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This topic comprises 3 pages: 1 2 3
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Topic: Sound Question what is X curve?
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Cameron Glendinning
Jedi Master Film Handler
Posts: 845
From: West Ryde, Sydney, NSW Australia
Registered: Dec 2005
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posted 10-10-2006 04:07 AM
Ok, Over the last 2 weeks I have asked a few people with life time worth of experience in the professional audio their opionon on the x curve.
So far on the phone I have talked to
1. John Bernet founder of Lenard Audio. Who were the world pioneers of the high power amp. When the rest of the world was making amps in the 10's of watts he was producing and selling amps in the hundreds of watts.
2.Fred Pickering, Who for decades worked in TV sound at Australias ABC Television. Fred is the sort of person who use to send the sound recordists to the school of the blind as work experience, he was the head of sound there when the now famous Neville Theele was the head of video tape there ect.
3.Richard Prittle has been contacted, designer of the Aussie Monitor Cinema Speaker range and even designed Dolby mixing suites ect
4. I am also keen to talk to a person who mixes movies for a living to see if they actually try to compensate for them ect.
One of the above is a friend of Tom Holmen and ultimatly his opinion will be asked as well.
First to respond is John Bernett who has given permission for this email to be reproduced here on Film-Tech. He has been researching the history of it.
The mysterious x curve
A variation of the x curve was originally described as the Academy curve, which came about as a convoluted entanglement from many different directions.
First we must understand that cinema evolved before radio and Television. Cinema was globally successful as the number one entertainment for 20 years before electronics evolved to enable sound systems to exist. This is the primary reason sound has always taken second place. We must also remember that the cinema industry has had an obsession that any new technology had to be backwards compatible, meaning that it was and still is contained by its past. This not right or wrong but simply evolution.
Sound appeared as a miraculous achievement and was applied when electronic technology was unfolding so rapidly that there wasn't time to for cinema to grow with it. Many of the decisions at that time were done with the best intentions to achieve consistency.
Before sound systems existed the largest auditoriums were cathedrals and the only way to increase the sound level was by controlled reflection of reverberation. Reverberation was therefore essential to maintain consistent sound level but it came at the cost of decreasing articulation and intelligibility. Early acousticians began with an imaginary ideal as a 100% reverberant environment enabling sound to be heard at the same level throughout the auditorium or cathedral.
Any sound absorbed was regarded as a loss described as 'room loss'. The aim was to provide minimum 'room loss' with zero loss of intelligibility which of course is a contradiction. Any reverberation reduces intelligibility. Therefore what evolved was the imagined ideal of correct reverberation. That is, reflective area around the stage of short distances opening up to larger area of longer distances. However this is not the case with cinemas because of the large screen.
It was noticed that the worst effect on destroying intelligibility was hearing distinct echoes reflected from walls and ceiling at large distances apart, compared to reverberation reflected from walls and ceiling at short distances (e.g. bathroom).
Lets compare a small and large reverberant auditorium made from the exactly the same building materials, and we fire a cap gun. Assume a period of 1 second in which sound will travel 1125ft (344m). In the small auditorium average distance between the walls and ceiling is closer, compared to a large auditorium. Therefore in one second the sound from the cap gun will strike the walls and ceiling more often in a small room compared to a larger room. This is described as 'Mean free path' or 'Mean free time'.
Each time sound strikes a wall or ceiling a % will be absorbed and a % will be reflected and so on and so on. Therefore in a small auditorium -more- sound will be absorbed by the walls and ceiling simply because sound strikes the walls and ceiling more often compared to a large auditorium, for the same period of time. However in most auditoriums the high frequencies are more readily absorbed that the low frequencies. Which means that in larger auditorium the echo reverberant sound will appear to be brighter.
The second explanation for why the Academy curve exists is to do with noise masking.
The first sound tracks on film were basic optical and the technology that existed at that time could only enable 2K - 6K max to be obtained. The first problem was the very noisy hiss from the optical track and the crude valve pre-amplifiers. Putting a filter at the output of the pre-amplifier reduced the hiss but also reduced the small amount of high frequencies information available from the film stock. The simplest solution was to boost the high frequencies when recording and allow the noise reduction filter to give a close to flat response again when played back. This became called the 'Academy Characteristic'.
The 3rd explanation for why the Academy curve exists is to do with speaker horns.
The early speaker systems consisted of a 12in or 15in speaker for the low frequencies and a compression driver and horn for the higher frequencies. Crossover at approx 700Hz which mostly consisted of a simple capacitor only. The compression driver and horn were approx 20dB (50 - 100 times) more efficient that the 12in or 15in speaker. The understanding how to correctly equalize both components so as to sound flat was very difficult as only a very few laboratory had the basic and crude test equipment to calibrate and acoustically align the drivers. The majority were aligned by ear and this was always in favor of the upper end of the voice. Also valve amps at that time were mostly 30 Watts or less. Often the horn was left almost at full efficiency so it could be loud enough for lost of people to hear with only a small amplifier power. The Academy curve also helped equalize the extreme level of the horn.
Conclusion.
We have to understand we are now living in a new century. Sound system power and fidelity is now unlimited, but unfortunately the will to act decisively is limited. Digital formats allow for a flat response 20Hz - 20KHz with almost zero noise. Electronic amplifying circuitry is now virtually silent. Auditoriums can now be made close to anechoic with almost zero reverberation and echoes. There is no longer any need for the Academy x curve or any external equalizing curves in the new millennia. We now have the technology and engineering skill and un-limited building resources to make every commercial cinema acoustically free of echoes and reverberation at all frequencies. Traditional cinema now belongs at home as 'Home cinema' where it can be trivialized and de-valued to any level we wish. But for the commercial cinemas survive in the new century they will have to be fully digital and provide the highest level of projected entertainment for mass audience. But to do so the industry has to let go of the past and invest in the technology that will enable it to maintain its position as the leader of mass entertainment
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Cameron Glendinning
Jedi Master Film Handler
Posts: 845
From: West Ryde, Sydney, NSW Australia
Registered: Dec 2005
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posted 10-10-2006 07:13 PM
Gordon, Johns research is not finished and he is very interested in learning more, your and other Film Tech members input is very useful to help demystify what appears to others, who do not work in cinema industry, an unusual practice.
Personally in the 90's when I was building speakers with wings/ baffles / modifying cinema speaker systems, I had the opportunity to get our 3 different service providers to each do an eq of the same auditorium, with very different results. So I have a good idea of the acceptable variation within the curves. It is not subtle. The way I deal with it was to only use the company that had provided the best and most natural sounding results.
Having worked in over 50 cinemas so far, I can also say that most auditoriums do not have good acoustics. With Digital Cinema promising uncompressed or IMAX quality audio, should cinemas start copying their dampened / dead auditoriums, Do IMAX use the X curve? What could be done to make it more consistent? if anything?
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