RCF TT08-A II at 44Hz/88Hz (F0/F1) early clipping and rattling

[jazz767]

I recently bought a pair of the the highly recommended (within this forum) RCF TT08-A II (MK2) Speakers. I bought them “almost new”, still with some warranty, from a professional, and they look optically like 100% new. They sound mostly wonderful even compared to a QSC K8.2 which I had available for testing at the same time, but with the exception of two notes: The F0 (44 Hz) and the F1 (88Hz). If these notes are played very loudly, the speaker rattles a lot, the F0 more the the F1. One semitone next to it hardly rattles at all, and one whole tone next to it not at all, no matter how loud I play. It is in a dynamic range in which the other tones in this pitch (and the full piano) still all sound very good.

I can rule out that it's the front cover as I took it off to test it. I made sure that the woofer screws were tight. Nothing on the speaker or woofer is wobbling. It seems that the rattling is coming from the woofer (also by feeling with my hands for vibrations).

For diagnosis, I generated various sine tones on the PC and sent them directly (audio interface) to the speaker (and I also tested another keyboard, from different brand). With the sine tone, one can already hear a clipping noise at 44 Hz (an additional tone an octave higher) when it is still quiet, and the same at 88 Hz. If you adjust the frequency just a little, at 42/84 Hz, this is gone. Only when the volume is much higher does the speaker start to rattle, because clipping naturally sends very strong impulses to the woofer.

So the speaker starts clipping very early at exactly 44 / 88 Hz, but not just a semitone off. I therefore get the impression that the electronics or the software (filter?) of the speaker is poorly tuned here and causes the clipping, and that the rattling is then, at higher volumes, caused by the clipping. Since both speakers in the pair behave in exactly the same way, I don't believe there is any mechanical damage.

However, other keyboard players should also have had or are having bad experiences with these sounds. That's why I'm reporting it here in the forum, and would be very grateful for feedback from other RCF TT08-A II (MK2) players. If it is the electronics or filters that are responsible for this, you should also have a different behavior between the first and the second (II, MK2) version of the box, as they differ due to the new DSP.

Also, I wonder whether I should make use of the warranty, or wether this is normal. Note that when playing any other kind of music, one can not observe (at leat not hear ) this, and it seems to be a kind of piano or keyboard-specific issue .

[cphollis]

There may be a problem with your units -- or not. You did the right thing and looked for vibrating elements within the speaker: screws, washers and whatnot.

Did you consider what might be nearby that might be rattling as well? Speaker pole or stand hardware perhaps? Something else that's always nearby and can make a lot of noise?

I ran into this with a pair of QSC K8.2 which put out a lot of power. I swore they were defective until I put them on a piece of foam and tested them that way. The speaker poles themselves had a resonant frequency which made them vibrate.

Best of luck!

[jazz767]

Did you consider what might be nearby that might be rattling as well? Speaker pole or stand hardware perhaps? Something else that's always nearby and can make a lot of noise?

Thanks for fast reply and this hint. I think I can exclude things nearby, as I tested the speakers in different positions within a room and even in a different room, on the floor and on different chairs. I don't use a speaker pole (yet).

[jazz767]

Just in case anyone stumbles across the same problem (maybe once your speakers get older): Here's how I solved the rattling problem: I got an RDNET Control 2 to connect the speaker's internal equalizer to my PC. In the frequency range where the rattling was occurring, I set a very narrow-band filter with the following settings: -12 dB at 92 Hz and Q=10 (so quite narrow).

The rattling is practically gone. When I play, I don't notice a hole in the dynamics in this range, not even when I play chromatic scales. When measuring with white noise, you can of course see a difference, but there is no hole, instead the drop in the bass range occurs at a few Hz higher. This is because the unchanged frequency response has a maximum at 92Hz and then drops off rapidly to the left anyway. You can see this clearly in the attached diagram, which is taken from the RCF specs:

Frequency_response_unchanged_RCF.png (27.88 KiB) Viewed 1074 times

I have no reliable explanation for all this. I can rule out room modes, as I have also measured in the garden (to the delight of my neighbors ). One guess is, it is due to wear on the woofer, which no longer damps as well as a new one. It could also be that the RCF engineers have overdone it a bit with the electronic bass boost and pushed it to its limits. It doesn't seem to me to be a coincidence that the rattling occurs precisely at the frequencies that still have full response (even a maxium) in the bass range. Or it's more likely a combination of both, or something else that I didn't understand.

I had to spend around 200,- € for the RDNET control, but I got my used speakers for a very fair price, and I am now very happy with the speakers.

[cphollis]

Glad you found a solution -- but that's a pretty severe notch you had to use.

I always used my 8" units with a small, inexpensive powered woofer and sent everything <120Hz that way. The RCF TT08as (first gen) were able to breathe much better with a more musical sounding result, and obviously any bass frequencies were better covered that way.

Doesn't have to be expensive, either -- just another piece of gear to lug around