Quote:
Originally Posted by PatronT4R
Awesome post! Quick question about your subwoofer. A month ago I replaced the factory JBL sub with the same rockford sub that you posted. When volume on the stock headunit is around 40, it makes a knocking sound (almost like 2 wood blocks clanking together) when the beat hits. I contacted crutchfield and they had no answers for me as to why this is happening. I am wondering, have you experienced a similar sound with your install?
Also, I replaced only the subwoofer nothing else. Before purchasing, crutchfield recommended it as a direct replacement to the factory subwoofer. Stock headunit and amp all remain.
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Patron, it's likely one of 2 things. Either you're clipping the JBL amp's subwoofer channels or you're bottoming the driver.
Years ago, someone did a tear down and identified the output chips in the JBL amp. IIRC, I think realistically, they only produced a clean 20-30 wpc into 2 ohms. The 45wpc claimed by Toyota was at 10% THD. So, as you can see, it wouldn't take much to run out of steam with the JBL amp.
The other problem might be the accuracy of R-F's specifications for the driver. Car audio marketing often plays fast-and-loose with specs. Assuming things haven't changed since I did the same JBL<>R-F driver swap 4 years ago, R-F claimed a little over 8mm of Xmax. There's no standard engineering definition for measuring Xmax and R-F doesn't state what they actually mean...so we can only simulate assuming it means usable cone travel. Without definition, they could actually mean peak-to-peak which gives only 4mm of one way (usable) excursion. Anyway. According to software simulations, the R-F should be able to handle about 50 watts per coil before reaching 8mm of one-way excursion in the factory .6 ft^3 box. However, if R-F actually means 8mm of peak-to-peak travel (4mm one way), then the JBL amp is capable of bottoming the driver before clipping.
When I initially installed the R-F, I got some heavy distortion at relatively low volumes. Apparently, the driver needed a little bit of break-in (loosen up the suspension), because the distortion went away fairly quickly, suggesting it was the amp clipping trying to drive the stiff suspension.
I would double check your mounting to make sure the driver seal is air tight. A small leak effectively removes the box's air spring and runs the driver infinite baffle, which it's not designed for and will bottom with very little power input.
I personally never go above mid-30s for volume myself...I'll try to push it next time I'm driving and see if I can drive it into distortion.