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Author Topic: Sound Question what is X curve? Cameron Glendinning Jedi Master Film Handler Posts: 845 From: West Ryde, Sydney, NSW Australia Registered: Dec 2005 posted 10-03-2006 11:31 PM                     I am currently being told that this X curve is a bad thing, on one side a speaker manufacturer of 4 way active systems and also by a person who designs Cinema Sound Mixing suites. I believe its a 3db roll off from 2 - 3 K, anyone with more details, are there other curves, has anyone heard a system flat? Any comment?  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 10-04-2006 08:31 AM                     The x curve is a 3 db rolloff in the room response stardig at 2K This was determined as to what a test group felt was a natural playback of a live orchestra that was then played back It was origin was with Bell Labs and Dolby defined it in the 70's The smaller the room the less of a roll off the larger the room the greater the rolloff If the room isn't designed to that standard then playback will not be what is intended just like anyone who records in a non standard suite would have a problem The ourepose of standards is an attempt to get a uniform yardstick  |  IP: Logged Robert Minichino Master Film Handler Posts: 350 From: Haskell, NJ, USA Registered: Dec 2005 posted 10-04-2006 10:14 AM                        The X curve (ISO 2969) comes from the way the ear hears the reverberant sound field versus the way something like a time-averaging real-time analyzer hears it. If you set up a large room to have a flat frequency response on an RTA, it will sound too bright as the ears add in the direct sound from the speakers to the reflected sound in the high frequencies, while at lower frequencies they average more like an RTA. The X curve is applied to the playback system, and not the recording, in an attempt to correct for the apparent response of the space.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 10-04-2006 04:47 PM                     A key to the discussion on the X-Curve is being left out...that is the X-curve relates to how continious sounds like pink noise will be heard in a reverberant room versus impulse type sounds that is more representative of actual program material. Since, as others have touched upon, continous sounds will allow the room to build up and have direct and reflected audio reaching the microphone (or ears for that matter), the X-Curve allows one to compensate for the fact that the direct sound will have subtractive audio from the delayed reflections added to it. Thus, the actual level will indeed be less. In theory, if one were to set up an average RT60 room with pink noise and follow the X-Curve and then remeasure the room using short tone bursts so that the direct sound is measured before the reflection...the response should take on that of FLAT. Thus, the X-Curve does not, in fact, roll off the response but actually allow one to have a flat response using conventional test material (pink noise) and test equipment (1/3 Octave RTA). Note, that with more reverberant rooms one is supposed to roll off more and with less reverberant rooms one is to roll off less...which is all part of ISO-2969 or ANSI/SMPTE 202M (Appendix A6 table A.1). Dolby's Technical Guidelines (available for download here and contains ANSI/SMPTE 202M of its year), Figure 2.3 expands upon that and allows for one to lessen the "roll off" for smaller rooms.  |  IP: Logged Christopher Seo Jedi Master Film Handler Posts: 530 From: Los Angeles, CA Registered: Jun 99 posted 10-06-2006 05:43 PM                     To put it another way: If you place the speakers in a free field (i.e., open space) or an anechoic chamber (i.e., 100% acoustic absorption all around), and EQ them for flat response on an RTA, then take the same sound system and install it in an "average" theatre, or at least the average theatre of the 1970s, then the RTA would show something approximating the X-Curve, but of course most theatres would show some deviation from the mean. So the X-Curve should be taken as a rough guide rather than an exact specification. It actually seems obsolete at this point. With the theatre acoustics, sound systems, and test equipment available in the 1970s, the improvement offered by the X-Curve standard outweighed its margin of error. But today, test equipment to measure direct sound is readily available, bypassing the guesswork of correcting for steady-state RTA measurements.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 10-06-2006 07:26 PM                     There are certainly alternative measurment techniques that have certain forms of validity and superstition assoicated with them...however when it comes to movie theatre room measurement, the current pink noise/RTA measurment has continued to prove to be the most reliable and consistant. Like all techniques that involve human reading, the person doing the interpretation has a great influence over the results. However, the statement about the X-Curve applying to theatres from the 1970s is flat out wrong. Furthermore, theatres of today are hardly superior...there are good ones and bad ones...in any era. The crux of them in the middle seem to be little better or worse. Note, despite much of the critism of THX...they (Tom Holman and others) did extensive research in acoustics in movie theatres. Furthermore, THX rooms are typically built better than the typical movie theatre in terms of RT-60 times and other anomolies. Even with such criteria, guess what...the X-curve applies due to the measurement techniques used. A movie theatre is not to be completely dead. Nor is it to be an echo chamber...there is a tolerable range given the volume of the room that will have the sound reproduced in a manner that most of the people can hear/understand it properly. Lastly, the X-Curve is reviewed every 5-years by SMPTE (people like Tom Holman are on the committee, A12) to ensure that a SMPTE standard reflects accurately what is proper to the best of everyone's ability to verify or submit changes.  |  IP: Logged Christopher Seo Jedi Master Film Handler Posts: 530 From: Los Angeles, CA Registered: Jun 99 posted 10-08-2006 03:49 AM                     Isn't it contradictory to say that pink noise/RTA is "reliable and consistent" and then that "the person doing the interpretation has a great influence over the results"? There's a lot of truth to the old joke that no one ever EQs the same room the same way, including studio and Dolby technicians. Part of the problem is the limitations of the 1/3-octave equalizers, but the other part is the ambiguity of the test data. I'm not arguing that the X-Curve is invalid for today's theatres, when using conventional measurement technology, but rather that the methodology itself is decades old and relatively primitive. There's no point in having a standard that no two people implement in the same way. If more sophisticated test equipment is available to improve precision and reduce ambiguity, why not take advantage of it?  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 10-08-2006 04:49 PM                     Nope, nothing contradictory there... it is indeed both reliable and consistant and the person doing the job has a great influence over the result....The facts are that there are no "modern" tuning methods that will result in as consistant or reliable results If one uses multiplexed calibrated mic system...odds are that if one were to tune the room...then another person were to come in and remeasure the room...also with a muliplexed calibrated mic system...the room will still read the same or so similar as to not be out of tolerence. The facts are...I've done just that. An arguement can be made about using 1/3-octave EQ versus parametric but...the bulk of the cinema techs can't handle parametric EQ...with 1/3 octave, there is a 1:1 mapping on the analyzer versus EQ. That has the best chance of working with most techs. When I end up going to DSP based crossovers (I don't necessarily take an analog signal, go through an A/D D/A conversion process unless the benefits outweigh the problems)...I will almost always use parametric EQ in the DSP that negates any need for 1/3 Octave EQ. However...the mere need of EQ, in my opinion admits failure to some extent. A speaker that is acoustically flat to begin with, in a room that is made properly...will always sound better than the system that require vast amounts of EQ to bring it into spec.  |  IP: Logged Richard Hamilton Phenomenal Film Handler Posts: 1341 From: Evansville, Indiana Registered: Jan 2000 posted 10-08-2006 05:21 PM                     quote: Steve Guttag However...the mere need of EQ, in my opinion admits failure to some extent. A speaker that is acoustically flat to begin with, in a room that is made properly...will always sound better than the system that require vast amounts of EQ to bring it into spec. Exactly. Otherwise you over EQ and try to make the speaker do something it isn't designed to do.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 10-08-2006 05:39 PM                     Christopher said"So the X-Curve should be taken as a rough guide rather than an exact specification. It actually seems obsolete at this point. With the theatre acoustics, sound systems, and test equipment available in the 1970s, the improvement offered by the X-Curve standard outweighed its margin of error. But today, test equipment to measure direct sound is readily available, bypassing the guesswork of correcting for steady-state RTA measurements." 1st it is a standard not a guide and it has a tolerence factor in it It maybe an old standard but it is still very valid As to measureing direct sound it is irrelevent as in a large room one is in the reverberent field not the direct field Also Parametric EQ is vastly superior to 1/3 octave EQ but even 1 octave eq is better than badly done parametric EQ Many a Starscope has sounded better when someone who understands a parametric eq aligns it  |  IP: Logged Jack Ondracek Film God Posts: 2348 From: Port Orchard, WA, USA Registered: Oct 2002 posted 10-08-2006 06:50 PM                        So... if a processor was intended only for personal use through a set of high quality headphones, one would use no equalization at all?  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 10-08-2006 11:16 PM                     You'd want it set flat if you're going to do critical listening with phones. The response of most headphones already take into consideration most of what was done at Bell Labs regarind the ear's sensitivity at different frequencies. I have EQ'd ramp systems to somewhat of an X curve though and they did sound alot better than they did flat.  |  IP: Logged Richard Fowler Film God Posts: 2392 From: Ft. Lauderdale, FL, USA Registered: Jun 2001 posted 10-09-2006 10:59 AM                     Parametrics are especially great some situations such as live sound, where notching with a 1/3 EQ doesn't do the job. X curve is a good standard; most of the problems I see are in very small rooms or dailies sound playback situations where the roll off is not adjusted for the space....or the sound mixer says "run it flat" for playback, the end result ear splitting high frequency  |  IP: Logged Christopher Seo Jedi Master Film Handler Posts: 530 From: Los Angeles, CA Registered: Jun 99 posted 10-09-2006 03:24 PM                     Sure, it's a standard, but it's also a rough guide. It's a rough guide because it's not a direct specification of what one wants to achieve, which is flat loudspeaker response. What is the point of having a standard that Dolby technicians themselves tell you cannot be followed to the letter? What is the point of having a standard which allows a tolerance range of 6 - 10 dB? SMPTE 202M-1998, Section A.11 ("Future work"), lists the following as useful future inclusions to the standard: "a) reverberation time vs. frequency; b) loudspeaker radiation pattern vs. frequency; c) the consequent direct-to-reverberant ratio at listening positions within the space due to a) and b)". Can these factors be accounted for by traditional measurement methods?  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 10-09-2006 03:40 PM                     A standard is a standard and the tolerence range is less the 10 db it is +-3 db which is the norm for rateing most audio devices and THX is +2 -3 as a norm  |  IP: Logged

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