Speaker Testing Suggestions?

[Jecarroll7]

Got in a 6x10 cab, with Jensens. One of the speakers is making a very intermittent low level buzz noise after 20-30 min of playing, but I haven't been able to narrow it down to which one.

I've been giving some thought to a speaker test station anyway and this cab renewed my interest.

I know there are some high end products out there that do a great job testing for rub/buzz, etc., but I'm trying to keep it within a DIY budget.

My speaker repair guy suggested this product,

woofertester.com

But was wondering if anyone else had a suggestion?

[Tom Phillips]

The "speaker tester" you linked is advertized to measure the parameters of a raw speaker. A cabinet designer would then use those parameters to design a cabinet. It doesn't seem to be what you need to solve a buzzing speaker problem.

Buzzing speakers in a multi speaker cabinet can be difficult to identify. You need to isolate the individual speakers. Note that, even if you unplug one speaker, it can still resonate and make noise when the other speakers are driven. Shorting the terminals of the disconnected speakers suppresses that effect and often allows you to troubleshoot the speakers while they are still mounted in the cab. If that doesn't help you find the offending speaker then you may need to remove them from the cab and drive them one at a time.

Something else that works for me is to drive the amp with a sine signal generator and tune the frequency to the exact value that causes the buzz continuously. When I get the speaker to buzz I reach in and push carefully on the spider of each speaker with my finger until I find the one that stops buzzing when pushed on. Only do this if you feel comfortable with the process and feel that you understand what is going on so you are sure you won't cause more damage. It would be much easier to demonstrate the process than to explain it. The volume level doesn't usually need to be very high and can often be reduced after the frequency is tuned to cause the continuous buzz. This process works great to isolate cabinet buzz too.

[Mick Bailey]

I also use a signal generator and amp. With test rigs some care needs to be taken to protect the driving amp from a speaker (or wiring) that shorts intermittently, so I include a 4 ohm resistor in series where this is a possibility. The volume level usually isn't even enough to disturb my snoozing cat. Buzzes that are difficult to elicit at a working frequency and volume will often reveal themselves at very low frequencies. Sweeping a signal generator also makes it easier to hear if a passive crossover is working.

The harder thing to locate is a tweeter that only breaks up at high volume.

[Audiotexan]

If you have an Android based phone, here's a handy alternative if you don't have a function generator already: FuncGen

The Crescendo Systems function generator can be used to generate sine, triangle, square and non repeating noise signals.
Additionally these carrier signals can be AM, FM or PM modulated or swept in frequency. The modulations can be enabled on their own or in any combination.

The square and triangle signals have a duty-cycle that can be set from 1% to 99%.

The ArbGen app adds frequency resolutions below 1Hz, automatic and customized saving of all parameters and the arbitrary function generator.

Absolutely wonderful app and it's free!
Just need to add the cabling as needed.

In this case, 1/8" stereo male (to phone), to 1/4" mono (to working amp)
and then plug your cab in and sweep as instructed above! =)

[Jecarroll7]

Some great suggestions, will try them all, thanks guys.

I found this case study on detecting loudspeaker rub & buzz from NI.

http://www.ni.com/white-paper/7289/en/

Test jig is simple enough, bungee cords to suspend the speaker, and a mic stand. wondering if an o'scope could be used to monitor the output of a mic amp as you do the freq sweep? Again, looking to compensate for older ears.

[nashvillebill]

I've got an old HP200CD sig gen, and it can drive a woofer by itself with (just enough) power to check for voice coil rubbing. Take the driver out of the cab, set it facing up on a table, hook it up to the sig gen, set the frequency to a couple of hundred Hz, and crank the output voltage up. Listen to the speaker and then slowly turn the frequency down. If the voice coil is rubbing, you can hear it as you get below 100 Hz. No external amp or o-scope required.

[g1]

Some excellent tips/methods here. Just wanted to mention that for things like this, (and mechanic type work), I find the stethoscope idea extremely useful. I use the poor man's version, just a piece of plastic hose held up to the ear. I suppose a funnel would make a good earpiece.
If you've never tried it, you wouldn't believe how much it isolates sounds.

[52 Bill]

I'm with Tom here, physical touching of the different parts of the speaker will find lots of buzzing problems. And you don't need to press that hard to make a difference in the noise. This works in single cabs or multiple cabs.

I've found loose voice coil dust caps, spiders that have come unglued from the basket, cones that are loose around the compliance, etc. just by feeding a signal through the amp and then lightly running my hand across the different sections of the speaker. Just the lightest touch will cause the part that is buzzing to change frequency or to stop buzzing all together. And if you're lucky, a little dab of glue will solve the problem.

