[John Casler]

Strangely enough, many read of "putty pinching" of the Passive Radiator of the Floor Standing Ribbon Monitors and Subs, and seldom ever think of "adding" putty.

The way the "putty system" works is that the "active woofers" in the system move in and out to produce bass.  This then creates pressures, both positive and negative within the cabinet.

These + and - pressures then cause the Passive Radiator to also move in and out in response.

This response to the internal pressures also acts on those pressures as opposed to a "stiff" sealed cabinet, and in doing so, allows the dynamic woofers to perform differently.

This means that all the bass drivers act as a "system" in collectively producing the bass notes.

Now collectively the system has a mass, and coil and recoil elasticity that creates resistance or assistance to the movement of each.  The larger mass, and amount of resistance the system encounters when attempting to reproduce the wave is what causes distortion, since no system can respond instantly.

However, many factors go into the response.  The idea is to have "instant" acceleration, and deceleration of all driver cone in relation to the signal.

But the weight of the drivers, the compliance of the surrounds, the compression and decompression of the air inside the cabinet, the stiffness of the cone material, and even the electrical signal can all affect the response of this system.

To that end, adding and subtracting putty which is mass, from the Passive Radiator can "tune" the response of the total system.  This tuning has one singular purpose. . . . to make the system respond as accurately as possible.

By adding or subtracting mass, it allows you to adjust the system to your electronic signal, and to the specific condition of the system as a whole.

Now if you make the system "TOO" light by removing too much mass, it will "over respond" to the musical signal, and the sound will be "woolly, and lack definition" because the drivers will slightly "overshoot", and travel too far and fast.

If you add "TOO" much mass, then the system responds on the sluggish side and again the sound is slow, sluggish and "woolly".  So as B, has often said, too much and too little actually "sounds the same".

However, when you get the system "in balance" and "fine tuned", you discover a world of bass detail and clarity that few ever get the chance to hear, or even know exists.  This is also accompanied by distortion reduction which is typically quite high in most woofer systems.  For many it is acceptable to have bass distortion into the teens.

B, recently said that he had measured the distortion of the VLA at 1.7% at 20Hz

That is clean bass.

Now the reason for all that explanation is that for many "tuning" the VMPS system entails "removing" small amounts of the putty. 

However, the tuning need be approached from both possible positions.  It may need added putty, or you may need to pinch.

In the beginning the speaker comes pretty much "pre-tuned", but with your amp, and wire, and then the subsequent "breaking in" of the drivers and sourrounds, you may need to slightly adjust. 

(Oooops. I have run out of time   I will continue this when I get back)

[DFaulds]

In general, adding weight to a PR lowers the resonance frequency of the PR.  A simple explanation of the PR system can be found here http://www.diysubwoofers.org/prd/ although as John notes the final tuning can be quite subjective.

[John Casler]

In general, adding weight to a PR lowers the resonance frequency of the PR.  A simple explanation of the PR system can be found here http://www.diysubwoofers.org/prd/ although as John notes the final tuning can be quite subjective.

Hi DFaulds,

That was the reason for this thread, and thanks for the link.

Many VMPS users have a "tendency" to continually "remove" small amounts of putty, but seldom engage in "adding".

Obviously it is a natural thing to continue doing what might have worked in the beginning, which for many is the removal.  The general reason removal might have worked in the beginning is that prior to "break in" when compliance of all the drivers was at its lowest (meaning slightly stiff) the system was already damped to the degree that that removal might have allowed it to "respond" a bit quicker.

But as the system as a whole, breaks in the opposite is true.  Most will find that as the speakers get 100hrs or so on them, the bass "blooms" a bit more.  This means it becomes a bit warmer, and goes a bit lower.

Now, the element of tuning that should be known is what the article suggests, and why I started this thread.

Adding putty "deepens" the perceived bass from the system.

Why?  Because the PR plays "slower" and lower than the active drivers, because it doesn't respond to the electrical signal, but the air pressure changes within the speaker cabinet, and they are dictated to a large degree, by the larger excursions of the active drivers.

