Vas variation with temperature?

[AndrewT]

Hi,
I have been running a big driver for 3days (@250mV) to run in the surround ready for testing.
I set it to free air resonant frequency of 43Hz and came back to it today. Fs is now 69Hz.
But, the workshop temperature has dropped to 14degC from the 20degC it was at the start.

Could the temperature have affected the Fs or is the change more to do with running in?

Tried searching but could not find anything similar.

[Svante]

It would mean that Cms has changed by a factor of (43/69)²=0.39. That's a huge change...

Sure there is some sensitivity to temperature, and I would guess that the suspension would get stiffer with sinking temperature, but not that much for that small a change. Breaking a driver in would typically lower the resonance.

Are you sure about those measurements? How were they done?

[stokessd]

That's a Huge change in Fs in my experience. I have a set of Vifa PL18 whose Fs rose by about 4 Hz over 5 years of sitting in my basement, even after flopping them on a function generator for a couple days.

FS should go up as temperature goes down, but that seems like a small temperature change and a large Fs change.

Sheldon

[augerpro]

Newb opinion here but I just started measuring T/S with SE a few weeks ago and I noticed a large difference in FS if the drivers were cold. By cold I mean middle 30 degree F. It's winter here in Denver and my measurement rig is out in the garage. I had to make all my measurements with drivers warm and sitting inside the house. Also the Re was about .1 or .2 ohms lower in the cold drivers.

[Circlotron]

When I was was messing with T/S measurement a couple of years ago I found the Q of the resonance dropped substantially if the driver was simply held or suspended in mid-air. If the back of the magnet was held against a large (non-magnetic) mass so as to prevent the mass of the frame moving the opposite direction to mass of the cone, then much higher and more consistent Q measurements were had. I can't see it affecting Fs but it's worth keeping in mind. Just don't block off any vent at the back of the centre pole piece!

[AndrewT]

Hi,
this first driver was tested in my normal way. Suspended in a vertical position, but very near a wall.
Oscillator into series resistor and then to speaker. DVM across the driver test leads.
This speaker is secondhand but a lightly used small venue PA bass unit. I had not expected any change due to running in but maybe it stffens up when left idle for a while so that's why I decided to try long term running.

I checked Fs some weeks ago and last week's Fs was near identical. Left it for three days (too busy with other jobs) and in the meantime winter arrived properly. I measured Q=0.255 when Fs=43Hz. But I see little point measuring it at that low temperature (it will never be used like that).

I noted that the voltage on the DMM was low.
Odd, that's not the way I left it.
Sure enough as I raised the frequency, the driver voltage came up and Fs turned out at the higher figure.

I have it's partner to measure & I'll repeat the initial check and after some days running in.

[AKN]

Hi,

Zaph has done some testing on a driver in different temperatures.
http://www.zaphaudio.com/temp.html
He tested with large temp differences but the fs change is substantial.

Could it also be that your instruments and amp got affected by the temperature change?

[AndrewT]

Hi,
I don't think the instrumentation was wrong or had changed.
No amplifier, oscillator drives 220r+driver directly.
Setting on the oscillator matched the frequency readings from the DVM.

Thanks for that link, he used a much wider temperature range but is confirming BIG changes in these two parameters.

[richie00boy]

Maybe not really part of the issue but are you sure you are only driving the speaker with 250mV? That's hardly enough to run in IME.

That's a big change in Fs, I think temperature would have an effect, but maybe not that much.

I have found that Vas seems to be affected by humidity as well.

[AndrewT]

Hi,
the surround is pleated fabric with some shiny black goo that is very slightly tacky. They appear very stiff when I try to push the cone in towards the magnet. But I think that's fairly normal for a PA type driver aimed at high efficiency. The pair are claimed at 103db/2.8V (8//8ohm) in the cabinet.