The Best Bass Ever? A Subwoofer Discovery

I want to share something I found by tinkering with car audio subwoofers. I see no reason it wouldn’t apply to home audio as well, and maybe this is already known but it’s new to me and I wanted to share:

A couple years ago I got a 15” sub in a prefab ported box for my car. It has a slot port with a 90 degree bend in it with the port terminating next to the sub on the front surface of the box. Tuning is probably mid to high 30s. This is my best guess but I don’t know for sure as I haven’t taken the time to model the box. I went back and forth between ported and sealed by closing off the port with wood. I wasn’t quite happy with either configuration. Sealed had great transients but it needed gobs of power and it seemed kind of anemic with some of the low notes. On the flip side, the ported configuration didn’t require as much power and hit the low notes with authority. The downsides were that the transients were a bit rounded off and it was a bit boomy with excess overtones / harmonics / high frequency “ringing” or whatever you want to call it. It sounded kind of nasally.

I attempted to tame the ported box by putting some polyfill in the box and then put some insulation in the port (this is the secret sauce that inspired me to make this post). The insulation I used was a sheet of insulation approx. 1-2” thick from a Hello Fresh order to be precise. I only put it in half the port which was in the rear of the box up to the 90 degree bend and leaving the port exit and the length after the 90 degree bend empty. The width of the port is about 3-4 inches so the insulation takes up about half the cross sectional area.

The sound, in my opinion, is beyond amazing. It’s tight and controlled while going deep and loud. It’s like I’m getting the benefits of sealed with the output and low frequency extension of ported with hardly any of the “nasally” ported sound. I much prefer it to either sealed or ported (without port insulation). The blend with the main speakers is also excellent.

Here’s what I think is happening from my limited understanding of the math behind ports: First, the air inside a port resonates as the subwoofer moves and because it’s just a slug of air in the port there isn’t much to dampen it until we add some insulation. I think the insulation acts as a dampener on the air mass which gives it such good transients and reduces the nasally, ringing sound that ported subs can sometimes have by making everything settle down quickly after the bass note stops. Second, I think the insulation lowers the tuning frequency of the box by reducing the cross sectional area. Normally this could be a problem because it can introduce chuffing noise, but since the insulation is only in the back of the box/port, the velocity at the port exit is still normal and I don’t notice any chuffing noise.

I’m only sharing this now because I tried a different subwoofer (supposedly much better) and ran into the same issues with sealed and ported sound until I did the insulation trick which made me realize just how important that step is. I’ll never run a sub without some type of port insulation strategy again. 

Has anyone else had similar experiences or have anything to add?


You should be measuring as much as possible, and consider a miniDSP for your car to let you tune the subwoofer.

You should probably go to DIYaudio and make your comments there.  Lots of speaker builders there ready to help you who have experience tuning resonant ports with precision.


Plastic straws work really well. They reduce turbulence and increase speed.

Also experiment with extending out of port a couple of inches. If the tube is folded don't interfere or restrict the fold. Tom D

Great experiment!

Anything placed in the port tube will disrupt the airflow and inhibit the resonance tuning from occurring. Altering the air flow in the port removes that 'bloated' one note bloom from occurring yet still allows the speaker full excursion. 

If you completely plug the port you are changing the internal air pressure inside the box and restricting cone movement. Inserting a breathable medium you are not sealing the box and yet allowing the driver to still have full extension. 




FYI... you can just take a rolled up piece of paper and insert half inside the port tube and extend the length of the tube. Open breathing, no restriction and no more one note bloat sound. Straws also eliminate the bloat note and allow for open breathing.

The frequency would be lowered with a longer tube. The same way a trombone works.

For home bookshelf speakers two of my favorite tweaks are stuffing the ports and turning the speakers upside down.  Some pretty dramatic changes occur with these two tweaks - whether they are an improvement or not is up to you.

Yup, done this for years with ported speakers, it just doesn’t have to be subwoofers that you tune a port. For audio, some of the best sounding subs have no ports, or they use passive radiators in place of a port. Home theater purposes, get the biggest subwoofer with 1 or more ports and let the rumble/shaking begin.

Be careful of the type of insulation you choose for damping material in any non-sealed enclosure. Airborne glass or mineral fibers will be inhaled and cause lung disease. 

On tube length(s), tonight’s (well, this a.m.*g*) cheap trick:

-Select the outside diameter of an off the shelf PVC size, noting the I.D.

-Cut a short length, and temporarily affix in place.

-Make out of thick paper or similar, an overlong tube that fits Into the PVC temp.

-Adjust length as desired, note, and do the final tube in another PVC piece.

-Viola’! +

PS, play more than 1~2 test samples; results will vary and you should too. ;)

+ = or violin, Strat, trombone.....bass guitars & drums, obviously....

Straws can make you crazy...(how many, not enough, keeping them corralled left for the post-fab drink is a plus... 

@sleepwalker65 ....Pillow fibrefill.....

If you spend a lifetime sleeping on it....🤷‍♂️

A little....a lot.....plastic window screen cones to keep it out of the speaker basket.

The inside need not be pretty...(unless you Must Go There...)

EDM makes for good test tracks...mho....

Brace Everything as much as melt glue guns make this a quickie...lock washers on driver hardware....

Enjoyed the car-audio leading to home audio story. My car-audio experiences lead me on a journey to open-baffle in 2-channel system. At this point, I'm convinced that open-baffle is the most cost-effective path to greatness - especially true for subwoofers.part of that has to do with how they minimize room effects (which in most cases are difficult/expensive to fix.

Getting back to car-audio subs... I use a custom sealed box (my design & build) with a stereo (because I prefer not mono) pair of 12" Digital Designs dual voice-coil drivers powered by Orion HCCA275-G4 (last gen made in USA). Signal to the amp comes from dedicated 2-ch DAC. DAC receives signal by S/PDIF coax from miniDSP NanoDIGI (2x8), which receives signal (also by S/PDIF coax from Android head unit. DSP provides abundant control of sub and all the other 6 channels.

The reuse of paper rolls for me work great when filled with straws.I wrap with blue tape and then spray the outside with black paint so to seal the paper from breathing. I use the same port devices placed strategically on my audio rack as well as placement on and under my speakers. Placement improves focus within the stage. TomD

Hey I think you may have found the perfect way to go deaf as fast as you can... a 15" woofer in a closed car should do it.

Port noise in part has a great amount of distortion.

With the port tube as described and filled with straws you will hear the back sound from the speaker sound much like the front making it nearly a dipole. The tube can be to long changing the bass cut off but it's fun to hear the more open midrange trapped in the box being utilized.Tom D

I love My JLAudio sealed 10 in. SW in my car --driven by a used JLAudio 250w monoblock ( Cost to me ~$200)

At home I have, and really like, my Dynaudio 18s - self-powered 500w - sealed, twin opposed 9.5 in drivers ( virtually eliminating any vibration) - not expensive either

Paying a little for the extra amp power to compensate for the inefficiency of sealed designs I have found to be worthwhile to get the control and lack of coloration vs ported designs.