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is it normal for a car sub to rattle in free air?

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Posted on December 28, 2016 at 15:35:54
thump
Audiophile

Posts: 416
Joined: April 19, 2016
 until i get the sealed box i'll be using for my JBL GT5 15" poly subwoofer, i've been breaking it in a little at low volumes in free air. today, i turned the volume up a little while checking some new tracks out that i'm planning on putting on my media player. i noticed that the cone was buzzing, even rattling, on tracks with really loud and deep bass. it sounds plastic like the dustcap is loose or something, but i'm wondering if maybe it's just an artifact of playing in free air or resistance from the stiff surround breaking in.

it sounded REALLY bad at first with the sub sitting on the corner of a flimsy milk crate along with the box my headphones came in and a pice of paper sitting on the crate too. i took them off, and put a thick piece of wood under the woofer and some camping mat style foam under the magnet leaving an air gap for cooling, and the buzzing seemed tamed, but still apparent. finally, i moved it to the tiles over cement floor on top of the pieces of foam, and can still hear the buzzing, but it seems like it might be decreasing.

is it normal for a sub to buzz like that in free air or as it's breaking in? it sounds a little like when a car body vibrates to bass, but higher pitched like it could be related to the poly cone and dustcap. i tried tapping it to see if i could hear anything loose, but couldn't

 

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RE: is it normal for a car sub to rattle in free air?, posted on December 28, 2016 at 15:53:45
PAR
 As playing a speaker drive unit in free air is a classic way that manufacturers check for gross manufacturing errors (rubbing voice coil etc.) your story is unfortunately not good.

 

RE: is it normal for a car sub to rattle in free air?, posted on December 28, 2016 at 17:30:50
airtime
Audiophile

Posts: 11287
Location: Arizona
Joined: February 4, 2003
 A driver should not rattle under any circumstance

 

RE: is it normal for a car sub to rattle in free air?, posted on December 28, 2016 at 18:17:12
thump
Audiophile

Posts: 416
Joined: April 19, 2016
 hmmm...
well, the speaker is still under warranty at least.

i can't say that it's ACTUALLY rattling, it's just making a noise that it shouldn't, but from both responses, it's making that noise because something's wrong

 

i started a ticket with JBL for repair or replacement, posted on December 28, 2016 at 18:44:51
thump
Audiophile

Posts: 416
Joined: April 19, 2016
 it hasn't even been 30 days yet.

if i didn't know better, i'd swear someone is breaking into my apartment when i'm away and sabotaging my system! my xduoo media player can't hold a charge for over 4 hours use, my sub is busted even before i installed it, and BOTH the battery & charger i bought for the system have corrosion already and the lights aren't working on the charger!

i've only used the system (except the media player) at low volumes, and always hooked the charger up whenever i was listening to music and unhooked it when i was done except for one night that i left it on because it was showing an 80% charge

 

RE: is it normal for a car sub to rattle in free air?, posted on December 29, 2016 at 20:15:18
Mr Peabody
Audiophile

Posts: 1109
Location: St. Louis, MO, USA
Joined: August 14, 2010
 I agree, you did the right thing by exchanging it or sending it back.

I was just curious what amp you will drive the sub with? You know a sealed box will require more power opposed to ported? Also, JBL should have given specs on what size box to use, or, various port designs for frequency response.

 

RE: i started a ticket with JBL for repair or replacement, posted on December 29, 2016 at 20:19:55
fantja
Audiophile

Posts: 15518
Location: Alabama
Joined: September 11, 2010
 keep us posted- thump.

