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Author Topic: The X-curve for theatres Sumit Guha Film Handler Posts: 1 From: Mumbai, Maharashtra, INDIA Registered: Sep 2003 posted 09-14-2003 10:24 AM                     I would like an explanation of the X-curve that is used to check the frequency response of loudspeakers in cinemas. If a loudspeaker with flat frequency response is used, why does the curve show a roll-off from 2Khz onwards?  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 09-14-2003 10:39 AM                     First I would highly recomend that you read Ioan Allens articles on the development of the x curve They started with research by Bell Labs in the late 20's of what the psycoaccoustic properties were of reproduced sound in large rooms By using large concert halls and transmitting orchestras to them they used audiences to determin what electro accoustic response the mean audience group felt was natural in a large room It was discovered that the brain in a large room expects a certain attinuation of high frequency sound and the x curve eventually derived from that The large the room the more rolloff the smaller the room less roll off  |  IP: Logged Manny Knowles "What are these things and WHY are they BLUE???" Posts: 4247 From: Bloomington, IN, USA Registered: Feb 2002 posted 09-14-2003 12:59 PM                     So, what's considered a "small" room? Are there any commercial theatres that are so small they don't require any HF rolloff? Most of my theatres are in the 200 seat range. I have a few that are 100 seats. Our "big" houses are about 300 seats or so.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 09-14-2003 03:13 PM                     The 150 seaters I tend to leave a bit brighter and those over a 1000 a little duller otherwise I follow the curve  |  IP: Logged Christopher Seo Jedi Master Film Handler Posts: 530 From: Los Angeles, CA Registered: Jun 99 posted 09-15-2003 06:57 PM                     Gordon, Could you point me to the articles you're referring to? The Dolby manuals reference papers, but they (or the ones I could find, anyway) are all either on the development of optical stereo or noise reduction. I haven't read anything that specifically and thoroughly addresses the research that went into the X-curve. Thanks.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 09-15-2003 09:20 PM                     There is a booklete put togather by DOlby by Ioan Allen on cinema design and it historys the x curve Also there was several article in SMPTE over the issue around the time Quintaphonic came out  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 09-16-2003 10:13 AM                        Links about the X-Curve: http://www.robertmargouleff.com/publications/surround_0100b.htm http://www.genelec.com/ht/pdf/aes23rd.pdf http://www.genelec.com/ht/pdf/aes114th.pdf http://www.realspace.com/SigTech/aes_90sf.html http://www.thx.com/mod/techLib/pdf/guidelines.pdf http://www.americanmusical.com/Manuals/JBL%20Pro/CinemaSoundSystem_manual.pdf  |  IP: Logged Ray Derrick Master Film Handler Posts: 310 From: Sydney, Australia Registered: Sep 2001 posted 09-16-2003 10:54 PM                     The standard X-curve that we all know is optimised for a 500 seat theater. For smaller or larger rooms a correction factor is made to the slope of the curve above 2kHz. For example, for a 1,500 seat room you increase the slope of the curve by 1dB at 4kHz and 2dB at 8kHz. For a 30 seat room you reduce the slope of the curve by the same factors. Some RTA's provide X-30, X-500 and X-1500 versions of the curve, otherwise you need to interpolate the amount of increase or decrease at the other frequencies above 2kHz. I have edited this post to add, for those interested, the rolloff slopes specified in the standard for various theater sizes (the number after the X being the number of seats): X-30 2dB/octave 2kHz to 10kHz, 4dB/octave above 10kHz X-150 2.5dB/octave 2kHz to 10kHz, 5dB/octave above 10kHz X-500 3dB/octave 2kHz to 10kHz, 6dB/octave above 10kHz X-1000 3.5dB/octave 2kHz to 10kHz, 7dB/octave above 10kHz X-1500 4dB/octave 2kHz to 10kHz, 8dB/octave above 10kHz X-2000 4.5dB/octave 2kHz to 10kHz, 9dB/octave above 10kHz [ 09-16-2003, 11:56 PM: Message edited by: Ray Derrick ]  |  IP: Logged Manny Knowles "What are these things and WHY are they BLUE???" Posts: 4247 From: Bloomington, IN, USA Registered: Feb 2002 posted 09-17-2003 10:32 AM                     Thanks for all the info. Can a half-dB really make that much of a difference? (I guess I'll just have to go listen and find out!)  |  IP: Logged Antonio Marcheselli Phenomenal Film Handler Posts: 1260 From: Florence, Italy Registered: Mar 2000 posted 09-17-2003 06:17 PM                        Ray, Should't I lower the highs in larger rooms? Bye A  |  IP: Logged Ray Derrick Master Film Handler Posts: 310 From: Sydney, Australia Registered: Sep 2001 posted 09-17-2003 06:24 PM                     Yes, Antonio, "increase the slope of the curve" means lower the highs.  |  IP: Logged Richard Fowler Film God Posts: 2392 From: Ft. Lauderdale, FL, USA Registered: Jun 2001 posted 09-17-2003 06:26 PM                     To a trained ear a 1/2 db is noticeable. I supplied dailies for a Disney movie where the production manager insisted on Flat reproduction for the screening room. I EQ for his spec and for the proper rolloff similar to the X-30 curve...after a 3 minute preview with the FLAT horns piercing the room, the sound mixer and production manager went for the X-30. For a Paramount movie, the mixer was able to hear a 1/2 db change between projectors and with my test equipment, he was exactly right on the level  |  IP: Logged Ray Derrick Master Film Handler Posts: 310 From: Sydney, Australia Registered: Sep 2001 posted 09-17-2003 06:51 PM                     Also we are talking 1/2dB "per octave" not 1/2dB across the board. Audio rolloffs are often expressed as so many "dB per octave" which means for each doubling in frequency, the attenuation increases by the number of dB. So at the first octave, say 4kHz (if you are starting at 2kHz), the attenuation is 1/2 dB, but at the next octave (8kHz) it increases to 1dB, and so on up the spectrum. So the effect is more pronounced the higher you go up the spectrum. In the case of the X-curve(s), the slope doubles above 10kHz, so the effect is even more pronounced above that frequency. Of course "dB per octave" also implies that the in-between frequencies are proportionally attenuated. So for a 3dB/octave slope starting at 2kHz, the attenuation is 1dB at 2.5kHz, 2dB at 3.15kHz, 3dB at 4kHz (1 octave above 2kHz), 4dB at 5kHz, 5dB at 6.3kHz, 6dB at 8kHz (2 octaves above 2kHz) and so on.  |  IP: Logged Antonio Marcheselli Phenomenal Film Handler Posts: 1260 From: Florence, Italy Registered: Mar 2000 posted 09-18-2003 07:06 AM                        Ray Sorry, I didn't note the "slope"! Very interesting this thread! I've read some of the John's links. But finally, the eq curve is attenuated to highs for a psicological matter or for a physical matter? It is not clear to me... Bye! A  |  IP: Logged Ray Derrick Master Film Handler Posts: 310 From: Sydney, Australia Registered: Sep 2001 posted 09-18-2003 09:54 AM                     It is really not a matter of either. Look at it this way, it is all to do with how you measure it. In essence, the size of the room has very little effect on the high frequency response. It is the effect the room has on your measurements that varies. The various X curves are compensating for an imperfect system of measurement, and little else. Neither your ears, nor the sound changes (to any great extent) with the room size, what changes is the effect the room has on the accuracy of your measurements. It's as simple as that. I should add that there are some losses at high frequencies due to distance though the air and varying humidity but such loses are fairly insignificant in all but the very largest auditoriums.  |  IP: Logged

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