted on Flat reproduction for the screening room."
_________________________________________________________________
I was on way too many location dailies jobs where the sound mixer, these were some big time guys, didn't have a clue about proper room equalization or the X curve. It seems they spend most of their lives wearing a pair of headphones. After setting things flat at their request and they heard the sizzling bacon high end they would usually want to tune it by ear. When I checked it after they left it was almost always near the X-30 curve as well, however it sometimes was an odd curve that was derived from the fact that it was done by ear and I suppose they were attempting to approximate the sound of the headphones they used.
Mark

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Mark Seaton
Film Handler

Posts: 7
From: Glenview, IL, USA
Registered: Aug 2002

posted 09-25-2003 12:24 PM

Hi all,

Coming of lurk mode for a moment here...

I certainly agree that most any room other than the case of near field monitors will sound overly bright if you see a flat line on an RTA. What everyone needs to remember is what exactly they are measuring with the RTA. An RTA takes an average of energy per band over a selected quantity of time (slow/fast). In terms of acoustics, this time is very long. So, you are in fact looking at the total energy in the room over a given amount of time, which includes both direct sound from the speaker, and a large amount of reflected energy from the room. Rooms generally have more absorbtion at higher frequencies, and if you are lucky, or have a good acoustics designer, they remain fairly constant in the mid-midbass range. Cinemas have the benefit of using generally similar acoustics and layouts where we can correlate the sustained energy in the room (as measured by the RTA) to the direct sound radiated by the loudspeakers.

In fact most would be quite suprised if they saw a measurement of a system voiced to an X-curve then measured by a system which can gate or window out late arriving energy. An MLS, TDS or Log-Swept-Sine type measurement all are capable of this in different manners. When doing any measurement, the most important bit to know is exactly WHAT you are measuring, and what your measurement does and does not tell you. In fact many system would likely measure much closer to flat when looking at the direct sound than when looking at the RTA.

Unfortunately you are only looking at a snapshot of the response of the speakers and room with the RTA, as most speakers change in response and distortion quite dramatically as power levels increase beyond about 1/4 of rated power. Combine these many factors, and we can easily account for the "art" which is often needed in best balancing a system.

Regards,

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 09-25-2003 07:01 PM

Mark,
Obviously you are speaking in terms of a one mic spectrum analyzer here. What exactly does going to four mics through a multiplexer accomplish(as in the R-2)?
Mark @ CLACO

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Matt Zeiner
Expert Film Handler

Posts: 146
From: Windsor, CT USA
Registered: Sep 2003

posted 09-26-2003 09:17 AM

quote:
Unfortunately you are only looking at a snapshot of the response of the speakers and room with the RTA, as most speakers change in response and distortion quite dramatically as power levels increase beyond about 1/4 of rated power. Combine these many factors, and we can easily account for the "art" which is often needed in best balancing a system.

Yes... and to complicate matters further, most amplifiers and all analog filters will respond differently to an instantaneous transient versus a steady-state of the same amplitude. Steady-state measurements can be spot on, but the system may still sound less than stellar. Ears are key.

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Manny Knowles
"What are these things and WHY are they BLUE???"

Posts: 4247
From: Bloomington, IN, USA
Registered: Feb 2002

posted 09-26-2003 04:20 PM

Let's make it "calibrated" ears and we have a deal! [Wink]

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