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» Film-Tech Forum ARCHIVE   » Operations   » Digital Cinema Forum   » Do high-frequency speakers deteriorate over time? Or is it just my ears? (Page 3)

 
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Author Topic: Do high-frequency speakers deteriorate over time? Or is it just my ears?
Sam D. Chavez
Film God

Posts: 2153
From: Martinez, CA USA
Registered: Aug 2003

- posted 03-25-2012 06:11 PM      Profile for Sam D. Chavez   Author's Homepage     Send New Private Message       Edit/Delete Post 
The Dolby screening room was very well done, but there were no tricks of any kind. Store bought CD horns, HF drivers, bass bins etc. The idea was to show what could be done with off the shelf audio components.

The wall was built with no extra holes so even the old style Tru-Roll masking and curtain motors operate silently.

There is a matter of superb and somewhat adjustable reverb via bats in the wall spaces. More live for some apps, less so for film.

Yes, the wall does allow you to hear the precise imaging of each speaker as I mentioned previously.

The wall does work. It was not even a new idea to THX, Western Electric did a version of it many years ago. The fiberglass padding just makes it a little better.

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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 03-25-2012 06:37 PM      Profile for Manny Knowles   Email Manny Knowles   Send New Private Message       Edit/Delete Post 
I'm sure that Tom (Holman) could list many benefits of the THX baffle wall, but the two that I always recall him mentioning first were the two reasons Sam cited -- mitigation of behind-the-screen reflections and improved stereo imaging.

I recall Dolby produced a publication called "Guideline for Dolby Stereo Theatres" or something like that. They recommended draping the screen loudspeakers in heavy fabric and ensuring that the fabric created a kind of "seal" between each speaker and the screen.

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Trevor Anderson
Film Handler

Posts: 5
From: Fadden, ACT, Australia
Registered: Aug 2004

- posted 03-25-2012 06:47 PM      Profile for Trevor Anderson   Author's Homepage   Email Trevor Anderson   Send New Private Message       Edit/Delete Post 
Hi folks

Yes it is sadly true that our hearing degrades over time. But a sudden decrease in performance from your sound system sounds more like an environmental issue.

Does the return air from your HVAC system suck through the screen, or generally pull air towards the screen end? If so this will lead to a more rapid build up of dust in screen perforations. Likewise your masking may be the issue. If it is getting clogged up with dust it will affect your 1.85 screenings (assuming your masking covers the L&R speakers at 1.85).
At an extreme, I had a call from a complex once that their sound was very tinny in one cinema. Went in to check and someone had stolen all the LF speakers out of the boxes.....tinny sound indeed!
Have you recently changed your screen?
Different screens have different performance characteristics for sound. So you it is recommended to do a new B chain when replacing the screen. (All those silver screens going in everywhere!!!)

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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 03-25-2012 06:58 PM      Profile for Manny Knowles   Email Manny Knowles   Send New Private Message       Edit/Delete Post 
OK -- I just checked that "Dolby Theatre" publication -- Sections 2.2.3 talks about baffle walls and 4.1.5 offers suggestions in the absence of a baffle wall -- that's the section where the heavy draping fabric is mentioned.

The Dolby publication can be found here in the F-T warehouse -- which is awesome, since I lost my hard copy years ago.

quote: Trevor Anderson
Does the return air from your HVAC system suck through the screen, or generally pull air towards the screen end? If so this will lead to a more rapid build up of dust in screen perforations.
There's a wonderful story about this very thing happening at the Chinese theatre in Hollywood. The screen perf's had gotten clogged with popcorn grease and to compensate, over time, the HF was gradually cranked to the point where the HF drivers were destroyed. As the story goes, this was discovered after somebody "phoned THX" when "Godfather III" was being shown there.

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Chuck McGregor
Film Handler

Posts: 47
From: Bremen, ME, USA
Registered: Mar 2012

- posted 03-25-2012 07:19 PM      Profile for Chuck McGregor   Email Chuck McGregor   Send New Private Message       Edit/Delete Post 
Humidity is somewhere between about 5% and 40% can cause a loss of "crispiness." If it has it, failed humidity control in an HVAC system could cause this. The larger the theater, meaning longer "sound lines," the greater the potential problem.

