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3 4" Aeroports with only one causing strong resonance at certain frequencies when played very loud


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Hi,

I have a new box i just built with 3 4" Aerports made of PVC pipe, they are adjustable. The box is about 4.4 cu ft after all displacements and the Aeroports are tuned to 35.6Hz, its made of 3/4" MDF and has a double baffle with some 1" dowel rods inside for bracing. The 2 subs in the box are the 2nd gen Alpine Type R 12's running at a bit over 600w RMS @ 2 Ohms each from an Alpine amp each per sub.

The box sounds great and gets decently low for my liking, however I notice one song in particular that when the bass plays at a certain frequency for a long hold its causes a VERY loud grinding/ vibrating resonance sound thoughout the cabin in my 2007 Rav4, I notice the sound outside the car too vibrating on the roof with the door closed, so I inspected the box to try and isolate the problem and with the ports hanging out the back of the back of my car i notice that same resonance is coming quite pronounced from the lowest Aeroport at the bottom, the other ports are not making this sound as is nowhere else on the box. The sound was isolated to that port making it easy to diagnose as i had half the box outside the car with the back door wide open. I have tried separating the 2 halves of the port just a little to see if the inner joins weren't chattering against each other and this did nothing, same sound. I think the speaker wire used for jumpers and running to the terminals is probably wedged between the bottom and the middle port, I aimed for stuffing them under the lowest port during installation but that didnt happen. The jumpers kinda just hang down my the back of the sub, when installing. Im not sure that vibrating on the suspect port would be causing the sound though, it appears to be resonating down the walls of the port. I can watch the speaker wire from the other sub during the bass drops through the ports and they dont even move a whisper at all. I have tried tightening the screws holding the Aeroports on and this did nothing, btw there is thick weatherstrip all the way around where the aeroports are screwed in. Another thing the ports are all VERY stiff and more or less have NO movement/play in them if wiggled which is the only reason they are not braced. In saying that their could be a very slight chance that there is a VERY small amount (less than 1mm) of movement causing the resonance, when i stick my arm in there and carefully brace the port quite hard with my fingertips it still makes the sound (being careful not to let my forearm touch the inner walls of the port).

I notice if I pull the box way over to the right hand side of the cargo area so the ports are not so close to the inside LHS wall of the car (the subs face my back when im in the drivers seat) the resonance is dramatically reduced, however it is still there and a fairly annoying. I feel as though if I remove the resonance coming out of the suspect Aeroport it might cause less resonance in the Rav's cabin?

Btw I have some rings of ply where I screw the sub in to give a better grip when I tighten them down, works well, just so you know this is not a 3rd baffle.

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Any ideas on how I could get it to stop making that noise? Ideas why it may be doing that, would bracing the suspect port be worth doing? Something I could add to the inside of the box near the port to stop it resonating in that bottom RHS back corner?? Any ideas would be great.

Cheers.

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The noise you are getting could be from a couple things.  Your port area is a bit undersized for the power you are running, not drastically so, but still undersized enough that I would expect you are getting some port noise.  One way to isolate the problem would be to use a sine wave generator app on your phone and determine exactly what frequency you are getting the noise at while playing at moderate volume.  Take caution when doing this since sine wave tones played near your tuning frequency put a lot of heat into your subs.  

Another possible source of the noise could be a panel vibration somewhere.  This will also likely happen at/near your box's tuning frequency where stresses are the highest.  I would check and make sure both your bracing dowels are still firmly connected on both ends.  While testing you can try pressing on the outside of your box on different places and see if that changes anything.  

Let us know what you find and I can probably help direct you further.  

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"Nothing prevents people from knowing the truth more than the belief they already know it."
"Making bass is easy, making music is the hard part."

Builds:

U7qkMTL.jpg  LgPgE9w.jpg  Od2G3u1.jpg  xMyLoO1.jpg  9pAlXUK.jpg

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Hmm interesting, yeah I reckon it is port noise, there is a little but you can basically not hear any at all normally. Im not sure I would increase the port size to 2 x 6" Aeroports for another build as I do like there to be a little bit of compression in the air coming out of the ports. I think I just have to accept that there are some frequencies that will just be a bitch when it comes to causing resonance and causing things to vibrate, especially at very high volume, if I notice annoying sounds I just use the bass knob near the steering wheel to make it go away by turning it down a little. If its music from my folders I usually know what its gonna do when the song comes on anyways so Im sorta prepared.

