Back Loaded Horn (BLH) Full Range Speaker Build

[jgray]

I decided to tackle a full range speaker design and build to go with my 10W Class A amp. My goal was to create a floor standing speaker with good efficiency, keep the size reasonable, the cost down , and provide a fairly broad and flat frequency response.
I selected the Fostex Fe126en 4” driver, and decided to go with a BLH design to try and fill in the lower end. I came up with a design that measures approximately 7.5”Wx35”Hx15”D. I selected 19mm Baltic birch as the primary construction material, with a few internal baffles of MDF. This only required one 4’x8’ sheet of plywood, and a 2’x4’ piece of MDF.
I modeled the horn using HornResp, and the physical cabinet using Inventor 3D software.
I am happy with the outcome and think they sound great! It is hard to describe, but they sound really open and clean. Particularly with some piano music and female vocals you could swear they are in the room with you. Chamber music is also terrific with them. I find them to be plenty loud with my 10W amplifier.
Here is a picture of the completed speaker, with and without the grilles.

[jgray]

I measured the speakers using a homemade microphone based on the Panasonic WD-61A capsule which is supposed to be very flat.

It is coupled to a homemade preamp and then into the computer through a Behringer UCA202 USB soundcard. I used the Holm Impulse software to take the measurements. What a great tool! Here are the results.

The blue trace was taken right at the horn mouth. The brown trace was taken on axis with the driver at 12”, and the red trace is from 6’ away pointing at the driver, about 10 degrees off axis. I was surprised and pleased with how flat it was overall. The horn seems to fill in the lower frequencies nicely.

[jgray]

Here is a side view before finishing or “stuffing.”

It was a challenge to get enough length folded into the fairly small box, considering the choice I made for the compression chamber geometry. I particularly wanted to try the angled piece behind the driver in the compression chamber in an attempt to reduce reflections from coming back through the driver. I wound up lining the compression chamber with a plush felt to further this. I also continued the felt on one side of the first half of the horn length to reduce standing waves across the interior 6” dimension. I used some fiberglass and poly batting in a few other places to further reduce highs through the horn.

Biscuit joining was used for the main structure and most of the baffles, although two of the smaller ones are just glued.
To finish them, I first used a sealer, then a stain, and then several coats of polyurethane.
If anyone wants to try these out, let me know and I will try to help. Be forewarned; all the cutting of the angles is a pain. I can post pdfs of the build plans I came up with in a separate post. They may have a few mistakes in them, but they are pretty close to “as-built”.

Thanks to everyone on this forum. I have learned a lot by lurking and reading what you have done. Hopefully my experience can help someone else.

[rjbond3rd]

Fantastic -- congratulations! I would love to hear your further listening impressions as they break in.

[malibujeff]

Nice. Thanks for posting the measurements.

[ultrakaz]

Originally Posted by jgray

Here is a side view before finishing or “stuffing.”

It was a challenge to get enough length folded into the fairly small box, considering the choice I made for the compression chamber geometry. I particularly wanted to try the angled piece behind the driver in the compression chamber in an attempt to reduce reflections from coming back through the driver. I wound up lining the compression chamber with a plush felt to further this. I also continued the felt on one side of the first half of the horn length to reduce standing waves across the interior 6” dimension. I used some fiberglass and poly batting in a few other places to further reduce highs through the horn.

Biscuit joining was used for the main structure and most of the baffles, although two of the smaller ones are just glued.
To finish them, I first used a sealer, then a stain, and then several coats of polyurethane.
If anyone wants to try these out, let me know and I will try to help. Be forewarned; all the cutting of the angles is a pain. I can post pdfs of the build plans I came up with in a separate post. They may have a few mistakes in them, but they are pretty close to “as-built”.

Thanks to everyone on this forum. I have learned a lot by lurking and reading what you have done. Hopefully my experience can help someone else.

Great build! Thanks for posting your goal, build process, measurements, and conclusions. I especially liked how you stuck to the purist approach and went with a backloaded horn and no sub or super tweet. Not that easy given the diversity of options. Nothing like having real experience will full range drivers. Added bonus is that you are satisfied and happy with the speaker.

[freddi]

way KooL ! - nice work and nice measurements

[vacuphile]

Great project and excellent measurements. Really surprised to see how flat the curve runs.

[jgray]

Thanks for your comments everyone. I have to admit I was surprised at how well the curve flattened out. I guess it is good when the theory works out in practice. HornResp took me some time to figure out, but it is a powerful tool for gaining insight into how a speaker might behave. A big thank you to the developer for sharing that with the community!

As I have played around with listening and taking measurements, I have noticed that if I go say 30 or more degrees off axis I lose some of the highs. I have read of others observing this beaming effect, and guess it is pretty typical, but it was interesting to see the effect for myself.

I now find myself listening to every track critically for defects in the sound. Is this disease curable?

[vacuphile]

Beaming is physics at work and you can't f*** with mother nature, as the saying goes amongst engineers. When the circumpherence of a sound radiating surface is > than the wavelenght of the frequency it is radiating, beaming sets in. Full range speaker manufacturers such as Fostex came up with solutions such as wizzer cones etc. to minimize the area radiating the high frequencies. but in the end, you can't win from the laws of physics.