Playing with port tuning...

KJMO

KJMO

Feb 6, 2015
1,962
1,945
5,081
58
#1
Again, I have been playing around with the same old 10" genz-benz faitals in what was a sealed box. I cut a 4" hole in the cab in the front, left, bottom corner and used my phone to generate a sine wave. I put rice (actually quinoa) on the speaker and have it more or less level. Without anything in the hole the rice stops moving from about 50hz to 60hz. With a 4x4 plastic port inserted, it is more or less dead between 35 and 45hz. My questions are:

Why is it dead for such a large range? I wouldn't have thought anything could be tuned for a range that big. I mean, when you tune a guitar, it isn't tuned for a range, right? But then again, there's probably a reason the frequency response is plotted dB by frequency too.

If there is going to be a 10hz chunk missing, I have a hard time understanding where I would want that chunk. 35 to 45 doesn't seem great since that's where the fundamental of the E is. So would 50 to 60 be better? From what I understand, that's where the first harmonic of the B string is, so maybe that wouldn't be awesome either. Plus, to just have a chunk missing in the middle just seems wrong. I know in winisd you can make the port add some extension, but have a smooth curve. That seems like it would be the goal.

I've tried playing through it just to listen, and to be honest the cab doesn't seem to behave a lot different than when it was sealed, so I'm not sure what that would tell me. Also, I'm a little surprised there isn't more like a "puff" of air out the port, since I've noticed that with a lot of speakers with ports.

At any rate, does anyone have any ideas for how to proceed with my playing around?
 
#3
He's actually responded to questions I've had about this before, and said basically he's pretty sure I'll never get good results. I think he's probably right about that, but at this point I'm more just playing around for fun, so I kind of don't want to bother him, if you know what I mean ;-)
 
#4
Did you try a smaller hole first? A 4"port in a cab already fine as a sealed 10 might just be too big for that driver. Consider that the Shuttle 10t speaker (this may be the box designed for those drivers, IDK) only has two small triangular ports and is roughly 1cuft. If the box is much at all bigger, and the port is a bit large, you may be basically using the drivers in almost a "free air" environment.
 
#5
basscooker said:
Did you try a smaller hole first? A 4"port in a cab already fine as a sealed 10 might just be too big for that driver. Consider that the Shuttle 10t speaker (this may be the box designed for those drivers, IDK) only has two small triangular ports and is roughly 1cuft. If the box is much at all bigger, and the port is a bit large, you may be basically using the drivers in almost a "free air" environment.
Click to expand...
Correct, if that's a single 10" cabinet, a 4" port with a duct length of say 3/4" (the thickness of the baffle) is going to be way too large.

When you tune your guitar, you are tuning a simple system (like a pendulum), but when you tune a speaker, the parameters are distributed (speaker area and box area(s)/volume, so the Q of the tuning will be lower (meaning the bandwidth of the tuned "point" is no longer a point but a set of points).

Now, let's explore the simple guitar system a little deeper and you will find that it too is a set of frequencies and not just one frequency, there will be harmonics (which is why positioning of a pickup relative to the bridge yields such a difference in tones) as well as some frequencies very close to the fundamental (this is why pianos have more than one string to each note, very slight de-tuning results in a richer more complex sound). It appears to be a single note because the ear and brain are very forgiving.

Finding the resonant point(s) of a speaker system (like the OP did) doesn't have much value without considering all the other response parameters associated with the speaker. This is why we measure, calculate, and calculate a variety of parameters to be sure that we are getting what we want out of the speaker.
 
  • Like
Reactions: KJMO and basscooker
#6
It's actually two 10" speakers, so it doesn't seem like that would necessarily be too much port, but I have to admit it does seem to behave at least a little like it's "open". Do you think you would stick with the 4x4 inch (35-45), empty hole (50-60), or cut the 4x4 down little by little to bring it up a tad? Could I get more valuable info by taking db measurements at lots of frequencies (30hz up in increments of 2hz, maybe)? I'm thinking it would give me something like what winisd gives you, except for real?
 
#7
I think you would get a LOT more valuable info. by doing the math and studying the results. I suspect that making the port small enough will increase the air velocity at higher power, so tuning the air mass loading of the port with duct depth might be a much better option. How much? I don't know, but doing the math will reveal the answer.
 
#8
yes when doing the rice test the visible cone movement will have a 10Hz span of very little movement

this is the behavior of a reflex box
And in the modeling software you'll see the same thing, except the ppoint where there is little movement is more obvious on the graph.

you can get more exact frequency by turning your oscillator signal up pretty loud and just barely touch the edge of the speaker with your finger and feel where there is little movement. so basically yes visual gab is 10Hz and with your finger you'll a smaller more accurate 3Hz gab.
And yes this is the down side to reflex/ported boxes, as well as the good side if you understand how it works and what your designed the system for and what frequency range. transient response is lower at and near the tune frequency or fb
of the enclosure. the signal is no longer produced electrically by the speaker it's all port.

the other downfall is below the tune
frequency the cone movement is very high the speaker is almost operating like it's in free air without a enclosure. so yes you want to tune as low as possible so the fundamental is created by the speaker not port and the fundamental is gonna cause huge cone movement and reduce power handling.

ideal tuning would be in the 30s so yes a standard 4 string low e would still be created by the speaker not the port. And then the little cone movement you have observed now works to your advantage and is a benefit of reflex speaker. And works
as a built in brake to protect the speaker from over excursion.

