clay wrapped around voice coil

[SWhite]

I removed the dust covers in preperation of redoing the surrounds on a pair of woofers and found a substantial amount of clay wrapped around the voice coils.

I've never seen anything like that before and was curious what you thought.

I'm going to leave the dust covers off for a while and see how much difference there is with out clay and then decide which I prefer. I could use the increase in sensitivity.

An externally hosted image should be here but it was not working when we last tested it.

 

[Vikash]

Nice pic. I can only think that it is used to add mass to the cone and you would thus reduce efficiency and fs, and extend low end response. It would be strange not to just use a heavier diaphram though, so maybe it's a way for the manufacture to fine tune the same basic driver slightly. I'm just stabbing in the dark here.

The only other ring you would put around the top of the VC is a shorting ring, but it wouldn't be clay.

Or maybe the clay somehow aids heat transfer away from the aluminium former.

Depending on how much the clay weighs and consequenctly affetcs T/S params, it may change box requirements so you should keep that in mind. Worse still, if it is used to keep the VC cool, you could be reducing its power limits.

 

[sreten]

The clay will impact the high frequency response of the driver.

The upper frequency limit of a driver is when the voice coil
effective mass = the effective massloading of the cone, the
clay here is being used to limit high frequency response.

sreten.

 

[Svante]

The clay will impact the high frequency response of the driver.

The upper frequency limit of a driver is when the voice coil
effective mass = the effective massloading of the cone, the
clay here is being used to limit high frequency response.

sreten.

??? I have never heard this, could you elaborate on this a bit?

 

[sreten]

??? I have never heard this, could you elaborate on this a bit?

Just something I've known for a long time, can't remember
the source, but its certainly the main purpose of the clay ring.

Its accepted the effective radiating diameter of a cone decreases
above a certain frequency with frequency, and at some point the
mass of the voice coil will begin to dominate the response.

Hence a short coil in a long gap driver will have more treble than
a long coil in a short gap approach. I've also seen Tannoy 0.5
way bass drivers fitted with a large central plastic slug, this may
be extra mass loading and control of roll-off in the midband.

sreten.

 

[Salas]

mass

Just mass loading, with the usual affect on HF too.

 

[Svante]

Just something I've known for a long time, can't remember
the source, but its certainly the main purpose of the clay ring.

Its accepted the effective radiating diameter of a cone decreases
above a certain frequency with frequency, and at some point the
mass of the voice coil will begin to dominate the response.

Hence a short coil in a long gap driver will have more treble than
a long coil in a short gap approach. I've also seen Tannoy 0.5
way bass drivers fitted with a large central plastic slug, this may
be extra mass loading and control of roll-off in the midband.

sreten.

I don't understand how added mass could affect the FR. I can see that if the effective surface decreases, the radiated power will too. But that effect would be the same with and without the clay, right? I can see that the effectve mass of the cone decreases as the effective surface does, and that this would compensate to some extent the drop due to loss of area. Do you mean that the relative mass drop is smaller if clay is added and that this compensatory effect is reduced?

 

[pinkmouse]

I don't understand how added mass could affect the FR. I

The added mass will reduce the resonant frequency of the driver, however it will also decrease the efficiency as well, as the coil has to work harder to move the cone.

 

[Svante]

The added mass will reduce the resonant frequency of the driver, however it will also decrease the efficiency as well, as the coil has to work harder to move the cone.

Yes, of course, that I understand, but here the issue was the rolloff at high frequencies.

 

[pinkmouse]

Sorry...

Well, as far as I understand it, the compliance of the cone material acts to damp higher wavelengths from propagating out from where the VC joins the cone. This is why higher frequency drivers tend to be both smaller, and use thinner cone material than lower order drivers.

 

[roddyama]

Yes, of course, that I understand, but here the issue was the rolloff at high frequencies.

With any driver, as the frequency goes up, it become more and more difficult to accelerate the cone (higher frequencies means higher cone accelerations). So you reach a frequency where the cone can't reach the full travel for flat response and the response starts to roll off. If you add mass (clay) to the cone, this happens at a lower frequency.

 

[usekgb]

JBL has used mass rings on several drivers to lower the fs of the speaker. A good example is the 2235H. If you remove the mass ring, you get a 2234H, which has a higher fs. Another thing they did was to add somthing they called Aquaplas. This added mass to the entire cone of the speaker, lowering the fs. Anyone remember the old white cones on the 4311 studio monitors?

