Introducing Directiva - An ASR open source platform speaker project

[Rick Sykora]

Here is the nearfield on the right PR with different levels of damping...

First, is the new baseline when I pulled the Sonic Barrier from the cabinet (red trace). While you can see some small differences in spots, note the peaks between 2-4 kHz. These are significantly reduced with addition of Sonic Barrier (green trace), but overall only marginally changed by adding the Acousta-stuf. Replacing it with the cotton-based material yields little change in the lower frequencies, but some slight additional in the higher range of the chart.

Pretty sure what is producing the peaking around 2-4 kHz, and seems more clearly to be the woofer rather than my previously alleged measurement artifacts. The baffle shape difference likely plays a factor too. This made me go back and look at Purifi's specs and some 3rd party driver measures. Here is the Purifi one...

Note there is an on-axis shift around 1.2 kHz and comparable rippling up to 4 kHz. It gets nicer off-axis. Fortunately, as you may have heard, there is no major distortion in this frequency range. May be another reason to push the crossover frequency lower. Let's see what happens once I get the z-offset delay remeasured...

 

[tktran303]

Hi Rick,

very interesting.

what are your thoughts re: 600-800Hz.
They seem to be the highest in level.

it’s also an area where ports/vents have issues with resonances.

How do we tease apart whether it’s due to the active midwoofer or cabinet damping that’s affecting (or not) the loudest resonance, that is about only 10dB down from the passive radiators tuning?

can we definitely say that passive radiators don’t suffer from port noise?

Perhaps port (or passive radiator) noise is largely due to rear sound waves of the back of the midwoofer coming through the port/passive radiator.

 

[Rick Sykora]

Hi Rick,

very interesting.

what are your thoughts re: 600-800Hz.
They seem to be the highest in level.

it’s also an area where ports/vents have issues with resonances.

How do we tease apart whether it’s due to the active midwoofer or cabinet damping that’s affecting (or not) the loudest resonance, that is about only 10dB down from the passive radiators tuning?

can we definitely say that passive radiators don’t suffer from port noise?

Perhaps port (or passive radiator) noise is largely due to rear sound waves of the back of the midwoofer coming through the port/passive radiator.

Good catch. @ctrl pointed out the high level of the PR previously but was caught up in the phase issue and did not take note. Will look at that harder. Agree would expect that the PR output should drop off more above its tuning. Catching some dinner now...

 

[Zvu]

I've got good results by damping passive radiator's cone (no measurements saved, sorry). Use damping material (felt about 1cm thick) and apply thin coat of glue to the back of passive radiator cone. The added weight isn't huge so tunning shouldn't be to much affected (and you can recalculate frequency response with added weight). You stop or significantly diminish midrange leaking through PR's thin cone since 500Hz and up can be effectively absorbed by damping material. Also, since added weight is evenly distributed to the cone, you don't stress the spider and suspension as you would by screwing a metal weight at the bottom of the cone - as is almost always the case.

Since Purifi PR's are mighty expensive, you can experiment with Dayton PR to see what amount of damping material leads to the best absorption and at what frequency.

I think it would get rid of most of the hash seen on your measurements.

 

[tktran303]

I've got good results by damping passive radiator's cone (no measurements saved, sorry). Use damping material (felt about 1cm thick) and apply thin coat of glue to the back of passive radiator cone. The added weight isn't huge so tunning shouldn't be to much affected (and you can recalculate frequency response with added weight). You stop or significantly diminish midrange leaking through PR's thin cone since 500Hz and up can be effectively absorbed by damping material. Also, since added weight is evenly distributed to the cone, you don't stress the spider and suspension as you would by screwing a metal weight at the bottom of the cone - as is almost always the case.

Since Purifi PR's are mighty expensive, you can experiment with Dayton PR to see what amount of damping material leads to the best absorption and at what frequency.

I think it would get rid of most of the hash seen on your measurements.

fantastic useful info Zvu!

for a rear mounted passive radiator. I’ll have to try this, and take some measurements!

(In the past, in my own speaker with PRs when I experimented with putting polyfill in the speaker what sounded “best to my ears” was polyfill on the walls AND and directly behind the midwoofer; in front of the rear mounted passive radiator)

 

[ctrl]

Perhaps port (or passive radiator) noise is largely due to rear sound waves of the back of the midwoofer coming through the port/passive radiator.

