new speaker baffle design concept...good or not?

[thadman]

I was thinking today about wide OB and narrow OB and the benefits of both (wide=later rolloff, narrow=less baffle reflections) and realized an idea.

First off, for this to work at all, I need to know if quarter wavelength foam will absorb the entire frequency (it wont pass through the foam). If thats true then I could use a deep foam baffle (absorb all baffle reflections, but maintain width) and use a wood frame and then cover the whole speaker in grille cloth so the foam wouldnt be visible.

24" wide baffle with 7" deep foam across the entire baffle (the speaker will be suspended with a frame)

the 7" (1/4=28") foam would absorb all baffle reflections down to about 480hz where the dipole cancellation would take care of the rest.

[nunayafb]

Some problems you might encounter:
1. Driver suspended by frame, this will allow the sound that passes through the foam to cancel (front to rear)- the foam is not 100% efficient.
2. Driver mounted on a tube (7"), the tube will behave as a transmission line and cause resonance. So it wont be true OB.
3. High frequencies will be highly attenuated- the reason for this is that the higher you go in freq. the more effective the damping will be, the sound will pass through the foam and convert to heat and not be able to reach the baffle and reflect back to the listener.
4. 3. leads to another problem, time delay. If the foam is good stuff it will most likely lower the speed of sound, that coupled with 7" for the sound to reach the baffle and then 7" to get back out this delay will most likely be audible at alot of frequencies.

[thadman]

I dont know where you got the idea that I was using a tube?

How efficient is foam at absorbing sound?

Hopefully, this is a better explanation of what im trying to achieve. The baffle would be in essence a frame (everything cut away that doesnt severely compromise the structural integrity) with a few supports that support the mdf ring holding the speakers.


I would build a compound tower, composed of 2 baffles (each 24" x 24"). The base baffle would be angled upwards slightly (5*-10*) and would house a 15" midbass driver crossed below 560hz (~24) as to eliminate all baffle reflections in its passband. The upper baffle would be a 24" wide x 24" tall x 9" deep structure. The baffle would be placed within this structure in the very middle (4.5" deep) and all unnecessary surface area would be cut away, as to minimize reflections and allow the waves to travel from the front sheet of foam to the rear. A sheet of 4" deep foam would be placed inside the structure in front of the baffle, and another 4" sheet of foam would be placed behind the baffle. The frequencies radiating from the front would extend through the front piece of foam to the rear and the rear wave would do just the opposite. The midrange and tweeter would be placed within the very center of this baffle and foam would be trimmed accordingly as to allow 30* of off-axis response. I would then cover the top baffle with grille cloth as to shroud the foam and make it aesthetically pleasing.

[nunayafb]

Sorry, I thought the foam was attached to the front of a wooden baffle.

So now I get it, this method eliminates some problems but now you have some worse problems. 7" of foam will not stop low freq. from passing thtough. Think of it this way, if you had a 24" wide solid baffle you would get some cutoff freq. based on that width but you dont have an impermeable baffle so in addition to bass rolloff due to baffle width you will also have sound wave interaction through the baffle starting right at the edge of the driver frame. This fairly close to no baffle at all.

[thadman]

I realize the foam wont inhibit LF content (2 4" sheets only yield 418hz, I think 2 6" would do a better job as that would yield 280hz).

That is why I plan on crossing the midrange over to the midbass above this point. The midbass will be mounted on a 24" wide (solid MDF) baffle and will be operating dipole.