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Discussion Starter · #1 ·
Hello to everyone, I am a new member. Have been reading the posts for months. I got tired of having a speaker system with 35Hz to 17KHz but no imaging, presence or any of the other 50+ terms I learned working as a CET at a few audio stores.

I decided to build a "good" set of speakers. My wife defines "good" as small (?), goes with the furniture (?), does not cut too deep into the budget (under $100 each), and she can figure out how to use (no knobs, switches, etc).
So it could have no odd shapes, made of stained wood, (she could pick the stain), shorter than a 28" table, and less than a foot wide. For cost all the electronics had to be less than $75 and both speakers made from one piece of 3/4" plywood,(she did not require them to be lightweight)

None of this had anything to do with my definition of "good" (impossable ?). So I made my list; ultra low distortion less than (2%) THD & IM, a frequency response of 15Hz to 25K Hz. Although my amp is powerful enough, the distortion goes from almost nothing to painful as the volume is turned up, so had to be able to get 85db from only a couple of watts.

Been evaluating my new speakers with REW (Thanks!) Praxis, VA and others. They exceed all my (and my wife's) requirments. Guess what I built.
fwbutler


Read more: http://www.hometheatershack.com/for...ve-already-been-great-help.html#ixzz2McspTnKE
 

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SO what components are you using? Is this a 3way or 4 way speaker?

What amp do you have and what EQ are you using?
 

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Discussion Starter · #4 ·
The only way to get the low distortion and high efficiency was a horn, as it had to be small I designed a 40” ¼ wave folded exponential horn using as much of the box as possible. For added bass used bass doubling by setting them on the floor and rear firing to use doubling with the back wall. The main speaker, to help the SPL is front firing.

The speaker size was already determined. My wife said small or not at all. When she met me I had a pair of 5’ speakers with 12”. Sounded great but…I was replacing a 16” cube subwoofer and two 8 X 6” shoebox speakers. Not counting the 3” stand alone super tweeters this gave me 4, 672 sq inches for two speakers . 24” tall, 12” deep and 8 “ + or – 25sq inches

To have a low distortion, High efficiency, small driver at about $25, I chose the Dayton Audio RS100-8. A horn uses the driver as a piston so the low freq cutoff can be way below the f0 of the driver if it is low mass, long excursion, with a large magnet and phase plug for low simbiance.

I wanted to reduce Doppler distortion and IM as well as have the mid & hi fq SPL match the level of the horn I had to crossover to a horn tweeter. The Pyle PDB78 had the low distortion and 112db Sensitivity required and a price around the $25 target.

The horn simulation software and my own figures showed the bass horn to drop off at about 1000Hz and the tweeter would not come up until 2000 Hz so I decided to use a second order Butterworth to give the extra increase at the crossover freq. Then using the component formulas I split the crossover points 2000 for the main and 1000 for the tweeter. This eliminated the one octave notch. I bought a ready built crossover on eBay that included the binding posts and changed a very few components.

I have been analyzing them for two weeks using REW, VA, Praxis and a few other analyzer software packages. I hardly turn up the volume control for very loud audio ~ 86db. I watch a 10 Hz note coming back from my test microphone. The software programs say less than 1% distortion, and a frequency range of 10Hz to 25K hz, and only 3 weeks of intense math and $100 per speaker pictures and REM screen shots to come
 

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Discussion Starter · #5 ·
as for equalizers I do not use one. All tone control's are flat on the Behringer MX802 mixer I use for all the audio / video sources. As i use highly efferent components my amp is an old Opimus STAV-3270 80 watt / chan with .09% THD. I've had it on the bench a few times and it was flat to the edges of my equipment 8 to 30K, however under heavy load or output impedance mismatch the distortion goes up. But it is rare to get a speaker that is under 10% THD + IM. since I now live in a noisy environment, to keep the noise 20db below the signal I have had to turn up the amp hence another reason for effecency
 

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Do you have any pictures? I am curious what your horn looks like because if you want low extension with any real spl then your horn will be huge. Something small and horned with spl tends to have a cutoff of about 25hz for home use.

15hz-1000hz wont be a small horn at all but also will sound awful at higher than 200hz. Achieving the highs with 104db efficiency and higher is not hard. Just a matter of picking your budget. I have a horn design that would do 35-800hz very well but is not small at all. 40" x 40" x 18"

So any pictures or REW results?
 

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Discussion Starter · #8 ·
This is what convinced me that the speakers were able to reproduce the subsonic signals.

In order to make it a real life test, the tests are done in our living room.
Test signal, in red is mixed with music audio. Speaker output level is about 70-75 db. Microphone, in green is also picking up background noise.
Low frequency audio is from the horn which is delayed ~6 msec from high freq audio, due to 1meter long horn, + 12” to back wall, +12” & + 24” back to front of speaker. All audio (Green), is delayed 8 m sec, from input signal (Red) due to the 8’ to the microphone, so red and green traces only align on the test signal
 

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Cant quite tel in the graphs and pictures but how tall are the speakers?

And in your 0 to -30 scale for 0-25Khz what software were you using? Was this Audacity also?
 

