A little experiment

[augerpro]

In that thread about VC heating and it's effect I promised the guy I would run a test and post the results. Well I did and here they are. I'm using my center channel from my Bose Buster project: 3 way, Hivi M3N mid, Hivi M4N woofers, adn Dayton ND20 tweeter. In sealed box. Probably about the worst speaker to use for extended high SPL with all those little drivers and shielded motors to hold heat. Well the boxes are stuffed pretty well with acoustastuff so that should help. Anyhoo, the victim:






For music I used a 10 minute loop of Ministry- Filth Pig at about 99-102 dB. This is not easy listening music...control in red and after music in blue:





Hmmm...not bad. I'm happy to know this. But the next test isn't so easy. This right after the music test so no rest for the wicked little speaker. Five minutes of pink noise at 100-102 dB with 20v 8O input...my poor speaker...again control is red and after pink noise is blue





Well there is a difference. I blew up the scale adn it's roughly 2/3 dB at the worst. And my little mid's cone was very warm. Like a cup with hot coffee in it. A few resistors on the XO were hot and I could smell the hot glue I used on the components...I hope it was the hot glue...the only question I have now is how high of a level can the ECM8000 handle before the possibility of damage


Edit:Also the XO points are 300 and 3000 Hz. The MLS gate was 6.7 seconds so 160 Hz or so is the cutoff and up to roughly 300 Hz is rolled of more than it really is due to the gate.

[joecarrow]

I'd say that the difference is definitely there, but I'm surprised by how small it is. I think that part of the claim of trouble was the affect such a shift in parameters would have at the crossover frequency.

I see no major anomaly at 300 hz, but at 3khz I can see a slight deepening of the dip in response, perhaps one of the largest deviations. This could indicate a slight shift in summing due to a different Q in the filters due to higher DC resistance- not only in the voice coils, but in all components. Overall, I think that the difference between the two curves would be nearly inaudible.

Regardless, the original question was whether or not steep crossover slopes would be affected by this phenomenon. I believe that Jon answered this point adequately when he explained how he uses a sensitivity analysis for component tolerance to determine the effect on a finished design before he finalizes it.

The last thing that I think would be interesting to see would be a distortion sweep before and after such a stress test. If you feel like subjecting your gear to it once more, it might be interesting- if not, I think I'm satisfied. Thanks for carrying this out.

[augerpro]

Hey Joe, congrats on the job!

I was definitely expecting a bigger difference. I think RodE was refferring to higher order networks, not higher order slopes. Since it rarely takes a 4th order network to produce a 4th order slope I think that point is important to realize.

As far as another test, I dunno...I tend to look at the VC heat as similar to a lightbulb in that the temp probably drops very quickly as soon as current stops. You can see this effect in the control vs music, the change is almost negligible. It was for this reason that had to switch from pink noise to taking a measurement with maybe 3-4 seconds. So I don't think you'll see all that much in a THD plot after the first couple tones. Now that you have the idea stuck in my head i might have to test it... :g>

[Dennis H]

Brandon, I commented over on Jay's diyaudio rant but you deserve a more thorough thanks here. Beating on your speakers until they start to smell is going way above and beyond..... :T

[augerpro]

Brandon, I commented over on Jay's diyaudio rant but you deserve a more thorough thanks here. Beating on your speakers until they start to smell is going way above and beyond..... :T

Ha! Thanks Dennis. It seems that thread is getting crazy too so I guess we're not the only heathens on the web :rofl:

I do think the smell was teh glue. I think it was the resistors on mid notch filter that were pretty hot so they probably got the glue to smell. The speaker actually ok the whole time.

[JonMarsh]

Well, pinknoise is certainly a definitive torture test- much higher average power and heating effect than almost any kind of music (except maybe some older Stones stuff, and of course Santanna "Supernatural", which has absolutely NO dynamic range at all- that could have been such a nice album, but they mixed it purely for radio. Grrrrr!

Off to Raleigh/Durham tomorrow, then leaving for Austria for two weeks this coming Saturday, then another week on the road visiting customers (HP and others) the following week.


THEN, I've got ticket's to go out to Denver, and will be working there on the CC three way with RS52 and Peerless tweeter, finally! Woooooo hoooooo! :B

[TacoD]

Good news Jon!

Interesting read (imho that is ) about voice coil heating http://www.stereophile.com/reference/1106hot/index.html