 

RE: amps & specs, posted on December 29, 2016 at 22:34:11
thump
Audiophile

Posts: 416
Joined: April 19, 2016
 i'll be using it with a planet audio BB1200.4 in biamp mode with the rear channels bridged for the sub, and the fronts driving a pair of nice sounding 96dB proel flash 8Ps as mains. this is actually going to be a bicycle trailer system.

regarding ports, yes, i'm fully aware that they add distortion, i mean out of phase resonant harmonics that don't exist in a recording to give an illusion of bass, but i despise that bloated marshmallow sound. i like the tight thump of sealed speakers and and more than happy to sacrifice SPL for SQ.

it was a tiny pair of $100 4 1/2" infinity 2 ways a buddy demoed that gave me my very first taste of what my eras consider hifi despite numerous previous visits to 4 different shops. it was the first time i heard low distortion lightning quick tight bass and THAT was what wowed me (THUMP!), not big sloppy, put the boo in boom, klipshes etc. (that, and imaging)

this is just version 1.0 of the system until i find some part time work to be able to afford upgrades, and i'm thinking about a downward firing 4x12 fiberglass system with L/R mains on each side.

the JBL has outstanding specs for the 1.9 ft3 box i bought it for. JBL recommends 1.5-2.0 ft3 even though the Q is on the high side at 1.15, if memory serves me right. i already have a couple pounds of polyfill for an extra dB or 2, and will probably be adding bass boost to the sub.

i bought it as a compromise. the original system was intended to be 4x12, but would have cost a couple hundred dollars more, so i originally planned on getting a $20 MCM 15" for a quick fix and to move as much air as possible, budgeted for the JBL upgrade, then decided i'd stick with it for the weight and space savings once i had the brainstorm on how to turn it into a rolling lightshow without having to learn how to "arduino".

what i really wanted, besides cheap components to get started with is durability, so 96dB at 8 ohm mains and a 4 ohm sealed sub (hard to find a sealed box friendly single tap 4 ohm with a rubber surround and thiele small paremeters!) should be a relatively easy load on my amp.

i'm not looking to hit 150dB, just have a nice and clear sounding system for bike events, tailgate parties and downtown on club nights. i just went riding with "loud bike 2.0" a couple nights ago, and just like the ride i took before when he played funk, people were going nuts downtown and that bike just has a pair of 92dB peaveys and a 100w pioneer amp that wasn't even cranking.

my system will be louder and cleaner sounding with my really laid back but quick titanium waveguides, punchy 8" mids, and 93dB 15" with several times more power. it'll get the job done seeing all you really need for a little dance party is a boombox. my "boom box" will also have a sound activated RGB lightshow.

if you're at all interested in the system (haters hate bikes!) i already have a thread for it i'll add a link for.

 

RE: i started a ticket with JBL for repair or replacement, posted on December 29, 2016 at 22:56:46
thump
Audiophile

Posts: 416
Joined: April 19, 2016
 i'm kind of bummed out that i got the "bad unit" as reviews for the GT5-15 are almost universally positive and from what i could hear in free air, it had potential until i really upset it playing...

 

RE: i started a ticket with JBL for repair or replacement, posted on December 30, 2016 at 13:27:15
seas
Manufacturer

Posts: 138
Joined: December 21, 2006
 There may be absolutely nothing wrong with it. Subwoofers are not meant to be operated in free air. Without an air load behind the cone it is extremely easy to drive the woofer into over excursion. There are also some air noises to be expected from turbulence of air around the spokes, air venting through the magnet, voice coil, etc. Given that all the bass will cancel when used in open air you will overdrive it attempting to hear the bass. Drivers need enclosures to work properly unless they are specifically designed for open baffle usage. Subwoofers are particularly bad in this respect because their long excursions need to be controlled by the air load in the box.

