Monday, September 1, 2014

Bass Horn Project - Part Two

There is a lot to report today.  I'll get to that later.  First, before I could complete Bass Horn Project - Part One, which is the largest portion of this project, being the "mouth flare" of the horn, I needed to build the middle part of the horn.  This was necessary in order to fit the mouth flare to the middle part properly before installing remaining cleats and fasteners.  I do not have a photo of the middle section by itself because I went ahead and clamped the two sections together to begin the fitting process.  Here is a photo of the clamped mouth flare and middle section:


Bass Horn - Part-2-01
Bass Horn - Part-2-01
Of course, I couldn't resist preliminary testing.  The first thing I tried was listening to the world through the horn throat.  This worked pretty well with the garage door open and the horn propped up at an angle.  I could hear sounds through the horn that were inaudible to me without it.

I wanted to try a driver mounted at the throat to see how this might sound.  I tried a wide range GE S-1201D featured in an ad in Audio Engineering February 1952.  You will find an image of this advertisement on the page at The EH Dirt Road Special is Back for repairs posting.  The S-1201D I tried is in an old open back baffle originally part of a Califone record player.  Measured DC resistance of the voice coil is 8 Ohms.

Just about the time I got this lash-up ready, I realized that the power had gone off.  So instead of plugging anything into the wall outlet, I set up my 12 DC marine battery with a small inverter and plugged into that.  Here is a photo of the setup for testing:


Bass Horn - Part-2-02
Bass Horn - Part-2-02
This worked quite well playing music from one channel of my iPod through a Lepai LP-2020A.  I could have powered the amp directly with 12 VDC but decided to try out the small inverter instead.  Here is a picture looking directly into the horn with the Califone baffle and screen visible.

Bass Horn - Part-2-03
Bass Horn - Part-2-03
Here is how the driver and Califone baffle look from the throat end:


Bass Horn - Part-2-04
Bass Horn - Part-2-04
At this point, pleased with the preliminary testing so far, I removed the driver and baffle and continued to fit the two horn sections.  This work consisted of ripping and fitting four cleats to fasten to the flange at the smaller end of the mouth flare section and fastening the edge of the Masonite panels to the cleats.  In addition, I fastened the panels at the big end of the mouth flare section to the four longer cleats at the horn mouth flange.  Two of the beveled 3/4" x 1" cleats are visible in this photo:

Bass Horn - Part-2-05
Bass Horn - Part-2-05

Here are a couple of photos showing the interior of the horn after the eight cleats have been fastened:


Bass Horn - Part-2-06
Bass Horn - Part-2-06
Bass Horn - Part-2-07
Bass Horn - Part-2-07
This second close-up photo has the Califone and GE driver back in place.  The Masonite panels are much stiffer with the additional fastening.  By this time, the AC power had come back on, so I disconnected the inverter, put a little charge in the 12 Volt battery, and plugged the power supply for the Lepai LP-2020A into an AC outlet for further testing.

This project is about a bass horn.  However, preliminary testing with the vintage GE S-1201D with a variety of music indicates that, if you can live with a little directivity at the higher frequencies, this seems to work quite well as a full-range horn.

Sensitivity is extremely high.  While working in the garage today about eight feet away from this horn while it was playing, I had the volume set for a comfortable listening level.  Without changing anything, I connected an AC voltmeter directly to the amplifier speaker output terminals and measured peaks of about 200 millivolts going into the speaker.  At 8 Ohms, this is the equivalent of 5 milliwatts of peak electrical power.  While fastening the Masonite panels, I was essentially inside the horn listening to the music and I had to turn the volume down even further.

Before I set up the iPod music player, I tried a sine wave generator and got very strong output down to about 40Hz.

So far, this speaker is relatively lightweight.  I will try to weigh it tomorrow.  Using an SPL meter, it would be interesting to measure the efficiency of this speaker at several different frequencies.  Another test I would like to try is applying a square wave and watching the waveform in the air in front of the speaker with an oscilloscope.  Time coherence should be quite good because all of the sound is coming from a single driver without the need for any crossover.  I will probably take it outside for square wave testing to avoid room boundary reflections.

Even without any further testing, the musical sound from this speaker is really stunningly good, especially from well-recorded percussion instruments.

This bass horn project continues with Bass Horn Project - Part Three.










No comments:

Post a Comment