[Graham Maynard]

Hi Bruno,

Thats a nice group of investigation curves.

I must agree with Dave - the front cavity is radiating the main upper resonance, as can H frames, and (through the driver cone) U frames too.  Damping can help but affects output.

That is why I suggested making a toroid out of carpet and fitting it inside the cabinet around the driver apertures like a 100mm thick ring, though squashed to the outer edges at the front for maximum vent opening.
This should extend from the outer wood to between the cone edges but still be clear of them, and the carpet should deform resistively in a manner which will de'Q' the peak and minimise its recognisable reproduction characteristic.

The problem with this kind of peak is that although AF drive can be notched at this frequency, it is a resonant mode which can still be energised by large amplitude waveforms of lower frequency within the operating passband.

I am afraid that if I recognised a resonant peak like that I would choose to listen through LSs which would not oblige me to tolerate it.

Do you find it audible when compared to say listening to a flat mounted OB alternative, or a well damped boxed LS ?

Cheers ......... Graham.

[BrunoB]

Hi Bruno,

Thats a nice group of investigation curves.

I must agree with Dave - the front cavity is radiating the main upper resonance, as can H frames, and (through the driver cone) U frames too.  Damping can help but affects output.

Hi Dave and Graham,

if the front cavity is radiating the 200 Hz resonance, how do you explain the following?

1)  I measured much less resonance from the front cavity than from the sides

the black line line is the FR at the listening position when the front cavity faces the listening position whereas the blue line is the FR when the side cavities are facing the listening position.

2) adding foam in the side cavities decreases resonance.

That is why I suggested making a toroid out of carpet and fitting it inside the cabinet around the driver apertures like a 100mm thick ring, though squashed to the outer edges at the front for maximum vent opening.
This should extend from the outer wood to between the cone edges but still be clear of them, and the carpet should deform resistively in a manner which will de'Q' the peak and minimise its recognisable reproduction characteristic.

I tried two times to place some foam inside the slot. Each time I ended removing it because the bass was slower. Therefore, I would not add any sound adsorbing material inside the front cavity. If I would add a ring inside the slot as you suggested, I would use wood or another non adsorbing material.

The problem with this kind of peak is that although AF drive can be notched at this frequency, it is a resonant mode which can still be energised by large amplitude waveforms of lower frequency within the operating passband.

I am afraid that if I recognised a resonant peak like that I would choose to listen through LSs which would not oblige me to tolerate it.

Do you find it audible when compared to say listening to a flat mounted OB alternative, or a well damped boxed LS ?

Cheers ......... Graham.

I don't plan to try a flat OB alternative in the near future. The sub sounds pretty good thanks to the steep 80Hz low pass filter (200 Hz is 27 db lower than 80 Hz at the listening position).

Bruno

[Davey]

My comment was only regarding your out-of-phase impedance measurement.  That's obviously not a mode you would use so it shouldn't be of concern.  In that case the two woofers are always moving in the same direction so there is no pressure created in the cavity and thus you aren't "exciting" it.

I think you might try taking some close-up measurements to better define the radiation pattern.  I'm not sure it's as directional as you think it is.

Cheers,

Dave.

[Graham Maynard]

Hi Bruno,

You say that the bass sounded 'slower' when any front slot damping was inserted.  Clearly the characteristics of the bass reproduction were changed in some way, thus the impedance plot must have been changed too.
Did you try an impedance plot with the front cavity damped ?

Where impedance is not flat due to the development of any resonant peak, ( the Ripole has 2 ! ) and yet the SPL plot does not show that resonance, then it is the dynamic ( not steady sine ) reproduction which becomes affected.  Some enclosured subs can measure flat and sound impressive, yet where there is resonance within the listening passband their dynamic ( drum ) reproduction qualities are not entirely natural sounding;  ie. electrical phase changes lead to energy storage modes and modified sound reproduction.  I can't describe the differences I might well hear here, and recognition of those differences is not easy in isolation when there is not a better reference to listen to ( even headphones ).
I am not however saying that Ripole bass is not acceptable.

