Quarter Pie Horn Prototype and Testing

May 2020,

The mid bass horn idea has been tempting me for a while now. Despite being convinced that direct radiators are superior to truncated horns in the 50 Hz - 500 Hz range, (see my page: "bass vs Direct Radiator"), I kept wondering "what if".

I have little doubt about that a full size mid-bass horn would outperform a direct radiator. But a 50 Hz full size horn would require a path of 7 meters long, and be immense. That would require either folding (compromising the horn) or a dedicated unit built "in house". Some crazy people do just that. Build an immense horn into a house. I can't do anything like that being in a rental property, but perhaps one day...

I admit, I’d love to go hear the system shown below, but I’ve had no invitation yet. (I’d even bring cookies, chips and hard beverages…if invited)

Bass horn

I've decided than instead of thinking about it forever, I’d build a truncated horn prototype to find out the answer once and for all. I had some free time and these types of projects are interesting to me.

While some people would care to binge watch movies for an entire week, I decided to spend the same time building a horn. :)

Unlike the movies, building something is rewarding, and I can learn a lot from it.

Because it’s just a "test", attention to details was not overly adhered to.

I'm 99.99999% sure I’ll throw the horn in the fire pit after the test, so no sanding or any precision fitting was done.

So, I built-it only to evaluate the potential and investigate if doing a better design later on would be a worthwhile idea or a fallacy.

I've chosen a design called the "Quarter Pie Horn".

PROs:

It’s one half of a Klipsch MWM horn. (somehow a "proven design")
It uses a single 15" woofer (I had a suitable woofer in stock, so it was very cheap to try out).
It’s simple and easy to assemble.
It has an Fc of 60 Hz. (a tad high but perfect for my test)
It has "good reviews". (I rarely place much weight on reviews, as they’re almost always a false lead)
Real hard data is difficult to find on this type of project. I only had saw a few frequency responses that looked OK, and some biased rave comments, the OK FR was convincing for me to try.
Quite efficient. (like all horns)
Has only one fold, that would help with higher frequency quality compared to multi folded design.

CONs:

It annoyed me because of the two parallel walls (40 cm distance, which means we’ll have a resonance mode at 211 Hz)
Compression ratio of the horn is on the low side. I would had liked a bit higher, but a test is a test.
It’s a conical design (one of the worst flares you could pick, but it makes for very easy construction instead of bending multiple thin layers of plywood)
Size

Above simulation and plan from Internet

Below, my simulation (with Precision Device PD154 woofer)

Black is as per online model (1Pi)

Grey is same horn, but in an open field environment (2Pi)

I measured the horn outdoors, so my measurement would compare to the 2Pi model

Inside a room and placed in a corner, the 1Pi should be used

4Pi= Free air (like the speaker is suspended in air)

2Pi= Speaker sitting on floor (floor act as an acoustic mirror and reinforces sound)

1Pi= Against the floor and against a wall (even more reinforcement and smoother response)

0.5Pi= Corner placement (the way a Klipschorn speaker should be positioned)

My PD154 woofer doesn't simulate as good as the original design (EVM 15L). Tweaking the horn could be done, but this is only a test. If I was to make the quarter pie horn for long term use, I’d buy the recommended woofer for the horn (Eminence Kappa 15C or the vintage Electro-Voice EVM 15L). My PD154 is close enough for a valid potential mid-bass horn test

Used the router to create a 4mm deep groove.

At high SPL, the PD154 cone would have hit the baffle without it.

The PD154 has a travel of 7mm (one way, so 14mm of total Xmax)

The cone is recessed about 4mm from the frame. The 4mm groove gives me 8mm of clearance, 1mm above the PD154 maximum excursion.

Groove with woofer in place

Extra reinforcement added on the 45° reflector plate

The serious builder should consider filling the two cavities with sand or lead shot. I didn't.

1= 3/4" thick Baltic Birch Plywood

2= 3/4" construction grade Plywood

The space behind the 45° reflector is Baltic Birch Plywood.
The top cover will be 3/4" construction grade Plywood

No picture available, but I used 3/4 of a sheet of Roxul Safe ’n’ Sound behind the driver and on the front cavity, before gluing the top cover back on. Damping the rear wave of a woofer is always important, and I made sure I did it appropriately.

The finished horn weighs 120 pounds.

Outdoor measurement set-up:

Note to myself: What a pain to measure when the wind gets stronger by the minute... Wait for evening next time.

I've tried three positions: All @ 2 meters distance

(the house behind horn is 6.8 meters away)

The microphone at 16" height (yellow)
The microphone on the grass (Blue)
The microphone on a plank on the grass (green)

Best result seems to be #3 (I took a picture of this one)

No window used for the measurements.

I aimed for an average of 95 dB at 2 meters

Amplifier output voltage was about 4.4 VAC to obtain those responses.

This is about 2.4 Watts RMS.

First reflection from buildings at 6.8 meters (~13 meter return trip)

I used 33ms window for the next measurements just to be safe.

