[oldgears]

I've heard a lot about open baffle speakers and have always wanted to try.  A couple days ago, I removed the back panel of my vintage Altec Malibu speakers and gave them a listen; they sound much more open than having the back closed at the expense of low frequency response (which I am dont mind.)  Is this pretty close to open baffle design.  If not, are there any draw back for using them this way.

[ttan98]

open back=U frame.

You get resonance with open back.

[Rudolf]

My personal rule of thumb:
If the width of your "open box" is twice the depth (or more) it just works as an OB with folded back wings. No resonance to care for, just more bass without less WAF.
If the depth of the box is more than its width you certainly need to look after the "box" resonance.

Anything between is a matter of your personal hearing ambition or tolerance.

[ttan98]

My personal rule of thumb:
If the width of your "open box" is twice the depth (or more) it just works as an OB with folded back wings. No resonance to care for, just more bass without less WAF.
If the depth of the box is more than its width you certainly need to look after the "box" resonance.

Anything between is a matter of your personal hearing ambition or tolerance.

the width of my open back baffle is MORE than twice the depth(about 2.5 times), I still get resonance I need 4th order LR to reduce resonance to a reasonable level. Open back gives me better bass response than pure open baffle.

I still need a notch to remove it all together.

[Rudolf]

the width of my open back baffle is MORE than twice the depth (about 2.5 times), I still get resonance

ttan98,
could you provide more detail regarding your cavity resonance?

May be we are talking about different effects here. Every OB has a first dipole peak and the following comb filtering peaks and dips, which I would not call resonances but interference. Did you really find lambda/4 resonances as transmission line effects of the open box?

[ttan98]

1st resonance or peak is at 140hz, 2nd at 170/180 hz, are these peaks or resonance, they are actually there, I measure them.

[oldgears]

"If the width of your "open box" is twice the depth (or more) it just works as an OB with folded back wings. No resonance to care for, just more bass without less WAF."

Cool, looks like I have myself a nice set of Open Baffle and I dont even know it.

[Rudolf]

1st resonance or peak is at 140hz, 2nd at 170/180 hz, are these peaks or resonance, they are actually there, I measure them.

Sorry if I am a bit picky here, but I believe we should really make a difference between cavitiy resonance and dipole comb filtering.
For 140 Hz to be a resonance, your U frame has to be 45 cm deep at least and about 1.25 m wide (2.5 * 45cm). Does that comply with your dimensions?
John K published this SPL diagram of his NAO II U frame:



For this design the effective U frame depth is almost twice the frame width. So you really have to care for the 170 Hz rear resonance in this case.

[ttan98]

sorry i did not measure the 1st time, I estimated, I thought depth less than 1/2 the width. The MEASURED dimension is  width = 40cm and depth = 24cm, slightly more than twice.

you are right. thanks for pointing out, just bought a digital x-over I can remove the u-frame it causes more problems. I can now use my digital equalization in my x-over.

[johnk...]

U's are interesting with respect to resonances. We usually think in terms of the 1/4 wave resonance where the 1/4 wave frequency is C/(4L). Thus, for my 18" NaO U the 1/4 wave frequency would be 188 Hz. As you can see, it is actually significantly lower. However, when the u-resonance is correctly calculated, considering the length and cross-sectional area (assuming circular cross section) the resonances comes out very close to that measured.




When the length is about 1/2 the diameter of the u the resonances looks like this:



Lower yet in frequency and lower in amplitude.

If we look at a 6" long U, 1 foot in diameter we have:





The magnitude of the peak is set by the ratio of L to D, and the frequency by L and L/D.

The above simulations used a high Q driver to make the resonance peak more apparent. Returning the the NaO configuration and a driver with Q and Fs of the NaO U I get,




which is in pretty good agreement with the measurement. Adding damping yields





[edit... Please note that all the curves show only the response form the back exit of the u and not the summation witht he front radiation. ]