The REAL reasons behind open baffles 4" or less having no bass ?

aarvin2

Member
2012-09-02 1:41 pm
Hi guys I am very confused, I have read that the reason speakers need an enclosure is to prevent the low frequency waves coming from the front to interact with the low frequency waves(opposite phase) coming from the back and cancelling out.

Also that the quarter wavelength of the desired low frequency we would like to hit, must be smaller than one of the faces of the baffle.

The problem is that I made an experiment today and placed speakers (tang band 4" and a tang band 3" w3-1878) directly into the middle of a RW45 Rockwool pack (1200mm x 600mm x 400 mm).

The were no enhancement in bass, the only "sound" is one which is produced when all the holes behind the speakers are closed. You know, the boxy sound.

If that theory was really good, I believe all the low frequency waves coming from behind the speaker would be absorbed by this huge amount or rockwool, and that the waves coming from the front of the speaker would not be cancelled out.

But the sound was the same, no bass enhancement.


I think I must have done or understood something wrong and I would love to have some help.

Could anyone please explain why when a speaker plays on its own(without any enclosure) it produces low level of bass (sounds like a big roll off) ?

Many many thanks!
 
Bass enhancement in an enclosure comes from resonance either as a Helmholz resonator or as quarter wave / transmission line or a horn - all these methods require an output port. A sealed enclosure has no bass enhancement, it has very clean bass but is limited by the fs of the driver. This is similar to an open baffle. The bass you get will look very much like the manufacturers freq response curve. Any bass enhancement or 'gain' in bass comes from resonant amplification in an aero acoustic device.
 
You require something rigid to prevent the front wave and back wave from reaching one another. Damping material does little to absorb the lower frequency waves that are being cancelled when combined. Try a piece of wood and it will all become clear. :)

Damping material really only has value above what we think of as 'bass'. It helps prevent standing waves inside the cabinet and attenuates the 'midrange' frequencies that you don't want exiting the port.

Best not to use the 'enhancement' term here. It's got X answering a different question.
 

aarvin2

Member
2012-09-02 1:41 pm
Bass enhancement in an enclosure comes from resonance either as a Helmholz resonator or as quarter wave / transmission line or a horn - all these methods require an output port. A sealed enclosure has no bass enhancement, it has very clean bass but is limited by the fs of the driver. This is similar to an open baffle. The bass you get will look very much like the manufacturers freq response curve. Any bass enhancement or 'gain' in bass comes from resonant amplification in an aero acoustic device.

The problem is that what I hear doesn't come close to the frequency response graph of the manufacturer, in the sense that it seems to roll off way sooner (higher low mids). But 800Hz and more it is very representative of the graph.

I actually don't want bass enhancement but want it to sound like it it on the manufacturer's frequency graph.
 
Dipole will give a figure 8 polar pattern within the suitable pass band. Above the pass band they'll have a "ridged" polar response and below the pass band (bass) they'll have little to no output. The pass band is determined by the baffle width, which is typically very narrow or even baffleless in order to widen the pass band as much as possible. Are you listening directly on axis when you're experimenting? A microphone will help you understand what's happening here.

Can you explain more clearly what the problem is? The multiway forum may offer better explanations. Not many here do the open baffle thing. I'd love to give it a try someday, but I don't know much more about it than what I just typed.
 

aarvin2

Member
2012-09-02 1:41 pm
Dipole will give a figure 8 polar pattern within the suitable pass band. Above the pass band they'll have a "ridged" polar response and below the pass band (bass) they'll have little to no output. The pass band is determined by the baffle width, which is typically very narrow or even baffleless in order to widen the pass band as much as possible. Are you listening directly on axis when you're experimenting? A microphone will help you understand what's happening here.

Can you explain more clearly what the problem is? The multiway forum may offer better explanations. Not many here do the open baffle thing. I'd love to give it a try someday, but I don't know much more about it than what I just typed.

I was indeed listening directly on axis :)

The problem is that people explain the lack of bass is due to low frequencies from the back interfering with frequencies coming from the front as they are out of phase.

But in my experiment where I placed the speaker in a big rockwool pack, which at this thickness and size have a huge absorption coefficient, and the result was as I explained in the original post.
 

aarvin2

Member
2012-09-02 1:41 pm
The point that was being made is that rockwool or any other absorptive material will have the least effect on the BASS. At low frequencies, your rockwool pack is entirely transparent. You need to use a board.

Bob

Hi Bob, thank you for your insight, but Rockwool is used in studios to absorb low frequency resonances isn't it(and they are using relatively thinner panels), why wouldn't it work behind a speaker for example ?
 

aarvin2

Member
2012-09-02 1:41 pm
Rather than try to conduct a tutorial here, I suggest that you look into the coefficient of absorption of various materials. Start with http://www.bobgolds.com/AbsorptionCoefficients.htm


There is nothing with regards the RW45 :(

But the coefficient of absorption of 3" of RW3 to be 0.34 and the coefficient of absorption of 3" of RW5 to be 0.40 at 125Hz ... The RW45 should be in between, and now I didn't use 3" but actually 15" of this material(the whole packet)... which should take the coefficient of absorption to be more than 1, if I am not mistaken.

Why didn't it work ?

Correct me if i'm wrong :S
 

aarvin2

Member
2012-09-02 1:41 pm
You can't absorb low frequency with thin panels of rockwool. They will only work 500 Hz. and up. You need bass traps. If these are made of rockwool you would need at least 100 cm. thick rockwool traps. There are other ways to absorb low frequency as well.

As I said I actually had a 120cm x 40cm x 60 cm packet of rockwool :)

Would you suggest other more absorbant materials please ? I would love to experiment
 
I guess you made a hole for the driver on the front rockwool sheet and stacked the other sheets behind. That probably means you left a very small volume behind the driver and Fs went to moon. For a couple of years I have been playing with rockwool baffles (not enclosures) with mixed success and I just don't believe 2+ inches thick rockwool will be that transparent below 500 Hz. Of course one cannot expect to hit buch below 200 Hz or so...
 

Scott L

Member
Paid Member
2008-12-27 12:32 pm
Knoxville, TN
I guess you made a hole for the driver on the front rockwool sheet and stacked the other sheets behind. That probably means you left a very small volume behind the driver and Fs went to moon. For a couple of years I have been playing with rockwool baffles (not enclosures) with mixed success and I just don't believe 2+ inches thick rockwool will be that transparent below 500 Hz. Of course one cannot expect to hit buch below 200 Hz or so...

The solution is a massive build reaction-cancelling enclosure with 2x15's !

Murat, how about a nice cup of coffee ? :)