Reflection start at 34.3ms.

Frequency response isn't too bad, but it isn't great either. Reaching 500 Hz would prove impossible.

Gradual roll-off start at 300Hz

EQ would be needed and still...

Big null at 550 Hz, and not sure why

Calculated resonance at 211 Hz shows as a smaller null.

Distortion also shows an anomaly at 211 Hz.

Spectrogram from the horn mouth

at 16" height, measured outdoor.

Spectogram from 2 meters distance

Green is a repeat of the 2 meter outdoor measurement.

Red is indoor measurement with the microphone at the mouth exit.

The 500 Hz - 600 Hz frequency null isn't present on the simulation, but the null I obtained is real.

The red circle is where the vibration on the horn seems the strongest. (hand touch)

It’s about 1/3 into the wood piece. Doubling the thickness of that piece is highly recommended.

Existing opening angle (single piece)

Test of a new opening angle

I'll add a new piece at this rough angle.

The new piece of wood will effectively reduce the horn mouth a bit, but from the simulation, this will be pretty much negligible.

The piece of wood will prevent sound, produced from the vibration of the wood panel behind, from reaching the listening position (it points directly at your ear, so a very small vibration from this panel will color the sound a lot)

Sound dampening material will be added into the created cavity

Sand or lead shot to fill the cavity, would be an even greater idea here, but I won't do it for now.

Dats impedance test of the Qpie horn with extra piece of wood as shown above.

Pretty much no change as per before.

As expected, the addition of the extra reflector panel helps a lot.

Stock design = Green trace
Reflector plate = Red trace
Polyester batting and reflector = Blue trace

The peak at 160 Hz is eliminated with the extra reflector (I expect the panel resonance I mentioned above, was radiating sound at this frequency)
The null at 131 Hz is eliminated as well, with the reflector.
The polyester absorbent smooths out the frequency response a bit more.
Subjectively, the polyester material and reflector together sound better.

I spent about five hours listening to the Qpie, I was thinking that instead of writing my subjective comments, I’d do something interesting and shoot a few videos of the Qpie in action. Hopefully, the drastically different sonic character of a single 15'' sealed vs the Qpie horn will translate on the poor quality videos. I came with my own conclusion of the Qpie test.

Below are for videos I took to compare the sound of the Qpie horn to my single 15" sealed cabinet.

See my other page about my sealed cabinet for more details about it.
Test aren't scientific. I was in my garage and just put something together in few hours.
Obviously, the music was down-mixed in monophonic.
Both bass cabinets use the same Precision Device PD154 woofer, which makes comparisons very interesting.
I recorded the videos with my Android phone (microphone quality is low, and YouTube converts all audio to MP3...)
I selected four different types of music, and used heavy equalization to match the Qpie horn as close as possible to the sealed cabinet.
The JBL 306P plays from 450 Hz and up. (450 Hz being the upper limit of the Qpie horn)
The sealed or Qpie plays from 52 Hz to 450 Hz, with a 48 dB/oct crossover between them and the JBL 306P.
The sealed cabinet could play much lower, but I cut the bass, so the comparison is more valid.
Halfway through the song, I changed the bass bin and point, to the one playing.

Notice that both frequency responses match pretty well. (from EQing both bass bin as close as I could)
The microphone was placed at the listening position (where I was holding the phone)

I’ll let you draw your own conclusions from the videos.

Hopefully, copyright owners and You tube will not ban the clips.

The Qpie is a compromised truncated horn design, but still exhibits dynamic that the single direct radiator sealed enclosure using the same driver can't match.
The Qpie also has coloration on voice. Not sure if it’s because of various panel vibration or the 90° bend, but it does, even with the added reflector I installed.
Pretty much every panel vibrates despite the horn weighting in at a solid 120 pounds. Lots of bracing and extra weight would be required to tame the Qpie down.
The mouth would have benefited from a double thickness top and bottom panel and more cross-brace.
The cavity I created by adding an extra reflector could be filled by sand or lead shot to damp even more resonances.
The sealed kind of sounds 'thick and slow' but requires much less EQ to obtain flat response than the Qpie does, on a simple passive crossover, this is a big advantage for the simple sealed.
A quartet of 15" like my main bass bin use, can make a single 15" sound like a toy. Comparing the Qpie horn to a double or a quartet of 15" is fairer, as efficiency would be similar.

I've learned a lot from this Qpie project. It was fun…but the party’s over.

Qpie… meet sledgehammer! :)

Sledgehammer win...

Perhaps, I should have taken a video of that too?

Conclusion.

I may venture to a better design mid-bass horn later in my audiophile life, but for now the quest is over. A single 15" feeding a horn won't be in the plans for sure. If I do a truncated design, it won't be 1/4 size, as I’d move up to a 1/2 or even full size. I’d opt for a dual 12" or even a quartet of 10", arranged in a push-pull, to try and deal better with the vibrations. It won't be a conical flare for sure. If there must be a fold, it would be rounded, not with a straight reflector.

Me in few years...