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Louis Bornwasser
Film God

Posts: 4441
From: prospect ky usa
Registered: Mar 2005

- posted 03-25-2012 09:11 PM      Profile for Louis Bornwasser   Author's Homepage   Email Louis Bornwasser   Send New Private Message       Edit/Delete Post 
According to JBL, ALL SPEAKERS require the baffle wall to make spec. This is not limited to Cinema; all speakers. From the smallest to the largest are tested on an elevator at JBL that uses the roof of the building as a sound wall.

"Bookshelf speakers" require the books to make spec. I once installed a cinema system in a theatre where the owner had "every classical LP ever made." They were stored numerically behind the cinema screen. We used the mass of the record albums as a sound wall.

btw: Thx promoted a sound wall; too bad it was only just a copy of the old Western Electric sound wall of the 1930s. Louis

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Mike Blakesley
Film God

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From: Forsyth, Montana
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- posted 03-25-2012 09:12 PM      Profile for Mike Blakesley   Author's Homepage   Email Mike Blakesley   Send New Private Message       Edit/Delete Post 
Baffle wall questions.

How tall does it need to be above the speakers? All the way to the ceiling?

And:
quote: Chuck McGregor
So to be effective to say 50 Hz the baffle/enclosure width must be about 10 feet centered around the loudspeaker.
Our screen is wall to wall...how does that effect a baffle design?

Currently we have this "blanket" made out of 1/2" insulating material hanging behind the screen and surrounding each of the speakers. When the insulation showed up it was white in color, so we painted it black to minimize reflections. (I don't know if that was necessary or not.) Then later, somebody told me that the paint probably negated any advantage of the blanket. I don't know if that's true or not and I don't know if the blanket does any good or not. I will say that I don't remember noticing a huge difference in the way things sounded after we put the blanket in (sometime in the late 80s, I think it was).

I would like to get rid of the blanket and put in a proper baffle, for sure.

TO answer the HVAC question, the returns are above the screen. The outlets are all in the ceiling and no air is sucked thru the screen. Our heat is hot-water so we don't use the A/C fans except in hot weather.

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Chuck McGregor
Film Handler

Posts: 47
From: Bremen, ME, USA
Registered: Mar 2012

- posted 03-25-2012 10:56 PM      Profile for Chuck McGregor   Email Chuck McGregor   Send New Private Message       Edit/Delete Post 
A wall-to-wall baffle is ideal. The theater I work with is multi-purpose with stage productions so a wall-to-wall is not an option, needing a clear proscenium opening.

I would agree that painting probably did the absorptive properties of that white material no good at all.

Regarding JBL loudspeakers, it depends which specification one is referring to. They test and specify sensitivities and frequency responses for both half and whole space, depending on the loudspeaker. To my knowledge they always state which. However, they specify beamwidth/directivity and impedance from whole space data. Professional loudspeakers are normally specified in whole space (mostly) but sometimes both and this includes professional bookshelf size (they don't need the books to meet spec). Cinema and ceiling loudspeakers are exceptions. I have no idea how consumer loudspeakers are specified, bookshelf or otherwise...and probably don't want to know! [evil]

quote: Mike Blakesley
I don't remember noticing a huge difference in the way things sounded after we put the blanket in
This would not surprise me, even if it were today. One of the things about loudspeaker systems that is a relatively unknown characteristic and certainly not specified is their resolution. Even some otherwise very good sounding loudspeaker systems can be fairly low resolution and that can mask subtler improvements that might be more readily apparent with a high resolution system.

