What is Resonate Frequency?

cavernplayer · Post by **cavernplayer** » Mon Feb 05, 2007 5:59 pm

Can you tell me exactly what resonate frequency is and how this applies to my problem? I own an Ampeg 115HP which has amazing sound however certain notes (just a few) causes a vibration or buzzing coming from the cab. I was told that it might be the resonate frequency of the room. What's that mean? Thanks.

cp

charlyg · Post by **charlyg** » Mon Feb 05, 2007 6:07 pm

Our church is in a metal building. You could look at it as a giant tuning fork I suppose! When I play a Bb, the note just keeps sustaining, as that is close to the resonant frequency of the room. In other words, the metal of the building, will vibrate and when you hit "resonance", the strings feel that vibration and they become "sympathetic", or self sustaining for a bit, if you will. Parts of your cabinet "could" exhibit the same behavior as my bass strings and the building. I don't get buzzing, I just get massive sustain at resonance, and you can feel a bit of the room vibration.

Somebody will happen by with a clearer definition.

beatlefreak · Post by **beatlefreak** » Mon Feb 05, 2007 6:30 pm

That's actually a pretty good explanation, Charly. Steve, as Charly has pointed out, it's called a resonant frequency, and a lot more than the room can exhibit it. Your speaker cabinet has a resonant frequency. It's one of the reasons sound damping measures (insulation) is used in cabinets - to kill the resonances. Even the speaker has an fR, where it will output better than other frequencies.

cavernplayer · Post by **cavernplayer** » Mon Feb 05, 2007 6:42 pm

Is something like that correctable or would it sound different in another room. Would I need to have the amp serviced or is it simply that way and that's that? Thanks.

blueflamerick · Post by **blueflamerick** » Mon Feb 05, 2007 6:51 pm

Resonance is the large amplitude variation that occurs when the frequency of some driving force, such as your bass, closely matches the natural oscillation frequency of the object on which it acts (the amp or the room.)

This is how opera singers can shatter a glass. They hold out a note that matches the resonant frequency of the glass, causing it to vibrate. Then longer the note is held, the larger the amplitude of the vibrations of the glass. At some point, the vibrations will cause the glass to break.

You could try stuffing a blanket or something similar in the amp. Before that, try moving the amp to a different location in the room or point it in a different direction. Try moving the amp off of the floor or putting some foam or that blanket between the amp and the floor. You could always tweak the EQ on the amp too.

jingle_jangle · Post by **jingle_jangle** » Mon Feb 05, 2007 7:04 pm

Whenever I play my 4003 through my small but powerful SS bass amp, and hit a "G" on one of the the top two strings, my old Hemosch archtop sitting on its stand nearby sets up an incredible, gnarly sympathetic resonance--you can actually SEE the "G" string on the Hemosch vibrating so violently it blurs.

charlyg · Post by **charlyg** » Mon Feb 05, 2007 7:11 pm

I forgot to mention that the Martin that is on "stage" gets to vibrating. It is the A string, and ANYTHING sets it off. It will just sit there and for what looks like no reason, it starts to "play" the A string. I did say that my Bb was close, well, it is probably a little closer to an A, although no other guitar exhibits this behavior. We have to make sure he wraps the strap around the strings when he sets it down and walks away. I can't count the number of times I have ran over to grab the neck to stop the dang thing!

jwr2 · Post by **jwr2** » Mon Feb 05, 2007 7:53 pm

the old fender bassman amps were notorious for vibrating and rattling at certain frequencies ...

Also amps and cabinets can have standing waves ... they will be louder for a D or G note than an A or E note ... a good manufacturer will size the cabinet and install baffles to minimize this ...

I had a small combo 1x10 bass amp that vibrated badly ... I took it apart and fond the problem ... I bent a small piece of metal and put some rubber inside and it stopped the annoying sound ...

johnallg · Post by **johnallg** » Mon Feb 05, 2007 8:14 pm

Good point, Jeff. I was going to suggest making sure all screws and bolts on the offending amp were tight.

teb · Post by **teb** » Tue Feb 06, 2007 5:51 pm

Another trick when you build speaker cabinets is to glue and/or screw a chunk of plywood (like the circular cutout scrap piece from making the speaker baffle) to the inside of any big expanses of cabinet (like the back). This acts as a damper to help keep that big pirce from resonating. If checking all the screws and attachments didn't produce results, I'd probably start sticking 8"-10" square or circular chunks of 3/4" ply on the inside walls (starting with the back) and not be terribly symmetrical about it.

jaybic · Post by **jaybic** » Wed Feb 07, 2007 5:28 am

Like many stories here, our practice space is a metal shed with a concrete floor. When I hit an open D - the place positivly rattles. And a low G gets the drums rattling a bit too.

johnhall · Post by **johnhall** » Wed Feb 07, 2007 6:18 am

Both buildings and amps can have resonant notes.

I've forgotten whether the Ampeg is open or closed backs, but any closed back enclosure is easily tunable.

Remove the back and install two or three diagonal strengthening braces inside the back. The can be something like 1 x 2 pine or strips of plywood, glued and screwed to the back along the thin edge. Don't make them parallel and don't make them the same length.

Basically, you're trying to make the back into separately resonating panels, each of which has a different size and shape, and therefore each having a different resonance. You can continue to tune the cabinet by trimming off sections of this new bracing; put your hand on the various sections to feel which one is vibrating most, for instance and trim that perimeter to raise it. Add bracing to lower it.

Over the years I've tuned dozens of cabinets this way and wondered why so many makers don't do this themselves to a greater extent.

drathbun · Post by **drathbun** » Wed Feb 07, 2007 7:41 am

That A string resonating in sympathy with a bass note is how the opening to "I Feel Fine" was invented.

edski · Post by **edski** » Wed Feb 07, 2007 1:58 pm

Cool thread. Resonances can be set up in many places. I've had places with D's and G's resonating, some E flats.

Back in college we had to watch footage of the Tacoma Narrows bridge in Washintong State. It was built in the 1930's and one day a few years (?) later a storm blew winds over the bridge that caused a resonance in the bridge. It took a few hours until the bridge was vibrating so much that it did the opera singer/wine glass trick.

blueflamerick · Post by **blueflamerick** » Wed Feb 07, 2007 4:58 pm

Actually, the collapse of the Tacoma Narrows bridge was due to aeroelastic flutter.