Now I don't know this for a fact, so it is just a pet theory of mine, but the PR has to operate "out of phase" with the to the Active Drivers, because it reacts in the opposite actions.  That is, it moves out, when the active drivers move inward.

Adding the mass to the PR, then lowers the tuning frequency of the "system" then by controlling the "speed" at which this reaction occurs.  The exact amount of putty, then would "synchronize" the two, and that is when the lowest distortion is realized and the the cleanest response is heard.

So the greater point to this thread is, "don't hesitate" to add putty and experiment.  I recently have added significant putty to all my PR's and found that the clean response in the lower frequencies is the result.  Too much however "pulls the wool".

[DFaulds]

Actually a PR, like a port, resonates at a specific frequency just like an improperly damped panel in an enclosure.  The idea is to get it to resonate at the proper frequency to extend the response of the bass driver without causing jagged response between the woofers resonance and that of the PR.  When the PR resonates, the change in pressure in the cabinet helps to control the excursion of the active woofer, thereby reducing the woofers excursion at and around the tuning frequency.  Interestingly, to accommodate tuning some of the extremely high excursion PRs that are now available have threaded bolts to allow the addition and removal of metal washers, or weights.

As you noted, over time as a PR is used its suspension will loosen up and will actually cause the resonance frequency of the PR to lower.  The same thing happens with woofers.

[John Casler]

Actually a PR, like a port, resonates at a specific frequency just like an improperly damped panel in an enclosure.  The idea is to get it to resonate at the proper frequency to extend the response of the bass driver without causing jagged response between the woofers resonance and that of the PR.  When the PR resonates, the change in pressure in the cabinet helps to control the excursion of the active woofer, thereby reducing the woofers excursion at and around the tuning frequency.  Interestingly, to accommodate tuning some of the extremely high excursion PRs that are now available have threaded bolts to allow the addition and removal of metal washers, or weights.

As you noted, over time as a PR is used its suspension will loosen up and will actually cause the resonance frequency of the PR to lower.  The same thing happens with woofers.

All true and all valuable info, as there is only "one" exact mass damping weight, and the closer one gets to it, the more accurate the output.

That said, and as I have mentioned before, those who have subs, that use them a bit "unconventionally" like I do, need to "compensate" for those changes.

For example, the slot loaded base also adds a slight resistive load to the passive radiator by containing the pressure produced by the PR.

If one reorients the sub so that the PR is now "vertical" (rather than horizontal) the back pressure resistance of the slot is removed.  As well, since the mass of the actual PR is supported differently, and it is moved in in a different plane, it would suggest that more mass need be added to the PR to accommodate each of these reductions in resistance.

I have found this true in practice, having used the subs in almost every orientation configuration imaginable.

And again the take home message is never be afraid to "add" putty when experimenting.

[Woodsea]

John,
I need to pick up some more, as I lost most of it in between Ulaanbaatar and Havana.  I know this has been brought up before, what is the replacement putty called.  I will be stateside in December.
Thanks,
Eric

[John Casler]

John,
I need to pick up some more, as I lost most of it in between Ulaanbaatar and Havana.  I know this has been brought up before, what is the replacement putty called.  I will be stateside in December.
Thanks,
Eric

It is called "mortite", but in a pinch well chewed chewing gum can work (but does not "pinch" well since it has a tendency to "hair" or stretch when it is pinched and leave a thin strand when pulled, rather than pinching cleanly)

[Woodsea]

Gracias, Hombre

[John Casler]

After further, continuous and regular experimentation I have to suggest that "if" you orient the PR's of any of the subs so that they fire "to the side" (as in vertical rather than horizontal orientation) that you may have to more than double the amount of putty.

This is (it seems) because it is far easier to set the PR in motion "pushing" the mass load when its weight is suspended, than for it to be pushed up and down when it is horizontal.

Kinda like suspending a weight on a rope, makes it far easier to move, as opposed to having to lift it by that rope.