 

RE: i started a ticket with JBL for repair or replacement, posted on December 31, 2016 at 00:35:18
thump
Audiophile

Posts: 416
Joined: April 19, 2016
 that's what i was thinking. there's absolutely nothing dampening the sub and it's nearly 100% cancellation. the cone was REALLY wobbling when it made the noise making me think it could have been reaching its maximum excursion, but i wanted to run it by someone more knowledgeable than me.

i have it bridged on the amp and probably turned the input level up so it would be audible. it could theoretically be hitting 600w, but that's giving the amp credit for its ambitious 4x300w rating. it could also be clipping.

it's really hard to know just what's going on without a cabinet, like you said. i might have been pushing it to lease breaking levels without knowing it because of the phase cancellations that are strongest at the lowest frequencies.

maybe before i assume it's busted, i should get the 1.9 cubic foot box i intend to use with it and test it at the same level outside some day. if it's excursion related, an air spring will totally cure that.

i did some break in with the 4x hifonic 12" subs i used to own playing 8Hz tones on my panasonic receiver and never heard anything except subtle air whooshing, but that was using less than 100w likely. i tried breaking the JBL in with an 8hz tone, but something in the signal chain was limiting the output so the cone didn't move over a centimeter if that where the hifonics wobbled like crazy.

even in free air though, BOTH subs can get obnoxious enough to bother the neighbors. when i had the issue, my mains were playing at a conversational level with the sub just adding a bit of weight to them.

i was skeptical that a sub rated 1200w peak would be causing a sub to buzz with half if that power, BUT then again, clipping could be playing a part too.

maybe tomorrow i'll switch the crossover to high pass and put my mains on them to get a better idea what i was pushing starting at a lower level. until i get a box where i can isolate the back wave, there's no way to know for sure how hard i was driving the sub, but by vague memory, it was hitting something like 30Hz with at least a half inch of excursion, which is more than 14mm, but my hifonics did that or more too with a lower rating i think.

this is why i created the thread. at lower volumes, there isn't any audible distortion. even at that same level, the buzzing was apparent only on a few tracks.

maybe it's just not that common for people to overdrive subs in free air. i want to believe that i was just pushing it past its limits, but, there isn't that much more of an air spring in a ported box either.

i could see the cone crashing on a backwave when it bottoms out creating EXACTLY the sound i heard. in fact, i'd say it sounds more like an impact buzz than a loose rattle.

before buying a big duffle bag so i can haul it in for repair, i'll try some diagnostics and test it in the box.

PS i'm listening to the "forbidden fruit" instrumental now on headphones at max level with EQ boost plus a level boost with winamp, and there's a subtle buzz going on there too, but just not a plastic sounding one.

thanks for your input.

 

it MIGHT have been a vent backwave distortion, posted on January 5, 2017 at 17:53:45
thump
Audiophile

Posts: 416
Joined: April 19, 2016
 i haven't taken them or the mains outside yet to test for distortion and volumes, but today, i tilted it down instead of propping it up on its magnet about an inch off the floor and even at low volumes, the bass was quite loud as i had the input pegged, but didn't hear any buzzing.

when i turned the input down to about 50%, there was still just a little bit too much bass on some tracks and apparently a lot of cancellation on others, but i didn't hear the buzzing.

MAYBE, just maybe what i was hearing was the sound of the magnet's vent bouncing off the floor or possibly choked a little by the narrow tunnels i created with the jigsaw puzzle edged foam pads i propped it up with. higher frequencies are less prone to cancellation than lows, and there's probably a venturi effect going on through the port too. at lower volumes, i thought i might be hearing a buzz coming out of the port.

before i go all over town looking for a duffle bag large enough that i'll be able to carry it on the bus with, i'll do some troubleshooting to see if there's a reasonable explanation for the anomaly.

 

my amp might have pushed the sub out of phase!, posted on January 18, 2017 at 02:32:13
thump
Audiophile

Posts: 416
Joined: April 19, 2016
 i just bought a multimeter tonight to test my battery voltage and found something REALLY strange when i decided to test what wattage i'm listening to right now on a pair of JBL northridge outdoor 2 ways i got a deal on.

the left channel consistently measured -.09v @20v setting and -.22a @10a or -3.98w if i did the math correctly and +.03v to +.04v and +.09a on the right or .72w.

the left channel is out of phase, so i might have been bridging L+ to R+ instead of L- to R+, and the woofer may have been firing out of phase and bottoming out.

does that make sense?

 

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