Saturday, February 28, 2015

A Better Arrangement of the Sound Transformer Horns

This is a continuation of The Sound Transformer - An Early Prototype.

With all three horn drivers in the same plane, a listener to this system is about seven feet further away from the midrange and tweeter horns than from the bass horn.  This location of the horns does achieve reasonably accurate "time coherence", i.e., the sound from each of the three speakers arrives at about the same time.  However, as a result the system "sounds" bass heavy unless you move away from the system.  That is not necessarily a problem if you enjoy strong bass.

Another consideration is the amount of space occupied by an eight-foot long straight bass horn even in a relatively large room in a house.  As it turns out, since 1934 when all of the early research on this was done at Bell Laboratories, it is now possible, with a modest investment in DSP hardware, to achieve "time coherence" between the woofer and the mid and treble speakers by introducing a time delay in the signal path.  Digital Signal Processing (DSP) makes it possible to do this in "real time" during playback.  With DSP time-alignment correction, it would be possible to place the mouth or delivery end of the midrange/treble and bass horns in the same plane and apply a delay to the midrange/treble horn signal.

To check this out, I mixed a recording in Audacity, the excellent open-source audio processing software, in the following way.  I only have one of the speakers so I have been working with monaural tracks.  In Audacity, it is possible to load a stereo track and convert it to a single monaural track.  After that, you can run a low-pass filter over the track and save that filtered track to a monaural track. I named this low pass track "track-lp-400Hz".  Then you can "undo" the filter and run a high pass filter on the same original monaural track and save that as "track-hp-400Hz".

Finally, you can load both the lp and hp tracks into a new project and assign one to the left and one to the right channel.  Now you have essentially built an "active crossover" into the program source file.  The last step is to use the "time shift" function in Audacity to slide the midrange/treble signal about 6 milliseconds to the right on the time scale relative to the bass signal.  This step builds "time-alignment" into the system.  The speaker horn arrangement looks like this:

Sound Transformer With DSP Time-Alignment
Sound Transformer With DSP Time-Alignment


Sound Transformer With DSP Time-Alignment
Sound Transformer With DSP Time-Alignment


Sound Transformer With DSP Time-Alignment
Sound Transformer With DSP Time-Alignment
Now, the balance among the three horns is really quite good no matter where you are in the room.  Also, with this arrangement, it would be possible to build the whole assembly behind a wall so that it would not obstruct the listening room.  By the way, no passive crossover is necessary in this hook-up.  The midrange and bass horn drivers are connected directly to the right and left outputs of the Lepai 2020A amplifier.  Only the tweeter horn is protected by a series capacitor.  In addition, the tweeter horn attenuator network is still in place.  As soon as I have more on DSP time-alignment, as applied to this system, I will post here.

Tuesday, February 24, 2015

Finally, The Real Sound Transformer Part Three

Recently, I built the throat section of the Sound Transformer.  It is 32 inches long and about 36 square inches at the driver end and 115 square inches at the opposite end.  I used some white-painted 1/4" Masonite panel board that I had found at the transfer station a few years ago.  All other materials except PL200 construction adhesive and sheetrock screws are from the same source.
 

Sound Transformer - Throat Section
Sound Transformer - Throat Section
Here is the throat section set up for a listening test using an 8 inch Altec Lansing speaker removed from a subwoofer cabinet.  The speaker has dual voice coils, one for left and one for right channel. I made the ribs much wider for improved rigidity thinking that the pressure in this part of the horn is likely to be quite a bit higher than in the larger horn flare.


Throat Section Set Up for Listening Test
Throat Section Set Up for Listening Test
This horn section actually sounds quite good even though this is only one-third of the design.
When mounted to the first two sections of the Sound Transformer, the total length of the horn is 96 inches.  Here is a photo of the first two horn sections ready for the third section.

Sound Transformer - Two Large Sections
Sound Transformer - Two Large Sections
This is sixty-four inches high.  This two-thirds of the horn actually sounds quite good even though it is not the whole design.  Here are all three sections connected together.


All Three Sections of the Sound Transformer Are Connected
All Three Sections of the Sound Transformer Are Connected
This just clears the barn tie beam overhead by about four inches.  While the horn was in this position, I slipped a couple of bathroom scales under it.  It weighs about 110 pounds.  After lowering it onto some blocks, it is ready for the driver and back chamber.

Sound Transformer Horn Ready for Driver and Back Chamber
Sound Transformer Horn Ready for Driver and Back Chamber
Here is the 12 inch KLH driver on a square baffle.  The back chamber is the black box under the speaker.


KLH Driver with Baffle and Back Chamber
KLH Driver with Baffle and Back Chamber
Part of the expense of this project was replacing the foam surrounds in two of these drivers.  Here is the Sound Transformer with driver and baffle fastened on and back chamber held in place with bar clamps.


Sound Transformer Driver and Baffle Fastened and Back Chamber Clamped In Place
Sound Transformer Driver and Baffle Fastened and Back Chamber Clamped In Place
Here is the whole assembly ready to hook up and test.


Sound Transformer Assembly Ready For Listening Test
Sound Transformer Assembly Ready For Listening Test
Here is the test setup.


Sound Transformer Test Setup Showing Lepai 2020A Amplifier
Sound Transformer Test Setup Showing Lepai 2020A Amplifier

Sound Transformer Test Setup
Sound Transformer Test Setup
And here is a photo taken from the opposite corner of the barn attic.


Sound Transformer Setup In Barn Attic
Sound Transformer Setup In Barn Attic
I listened to this for quite a few minutes playing various source material from an iPod.  I am quite pleased with the performance and it was well worth the effort.  I also tried manually sweeping a sine wave generator through the bass-midrange crossover point and noted a very smooth transition there.  Piano music sounds really good through this.  The bass is very clear, uncolored, and unrestrained.  Kick drums are also impressive.  I will try some more technical testing tomorrow.