Voice coil rubs can sometimes be found by lightly pushing and pulling on the folds of the spider, forcing the voice coil to move in the gap slightly. Fixing these is another story, but it can help in diagnosing the problem.

[Leo_Gnardo]

I'm with Tom here, physical touching of the different parts of the speaker will find lots of buzzing problems.

All good points. One of my methods, with an unmounted speaker, is holding the magnet/frame in one hand and lightly tapping on the cone with a finger on the other hand. If you hear a snare drum like noise, likely the speaker is rattly. Usually it's a rubbing voice coil, but occasionally it's a repairable loose dust cap or spider. I suppose it could be done with speakers mounted to baffles but 99% of the time I'm tap testing them unmounted.

[Mick Bailey]

I find the stethoscope idea extremely useful.

I have a proper mechanics stethoscope hanging up and every time I walk past it it dangles in my face! Why did I never think of using it on a speaker? Wouldn't think twice to listen to a rumbling bearing in an engine or transmission. Good tip. I'll promote it to the electronics bench.

[Tom Phillips]

I'm reminded of another cause of speaker buzzing that can drive you crazy but is easy to fix once diagnosed. It is caused by a small bit of magnetic FOD (Foreign Object Debris) held to the inside of the speaker cone near the voice coil or on the dust cover by the stray magnetic field. Under certain conditions just a small bit of metal can make a huge noise as it bounces on a hard dust cover. The first encounter I had was on a JBL D120F that sounded just like it was blown. When the speaker was removed from the cab I saw a tiny sliver of metal stuck to the aluminum dome. Plucking that sliver off with tweezers fixed the problem. Amazing how much noise that little sliver caused.

Just something to keep in mind.

Now I'm going to dig out the mechanics stethoscope stored with my old brake tools etc.

[Enzo]

When I need a stethoscope, I roll up a magazine and hold it to my ear.

As others mentioned, I might sweep the cab with a low frequency signal from a generator through an amp. usually you can find a tone that makes the buzz happen, so you can tube in on it. With a steady buzz it is easier to isolate it.


A test that usually works for when the speakers are out of the cab:

Hold the speaker in front of your face pointing away. Hold it with one hand by the frame, magnet facing you. Now ball up your fist and whack the back of the magnet. A good speaker will either do nothing or it will respond with a nice "tum" sound. A blown or rubbing speaker will make a sort of flap sound.

[Enzo]

And two other things. Sometimes inside the cab, a wire gets up against the back side of the cone. And particularly in combo amps, stuff can fall into the space between the frame and the back of the cone down in the bottom. Things like guitar picks, coins, odd buts of trash.


And sometimes debris gets between speaker and grille cloth.

[Leo_Gnardo]

And particularly in combo amps, stuff can fall into the space between the frame and the back of the cone down in the bottom. Things like guitar picks, coins, odd buts of trash.

True that! Some years back a young talented guitarist brought in the Super Reverb his pop played in the Blues Magoos. Besides the amp needing a lot of help, one of the speakers sounded shot. I took 'em all out for a thorough cleaning & exam. A very dusty Ruffles potato chip fell out from behind the frame of the "bad" one, then it was perfectly good. Smiles all around.

Sometimes you'll find a lot of lint captured between the grill cloth and cone. It's soft but if there's enough of it, can make a speaker sound lousy.

An example along the lines of Tom's post - our local recone guy related a rare mistake he made one day. A speaker sounded buzzy, as they do, a candidate for reconing. After he razored off the cone & spider, out fell a steel washer. "Had I looked closer, could have saved the hassle & looked like a hero." Careful observation can sometimes reveal a cheap trivial solution.

[The Dude]

An example along the lines of Tom's post - our local recone guy related a rare mistake he made one day. A speaker sounded buzzy, as they do, a candidate for reconing. After he razored off the cone & spider, out fell a steel washer. "Had I looked closer, could have saved the hassle & looked like a hero." Careful observation can sometimes reveal a cheap trivial solution.

Amen to that! I've witnessed guys hacking paper before even removing the dust cap and inspecting. I can't tell you how many speakers I've saved with a simple tinsel lead repair, removing some chunks of glue lodged in the gap, a surround replacement, or just re-gluing spiders or cones that have simply come loose. A customer will bring in a speaker for a complete recone and come back to pick it up with only a small repair charge for a little glue work or lead replacement. Never a bad thing.