I believe the primary reason is for phase alignment. If you go with the idea that the vast majority of acoustical energy is focused where the voice coil meets the cone and attempt to line up those points between drivers you often end up either tilting the cabinet back or mounting the tweeter on a seperate baffle that is further back than the main baffle containing mid/woofer.
If you make a side view drawing of your baffle and drivers, all mounted on a straight baffle perpendicular to the floor, you will notice that if you draw a line connecting the ends of the voice coil of each driver, and accept that line as representing the most phase coherent plane of the driver group, that the line tilts downward and thus many argue that the best sound your speaker has to offer is being sent to your lap rather than your ears. If you mount the tweeter below the mid or mid/bass driver then the line will tilt up, making the sound more coherent between a sitting and standing position in the room and/or allowing you to make your baffle shorter compared to the more traditional vertical alignment of mid above woofer and tweeter above mid.
A second potential benefit is that mounting a woofer up higher in the baffle and more towards the center of the room may produce a more even bass response as floor bounce and subsequent nodes may become less evident in ther adverse effects. Dynaudio once made a statement loudspeaker that had a very low mounted tweeter, I believe just a couple of feet off the floor, followed by an upper mid above it, with a lower mid above that, and then one or two final bass drivers with the largest driver at the top of the cabinet with was around 4 feet off the ground. It looked like a giant upside down speaker. It sounded great (I actually listened to it at some upscale audio store in Pennsylvania during a business trip) but I am guessing it had dissapointing sales because as you well know, that vast majority of audio equipment sales have little to do with performance and musicality and much to do with status and aesthetics.
There are some great examples of the tweeter mounted below the mid or midbas that come to mind, one being the PSB line from Canada wich has used this driver alignment technique to good effect, the other being the well received mini-monitors from the British brand Mission. There are many others but those two are the first to come to mind....
If you make a side view drawing of your baffle and drivers, all mounted on a straight baffle perpendicular to the floor, you will notice that if you draw a line connecting the ends of the voice coil of each driver, and accept that line as representing the most phase coherent plane of the driver group, that the line tilts downward and thus many argue that the best sound your speaker has to offer is being sent to your lap rather than your ears. If you mount the tweeter below the mid or mid/bass driver then the line will tilt up, making the sound more coherent between a sitting and standing position in the room and/or allowing you to make your baffle shorter compared to the more traditional vertical alignment of mid above woofer and tweeter above mid.
A second potential benefit is that mounting a woofer up higher in the baffle and more towards the center of the room may produce a more even bass response as floor bounce and subsequent nodes may become less evident in ther adverse effects. Dynaudio once made a statement loudspeaker that had a very low mounted tweeter, I believe just a couple of feet off the floor, followed by an upper mid above it, with a lower mid above that, and then one or two final bass drivers with the largest driver at the top of the cabinet with was around 4 feet off the ground. It looked like a giant upside down speaker. It sounded great (I actually listened to it at some upscale audio store in Pennsylvania during a business trip) but I am guessing it had dissapointing sales because as you well know, that vast majority of audio equipment sales have little to do with performance and musicality and much to do with status and aesthetics.
There are some great examples of the tweeter mounted below the mid or midbas that come to mind, one being the PSB line from Canada wich has used this driver alignment technique to good effect, the other being the well received mini-monitors from the British brand Mission. There are many others but those two are the first to come to mind....
The rational for a tweeter below the mid or mid/bass
is phase and/or time alignment.
On simplistic level :
The acoustic centre of the mid or mid/bass is behind the plane of
the the tweeter. The inverted configuration is aligned at an angle
above the speaker, ideal for a low standmounter or floorstander.
This is the theorectical justification.
On a more practical basis :
One disadvantage is you are necessarily listening well off the
tweeter axis and the top octave suffers. It is often simpler to
manipulate the phase at crossover to allow for the offset of
driver planes than physically compensating for the offset.
Inverted mid and treble is often used in 3 ways to allow the
expedient of using a single panel to form a triangular section
enclosure for the mid unit at the top of the cabinet.
Overall :
It certainly cannot be treated as a "general rule".
The speaker should be designed with this configuration
in mind to maximise off axis performance.
sreten.
is phase and/or time alignment.
On simplistic level :
The acoustic centre of the mid or mid/bass is behind the plane of
the the tweeter. The inverted configuration is aligned at an angle
above the speaker, ideal for a low standmounter or floorstander.
This is the theorectical justification.
On a more practical basis :
One disadvantage is you are necessarily listening well off the
tweeter axis and the top octave suffers. It is often simpler to
manipulate the phase at crossover to allow for the offset of
driver planes than physically compensating for the offset.
Inverted mid and treble is often used in 3 ways to allow the
expedient of using a single panel to form a triangular section
enclosure for the mid unit at the top of the cabinet.
Overall :
It certainly cannot be treated as a "general rule".
The speaker should be designed with this configuration
in mind to maximise off axis performance.
sreten.- Status
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