My thoughts also follow Dave's.  A radiation pattern made by rotating the cabinet would be interesting, at say 200Hz where that upper resonant mode arises.

Cheers ...... Graham.

[BrunoB]

I have the best bass I have ever heard. I made two modifications to the sub:
1) fill the small cavity with incompressible material while still allowing air flow
2) Using full length side walls

During  listening, I had to decrease the 20 Hz bass boost level.

I watched two times the same movie this weekend (Day after tomorrow, on Blu ray with soundtrack encoded in DTS-HDMA). The first time for the movie itself. The second time just to enjoy the new bass quality. In chapter 25, around 1:30, when the space station flies over the earth, some strong  drums can be heard. Theses are very fast, clear,  deep and ... scary!

I'll post picts and measurements next weekend (If I don't (re)watch too many movies  ).

Bruno

[BrunoB]

I built a stand for the sub.


This is a small table with wheels. A shelve holds the electronics.


Bruno

[BrunoB]

Here is how I filled the small cavity with incompressible material while still allowing air flow.


I built a U shaped piece of wood


I inserted the U inside the small cavity





Bruno

[Michael V]

Great pics, thanks for sharing.  With the addition of the side walls, how did that affect placement of the sub?  Do you still have it firing directly behind your listening position?

PS I built one a few weekends ago but haven't even hooked it up yet (embarrassing!).

[BrunoB]

Great pics, thanks for sharing.  With the addition of the side walls, how did that affect placement of the sub?  Do you still have it firing directly behind your listening position?

I still have the sub just behind the sofa. The full side walls seem to enlarge the bass sweet spot.

The main reason for using sidewalls is that the dipole resonance frequency moves higher (> 200 Hz). I will follow up on this with a more detailed post.

I made another modification that shifts the frequency of resonance even higher (about 300 Hz). I also have rotated my sub by 90 deg such that the front cavity is horizontal. I am still doing a lot of experimenting. This is why I haven't posted much lately.

PS I built one a few weekends ago but haven't even hooked it up yet (embarrassing!).

I hope you will hook up your sub soon and will try a similar set up. The bass is really that good.


Bruno







 

[BrunoB]

I built a stand for the sub.


This is a small table with wheels. A shelve holds the electronics.


Bruno

This little table with wheels is very convenient. Unfortunately, it is not as stable as the chair I had before. I noticed that bass was not as good:  slower. The solution I  tried yesterday is to place a heavy stone on the top of the sub. Works.

Bruno

[Rudolf]

You could even integrate the stone into the sub:

[scorpion]

Please Rudolf, remember this is a friendly forum, you shouldn't get stoned or perhaps more people should !

/Erling

[BrunoB]

...

The problem with this kind of peak is that although AF drive can be notched at this frequency, it is a resonant mode which can still be energised by large amplitude waveforms of lower frequency within the operating passband.

I am afraid that if I recognised a resonant peak like that I would choose to listen through LSs which would not oblige me to tolerate it.

Do you find it audible when compared to say listening to a flat mounted OB alternative, or a well damped boxed LS ?

Cheers ......... Graham.

Hi Graham,

you suggested two times to try a flat mounted OB alternative and to compare the sound to the ripole. It is a good suggestion but I haven't had the time to do so. What I did, is to measure the waterfall plots of the driver alone and mounted on the ripole. Here are the results:

1) The driver is placed on a small speaker stand and the microphone is placed very close to the membrane. The idea is to simulate the results one would obtain if the driver would be placed on a flat mounted OB.

2) Two drivers are mounted on the ripole and the mic is placed in front of the small opening. The small cavity is filled with the U (see my post above) and full length side walls are used.



The second plot is certainly noisier than the first plot. The ripole shows longer decays even at lower frequencies.
Note that I am not completely confident in these measurements. I have read elsewhere that WF plots are not reliable for low frequencies.

What surprises me is that the main resonances (34 Hz for open air driver and 22 Hz for the ripole) are barely visible.



