Car Speaker Box Using a 'Folded Horn' Design




vibe black death qb69 750 x 750.gifThis project came about because a friend of mine sold me some great Vibe speakers real cheap that needed repairing.

Folded Horn speakers work like a trumpet in that the sound comes out of an expanding tube/box. To make it fit into small spaces the tube can be folded. As long as the fold continues to get wider then it has a chance of working.

Step 1:

 The rubber surround had split in a number of places. So I repaired the splits by using a rubber/latex glue called Copydex together with a very thin but strong rubberised cloth cut from a cheap glasses drawstring case/bag.

This is a view of the gluing. Several layers were added, but not too much as to restrict the speaker moving. Its not very pretty but it works, it takes 110 watts RMS with ease.

Step 2: Cab Design


The design of the cabinet was inspired by cabs like this one shown at 

Mine had to fit in the limited space of the back of the car but I still wanted it to shake everything. I have heard folded horn speakers before and I knew that they could do that.

Folded Horn speakers work like a trumpet in that the sound comes out of an expanding tube/box. To make it fit into small spaces the tube can be folded. As long as the fold continues to get wider then it has a chance of working. There are formulas for this but I didn’t/can’t use them.

There are loads of these ideas on the net. A good place to start is -

Step 3: Theory (just a Bit)

The arrows in this pic show the path of the sound from behind the speaker in a folded horn design. An added benefit of using these designs is that you get sound form both the front and the back of the speaker. This means that less power is needed because the speaker is not compressing air in cabinet (to only produce heat) as a normal speaker does. All the energy is used to make sound, so you get about twice- ish the loudness you would get normally! Perfection.

Another bonus is that because the speaker is moving freely it sounds clearer, more detailed. Sound seems to travel further, like it is all around you, especially the bass. The only down side is that the speaker cone can travel beyond its limits if pushed too hard. But the vibe speakers I have are unlikely to do that with the limited amp I have.

Only bass comes out of the horn part of the speaker. LOADS of it. More than you would think possible. Horn speakers are brilliant.

No subwoofer will be necessary. The bass that I will get from these two speaker will out perform most of the closed box/ported subs I have heard, and with only 200 watts (these speakers will be very efficient in a folded horn design, a thousand watts are not needed).

This built speaker from the net shows the inside and outside, you can see how the internal port gradually gets bigger.

Step 4: Initial Ideas

A problem was the Vibe speakers were deeper than I realised.

So some compromises were made, but as long as there was a chamber behind the speaker and the port gradually gets wider and wider then it will work.

So some compromises were made, but as long as there was a chamber behind the speaker and the port gradually gets wider and wider then it will work.

Step 5: Final Design

Speaker Simulators

I didn’t use any of the simulators available on the net (for free) because I couldn’t be bothered to work out how to use them, and also with past experience folded horn speakers have all worked well so far. I’ve made 2 sets for the house in different rooms and one large pair (6 foot tall) for the band I play in.

This is it for the car. 

Next I had to make sure that it fitted in the boot so precise measurements were taken and clearances were added. It would need to be tilted into the boot space. A cover will be added so that isn’t possible to see it was there. 

Step 6: Car Issues

car interior.jpgThe amplifier was screwed to it at the back where there was a space because the seats tilt back. there is still lots of room so that its stays cool. 

Step 7: Inside, How It Works

speaker flow.jpgYou can see here the way that the sound comes out of the front of the speakers and also travels around the inside of the box. The waves then join together to make ONE BIG wave.

This is the space it has to be big in.

Step 8: Construction

Here it is before paint and with the side missing so you can see the path that sound will have to take.

All the panels are screwed. No glue was used until we were sure it worked.

These Vibe speakers also have a crossover to fix inside the box.

Step 9: Tried in the Car

Just a few adjustments then it was in. A cover will be made later, just a frame with black cloth stretched over (it was on old T shirt).

Step 10: Finished Item

Now its painted with water based acrylic from Ronseal.

The sound is more than we had hoped for. Friends want to know where the secret 15 inch sub is hidden.

It fits in a Corsa and is 12 x 17.5 x 33.5 inches.

It moves hair!

Car Audio Challenge

First Prize in the
Car Audio Challenge



    • Pets Challenge

      Pets Challenge
    • Backyard Contest

      Backyard Contest
    • Frozen Treats Challenge

      Frozen Treats Challenge

    9 Discussions


    5 years ago

    I'm sure there's a term for it, but wouldn't this terrible box design just give you some kind of insane amount of delay between the sound coming off the front and the sound from the rear? I've seen folded horn subs before and this isn't right.


    6 years ago on Step 3

    nice looking speakers u got wattag and independince?


    7 years ago on Introduction

    This seems an impressive project, I want to make one.

    But I wonder if you don't lost an important part of stereo effect. It is to say, the channels separation.

    3 replies

    Reply 7 years ago on Introduction

    Yep, you do lose a bit, the back seat passengers get the best deal, but the original car speakers are also connected to the music source, so they put back the stereo and the limited high frequencies.


    Reply 7 years ago on Introduction

    Perhaps the loss would be somewhat less if you manage for leaving mainly bass thru folded horn. That could be achieve reinforcing the inner filling of the walls, so that absorb the treble.

    Mind you, I'm guessing, would have to be tested in order to be sure.


    Reply 6 years ago on Introduction

    A true folded horn (which this is not) or transmission line acts like an acoustical filter and filters out much of the high frequencies. Fibre fill, and the like is not used to absorb high frequencies, but to reduce internal reflections. Some theory suggests that fill in an enclosure design as suggested would actually ruin the way the acoustics work.

    Also there would be no "loss of stereo separation" here, since low frequencies are omnidirectional, and quite long, which goes along with the acoustic filter that I mentioned already.

    The stereo separation effect that our minds recognize come from much higher frequencies than what would be passed through an enclosure design like this, from the back wave of the speaker.


    7 years ago on Introduction

    This not a folded horn. It's some sort of transmission line based on a wild guess and the idea that it looks vaguely familiar to something the author saw once. Just imagine the amount of slammin bass, yo, if there was a single ounce of reasoning in the layout...

    This is meant to instruct people on how to produce something useful. It will result in dismal performance (one-note bass, massive group delay and distortion).

    1 reply

    Reply 6 years ago on Introduction

    I agree with this.
    Folded Horn uses the front wave of the loudspeaker, not the rear wave. This is sorta based on a transmission line that uses the rear wave of the loudspeaker. The problem with the folded horn and transmission line designs is that they are not very forgiving, and need to be designed specifically for the drivers being used, among other requirements.


    7 years ago on Introduction

    Good call on the deckert design, Bummer hid setup didnt suit your space needs. i have had the chance to be privvy to 3 of thode builds and WOW ! impressive. Kudos on taking his design a step further .