Good catch. @ctrl pointed out the high level of the PR previously but was caught up in the phase issue and did not take note.

Looking at the overall near-field measurements with baffle-step correction, the interference around 600Hz of the PR is about 15dB attenuated. This is actually quite good.

You could try to reduce the rear sound radiation of the woofer by inserting damping material near the woofer frame, but this usually costs sound pressure in the low frequency range too.

 

[Zvu]

@tktran303 Since you would be gluing the felt to the back of the PR cone, i see no aesthetic problem regardless of PR placement.

 

[tktran303]

@Zvu

Have you seen any studies of differences between what kind felt affect things differently?
Eg. carpet underlay (cheap), flooring foam (eg. Yoga mat- readily available, polyfill/Dacron or Sonic Barrier ($$)

 

[Zvu]

Most similar pic of what i've used is this:



Felt made out of recycled cotton fibers. Salvaged from my vintage Sansui loudspeakers.

There are lots of studies about material absorption coeficient. Use what's best for the problem you have.

Felt from pic above acted almost like a black hole for sound

 

[MrPeabody]

Here is the nearfield on the right PR with different levels of damping...

View attachment 119586
First, is the new baseline when I pulled the Sonic Barrier from the cabinet (red trace). While you can see some small differences in spots, note the peaks between 2-4 kHz. These are significantly reduced with addition of Sonic Barrier (green trace), but overall only marginally changed by adding the Acousta-stuf. Replacing it with the cotton-based material yields little change in the lower frequencies, but some slight additional in the higher range of the chart.

Pretty sure what is producing the peaking around 2-4 kHz, and seems more clearly to be the woofer rather than my previously alleged measurement artifacts. The baffle shape difference likely plays a factor too. This made me go back and look at Purifi's specs and some 3rd party driver measures. Here is the Purifi one...

View attachment 119588
Note there is an on-axis shift around 1.2 kHz and comparable rippling up to 4 kHz. It gets nicer off-axis. Fortunately, as you may have heard, there is no major distortion in this frequency range. May be another reason to push the crossover frequency lower. Let's see what happens once I get the z-offset delay remeasured...

I have barely any experience with passive radiators, but I've often wondered about what happens when people add mass, because there's no good way to increase the damping to keep it critically damped. It has seemed to me that if you add mass, you end up with an underdamped oscillator. Also, it seems to me that even using light mass, the passive radiator can still be tuned as low as desired by using weak damping, but that in this case you would have a passive radiator that will behave almost like a big opening in the enclosure, due to insufficient impedance to the transfer of energy. The impedance to transfer of energy needs to be fairly high, in order that the PR will not behave like a big opening. And in order for the impedance to be adequately high, the suspension needs to be adequately lossy, which I think implies the need to achieve the desired resonant frequency by appropriate tweaking of the mass and the suspension stiffness (spring effect, distinct from damping). The difficulty, I think, is with the suspension being sufficiently lossy. By comparison, in active drivers the suspension provides stiffness but contributes only a small amount of damping. With most active drivers, the bulk of the damping is the electrical portion, represented by Qes. In a typical active driver, Qms will be a very high Q value, corresponding to minimal damping, while Qes will be very nearly equal to Qts. I've pondered these questions in the past and have wondered if it wouldn't be better to use an active driver and use a power resistor to make a closed circuit. Motion of the coil within the magnetic field will generate an EMF and current, with energy lost to heat in the power resistor, i.e., damping. But no one does this with passive radiators, because the fundamental principle with both passive radiators and ports is to use a low-loss radiator at very low frequency in order to improve efficiency at very low frequency, and if the damping is too great, the intended effect will be defeated. With a port, you can increase damping by using a long and wide slot instead of a circular port, but then you have the issue of distortion due to power compression, because the drag increases as the square of the air velocity, and of course the air velocity increases with increase in pressure amplitude (possibly as a function of the square of pressure amplitude - I'm not certain of that).

Regrettably, I don't have any advice to offer. I wonder, though, if it might be worthwhile to experiment with a different PR, if there are any with the same diameter that might be an easy swap.