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Discussion Starter · #11 ·
The speaker is 24" high X 8" wide X 12" deep. here is a picture with a yard stick
here are the oscilloscope pictures of the low freq signals
I duplicate tests with 4 different signal generators and 5 analysis programs to insure accuracy
REW, VA, Praxis Audasity, soundcard scope, sound ruler,sample champion, sound tone generator, ringbell tone generator, and others. I have tried many others but come back to these

I have been shocked by these results as a bass horn with a mouth of 16" sq should not go below 50Hz. However I have done a few (many) things differently. Hi compression ratio, Throat horn loaded on both sides, baffel step horn (bevel), and others.
 

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Discussion Starter · #12 ·
I thought everyone would see what I did that made these work so well. I insured that the acoustic impedance discontinuities were minimal.

The throat is horn loaded on both sides. First it reduces the distortion as it balances the acoustic impedance on both the push and the pull of the back pressure on the driver.

Next it increases the efficiency as there is a compression change leading to the throat, of 28 in ^2 from 7 in ^2. The “Throat Horn” works like the old ear horns.

When air goes over a big discontinuity many extraneous harmonics are produced, that is how a whistle or an organ works.

When a wave meets an impedance discontinuity some of the energy is reflected back (or in all directions), either in phase or opposite depending on whether it is an increase or decrease in impedance. The amount of energy depends on the amount of discontinuity and the frequency of the wave.
The acoustic energy traveling from the Driver normally meets a large Impedance change when it meets the throat. Not with this modification.

Each speaker uses 20 pieces of wood. The extra pieces insure that the sound wave remains coherent around corners. When sound has to reflect or take a longer path then recombine, harmonics are produced. Which harmonics depends on the time difference, distortion is the result.

When the cross section of the horn’s corner folds are not kept constant, impedance discontinuities result. Out of phase energy is reflected back, and then hitting the large impedance change at the throat, reflect again. By now the sound would be getting garbled

Discontinuities rob the base frequency of energy and put it in side-bands and harmonics.
 

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Discussion Starter · #16 ·
These speakers that I designed and My good friend, and craftsman Bob and I built are better than they should be!

Here is what I mean.
The math that describes the performance of a horn speaker, defines the lowest frequency sound the horn will reproduce as: a frequency {complete cycles in a second}, who's wavelength {how far sound travels during a complete cycle} is equal to:

1. The circumference of the mouth. The horns we built have a mouth which is 8" wide and 14" high. This gives a circumference of 44". Sound travels 13200 in/sec, so 13200 in/sec divided by 44" is 300 cycles/sec or Hz.

2. The length of the folded horn. The horn is about 40" long. 13200 / 40 = 330 Hz.

But no lower than 4 times the The length of the folded horn! 13200 / (4 X 40) = 82.5 Hz

However the Horn radiates into "3 (pi) Space". That is, 1 front-back, 2 left-right, 3 up-down. If you put a mirror next to a window it makes the window "look" twice as big. Acoustically, if you put a wall up next to a horn's mouth it makes it "sound" twice as big. As it “is” twice as big it doubles the lowest wavelength. It goes from "3 space" to "2 space". The floor also does this. a corner is 3 walls.

We are using the floor and the back wall. Using the rules for base doubling...300 Hz / 2 = 150 Hz (the floor), then 150 Hz / 2 = 75 Hz (the back wall). This says that 75 Hz or 82.5 Hz is the lowest they should go.

But I have monitored electronically; 40 Hz, 20 Hz, 15 Hz, and even 10 Hz sound coming at a reasonable level from these speakers. IMPOSSIBLE ? I saw it, I can hear and feel it, but I did not believe it. I Thought it was a fluke, maybe a result of the microphone I was using to test them. So I did a little research, then I sent away for the parts to build a very accurate, electronically shielded electret microphone.

Used a 9 v battery for low distortion, matched the load resistor to the input impedance of the “mic input” of the mixer. Matched the DC blocking cap’s impedance to the mixer’s input impedance at 10 HZ to insure a wide bandwidth. Lined mic enclosure with foil with a wire attached. Covered the foil with clear shipping tape to eliminate shorts. Used a two conductor plus shield cable. Put a TRS (Tip, Ring, Sleeve, aka stereo) plug on the cable with the tip hot, ring common, and sleeve to ground wire on shield.

But I got the same results with it!!!

I loaned our son, my sub woofer. It was no longer needed. But I am not sure why? could it be the "ear horn" in the throat?
Any Horn scientist out there that can advise?
 

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Discussion Starter · #17 ·
I made a mistake in the description of doubling.
4 Pi is a sphere.
2 Pi is a hemisphere, using the floor
1 Pi is 1/4 of a sphere, using floor and one wall.
1/2 Pi is 1/8 of a sphere, using a corner & floor
 

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Discussion Starter · #18 ·
Here is what I am inquiring about. The low frequency response below cutoff is so good that with a 1.3 cubic ft speaker I no longer need a sub woofer.

Adjusting the low freq equalizer setting just a few dB gives an almost flat response. Most music is already boosted in the low end.

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