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Old 20th January 2006, 05:54 PM   #1
2litre is offline 2litre  United States
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Default Driver mounting.

Hi All,

I'm curious how certain drivers are designed to be mounted. The deeper I get into DIY the more apparent things become but also the question "why?".

I just recieved a pair of FR125S's and they obviously are designed to be mounted in a shallow resess so they become flush with the front baffle.
In general, what effect on the sound quality would happen if I either mounted it straight atop the baffle or mounted it from the back of a baffle. (I have a Jasper Jig and router and will do it right but, what if...)

Other drivers I've seen look to be designed to be rear mounted from the back of the baffle. They usually have very thick, layered fiberboard 'pads' covering the the cone to frame edge. Often they aren't round but have small ears or bumps where the mounting screws go.
Do you mount them from the rear for optimum results and then round over only the front cutout edge? What if you just mount them atop the baffle without building up around them?

Some drivers could go either way. Like the NSB's or the 3" Goldwood tweeter/high freq (PE). They have a real thin mounting flange (top mount?) but also have phenolic or compression ring that makes you think they should be mounted to the rear of the baffle.
Does it realy matter on these mid and high frequency drivers? Maybe the 3/4" depth difference gives you the ability to affect imaging and delay?

I've read that these little things make audible differences in the sound quality. What are some good rules of thumb?

R/

Jim
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Old 20th January 2006, 06:02 PM   #2
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Surface mounting a driver designed for flush mounting can have a significant effect on the sound. Diffraction effects can cause big variations in the frequency response.
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Old 20th January 2006, 06:39 PM   #3
2litre is offline 2litre  United States
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gfinlayson,

So I guess it would be even worse for a driver that looks to be designed to be mounted from the back of a baffle?

The FR125S has a relatively thin, nicely built flange. The drivers with the heavy fiberboard pads have a much thicker hump to overcome.

Say you're using 3/4" ply, would you mount them from the back and then use a 1/2" roundover on the front cutout face?

R/

Jim
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Old 20th January 2006, 07:16 PM   #4
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Jim,

For drivers designed to be mounted from the rear, a decent roundover would be a good idea. This will help to reduce diffraction. The worst scenario is leaving a square edge.

Adjusting the depth of a tweeter relative to a woofer can help with time alignment and improve the relative phase differences at the crossover frequency. The same effect can be achieved by tilting the baffle backwards a few degrees.

Depending on how well the crossover is designed, the crossover can have a significantly greater effect on relative phase than simply moving the tweeter a little.

I'd mount the Fr125s flush as they're intended to be mounted. I would expect that mounting them from the rear of a 3/4" panel will cause big diffraction problems around 3000 - 4000 Hz.


This article gives a bit of explanation and quantifies the effects of surface mounting.


Graeme
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Old 20th January 2006, 08:01 PM   #5
2litre is offline 2litre  United States
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Graeme,

Thanks for the input so far.

I was actually asking because I also own some old drivers that look like they should be mounted from the back of the baffle, like this:

Prometheus rebuild

I already know that when I start to use my FR125S's they will be mounted slightly recessed, like the CNC baffles enclosed in my kit suggest they should be. Should I decide to use bigger baffles I'll just replicate the cutouts using my Jasper into the new baffles.

At the risk of sounding dumb or flippant, if Bastiani mounts their large driver from the back and chamfers or rolls over the outer edge, then this must be a good idea as far as sound quality and freq response goes?

-my current pics are to big to post. I'll shoot some smaller files and then see about posting them so we can see what I'm talking about-

R/

Jim
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Old 20th January 2006, 09:03 PM   #6
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With large drivers rear mounting is OK. It's on smaller drivers that the problems occur.
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Old 20th January 2006, 09:13 PM   #7
Vikash is offline Vikash  United Kingdom
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Large drivers (bass) have wavelengths that make flush mounting irrelevent. Drivers which produce mid to high frequencies should be flush mounted to avoid edge diffraction of the driver edge and rounding the cabinet edges will tame edge diffraction too. The larger the roundover radius the better.

Defintely flush mount full range drivers like the FR125S.
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Old 21st January 2006, 01:24 AM   #8
2litre is offline 2litre  United States
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Thanks Graeme, hopefully the pic I attached works. It is of the older drivers I'm talking about.

Notice on the 12" and the 8" there is a thick fiberboard pad. On the 3.5" the pad is only 1/16" tall. With the metal and the pad it's a touch over 3/32" thick.

I know this is an odd presentation but hey, it's all about DIY and learning

Thanks Vikash for pointing out that I shouldn't worry about back mounting the 12". It's crossed at 125Hz. What about the 8"? It rolls off at 3K.

R/

Jim
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File Type: jpg old drivers.jpg (58.9 KB, 181 views)
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Old 21st January 2006, 01:29 AM   #9
2litre is offline 2litre  United States
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All,

Here is a pic of my take on the Visaton NoBox.
I'm using 3/4" 7ply Birch and old console pull Alnico drivers.

They sound nice and are cheap enough to let me learn about all the details of DIY speakers without it really hurting my wallet. Yet!

So now you see why I was asking about different types of drivers.
R/

Jim
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File Type: jpg nobox9.jpg (47.4 KB, 186 views)
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Old 21st January 2006, 07:04 AM   #10
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The 8" won't be a problem either. At 3khz, the wavelength is only 4". So coming from an 8" driver you won't have a problem. In any case the driver will start to 'beam' at about 1720 Hz, so edge diffraction from the cut out won't be a problem.
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