So if you change orientation (of your sub, not you ) be prepared to put a lot of putty (or ABC Wrigley's) on the PR, if you want clean LOW BASS.

[easyrider2020]

Would it be possible to tune the VMPS PR mass in a more objective way?
THD measurements are now possible with inexpensive iPhone apps (Studio Six Digital)
or free PC downloads (REW - Room EQ Wizard) that work with the Radio Shack SPL meter
and your PC's soundcard.

What would be the preferred method to adjust (pinch) putty mass if you had
the ability to measure THD + N? I remember Brian stating the 2nd and 3rd harmonics
in particular will drop dramatically at 20 Hz when the putty has been tuned properly.
Would it be best to use a pure 20 Hz sine wave or LF sweeps while measuring THD?
Any tips on how to do the tuning this way?

I've large hands/wrists that barely fit thru the slot to add/remove putty.
Also, should I decide to upgrade my original subwoofers to use the VSS 12" driver
and new PR, I'll need to tune the putty again.

Any thoughts on this John or Brian? Some of us are frustrated with this tune by ear process.

 

Thanks!

[Brian Cheney]

If you have a very good measuring mic and distortion measurement capability you can certainly tune the PR for lowest THD.  However, the null is most easily heard by ear, so do that as well when you're finished with measurements for best results.

[Wayner]

Do not use Mortite as it is caustic and may eat away at your driver. I suggest using a product called Plasticlay, which is simply a non-hardening modeling clay. This stuff never hardens and will help reduce resonances and other "unwanteds".

Wayner 

[lowtech]

Would it be possible to tune the VMPS PR mass in a more objective way?

The correct answer is "yes" and it doesn't involve using a mic to measure the acoustic response of the system.  The impedance curve will clearly show you if the tuning is correct.  A tuned port (or PR) system should yield two peaks in the Z curve and each should have the same amplitude.  If they are not equal, the tuning is off.  There are several ways to measure impedance accurately and a web search should provide you with some ideas.

This is what the impedance of a properly tuned ported/PR system looks like:

[Zheeeem]

Do not use Mortite as it is caustic and may eat away at your driver. I suggest using a product called Plasticlay, which is simply a non-hardening modeling clay. This stuff never hardens and will help reduce resonances and other "unwanteds".

Wayner 

This would be good advice except plasticlay will not stick to a PR.  Mortite is the recommended putty. 

[Wayne1]

A tuned port (or PR) system should yield two peaks in the Z curve and each should have the same amplitude.  If they are not equal, the tuning is off.  There are several ways to measure impedance accurately and a web search should provide you with some ideas.

One of the ways is Parts Express' Woofer Tester 3



They even offer a package with a scale to measure the amount of putty added or removed.

Woofer Tester 3 bundle

[Brian Cheney]

The impedance curve test does indeed tune the PR to its enclosure to a great degree.  However, I consider this a "coarse" adjustment.  Putty pinching is a "fine" adjustment involving changes of moving mass  less than 1g, too small to influence modulus of impedance.

[lowtech]

The impedance curve test does indeed tune the PR to its enclosure to a great degree.  However, I consider this a "coarse" adjustment.  Putty pinching is a "fine" adjustment involving changes of moving mass  less than 1g, too small to influence modulus of impedance.

Then I suggest the putty application is there more to satisfy (?) the audiophile buyer rather than to tune the speaker.  Additionally, I cannot fathom how adding minute amouts of putty changes the THD that (you) can measure.  Anyway, I'm probably in the wrong forum, so please allow me to excuse myself... now.

[Wayner]

I'm pretty sure plasticlay will stick to anything. It sure sticks to my fingers!

Wayner

[tbrooke]

I am glad to here about the impedance measurement. I have played around with a lot of in room frequency response measurements and I couldn't see where the putty had much effect. But if as, Brian says, it can be used to get in the ballpark that would be great. I have two subs that have had the upgraded woofers and the passive radiators have been reconed so just getting in the ballpark would be a big help. I think I'll pick up a WT3

Tom

[Housteau]

This software does look like a good tool.