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Old 4th January 2008, 05:00 PM   #1
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Default Puzzled by listening tests

I don't know much theory but have been experimenting with some simple listening tests and port lengths for the GM MLTL. Original build has a 2 in x 4 in. port out the bottom.

Opposite channel has a 2 in x 3 in port also on bottom.

24 gauge solid copper wire (high resistance) used both sides. Interior treatment the same: above driverr with pillow batting fiber fill and along one wall.

What's puzzling is one sounds louder than the other on test tones.

I'm 60 with is another variable for hearing loss.

What would show a noticeable difference in volume of like units installed in the same design with only wire and port length dissimilar?
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Old 4th January 2008, 05:10 PM   #2
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Are all frequencies louder on one channel? I have been fooled in the past. First I turn around and listen with them at my back. If that doesn't work I swap the speakers and try again.

According to your post the wires are both the same and different. Did I miss something?
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Old 4th January 2008, 05:26 PM   #3
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Quote:
Originally posted by Cal Weldon
Are all frequencies louder on one channel? I have been fooled in the past. First I turn around and listen with them at my back. If that doesn't work I swap the speakers and try again.

According to your post the wires are both the same and different. Did I miss something?

Yeah, foolish me. After I made the post I thought about reversing the amp connection. I just don't think my hearing is that off.

As I raise the test tone frequency, I can hear the 'quieter one' come up in volume but always always one is louder on a side to side test.

I'll reverse 'em and check again.

Also puzzled by the original port design which is smaller and shorter. I did not care for this and when I hit on 2in x 4 in. long I stuck with it. GM said that it was to achieve lower frequency but I found it to be muddy in practice. I have no test gear and am fairly theory illiterate (even after having been on forums for some time.)
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Old 4th January 2008, 10:18 PM   #4
jnb is offline jnb  Australia
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Try swapping at the speaker terminals on the amp. Next try physically swapping the boxes to the opposite sides of the room. Thirdly, try swapping the drivers between boxes. You should then have your answer.
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Old 5th January 2008, 08:48 AM   #5
Nanook is offline Nanook  Canada
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Default Helmholtz frequency

Click the image to open in full size.

this is the equation that "governs" the behaviour of air in port, the change is "L" , the length of the port

Everything else being equal, sqrt(1/3)>sqrt(1/4) by some 15%...so the tuning frequency will be lower by 15%.
An example... if the L=4" port speaker is tuned to say 40Hz, then the L=3" port would be tuned to 34Hz. This difference is easily enough to hear.

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Old 5th January 2008, 12:31 PM   #6
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You don't specifiy which MLTLs (or drivers) you've got currently, but by shortening the vent, you will raise both the tuning frequency & also reduce the damping of the cabinet as less resistance is placed on the internal standing waves.

So, the cabinet with the shorter vent will indeed sound louder in the LF at least (though it won't go as low), probably exhibit a little more ripple higher up as there is less attenuation of pipe harmonic modes, and perhaps a little less compression of the LF, though those last will be very subjective.

For example, here's GM's MLTL31 cabinet (IIRC he advises a 2in x 5in vent for a max-flat response & adjust to taste from there for that particular box) with a 2in x 4in vent:
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File Type: gif 4in port.gif (5.6 KB, 440 views)
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Old 5th January 2008, 12:33 PM   #7
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And 'ere be the same box, with the vent length reduced to 3in:
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File Type: gif 3in port.gif (5.6 KB, 430 views)
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Old 5th January 2008, 12:34 PM   #8
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And for completeness, with the 2in x 5in vent:
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File Type: gif 5in port.gif (5.6 KB, 428 views)
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Old 5th January 2008, 02:10 PM   #9
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It's my hearing.

I did the simple test of turning my back to the speaks and doing a/b tests that way. It verfies a simple hearing test I got a while back. the test was not thorough-- it was a promotional freebie.


The MLTL is the GM design for FE127e. 'Zilla made that with a side firing port. I used the example from the Jordan GM design and decided that the bottom firing port (and sticking out the bottom as well) gave the best effect. I tried an angled baffle at 45d at the port but that was disappointing too.

The idea of top firing or bottom firing ports comes from Martin King's notes. I have not read Augsburger. In his experiments with transmission line, King states (and correct me anyone if I got this wrong) that the air energy at the port is zero and the dispersion is at the terminus, not ballooning out as the front wave does. So getting the port in the horizontal plane rather than the more typical vertical (rear mounted, front mounted) should give better sound to the ear.

What I was trying to get was a shorter port on the bottom, protruding out, but able to sit on a low speaker stand or table height typical for a larger tv.

My method of setup for listening to this port out the bottom MLTL is to insert the port through a hole cut into a cheap screw together shelf stand typically sold in big box stores for $10 US. This has worked out amazingly well. Not ready for Better Homes and Gardens but this was never my goal.

Soon I will have the last side on the Harvey. Since the port on the Harvey is radically different from the MLTL I've heard a radical difference in sound. But that discussion has to go into the Harvey builder's thread.

The last listening test I did with the Harvey as dry clamp job I was using my 'good' ear side.



Properly sealed, I'm hoping to get it in shape to sound comparable or enhanced in relation to the BIB-- also for FE127e.
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Old 7th January 2008, 02:09 PM   #10
Colin is offline Colin  United Kingdom
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Thanks for those graphs, Scott - I like a person who works in pictures. I do worry sometimes, though, about your ability to generate graphs so fluidly. Are you directly hotwired to MathCad?
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