problem is the speaker itself has a huge impedance spike also known as the resonant frequency or fs. And in a reflex box you usually tune no more than 10Hz below or above the resonant frequency.

because there is a huge impedance peak already at the resonant frequency of the speaker and there will be another huge impedance peak where you tune the box. your working against two huge impedance or resistance and frequency response is going drop at this point no matter what.

so you simple just lay these too problems on top of each other instead of spreading them out. you put the mechanical impedance of fb right on top of the electrical impedance of fs. And that's it.
basically if you want deeper bass or more punch/ transient response on the low end you find a speaker with a low resonant frequency or fs that's below the fundamental of the signal your designed the speaker system for. in this case it's bass guitar and your well aware of those frequencies, excellent observation by the way.

big big annoying mistake by many is they high tune. this will actually reduce cone movement above the tune frequency. so they think they have improved power handling of the system. typically with 15s with a fs of 45Hz they high tune to 55hz and this puts the no cone movement up in the A string frequency area. And transient response is reduced now on the a string and the natural frequency drop off at 45Hz by the fs of the speaker effects the e string. And the fact there is more cone excursion caused below tune frequency. power handling is reduced not increased for the E string.

now you got poor transient response on a and flapper excursion on e.
And if your using a tube amp you just created a huge impedance spike on your common used strings and amplifier power and impedance match for the tubes is poopy doopy

most players don't notice the transient response drop of ported systems and are happy with the extended bass response compared to sealed. so finding a speaker with low fs and simple tuning to fs creates a more punch and response.

there is plenty of guys more than happy with 12" ported speakers at 55Hz to 60hz. I don't care for them cause after you've played bass for 20 or more years through ported 12s or 15s you start to notice the response of them. this is why so many swear by sealed 10s cause they are more punchy cause they don't have the poor transient response of ported. And that magic mid range isn't the speaker size. it's electrical reproduction of the A string not a high tuned system in the 55/65 Hz range.

so we live with it and just put a 15 inch in 3 cubic feet and tune to 40hz....sound familiar. yeah TL 606 this is why it's so praised and claimed to magically work for almost every common bass 15" driver. for ported

you want punch and accuracy sealed 10. sick of the bloom from 1 cubic foot 10s go to 2 cubic sealed 12. you need four of them so you can over eq to get lowend. 4x10 or 4x12. drummer to loud add four more 8x10 or 8x12 yes they figured it out like 60 years ago.

otherwise the higher power ported systems available now work fine and 85% of most players don't notice the transient response of those systems blah blah blah it's pretty simple.
 
  • Like
Reactions: bass_69
#9
Many players just prefer the response, or response versus size tradeoff, of ported systems compared with sealed box systems, those players should play whatever makes them happy for their on particular needs. It doesn't reflect on anything other than their own preferences.
 
#10
I actually had already done the winisd before, and it makes sense as far as the vent tuning. It says to get the port tuned to 56hz (which happens to be the Fs of the driver) I need a 4in port with no length. Since I measured it that way and got a dead spot at 50-60hz, that makes sense, I think. So is that what I want? The graph shows -3db to be at 45hz instead of at 100hz with the cab sealed. What am I losing to get the bass extension? Here's the graph:

genz benz sealed and ported.JPG
 
#11
You are losing a lot of usable overall low frequency energy with the sealed cabinet.

Another way to look at it is the area under the curve method. If you consider 30Hz to be the lowest frequency you might care about (just an example), the area between the curves above 30Hz is what is gained by going with your ported example.

As an experiment, try 3.75" and 4.25" port dimensions, see what the two curves look like, one that should get you to a critically damped response. The set of curves will all be similar except that (primarily) the Q of the filter response will vary.

I don't know what you are using for parameters, your drivers are NOT stock Faital's so you can't use the 10PR300 parameters (though as a learning exercise it's ok).
 
  • Like
Reactions: downlowuponit
#13
Ok, one thing I was confused about (embarrassing, but I'll take one for the team) was that I thought that since the speaker wasn't moving much at the tuned frequencies that the cab wasn't producing much sound at those frequencies (even though I could hear that it was). So that is what Bogeybass was talking about when he said the box and port were producing those frequencies, right? Ok, so I was confused that there weren't any dips in the graph where the port was tuned, but now I understand that the speaker isn't moving, but the frequencies are being produced.

At any rate, I made a mistake, and actually to get the port tuned to 56hz, I would have to have a 4" port that is negative .19in long, so rather than try to do that ;-) I think I'll tune it to 46hz with a 1.5in long port, since at that point there is no hump above the tuned frequency (there is when tuned higher) and it seems to drop pretty nice from 50hz. If I tune lower, I gain output in the lower range (e.g. 20hz), but lose it in places I think are more important (50hz, etc.), so I'll avoid that.

If you change port size, you seem to be able to get the same curve by changing the length of the port. Is there a reason to prefer changing port size instead of port length, or vice versa? I've seen those speaker designs with super long built in ports, which is interesting.
 
#15
Whoops! That would be funny if it didn't make me feel like an idiot :-) . I guess the only way I could tune to 56hz would be with a bigger port, huh. I don't know why bigger, but that's what winisd says, I guess.
 
  • Like
Reactions: agedhorse
#16
Making the port bigger and longer will let you move more air before you get "chuffing". With a smaller/shorter port, tuned to the same frequency, you will very quickly reach an air velocity in the vent which will make noise (the chuff).
 
  • Like
Reactions: KJMO
You must log in or register to reply here.
Top Bottom
Back