I dont really want to change tuning as I like it right there as I like it to rattle my head a bit at some of the higher bass frequencies, its a win-win really as the tuning is a bit high but the size of the box makes it pretty damn low anyways, not if I go to 37.5Hz though the lows are really depleted, but SQ is ALOT better. I could no longer feel it deep in my gut so I found this to be pretty annoying - sounding, feeling like I had been cut-off, just thinking about going to 36.5Hz right in the middle just made no sense because I had realised that 35.5Hz would be perfect for what I want. The more of the boominess it eliminates it just totally cleans up the sound, at least with those subs its actually pretty noticeable, but I found I was enjoying music less because the lows were gone too much.

I tried pressing on lots of areas on the box but yeah did nothing, its a very solid box, pressing on it just feels funny like it just wont do shit to any sounds or vibrations as its so solid. Its all sealed up with Liquid Nails on the edges inside, Gorilla Wood Glue and screws holding it together. There is lengths of electrical tape of the ends of the joins on the PVC Pipe pieces depending on how many were necessary to create a strong hold inside the PVC coupling piece but not too strong so I cant separate them. I reckon your spot on with the port noise bit as there is a tiny bit of port noise at high volumes but it is so minimal and really not a problem but it is that sound that you can tell turns into that hellish Port Noise eventually when it cant breathe properly. Just sorta sounds and feels like a soft breeze atm, but yeah definitely would turn into chuffing if conditions were worse. I guess the resonance is in that bottom port because that is where the box is making contact with surface area or maybe because of how the box is designed it just chooses that port to create the resonance it is so eager to create when that frequency plays, manifesting because of port noise causing restriction. Maybe it just chooses that port to make the sound and not the others because it has chosen that port to make the sound due to slight variations in the box and the other 2 are happy not to do it as that one is doing it first for them, so-to-speak? It really resonated with me when you said the 2 (Port Noise & The Resonance) are probably related as I could remember thinking the 2 are probably related when I put my arm in there to try and brace the port, its hard to explain but you could just sort of tell. The resonance sound is kinda tearing down the port as apposed to the port itself actually vibrating.

 

Btw the song that has the sound that is causing the extreme resonance is "Drake - Wu-Tang Forever" - not that I really like this song especially after I heard Inspectah Deck's opinions on it but it was just randomly in one of my folders I was flicking through when I was setting up the subs for the first time and got intrigued by the sounds it made and had to work out how to fix them. Its where the bass does a sustained higher freq note right before it lets the really low sustained bass note drop. The bass drops in that song are actually not bad, I think you will know the note when you hear it I find it doesnt really resonate at all if I just turn the bass knob down a bit. I think its just the type of frequency that will vibrate shit like a MF if its turned up too loud, depending on where your tuning is at I suppose. The same song did it in my car with my last box I had running just one of the Alpines. Anyways just thought I would add all that so you know exactly what sound I was talking about. If you dont know which bit I mean I'll find the exact time it first happens in the song.

 

Some pictures for perspective too...

 

 btw the cargo blind is just there so thieving passers-by cant see what is in the back as the car looks extremely normal from the outside, except for black rims (which is getting a pretty normal look on white cars around here in Australia atm) It doesnt vibrate or rattle at all, there is actually no difference in rattles if I take the cargo blind out. There is only about a cm between it and the box but it doesnt even make this resonance sound or anything.

 

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A bit of a lengthy reply there, just to give you a better idea on what is happening and what is causing the issue and what my setup is like. In short I guess Im saying that that I agree that port noise is more likely the culprit as I tried pressing on the panels but this did nothing at all.

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It does sound like the port/s is the cause of the unwanted noise you are getting.  

Due to the fact that you are a little undersized in port area anyway, and you are experiencing problems because of this, my recommendation would be to rebuilt the box using a slot port.   You will then be able to get exactly the port area you need.  If you decide to go this route let me know and I can help you with a design if you need.  

"Nothing prevents people from knowing the truth more than the belief they already know it."
"Making bass is easy, making music is the hard part."

Builds:

U7qkMTL.jpg  LgPgE9w.jpg  Od2G3u1.jpg  xMyLoO1.jpg  9pAlXUK.jpg

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hey gday mate, cheers for helping me out. sorry for posting too much irrelevant shit before i guess i just got a bit too carried away...

 

ok so im actually hearing what your saying now, if I increase the port size the resonance will go away. I guess it would be in my interest to design a slotted port to get the exact size right, first question though as to be honest Im kinda set with using Aeroports if I can get away with it, you said that my ports were slightly undersized - how much are we talking? Would 2 x 6" Aeroports at my tuning (35.5Hz) be too much? If so by how much and would this impact me negatively? I guess 4 x 3" Aeroports is still an option?

