# Loudspeaker Drivers and Gravity

#### René - Acculution.com

##### Senior Member
Technical Expert
I was recently asked what the effect of mounting drivers vertically (planar with the floor) versus traditional mounting with the driver axis pointing towards the listener. The former is for example the case for the BeoLab5:

In both cases gravity will inevitably act on the driver, and if a particular mounting is problematic, then it would be expected that others will be too (because of the shape of the driver and the torques involved), and it would seem empirically that no such issues have been reported.

Anyway, it is still good to do some modelling of this. I was asked if it would be easy enough to simulate with COMSOL, but for the case of a vertically mounted driver, it can actually be done much simpler.

The gravity will act on the distributed mass of the driver, and so one can look up the moving mass and do the calculations, first via Newton's second law:

F_mass=Mms*g

where g is 9.82 m/s^2. As this is countered by the stiffness of the driver setup (surround and spider), one can set the two equal with Hooke's law with the stiffness Kms=1/Cms:

Mms*g=Kms*x

such that

x=g*Mms/Kms

So, for a particular case that I simulated years back, I find that

x=9.82 m/s^2 * 8 g / (0.43 mm/N)^-1

which is equal to 0.033 mm.

This can also be simulated via a Finite Element Setup, and so I made a static study with gravity applied to the old case in question:

As can be seen, the displacement value found via this numerical setup gives a similar value of 0.03 mm static displacement.

This initial displacement that the entire frequency response now is a perturbation to is not likely to affect any conclusions, especially not when taking into account the manufacturing tolerances.

The actual displacement of course needs to be evaluate on a case-by-case basis, but nothing alarming is seen in this quick investigation.

---

Simulations are great, and I typically trust them (mine, at least) over measurements, and have often found via simulation that measurements had to be wrong. But any simulation should be accompanied by a simpler analytical model or description for sanity checks, and for the present case an analytical approach is much faster and just as a accurate as a full-blown simulation.

Merry X-Mas from Acculution!
René

The 10" driver has a stiffer suspension with VAS = 69.3 liter. The in-box resonance is not quite as high and better use is made of the available amplifier power. The higher stiffness also reduces the static displacement of the down firing cone to a mere 0.23 mm. Thus the 10" driver was chosen.

If there is any interest, I can look also at how the static deformation looks in a standard configuration, which needs then to be done in a 3D setup.

Hi Rene, thank for the analysis.
I'm not really concerned with driver mounting orientation when the driver is under the control of the motor. I know there may be some slight decentering and asymmetrical excursions, but seems like those are probably very fine details.

I am concerned however, with voice coil decentering that results from static cone sag when drivers face upwards or downwards.
I had some 18" BMS sub drivers I needed to mount that way, and asked BMS about it. They said in a perfect world, drivers would be playing 24/7 and solve cone sag
But over time the 18n862's would definitely decenter, sooner or later.
So I built an anti-gravity DC voltage source to lift the cones & coil when system was off. Has worked well.

I saw an old timers-formula somewhere as to when static cone sag should be considered an issue, but can't find it, and don't know it's validity anyway.
Is there any kind of analysis that can assess when a driver's size, weight, suspension, etc...make it susceptible to cone sag when mounted horizontally?

Thanks for the knowledge. What about angled drivers like 45 degree? I think rocking will be a concern. How bad would the rocking behavior be?

Hi Rene, thank for the analysis.
I'm not really concerned with driver mounting orientation when the driver is under the control of the motor. I know there may be some slight decentering and asymmetrical excursions, but seems like those are probably very fine details.

I am concerned however, with voice coil decentering that results from static cone sag when drivers face upwards or downwards.
I had some 18" BMS sub drivers I needed to mount that way, and asked BMS about it. They said in a perfect world, drivers would be playing 24/7 and solve cone sag
But over time the 18n862's would definitely decenter, sooner or later.
So I built an anti-gravity DC voltage source to lift the cones & coil when system was off. Has worked well.