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Christos Mitsakis
Expert Film Handler

Posts: 242
From: Ag.Paraskevi, ATHENS, GREECE
Registered: Sep 1999

- posted 03-26-2012 10:51 AM      Profile for Christos Mitsakis   Email Christos Mitsakis   Send New Private Message       Edit/Delete Post 
This is something of interest:

the THX patent

For the drawings see the accompanying pdf file

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Chuck McGregor
Film Handler

Posts: 47
From: Bremen, ME, USA
Registered: Mar 2012

- posted 03-26-2012 12:13 PM      Profile for Chuck McGregor   Email Chuck McGregor   Send New Private Message       Edit/Delete Post 
From the patent:
quote:
The sound absorptive means is frequency dependent and has a high frequency absorption characteristic complementary to the high frequency reflection characteristic of the screen, whereby the absorption increase as the reflection increases with frequency.
As I mentioned, you can't just apply some absorption if you want to do it correctly. The frequency dependent behavior of what you're doing is what you have to figure out to avoid unintended consequences. This is not a trivial task and the patent, of course, doesn't say how.

Some of the crossover stuff in the patent is often repeated today in various ways and still makes no sense.

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Sam D. Chavez
Film God

Posts: 2153
From: Martinez, CA USA
Registered: Aug 2003

- posted 03-26-2012 01:10 PM      Profile for Sam D. Chavez   Author's Homepage     Send New Private Message       Edit/Delete Post 
I think too fine a point is being placed on how much absorbtion is needed on a screen wall.

It's only used to mop up HF reflections off the back of the screen. In a perfect world all the audio would get through and there would be nothing to absorb. Also, each brand and type of screen is slightly different as to thickness, hole size...

An inch or two of black furnace duct has been standard practice for many years now.

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Dave Macaulay
Film God

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From: Toronto, Canada
Registered: Apr 2001

- posted 03-26-2012 01:48 PM      Profile for Dave Macaulay   Email Dave Macaulay   Send New Private Message       Edit/Delete Post 
Exactly. The quoted paragraph says just about nothing. Any acoustic treatment will be frequency dependent, LF absorption is much more difficult than HF absorption. The localization improvement comes from reducing the reflected HF from the screen back, the reflected sound has both source point and time delay variations compared to the direct sound from the horn. With some duct liner on the baffle wall, this reduction is accomplished: complete eradication is not required - psychacoustic perception just requires a much lower level than direct sound. The localization difference is very noticeable between a typical installation with no baffle wall and a THX system.
The wall also produces much cleaner LF reproduction. This isn't a localization problem, just having the LF radiators in half space improves their response and efficiency.

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Marco Giustini
Film God

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From: Reading, UK
Registered: Nov 2007

- posted 03-26-2012 03:44 PM      Profile for Marco Giustini   Email Marco Giustini   Send New Private Message       Edit/Delete Post 
Chuck,

Thanks for your post, very interesting indeed.
It would be nice to learn a bit more about sound analisys. RTA analisys is somehow a non comprehensive way to do it, since it does not keep into account the time variable.

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Chuck McGregor
Film Handler

Posts: 47
From: Bremen, ME, USA
Registered: Mar 2012

- posted 03-26-2012 06:38 PM      Profile for Chuck McGregor   Email Chuck McGregor   Send New Private Message       Edit/Delete Post 
To give some insight into some of that analysis, here's one of the tools I use. (The author was my boss for 7 years.) This would be an ideal tool for analyzing the effects of absorption on resolution. I used this to optimize the DSP for my screen loudspeakers.

Transient Analysis

The other half of the pie is measurements. Here's a paper I co-wrote (blush) on one aspect of that subject:

Directional Response Measurements (pdf)

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Robert Throop
Master Film Handler

Posts: 412
From: Vernon, NY USA
Registered: Jun 99

- posted 03-26-2012 10:46 PM      Profile for Robert Throop   Email Robert Throop   Send New Private Message       Edit/Delete Post 
I used to run a theatre that had been converted from a grange hall.The screen was about 1 ft. from the wall. The mono speaker (an Altec A5) was in a doghouse so there was a very effective baffle wall. The wall behind the screen was covered with fiberglass insulation. I had one customer who insisted there was a subwoofer.

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