Film-Tech Forum ARCHIVE: Speaker Pops on a JBL 2245H

This is topic Speaker Pops on a JBL 2245H in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Trevor Meusx (Member # 10737) on 03-02-2019, 04:28 PM:

I have a JBL 18 in sub. 2245h. Stickers worn off, but what I've read from JBL specs online, it's 600w 8ohms. I'm testing it out on a QSC mx 700 2 Chan amp, 150wper channel 8ohms, and when I push it to 💯 the speaker gives these pops. Like the coils hitting the back of the magnet or something. At only 150w?!!! Give or take. Definitely No clip at the input. The speaker seems fine otherwise. No scraping noises in the coil when I depress the cone. No visible tears. I thought it may be an amp issue, so I tried another amp. Same thing. Although it did seem to take more wattage to replicate. No more than 300. Sorry for the long story thinking you may have some sage wisdom regarding. I will test the speaker again with a low frequency sweeper more in depth but I was mainly wondering if there's a good way to test the output of the amps for spikes without using an oscilloscope.
If you've read this far...thanks for your time. Ha!

Posted by Carsten Kurz (Member # 5396) on 03-02-2019, 06:31 PM:

Is that speaker built into an actual cabinet - or do you test it 'as is'? I don't know your background, but, depending on your knowledge level, you are probably doing a few things fundamentally wrong.

- carsten

Posted by Steve Guttag (Member # 268) on 03-02-2019, 08:00 PM:

The 2245 isn't a 600W driver on a good day.

You need to learn how to read the specs. 600W "program" is a made up number for power inflation. It is nothing more than double (+3dB) of continuous sine wave. So it is a 300W driver.

A driver has to be loaded (e.g. be in a cabinet that loads it). Once you go below the loading frequency of the cabinet (e.g. the port frequency), it is the same as that driver flapping around outside of the cabinet. It isn't that the voice coil can't handle 300W, it is the mechanical nature of the driver, unloaded will not operate properly.

If you were to look at the specs of the cabinet, as used in Cinema, you had the 4645 (no suffix, 9ft^3, three port tubes), it is to be high-passed at 30Hz.

http://www.jblproservice.com/pdf/Theatre%20Series/4645.pdf

The system is down 3dB at 35Hz and 10dB at 25Hz so you can see it is falling pretty fast and hence the recommendation of high-pass at 30Hz as that is likely where the port tuning is (give or take 30% of the port tuning frequency "fb"

The standard test for the 2245H out of the cabinet is to only apply 10V sine wave and to sweep from 20Hz to 1.2KHz, at mere 12.5W! Again, it isn't the voice coil power handling/cooling that is the limit.

http://www.jblproservice.com/pdf/Discrete%20Sales%20Models/2245H.pdf

Speaking of which, if the driver is out of the cabinet, be aware, it vents from the rear and one has to prop the driver up a bit so it can breath out of the rear vent.

If you look at the T-S parameters of the 2245, it actually has a pretty low resonance frequency (fs), it goes down to 20Hz! However, you need something akin to a 12ft^3 box to do that with appropriate port and B6 alignment.

Note, for subwoofers, ALWAYS use the continuous sine or preferably pink noise rating. You can get away with the program rating on stage/surrounds as that more accurately reflects what goes to those channels. Subwoofers are typically fed continuous deep bass in cinemas for effects.