I saw an old timers-formula somewhere as to when static cone sag should be considered an issue, but can't find it, and don't know it's validity anyway.
Is there any kind of analysis that can assess when a driver's size, weight, suspension, etc...make it susceptible to cone sag when mounted horizontally?
So in the above the driver is mounted up/down, without any regards to the motor, and so the displacement shown is the 'sag'. But perhaps you are thinking of creep, in that over time the displacement increases? I did calculate the DC voltage needed to raise it up, but I would imagine that that is not a great idea considering the heat that is produced?

Thanks for the knowledge. What about angled drivers like 45 degree? I think rocking will be a concern. How bad would the rocking behavior be?
So here I am looking purely at static conditions, and so a rocking investigation would require running a frequency sweep with gravity pre-stressing the setup. I can be done, but first it would be most informative to look at a 3D static simulation of the static displacement with that angle. I might get around to it at some point in time.

So in the above the driver is mounted up/down, without any regards to the motor, and so the displacement shown is the 'sag'. But perhaps you are thinking of creep, in that over time the displacement increases? I did calculate the DC voltage needed to raise it up, but I would imagine that that is not a great idea considering the heat that is produced?
Not sure what you mean by creep...maybe it's what i'm trying to describe, which is a permanent shift of the voice coils resting position in the motor.
Increases over longer time period. And decenters the voice coil even when returned to normal vertical surface mounting.

Maybe this will help describe.....
A thread on the sub and the voltage applied to offset gravity. (i'm the poster)
As you'll see, voltage and heat are pretty trivial.

So in the above the driver is mounted up/down, without any regards to the motor, and so the displacement shown is the 'sag'. But perhaps you are thinking of creep, in that over time the displacement increases? I did calculate the DC voltage needed to raise it up, but I would imagine that that is not a great idea considering the heat that is produced?
Curious about how much DC voltage it takes as many amps have some DC offset.

Back in the day Adire Audio had a nice writeup on this:

Not sure what you mean by creep...maybe it's what i'm trying to describe, which is a permanent shift of the voice coils resting position in the motor.
Increases over longer time period. And decenters the voice coil even when returned to normal vertical surface mounting.

Maybe this will help describe.....
A thread on the sub and the voltage applied to offset gravity. (i'm the poster)
As you'll see, voltage and heat are pretty trivial.
If there in addition to the static displacement from the mass is a displacement over time then that is creep.

Curious about how much DC voltage it takes as many amps have some DC offset.
Voltage is Kms Re / (Bl x).

I meant how many volts. Do you have an actual voltage?

I meant how many volts. Do you have an actual voltage?
Depends on the driver. The one I looked at does not exist physically. Maybe a 100 mV for some drivers.

The big drivers (of yore) from JBL & Altec often benefit from an occasional 180 degree reorientation to minimize the effects of gravity on their suspensions.

^^^ Altec 515B @ 26 lbs per https://www.lansingheritage.org/html/altec/specs/components/515b.htm

Lowther twincone "fullrange" drivers (at least the classic ones with enormous Alcomax magnets) were (are) notorious for sagging issues and were/are reputed to require periodic reorientation so as not to drag with audible consequences. Some of the more modern Lowthers use rare-earth magnets and are presumably less massive.

In volts?
I use 0.8V for BMS 18n862.
Has proven effective over several years, in countering cone sag.

Last edited:
The big drivers (of yore) from JBL & Altec often benefit from an occasional 180 degree reorientation to minimize the effects of gravity on their suspensions.

^^^ Altec 515B @ 26 lbs per https://www.lansingheritage.org/html/altec/specs/components/515b.htm

Lowther twincone "fullrange" drivers (at least the classic ones with enormous Alcomax magnets) were (are) notorious for sagging issues and were/are reputed to require periodic reorientation so as not to drag with audible consequences. Some of the more modern Lowthers use rare-earth magnets and are presumably less massive.
That is interesting to see. I did think that if an up/down orientation is problematic, then so are any other orientations, as you show here.

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