 

Also I noticed with the Alpine Brand sub box you get with my subs I immediately get harsh cracking-style rattles in the cabin when the bass kinda hits hard like impact, it was almost like the slot which is under the box is getting hit, then hitting the moveable floor (which would normally cover a spare wheel or whatever, with the Rav it just covers other shit) which the sub box has to sit on in the cargo area behind the rear seats. I mean thats where the cracking starts I think but its VERY noticeable around the dash and sunvisor area, and headliner too, which is where it really annoys me. And when I swapped back over to my pre-fab box with a single 4" aeroport the cracking stopped immediately, only so by the difference in the way the aeroport plays the bass smoothly as opposed to flat (slotted port), not to try to pick on Slotted vs Aeroports at all, im just saying that this difference seemed the best way to go with my build as the slotted port seemed to really highlight all the rattley spots in my car MUCH worse. Am I right in thinking this is my problem there with the Rav4? Keep in mind I have already gone over the car fairly extensively fixing rattling areas with decent success before comparing the Alpine slotted box that I got with the 2nd Alpine Sub, so I kinda just immediately went f*%$ well I dont really want a slotted design then do I... It just seems to me that air wants to really crack out a slotted port very hard no matter where it would be positioned on the box, not necessarily a bad thing but I think with the Rav4 this might be not a good thing basically? Not sure?

 

Oh and I also have Alpine's Subwoofer System 2 always turned on so I guess that might add to excursion a fair bit, its a Navigation Head Unit from 2010 not a single din one.

 

My trick with the removeable floor rattling in the back is to pull the box right back nealy to the rear cargo door and it doesnt seem to make much noise if you do that, btw this did not work with the Alpine Brand pre fab box. Any ideas on how to make a floor like that no so rattley?

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So tuned to the same frequency and appropriately sized, there is NO difference in performance between an aeroport and a slot port.  The difference you heard was almost certainly due to different tuning frequencies and/or port areas between the boxes you were trying out.  

In general, my preference is for slot port boxes because I can get whatever port area I want and they usually make for a stronger box.  However, if you want to stick with aero ports, that's totally fine and you can certainly get good results with them too.  Two 6" ports would work very well for your setup.  They will have to be a good bit longer than your three 4" ports were to get the same tuning, but not so long as to create design issues.  

As far as rattles go, careful application of sound deadener is usually your best bet, that's kind of outside my area of expertise though.  

"Nothing prevents people from knowing the truth more than the belief they already know it."
"Making bass is easy, making music is the hard part."

Builds:

U7qkMTL.jpg  LgPgE9w.jpg  Od2G3u1.jpg  xMyLoO1.jpg  9pAlXUK.jpg

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ok cool so a slot port kinda seems as though it might not be a bad idea then. Ive gotta admit though I do really like the Aeroports now and will try to stick with them in future builds if I can get away with it, just something about them, fun to work with i guess, I really like them.

Ok so going with the 2 x 6" Aero's (which I was going to do but by the posts I read I decided on 3 x 4", people were suggesting that the 2 x 6" might be a tiny bit too big, kinda with not really saying it in so many words though - I guess cos Im giving them a fair bit more power than probably what they were talking about?, plus I can give em alot more gain before clipping as the amps have really clean power - MRD-M605's birth sheets show around 660w @ 2 Ohms) what are any of the problems I might run into with them being too big like what about when Im listening to music at low levels or anything like that is there any advantages at all to have the 3 x 4" Aeros at like would the bass be a little bit tighter or something, is it possible the the 6" aeros would be a bit too loose in some situations? Actually I think Im getting what you mean now (just while i was typing that lol, still answer the question though please), even at lower high volume levels i notice some of that grinding resonance but not too much, its kinda just subtle, so that is literally starting out of the port from just that little bit too much port restriction?? OMG I get it now, just please answer all that questions though as if the sound will be a bit too loose im not sure about building it.

So I can just modify my design very slightly to accommodate the wider ports behind both subs, man this thing is going to look badass, i will nearly be able to fit my head in them lol. Is there really any downside for me to go this route, like will bass be too boomy, less SQ, too much free air excursion, less tight bass impact feeling in my back when im in the drivers seat, i really like it to rattle my eyes and mirrors as much as possible if i was to loose that it would be not really worth it IMO - like when i tuend it to 31.6hz i noticed i lost the rattling of my head and mirrors too much so i stayed away from that - i dont like it, etc, anything you can think of will be appreciated.

Oh and TBH I am already planning to upgrade the 2 x MRD-M605's to 2 x MRD-M1005's and change out the subs to 2 x Alpine Type X 12's so i need to build the box for these too really even though I am meant to be slowly accumulating the amps and subs over time, i already have my eyes on 2 of the amps and looking for ways to purchase them this month. will these subs go better with the 3 x 4" ports or the 2 x 6" ports? Like can I sell this box with the 3 x 4" aeros or should i keep it for the type x's? Cos there is really no point me building anything that wont be absolutely perfect for the type x's as well. everything else in my setup will be exactly the same.

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