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This topic comprises 6 pages: 1 2 3 4 5 6
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Topic: Do high-frequency speakers deteriorate over time? Or is it just my ears?
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Marco Giustini
Film God
Posts: 2713
From: Reading, UK
Registered: Nov 2007
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posted 03-24-2012 06:33 PM
Why do you have your EQ re-done every six months? Properly tune a sound system requires a lot of time, and several subjective tests to obtain a good result. Unless this is done every six months, I would not recommend to bother a properly eq'd sound system every 6 months. Unless the same long procedure is done by - possibly - the same person.
Now, don't get me wrong, I'm not saying not to touch your sound systems for years. What I want to say is that I've seen too many people around checking a system in half a hour, placing ONE microphone 2/3 away from the screen, dialing the perfect X-curve and winding up the cables without testing the result.
That is not checking/equalising. That is asking for random results depending on the microphone position and auditorium acoustic.
What I'd do, is to have the auditorium properly eq'd, then maintain it every year: you check the levels (with the microphone in the same position!), and channels. If no major disruptions, happy days.
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Manny Knowles
"What are these things and WHY are they BLUE???"
Posts: 4247
From: Bloomington, IN, USA
Registered: Feb 2002
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posted 03-24-2012 10:53 PM
Hearing does tend to deteriorate over time -- usually starting with the highs. We did some informal testing in the Norris theatre at USC and most of the adults couldn't hear anything above 16kHz, whereas a few of of the younger "kids" could hear 17kHz.
Even so, your dialog should "suddenly" sound funky, unless you recently experienced some kind of trauma that could explain why your dialog suddenly sounds lousy.
There's an old saying that goes "if three people tell you you're drunk, lie down!" If you and several of your patrons are hearing this (all of a sudden) then there's a good chance that "it's not just you."
As Sam pointed out, don't rely on pink noise and your RTA -- it is possible to "tune" a busted speaker to the X-Curve. Amazing, but true.
This is where a test reel is worth its weight in gold. It should be a test reel containing dialogue (male and female) that you have heard before and you know what it ought to sound like when the system is functioning properly.
Dialog is important because we are very good at detecting poor reproduction of human speech. We don't *really* know what that dinosaur is supposed to sound like.
Even if you don't have DTS, I'd still recommend the DTS "Buzz and Bill Show" reel because it has dialog, it has a very quiet scene (from "Far and Away") and a very loud scene (from "Backdraft").
The Dolby "Jiffy" reel has sweep tones, but it suffers because the male voice (Ioan Allen) only appears at the very top of the reel (he reads the slate). And the woman's voice is a little too "smoky" for me. Distortion and hiss can easily hide in a voice like that. I've grown to really LOVE that goofy (Wendy Carlos) music that closes the reel.
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Chuck McGregor
Film Handler
Posts: 47
From: Bremen, ME, USA
Registered: Mar 2012
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posted 03-25-2012 02:11 PM
Greetings,
This is my first post having just joined Film-Tech after reading the Forums for the better part of a year. I am ever-so grateful for the all the knowledge I've thus far gained from the members and this site.
quote:
Cameron wrote:
A baffle wall or even just some very solid wings added to the side of a speaker box can make a huge difference to the sound.
Huge is a relative term, however it will make differences. The question is what ARE the differences?
quote: Cameron Glendinning
The wall controls the air so that the formation of the soundwave is far more accurite and realistic than when the wave is distorted as it wrapes around the box and then into the backstage areas.
I would say instead that the dimensions of the radiating device determine its power response. The power response determines how the sound from a loudspeaker (or any source) couples to a room. If the power response varies with frequency then, among other things, the direct-to reverbrant ratio will vary with frequency. With loudspeakers this invariably means it decreases with frequency.
A baffle wall or wings increases the directivity with decreasing frequency from the frequency where the normal beamwidth starts to exceed 180 degrees. The maximum increase is where the frequencies of the "unaided" beamwidth would normally be 360 degrees. This means the loudspeaker's axial output also increases with decreasing frequency from 0 dB to 6 dB.
This is ONLY true if the width of the wings plus enclosure is at least 1/2 the wavelength of interest to be effective. There is a sharp discontinuity below this frequency transistioning rapidly from 2 pi to 4 pi radiation. Above this point the frequency response of the system will change with increasing frequency until the natural beamwidth of the system is reaches 2 pi radiation. Above that the baffle will have essentially no effect on directivity. So to be effective to say 50 Hz the baffle/enclosure width must be about 10 feet centered around the loudspeaker.
However this is not all a baffle can do. As Cameron noted, a baffle helps keep sound from the loudspeakers from reaching areas behind the screen. Any of this energy that is reflect back into the audience, whether discrete or diffuse, can cause all kinds of unwanted audio anomolies. I've got such problems in the theater I work with.
Another thing baffles can do is improve the performance of horns. When a wavefront leaves the mouth of a horn, the change from the horn wall to the "open air" creates a discontinuity in the wavefront. This diconinutity causes some of the energy to be reflected back into the horn. Then this reflection also gets reflected back out the horn, obviously later in time. Some of this reflected energy is also reflected back into the horn and so on. The frequencies most affected are related to the length of the horn. This casues a buildup of energy over time creating a sonic effect commonly referred to as horn honk. Obviously if a horn is used only above those frequencies this is not an issue but this is rarely true.
A baffle wall or wings can mitigate horn honk IF, repeating IF..., there is a relatively smooth transition from the plane of the horn walls at the horn mouth to the baffle surface. The smoother the better. If there is a angle between the horn mouth (excluding any mounting flange) and baffle surface this benefit will be reduced or relatively non-existent.
quote: Cameron Glendinning
The use of fiberglass on the baffle wall also helps with seperation between channels.
This will also have a frequency selective effect on the linearity of the loudspeaker's power response and thus frequency response This will depend on the above effects and the material's absorption characteristics. At frequencies above where the normal beamwith of the the loudspeaker goes below 180 degrees, the absorption will have little effect. At frequencies below it will have an effect but it will increase then decrease with frequency. This is not something I would do without modeling its effects (and they can be modeled) in combination with the wall or wing effects on the particular loudspeaker.
Overall baffle walls or wings can be good things. A wall is most effective while wings can introduce more frequency selective effects and these effects must be considered. Some of the benefits will depend on loudspeaker design, others not so much.
I just happen to be in the process of determining the effects of adding wings to the loudspeakers in the theater I work with. One fly in the ointment in figuring this out is that the modeling must take into account a rather arcane "soffit mounting" effect as the wings will only be as tall as the loudspeakers. Just another parameter to take into account.
While the results may be well worth it, achieving optimum results, especially with just wings is not just a matter of installing wings and calling it a day. The "logic" behind it does matter in understanding the impact on the sound projection and therefore what to expect.
The best measurement system in the world (I use Smaart and a proprietary loudspeaker measurement and modeling program called Rayleigh) is only as good as the analysis of the data collected. One really should know what to expect by adding wings or a baffle wall in order to properly interpret the measured results and make the proper corrections for the expected differences. This is especially true if the results are spatially variable.
How to predict, measure, and properly correct for any of these effects is whole other story... ![[Smile]](smile.gif)
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