microTower Port Tuning

[cogitech]

Based on an off-topic discussion in the CHR-70 destruction thread, I have taken 5th Element's advice and played a bit with port tuning on my Castle microTowers.

Based on his calculations and some of my own playing around in WinISD, I decided to extend the ports from the stock 4" long to 9.4" long (24cm) in an attempt to tune the port somewhere in the 40hz range.

I accomplished this with 5.4" lengths of ABS pipe and some duct tape. The change was simple to make since my ported bottom panels are simple to remove.

The result? Well, I asked my wife and her response was "....mmm...deeper."

Indeed, deeper. Even with the left end of the EQ sliders down about 2 or 3 dB (which results is less escursion) the bass from 50-ish hz through to the mid 30s is quite obviously "better", smoother perhaps.

Subjective, I know, but if my wife can hear the difference then it is not subtle. I am pleased. I was happy with my microTowers as they were, but the low end is just that much better now...

Anyone else ever play with port tuning their microTowers? I'd love to hear about your setups and your results.

[planet10]

WinISD is not capable of predicting the response of the microTower because it does not take into account the TL action. The stock tuning is 42 Hz. That is already pushing the limits of what is same (since Fs is about 70 Hz)

dave

[cogitech]

Originally Posted by planet10

WinISD is not capable of predicting the response of the microTower because it does not take into account the TL action. The stock tuning is 42 Hz. That is already pushing the limits of what is same (since Fs is about 70 Hz)

dave

As you know, I have really no idea what I am doing

I know TL is Transmission Line, but I have no idea what that means. Also, the Fs is 64Hz, according to Mark's pdf.

Is there a danger in doing what I have done? Does it make any sense that it sounds "deeper" and/or "smoother"?

[planet10]

If you tune too low you usually get a dip up higher and more group delay.

I just measured 16 EL70s and that is the number i used. That it is that close to Mark's done with pro kit, implies that the EL70 T/S (Fs) curve is fairly horizontal.

dave

[cogitech]

Originally Posted by planet10

If you tune too low you usually get a dip up higher and more group delay.

I just measured 16 EL70s and that is the number i used. That it is that close to Mark's done with pro kit, implies that the EL70 T/S (Fs) curve is fairly horizontal.

dave

By "a dip up higher" do you mean in the upper bass, or somewhere in the midrange, or...?

I have no idea what group delay is, but I'll try to google that...

I'm listening to Rage Against the Machine right now and I must say that the best way to describe the change that this longer port offers is "more balls."

[cogitech]

OK, I understand what "group delay" is, but I have no idea what it might sound like. Perhaps "more balls"?

[planet10]

definitely not more balls. But less fidelity.

dave

edit: added missing word.

[5th element]

I'm surprised that the micro tower actually shows any significant TL loading considering it looks in essence like a large ported cabinet. Sure the overall height of the thing will produce Quarter wave loading at something like 100hz, but then every floor standing ported speaker surely would too. Maybe they do and it's neglected in some way by design.

I have to admit though I've never looked into MLTLs before as I've never wanted to build one. A quick browse through a couple of papers seems to indicate that for a system to be a MLTL, the port has to be placed at the end of what would otherwise be the opening in a traditional transmission line. Then as the port changes the loading presented by the transmission line it pushes the tuning down in frequency.

Now from what I understand a TL doesn't really have a 'tuning' it more controls the air load presented to the driver and as a result controls the drivers behaviour. This occurs because the air in the line sets up standing waves at various frequencies that alter the pressure within the line and loads the driver so as to control it. Once you go below the lowest frequency the line can offer any control over though the driver excursion should shoot way up, much like a ported loudspeaker unloading.

Now the thing I am wondering is that the transmission line behaviour occurs because of the length of the pipe relative to the wavelength of sound going through it. So if the microtowers 1/4 wave is @ ~100hz how does the port go about altering this down to something ~40hz? Does the change in loading presented at the terminus of the line, by the port, set up a resonance that now occurs much lower in frequency where the port and the tower height actually end up working in sympathy? In a ported cabinet this ends up happening, but when it occurs, quarter waves aren't setup like they are in transmission lines. So does the 'mass loaded' presence of the port at the end of the line, actually allow quarter wave resonances @ ~40hz, to start occurring down the length of the line?

The tuning of the port in an MLTL must be quite critical I would imagine. If you keep altering the tuning is there a point where the cabinet actually stops behaving like an MLTL and transitions more towards behaving like a traditional ported cabinet? And does the Helmholtz resonance still occur in an MLTL or does the quarter wave loading act to suppress it?

Group delay is fairly simple in definition but probably a lot harder to actually know where to draw the line at. Group delay simply means how long you have to wait, after applying a stimulus to a system, for it to respond and produce something on the output. If this delay were identical for the entire system at all given frequencies then there wouldn't be a problem. The trouble is that in some systems the group delay differs depending on the frequency involved and this will/should degrade sound quality if large enough.

[trumper]

Has anyone thought of putting a baffle, similar to the bd voight pipe, into the microtower?

[weltersys]

Originally Posted by cogitech

OK, I understand what "group delay" is, but I have no idea what it might sound like. Perhaps "more balls"?

More response down low will translate loosely to "more balls", the ear perceives a 10 dB change around 1000 Hz as twice or half as loud, while down at 20 Hz it only takes a 5 dB change to sound twice a loud.

Generally, tuning below Fs is "less bang for the buck", the output goes deeper, but is not as loud. The lower the tuning below Fs, the more the speaker sounds/looks like a sealed cabinet.
You can easily find the cabinet tuning (Fb) by sweeping down with a sine wave tone generator, the cone movement (excursion) will be at minimum at Fb, it will go up rapidly below Fb, and will also rise above Fb.

If you don't mind putting a dot on the cone, it makes it easier to read excursion. If you are using very low test level, where cone movement is hard to see, you can use a piece of paper (or a finger) next to the cone, at Fb the paper won't be hit, but above and below Fb you will feel movement of the paper, or vibration of your finger.

Using this technique it will be easy to adjust the length of the port to get the Fb close to the speakers Fs.

If you don't have a sine wave generator, there are many test CDs and online free down loads of sine wave tones.

This will also get you familiar with the safe level of excursion vs. frequency your speakers can operate in, simulations often don't always relate all that well to the real world.

Have fun,

Art