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This topic comprises 3 pages: 1 2 3
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Author
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Topic: Altec 802 and power handling math session
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 10-08-2001 09:57 PM
Hi all. One of the nice things that came out of the CP-50/CP-55 discussion between Paul and myself is that we got to hash out a few things...one of them was the good old Altec 802 and the Altec A-7 in general. As per his request, and for the benefit of the Film-Tech memebers here is some potentially useful information, use at your own risk.First off the Altec 802 was used in a great many Voice of the Theatre systems throughout the history of Cinema. It is a 1" compression driver and can normally be found mounted to the Altec 511 or 811 Sectroial horns. Most of the old-timers know it as being one that blew out a bunch and the newbies normally just see it as old. Here are some of the facts on the 802. It came in 5 different forms and if one follows the Altec line, there are near identical drivers called 804, 806 and 902...the differences being in the magnet (ferrite or Alnico), phase plug and other evolutionary changes though the basic motor structure has been unchanged thoughout. The 802 (and it's varients) handle a mere 15 watts AES continious pink noise when installed on the 511 horn (500Hz crossover). There was a small period (1977-1980) where the 802G series was available and that one used a different diaphragm that only handled 10-watts! (Close to Paul's recollection of 9-watts). In 1980 the 902T came out and the rating returned to 15-watts. If this sounds like low-power handling...it is...espeically for theatre applications. As such, Altec also had, starting in 1969, the 807 and 808 series drivers (theatres using the 808, MI using the 807). The 808 is identical to the 802 except the diaphragm is Altec's Symbiotic and has an AES rating of 30-watts (double that of the 802). There was a cost of extreme high frequencies but in theatres, the screen is already killing that. In 1988 Altec came out with it's "Pascalite" series of diaphragms and the A-7 could get the 909 series of driver...identical to the 902 but with the Pascalite diaphragm. The 909 is also rated at 30-watts AES power. The Pascalite diaphragms are an aluminum alloy so they have the aluminum sound but the power handling of the composite symbiotic diaphragms. Since all of these drivers can take the 800/900 series diaphragms, one can substitute a preferred diaphragm in an exisiting driver to improve performance and power handling. Currently, the standard Aluminum diaphragms (8 and 16-Ohms) as well as the Pascalite (8 and 16-Ohms) are supported with replacement parts...the aluminums are rated at 15-watts and the pascalites are rated at 30-watts AES. So what does this mean to the A-7 owner? Well now is the math part of this discussion. Presuming one already has the A-7, then just how big a room will it work in? If some recall a previous thread on a JBL subwoofer, I wen't through how to calculate how much power is needed to drive a cinema speaker (the formula is posted in that thread and in Dolby's document on "Technical Guidelines for Dolby Stereo Theatres". However, this is a different look at the same problem...we have the speaker and we want to know if the room and speaker combination will work. I have rewritten the formula so we are looking for room depth Room Depth (max) = 10 raised to: (Sensitivitiy - SPL needed + 10Log(Power Handling))
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20 Now this gives use the distance in meters to the back of the theatre. If we multiply by 7.38, we get the distance in feet to 2/3 of the way back in the auditorium and take into account that the HF driver is not playing the entire audio spectrum but just the portion from the crossover point (normally 500Hz) on up. Most drivers are rated in a few different ways...above I have used the "AES" rating that is generally the most fair. It is a pink noise source over a specified bandwidth for a given length of time. For bass and subwoofer, it is the best number to use. For high-frequency drivers, it isn't a real-world situation except during tune-ups (and the occasional effect). However, we tune up at 85dB but we want to know how well the driver will do with program material that varies in level and frequency but may go as high as 103dB (measured from 20-20KHz). Most manufacturers give a "program" rating, the "Continious Program" rating is just the AES rating multiplied by 2. Thus for the 802, the power rating (program) is 30-watts. Now if you are going to do some RT-60 measurements...watch out...normally the level of the channel will get closer to 95dB to get a good reading and that is continous pink noise when the room is excited so the AES number is the one to use. To be on the conservative side the AES rating should be used since effects like in TWISTER could have sustained HF information at high levels. One last number we need is sensitivity. The 802 mounted on an Altec 511 horn comes out to 106dB at 1-watt, 1-meter OK lets see how we do: First, lets see how the speaker will do running good old Dolby Stereo (type-A NR) (91dB). We plug in the numbers and we get: A whopping 161 feet. No too bad from 15 watts. But what if the fader gets raised? Give ourselves 3-dB of margin and we are down to 114 feet. Now lets put the speaker though the test with Dolby SR (94dB) Well 114 feet is absolute maximum but again a 3-dB safety margin would have the figure be 80 feet. The odds that an A-7 would be in a theatre 100 feet long is quite good since in it's day most theatres were in the 100' plus range. So even with SR the speaker is being taxed. What can be done short of changing the speaker? Try the 909 series diaphragm... we get...114 feet. This puts