Bruno

[BrunoB]

The plot below shows the effect of the sidewalls and the "U" on the FR of my sub.



The mic is placed just in front of the small cavity.

The red line:  no side walls, no U.
The green line: + full length side walls.
The blue line: + full length side walls  + U

Adding the "U" increases the 20 Hz level by 4 db.

Bruno

[Graham Maynard]

Hi Bruno,

I cannot comment on the waterfall 'ripples' because I do not know the how and why of their origins.  However a 4dB relative increase clearly shows at 20Hz wrt 100Hz for the Ripole mic compared to single driver mic measurement.

Resonant development requires 'Q' and a steady sine drive waveform continuing for more than one full cycle. 
With low Qes LS drivers and SS drive the 'one-note bass' resonance cannot develop.
Insert series R or use a SET amplifier and it will though !

So the front 'U' insert improves LF output and gets rid of the notch at about 420Hz.
Wonder what rear 'U's would do ? 
Or rear aperture damping - maybe reduce the new 270Hz SPL peak ?
Rear damping might reduce some of the Ripole waterfall 'noise' too ?

Cheers .......... Graham.

[BrunoB]

I also tried a Linkwitz configuration of the two drivers http://www.linkwitzlab.com/woofer.htm.

.

More details later.


Bruno

[BrunoB]

Here are the measurements of the sub with a Linkwitz configuration.
Frequency response:

The red line:  no side walls, no U.
The blue line: + full length side walls  + U
The black line: + full length side walls  + U + Linkwitz configuration
The black line shows clearly that the sound level in a Linkwitz configuration is decreased. However, it is actually not as strong as shown on the plot.
I made a mistake when mounting the driver in the Linkwitz configuration. The driver hole was slightly too small and there was an air leak. I measured afterwards (after fixing the leak) that this leak accounts for a loss of about 2 db at 20Hz and 1 db at 100 Hz. Still, even taking the effect of the leak into account, there is still a slight decrease of the output level.


This is the impedance plot of the sub described above (see my post above):


The secondary resonance moved to a higher frequency, almost 300 Hz.

The waterfall plot (not shown) looks slightly better in the region < 150 Hz.

I don't know yet which configuration sounds better.

Bruno

[Graham Maynard]

Hi Bruno,

Clearly running drivers face to face with your centre 'U' section improves output and reduces cavity resonances.

I am tempted to ask what it would be like with 'U' sections at the rear as well, but both the impedance and SPL plots already look pretty clean.

Cheers ........ Graham.

[BrunoB]

Hi Bruno,

I cannot comment on the waterfall 'ripples' because I do not know the how and why of their origins.  However a 4dB relative increase clearly shows at 20Hz wrt 100Hz for the Ripole mic compared to single driver mic measurement.

Resonant development requires 'Q' and a steady sine drive waveform continuing for more than one full cycle. 
With low Qes LS drivers and SS drive the 'one-note bass' resonance cannot develop.
Insert series R or use a SET amplifier and it will though !

Hi Graham,

This is why I am using a digital amp and very short and  thick cables.

So the front 'U' insert improves LF output and gets rid of the notch at about 420Hz.
Wonder what rear 'U's would do ? 
Or rear aperture damping - maybe reduce the new 270Hz SPL peak ?
Rear damping might reduce some of the Ripole waterfall 'noise' too ?

Cheers .......... Graham.

I have prepared two "U"'s made of relatively rigid foam for the sides. I haven't had the time to install them yet.


I'd like to share another finding. I moved the side walls closer to the driver such that the volume of the two side cavities decreases slightly. I measured the impedance plot and found that 1)  the secondary frequency of resonance increases slightly (>= 300 Hz), 2) the main resonance decreases slightly in frequency (-1 Hz) and intensity (-2 Ohms).



Bruno

[iON]

Thanks for sharing your experimentation Bruno. It's is interesting to note that the cancellation of non-linearities in the push-pull mount is so clearly visible! (seems to work then ). It looks puzzeling that sensitivity on that particular measurement is lower though...