Flush Mounting Rear Port Monitors
ausrock said:
Can you give some detailed advice on flush mounting monitors with rear ports (i.e. Tannoy Reveals) as I have read your posts on an enclosure for flush mounting but I presumed it would be different with rear ports.
ChrisO,
As you see from my posts, the main point of flush mounting is to increase the baffle size of the monitor. You can still do this with rear ported speakers, but there are a few problems you might run into with the design: 1. You cannot have a rear cavity to the mounting otherwise you will isolate yourself from the acoustic output of the port. 2. You don't want standing waves or enhanced bass loading behind the baffle. 3. You don't want to create coupling problems between the port and the woofer. I'll explain these further.
1. No rear cavity - The problem with this one is self evident. The baffles must be "free floating". You can either suspend them from the ceiling or from an open frame mount.
2. Standing Waves and Loading - The attached file shows a floating baffle which is pulled away from the rear wall. This helps reduce loading effects from the quasi cavity made by the room boundaries and the baffle. It's tilted in both the horizontal and vertical directions to minimize standing waves. It might also be worth while to put acoustic foam and the rear of the baffle if the walls are no treated.
3. Woofer/Port Interaction - In order to understand this it's good to know basically how the port and speaker interact so you can avoid running into problems.
The attached file also gives a schematic representation of the acoustic output of a ported speaker. The green curve is the total output, the blue curve is the speaker cone, and the yellow curve is the port. You can see at frequencies near the lower cutoff most of the output comes from the port. For about an octave above that the port and speaker cone work in conjunction. The port and cone are in phase and since the box is small relative to those wavelengths their sum is constructive.
The problem you can run into when adding a baffle is that you increase the path length between the port and cone. If the path length corresponds to roughly 1/4 wavelength anywhere in the frequency band where the port and cone are working together it will result in destructive rather than constructive interference between the two. This will cause a frequency response dip(s) around the associated frequency(s).
The best way to minimize this is to make the baffle rectangular rather than square (circular would be the worst case) and mount the monitor slightly off center horizontally and vertically. Also you want to avoid having the baffle dimensions close to a 1/4 wavelength of a frequency very close to where the port and cone share the output equally. This could result in the largest potential response dip.
Without measurements all of this would be guessing. I can explain how to collect rough but useful data with a microphone and wave editor with a spectrum analyzer. In any case there will probably be some compromises in the design. Then, in an average room there are always problem and compromises. You might make things worse, but you could make them better. It's worth a try.
Hope this helps
barefoot