us over the top. Now's the real test...Digital(103dB) We are down to 40 feet! That is a pretty small theatre by any standards! The A-7 in the digital era is now religated to screening rooms. Does this mean that in a 60 foot long theatre the speaker will die instantly? No but it is being driven very hard and will tend to fail a bit often, depending on if your theatre sets is fader to 7 and what films you tend to run...art films arn't as taxing as whiz-bang special effects laden films. Oh, and if you give yourself a 3-dB safety margin...you are down to about 28.5 feet! Dropping in a 909 diaphragm will get you about 57 feet without the safety margin or about 40.5 feet with the safety margin. Your last ditch effort would be to raise the crossover point to 800Hz...that will get you about a 33% increase in power handling...so with the 909 and at 800Hz...with a 3-dB safety margin...46.68 feet. One last point on the above...all of this presumes a "FLAT" EQ setting...no boosts on any EQ...boosts on the EQ come off the power handling! How many of you find you have to boost the high-end to push the sound through the screen? If you are like me and find that yup, even in the best environments a little treble shelving is needed to offset the screen loss...you have even less to work with. Compare the above with the Altec 299 (1.4") with a MR94B Mantaray and you get out to 147 feet or 104 feet with a 3-dB safety margin. Check out the Lyric's photos in the galley to see the Altec 229 on an MR-94B For those of you that have the old barn of a theatre and the dual driver 228C on a multicell....with a safety margin you are good out to 132 feet! The Altec 299/MR94 combination, powerwise is on par with the JBL2446/2360 combination with the edge going to the JBL on power handling and, in my opinion, the sound quality going to Altec. As a side note...the Altec Lansing badge has been retired from pro-Audio but the Voice of the theatre products are still supported via Telex and authorized repair facilities...diaphragms and bass driver recones are still available (and into the long forseeable future) with genuine Altec factory parts. Hope this helps someone. Steve ------------------
"Old projectionists never die, they just changeover!"
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John F. Allen
Film Handler
Posts: 54
From: Newton, MA
Registered: Nov 1999
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posted 12-16-2002 04:22 PM
Every manufacturer encounters false rumors from time to time. It seems that the most common one is when someone quietly spreads the idea that a rival company is going out of business or is in some kind of trouble. When the targeted company then continues in operation and the rumor is proven to be nothing more than an attempt to harm a competing business, everyone is damaged. The slandered company needs to spend time and resources to explain to their customers as well as their vendors that they are not in danger of going away, their customers are needlessly alarmed and, finally, those who are responsible for spreading false information are discredited.
So, aside from being irresponsible, it really does no good to write or say things when one has no direct experience, let alone proof, that the statements are true.
Like all manufacturers, I occasionally see or hear misinformation about my company. Often I blame myself for not doing a better job "educating" the industry about the things we do and how we do them. But since I have written and published more on the subject of cinema sound than anyone else I know for the past 22 years, it's hard for me to think that I haven't at least tried to communicate.
When I see a dismissive remark that pronounces that it takes me days to tune a sound system, I really can't believe my eyes. How could someone I have never had the pleasure of meeting or working with claim to know such a thing? Further more, how could I, or anyone so slow, remain in business if such a thing were true, or even remotely true? The statement is ridiculous and false on its face. Yet, as with any rumor, it damages everyone involved. If no one has any objection, I would like to set the record straight.
While I am proud to say that Klipsch did indeed build speakers for me some years ago, at the moment they are not manufacturing the speakers I use. However, they are my friends and my relationship with them remains strong.
I have been in the cinema sound business for over 22 years. My company offers unique products and advanced methods for creating the finest motion picture sound systems in the history of the industry. They are also the most reliable. Not a single driver in any HPS-4000® system has ever "blown" or been damaged by the demands of any soundtrack. I do not believe that any other sound system manufacturer or designer could make such a claim. In addition, of the 17 major "B" chain improvements introduced in these 22 years, from three-way and four-way speakers, to screen speakers with beamwidth correction for the screen, to the first feature film exhibited with digital sound, I have introduced all 17 of them. The industry as slowly copied about 10 of these enhancements so far, though often taking 10 to 15 years to do so. Some may wish to quibble, but those are the historical facts. The complete list of these firsts can be seen on my web site.
The sound systems we provide are each designed for their specific auditorium. Not only is each system carefully designed to match the space it's in, each system, wherever in the world it may be, is personally set up by me to ensure maximum performance and smooth frequency response. The setup and tuning methods used have been developed and improved over the past two decades and represent the most predictable, reliable and (most importantly) repeatable methods in use today. Typically, the "tuning" portion of the setup procedure takes about 20 minutes.
