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Shown below are the relative responses of various mounting methods. The cost of surface mounting a driver is the ripple in the response and these curves show that negative effect. Note: ignore all the garbage below 150 hz in a few of the measurements below. My house's furnace kicked in during those.

Surface mounted tweeter with 1/8" flange, relative to flush mounted

Surface mounting a normal 4" diameter tweeter is the worst thing you can do. A ragged lower treble response is guaranteed, and it's going to be even worse on tweeters with thicker flanges.
 
 

Surface mounted tweeter, 2" square small flange 1/8" thick, relative to flush mounted

Surface mounting a small, square flange tweeter also has negative effects, though not quite as bad as a traditional size tweeter. You may have to investigate the response of the tweeter to see if this makes a poor response worse or if it works out in your favor.
 
 

Surface mounted woofer with 1/4" flange, relative to flush mounted

Surface mounting a woofer however is not so bad. Some woofers are designed for it by having rounded over flanges, but the one above is a squared off flange. The real issues to consider are 1)looks and 2)changes in acoustic center. Bringing the woofer forward on a flat baffle may help with some crossovers, depending on the order and phase connection of the tweeter among other things.
 
 

Flush mounted tweeter, effects of overlapping woofer w/.12" flange

Placing the woofer closer to the tweeter can allow a higher crossover point while maintaining a certain off axis degree of lobing. Surface mounting and overlapping the woofer over a flush mounted tweeter flange will get the drivers as close together as possible. The woofer frame edge does have some interaction with the tweeter, but it's minor. 1/8" thick woofer flange.
 
 

Flush mounted tweeter, effects of overlapping woofer w/.25" flange

As above, but this time with a thicker woofer flange. The effects of the edge near the tweeter worsen a bit but are still manageable. 1/4" woofer flange.
 
 

Flush mounted tweeter, effects of overlapping woofer w/.31" ridge

As above but this time with an even thicker flange. The effects worsen to +/- 1 db but are still manageable.
 
 

Flush mounted tweeter, effects of surface mount woofer spaced 1" from tweeter flange

And for comparison, this is a surface mounted woofer and flush mounted tweeter, but there is a 1" space between the flanges. There is almost no interaction between the tweeter's output and the woofer's flange.
 
 

So what does all this mean?
Never, ever surface mount a 4" flange tweeter. Other than that, you have a little flexibility and your choice of driver mounting is mostly influenced by looks and acoustic center issues. Even those issues are minor.

If the typical 1/4" you gain from surface mounting a woofer is critical to aligning acoustic centers and maintaining a listening axis height, the crossover point may be too high for a given center to center driver distance. Use a smaller midrange that can mount closer to the tweeter, or use a tweeter that can cross over lower.

Surface mounting a woofer is going to look ugly unless the woofer was designed for it. The Vifa TC series for example have a rounded over flange that looks best surface mounted. Looks may not be as important as the time and effort of a router countersink operation however.

There you have it, a little insight into surface and flush mounting. Don't forget other important response affecting issues. You might want to round over your baffle edges to minimize diffraction effects. You also might want to round over the inside of a speaker cutout which will smooth the response and limit compression in smaller drivers where a large magnet and thick baffle can restrict airflow.

Page done by John "Zaph" Krutke © 2005
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