 

[Rick Sykora]

Thanks all! Time to review the sims and may have to build another box as I originally planned. I did a quick check to make sure the measurements were repeatable after resetting REW prefs. Was also worried that I may have had the mic to close. After a quick reset, there was no major change in the measurements.

Like many of you, have no significant experience with passive radiators (much less dual passive radiators). Have looked at some other designs like the DXT-mon and see that the tuning definitely pushes the initial floor 20 db below the primary output. Gathering these are with a crossover in place and mine are not. So may be worthwhile to try with one in place. More to come...

 

[Rick Sykora]

After trying a few things and consulting with some other designers, the PR behavior in post #341 is nominal. I did get a wider delta with the SB passive radiator in the SPK5 cabinet. I suspect I may have driven the internal pressure levels too high as I sought a lower cabinet volume. If so, would have to build another cabinet or modify the current one to try.

Am leaning towards modifying the current cabinet as could do by slicing off the front baffle and removing the brace. That should add about another 3 liters to the tuning and make it more comparable to the SPK5 with the PR.

 

[ctrl]

Am leaning towards modifying the current cabinet as could do by slicing off the front baffle and removing the brace. That should add about another 3 liters to the tuning and make it more comparable to the SPK5 with the PR.

Then the tuning frequency of the PR moves even further down. This should be avoided in any case, because a PR tuning frequency around 38Hz is already extreme for a 6.5'' driver.
Didn't see that you wanted to use the SBAcoustics PR.

 

[Rick Sykora]

Then the tuning frequency of the PR moves even further down. This should be avoided in any case, because a PR tuning frequency around 38Hz is already extreme for a 6.5'' driver.
Didn't see that you wanted to use the SBAcoustics PR.

No was leaving the SB PR as a lower cost option. Just using it for comparison.

 

[Rick Sykora]

After a Costco run for some food, removed all the drivers and internals and was prepared to cut off the front baffle. I then realized I had enough space to work with a Dremel to cut a big enough slice out of the brace to open additional volume. While not as much volume as removing the brace entirely, there is enough of it left to seal it back up if desired.

Time for some dinner and will put it back together afterwards and take some new nearfields on the passive radiator...

 

[Rick Sykora]

Well that was a bust (particularly for the brace!).

Added volume shifted the tuning slightly lower but did not impact the level of higher frequency resonances. Will try some different damping in the morning.

 

[Lbstyling]

If your having an issue with unknown source of resonance, add 30kg on top of the cabinet and remeasure. With standmounts, they often 'rock' forwards and back at a resonance frequency.

Physics dictates you have equal and opposing forces, and that bass driver MMS is very high.

 

[Rick Sykora]

Here is the visual comparing PR output in the smaller cabinet (11 liter) to larger cabinet (14l) volume...

Red trace is the smaller volume. So, as mentioned previously, the larger volume results in a bit lower bass form the PR. Maybe a bit of improvement at 2-4 kHz. Can ignore anything higher frequency as will be after irrelevant once a crossover is in place.

 

[MrPeabody]

Looking at it again, it looks like the amount of midrange emitting from the PR is maybe not a lot different from what is often seen with the port in 2-way ported speakers. Maybe it won't negatively impact the sound in the listening window measurement. It could even have a positive impact on the listening window measurement.

 

[ctrl]

Here is the visual comparing PR output in the smaller cabinet (11 liter) to larger cabinet (14l) volume...

So everything as expected, that's reassuring

This extends the already considerable bass range, but the sensitivity would deteriorate by another dB.


Due to the additional bass, the woofer cone excursion in the frequency range 40-60Hz increases significantly (and below 39Hz decrease) - at 50W input by about 1.5mm.

Since the active project has a high-pass filter that protects against too much excursion at frequencies below 40Hz, you have to decide whether low-frequency response or maximum sound pressure level has priority.

Maybe it won't negatively impact the sound in the listening window measurement.

Since the sound pressure level of the PR is attenuated at least 15dB, the contribution to the total sound level is less than 0.2dB.

But maybe Rick will manage to reduce the damping of the PR in the midrange a bit more. Soundwise it should not be decisive.