La Scala Rattle

[enigzenig]

I was playing some Patricia Barber and noticed a faint rattling sound coming from the right speaker on only specific high-mid-bass notes. At first I couldn't tell if this was in the recording or not. I quickly compared in mono the output from the left speaker. There is a slight difference between the perceived tightness of the two speakers (probably just a lack of rattle).

1. If I pressed the bass horn's mouth together with a few pounds of force, the rattle disappeared. Maybe the vibration of the mouth is causing the cabinet or another component to rattle? So I dug further.
2. I started taking off the back panel of the HF section and listened again ('86 industrial la scalas). It almost sounded like the cross-over was the source. I remounted the cross-over and bought some rubber disc pads to absorb the vibration. This doesn't seem to have resolved the issue.
3. I continued and removed the bottom access panel. If I pressed against the magnet during those particular notes, the rattle disappeared. I remounted the woofer and made sure the 4 screws are tight. Could it be the metal mesh that is vibrating?

I put it all back together and the rattle still persists on these particular notes. The cabinet appears to be structurally strong (no separating joints). This issue is not apparent unless you’re critically listening on specific bass solos. It still bothers me to know that it is there. Does anybody have troubleshooting tips? I could re-enforce the mouth I suppose to indirectly fix the issue.

Shawn

[Don Richard]

Check to see that all of the components on the crossover and the horns in the top end are tight. Make sure that the access door gasket is OK and that all those screws are tight and not stripped out. Check to see if there is a loose wire in the woofer circuit. Also, someone may have overtightened the woofer mounting screws causing a voice coil rub, be careful here because these are stamped frame woofers and the frame can warp if not evenly tightened.

[colterphoto1]

THis may sound simple, but if it happens in only one cabinet, switch speaker wires to the other cabinet. If the buzz is not present, then it's in cabinet #1. Sounds like you've tracked down all physical causes of buzzes except for any odd delaminations in the wood. I think it may be the voice coil of the woofer. It's a chore, but try swapping woofer, put the suspect unit in the 'good' cabinet and play the same passage.

If you have access to a frequency generator, set it to the offending frequency and that makes it easier to track down.