I was once involved with a demonstration of a new cinema processor. The client wished that it take place in a theatre with one of my sound systems. The setup time for that house had been the usual 20 minutes or so when the installation had been done. I arrived and did the setup for the new processor in about five or ten minutes. The settings were recorded. Then the processor company's chief studio technician gave it a try. He spent the next 2 1/2 hours "tuning" the system and, when finished, was quite satisfied with the measurements he had made as well as the work he had done. Then he, I, the senior executives of the processor company, their entire technical staff and a senior post production executive / film mixer all went into the auditorium and listened to their films with both my settings and their own. Their unanimous choice, after only one hearing, was to use my settings for the actual demonstration, settings arrived at in a mere few minutes -- not two to four days!
I have demonstrated these methods to audiences of industry professionals -- hundreds of people -- and compared them with typical practices. No one has ever contested the methods or findings. Indeed the SMPTE review of one such demonstration said "John explained and audibly demonstrated why these conventional formulae [tuning methods] may be flawed, resulting in unreliable and unnatural acoustic results. Several 2-3 minute scenes from recently released popular movies were screened, each with the system tuned to various industry-standard configurations. The final screening for each segment was tuned using John Allen's approach, yielding dramatic improvement in audible clarity, bass response and presence. In most cases, the audience heard sound elements such as birds, wind or rain which had not been discernible in the previous screenings. Audience reaction to this demonstration was enthusiastic."
John F. Allen
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John F. Allen
Film Handler
Posts: 54
From: Newton, MA
Registered: Nov 1999
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posted 12-16-2002 06:51 PM
Dear Mr. Fowler:
Apology accepted. Though we have never met, based on your contributions to this forum, I have considered you a gentleman and have often learned from what you had to say. Hence my surprise when I read your post.
I do not remember the question you referred to at the Boston ITEA seminar. However, based on what you remember, I believe that I was referring to my practice of doing the first four theatres of a 16 plex and then starting over. The reason for this is to ensure that all the theatres in the complex will have the same tone. Since every acoustic environment is different to some degree, I feel that I have obtained a good "handle" of the tone and acoustic behavior of the rooms in a complex after I have heard my reference materials three or four. In the event that this learned experience may allow me to make further refinements to the theatres I've already done, I revisit those auditoriums for another listen. When all the theatres are done, we will run film in all of them at once, if possible, and listen to all of them once again. I do not consider the job complete until all the theatres sound the way they should with the same tone, and until all the properly recorded films can play in all formats with the faders at 7.0 or 0.0 when the levels are correct in the theatres.
If allowed to work uninterrupted, a 16 plex can be done in a few days, both "A" chains and "B" chains. However, if I happen to be on the job before the complex opens, there are constant interruptions, as everyone knows, and the work typically takes twice as long or more to complete.
John F. Allen
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Mark Gulbrandsen
Resident Trollmaster
Posts: 16657
From: Music City
Registered: Jun 99
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posted 12-16-2002 07:40 PM
Having installed quite a few systems in past years with John as I'm sure he will acknowledge, I would have to say that I was never satisfied, or really impressed with the low end performance. It was always boomy, not the deep bass that I prefer. In fact one theatre repeatedly received written complaints from patrons as to the loud bass problem. While I hardly concur with all patrons as to loud sound, the fact goes that when someone takes the time to actually write a letter, they may have a ligitimate complaint. I assume that these letters were answered by the chain but that part was out of my hands at the time. I once EQ'd a large Klipsch system with my R-2, and all agreed that it was far better sounding than before.
Getting back to deep bass.....physically I doubt that the large horn can do much around 30 hz anyway, or even below 60 hz. The high end definition also left alot to be desired at least for me, and dialog on ocasion sounded like it was eminating from a horn. Cup your hands around your mouth and talk.....it sounded like that to some extent and seemed dependent on the film being shown.
After hearing a really good direct radiator system at Todd-AO studios and chatting with the mix down engineers, and then seeing/hearing a movie at the Hollywood Chineese #2 I was very convinced that the direct radiator/biradial horn/etc was a much better way to go. Great dialog, and all the nuances that I never heard come thourgh in the HPS system. In a nutshell, this has been my experience over the last 20 years. I have yet to be steered back to horn loaded systems. While his horn loaded systems may be a bit more efficient (although he certainly specs in as many power amps as a normal direct radiator system has) my impressions still stand as to the boominess and lack of high frequency detail. Granted these were systems that were done in the 80's...not his systemns of today. Perhaps they are better in these departments now. But I still have alot of respect for John and his systems have definately earned a place in the industry. What everyone out there needs to realize is that sound is a very subjective, personal thing for everyone. There really is no right or wrong system, or right or wrong way of reproducing sound. It all what you like!
Mark
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Buy the stuff from Klipsch and save some money. I wish cinema owners would give me 2 to 4 days to tune one room....sorry you can't open this multiplex for 3 to fours weeks while I tune. ![[Wink]](wink.gif)
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