Cheers,
Zach

 

[pinkmouse]

Anyone remember the old white cones on the 4311 studio monitors?

Yeh, and I nearly cried once when I worked with a keyboard player who had a pair as his on stage monitors, and he had cut holes in the cones 'cos he preferred the sound.....

 

[markp]

Yeh, and I nearly cried once when I worked with a keyboard player who had a pair as his on stage monitors, and he had cut holes in the cones 'cos he preferred the sound.....

In Texas you could legally kill him for that! It's called "the needed to be killed" law.

 

[Svante]

With any driver, as the frequency goes up, it become more and more difficult to accelerate the cone (higher frequencies means higher cone accelerations). So you reach a frequency where the cone can't reach the full travel for flat response and the response starts to roll off. If you add mass (clay) to the cone, this happens at a lower frequency.

Hmm... In my world, the acceleration is the same for a fairly wide range above fs, where the cone is mass controlled. The velocity and excursion decrease with frequency, but acceleration is the same. The cone is just as hard to accelerate, but it does not travel as far, due to the shorter cycles. (For the response, that does not matter, since sound pressure is proportional to cone acceleration, not velocity or excursion.)
Then, at a high enough frequency, the cone breaks up, and this frequency I would have thought was independent of the mass added by the clay. Obviously there is something here that I don't understand.

 

[roddyama]

Hmm... In my world, the acceleration is the same for a fairly wide range above fs, where the cone is mass controlled. The velocity and excursion decrease with frequency, but acceleration is the same. The cone is just as hard to accelerate, but it does not travel as far, due to the shorter cycles. (For the response, that does not matter, since sound pressure is proportional to cone acceleration, not velocity or excursion.)

... for flat response. Yes, I stand corrected,... even in my world.

Then, at a high enough frequency, the cone breaks up, and this frequency I would have thought was independent of the mass added by the clay. Obviously there is something here that I don't understand.

The greater mass lowers the efficency, as PM, suggested. Therefore requires more power (E*I) for a given SPL. That means the cone with the larger mass will require a higher peak voltage (and current) to produce the same SPL. If we assume the VC inductance to be constant, it will began to limit the dI/dt at a lower frequency thereby lowering the speakers high frequency BW. All this is ignoring the affects of having to dissipate more heat.

 

[sreten]

Obviously there is something here that I don't understand.

Well as I see it when the cone begins to break up it enters
a region where its effective area decreases with increasing
frequency, one way of looking at it is the mass of the cone
the voice coil is driving is decreasing.

When the mass of the cone = the mass of the voice coil you
have an intrinsic upper frequency limit, and varying the voice
coil mass will vary this upper frequency limit.

Everyone knows to get extended treble from a cone type
driver you need a lightweight voice coil assembly, and I've
always assumed that the above is self evident.

Other ways of extending treble are delaying the mass
reduction by doping the centre of the cone, or adding a
parasitic tweeter with a much more agressive profile
than the cone, maintaining loading on the coil.

sreten.

 

[Svante]

... for flat response. Yes, I stand corrected,... even in my world.

The greater mass lowers the efficency, as PM, suggested. Therefore requires more power (E*I) for a given SPL. That means the cone with the larger mass will require a higher peak voltage (and current) to produce the same SPL.

Yes, I totally agree with this, efficiency must be lower if the mass increases.

If we assume the VC inductance to be constant, it will began to limit the dI/dt at a lower frequency thereby lowering the speakers high frequency BW. All this is ignoring the affects of having to dissipate more heat.

Hmm... The cutoff caused by Le should be by w*Le=Re. Re does not change as a result of increased mass. I don't think this would have an effect either. The mass only influences the electrical impedance around resonance, right?

 

[markp]

At higher freqs the cone must reverse direction more often, right? Doesn't it make sense that it is harder for a heavier cone to be reversed more quickly? The distance might be less but the whole mass of the cone must be reversed. In addition to cone size, mass does matter.

 

[Svante]

Well as I see it when the cone begins to break up it enters
a region where its effective area decreases with increasing
frequency, one way of looking at it is the mass of the cone
the voice coil is driving is decreasing.

When the mass of the cone = the mass of the voice coil you
have an intrinsic upper frequency limit, and varying the voice
coil mass will vary this upper frequency limit.

So, there are two effects here, a reduced radiating area, which should make the level drop, and reduced mass, which should make the level go up. Are you saying that these balance each other, until the coil dominates the mass?