#1
Ive looked into and studied for a MECP licence and know alot of the basic stuff for building boxes and how sound works. Im a little rusty but my friend is in a band and he talked me into building cabs as i used to build custom subwoofer boxes. A question im stumped on right now is when building a cab how do you tune it? Ive done it with subwoofers before but not guitar speakers. If i were to tune a box what would I tune it to with specific measurements and a certain preference of how the guitarist wants the sound? Right now I am primarily looking at guitar speakers rather than bass which I will do later on.
#2
i could be wrong but i think with guitar cabs the process is effectively, "Do the speakers fit?"

The more advanced method is, "Hmmm, maybe i'll get more bass if I make it a bit bigger."
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#3
Hi, practical guitar cabs come down to a choice of three types only one type is tuned. Guitar cabs can be open backed, which aren't tuned at all, sealed cabs which aren't really tuned but which raise the resonant frequency of the speaker and shape it's bass response. The only tuned cabs are ported cabs which increase the bass output at or around the resonant frequency of the speaker. That's usually around 50-70Hz, the lowest note of a guitar is 80Hz and there is almost no output of fundamental from pickups on an electric guitar which would need to be in the middle of the string to pick up the fundamental well. Because it only makes a difference where it doesn't matter and makes design and construction more complex ported cabs aren't really what you want for a guitar. The only people who use them for instruments are bass players and keyboard players.

Sealed cabs give the possibility of slightly more bass output than open backed cabs and you can tweak a sealed cab a little to give a pleasing bass hump which will give an extra punch to the bass but most of the sound of a guitar cab is about the driver you choose, the cab really only affects the bass.

For a bit more detail you can read this https://www.ultimate-guitar.com/lessons/the_guide_to/cabinets_for_guitars.html
#4
Most guitar cabinets aren't tuned in any meaningful way, and most guitar speakers wouldn't profit from it in any case.

That said, there are a number of us out here who are actually using full range bass cabinets for modeled guitar ( google fEARful 15/6/1 and fEARless F115 for examples). We're also using enough power (I'm using 1500W RMS power amps per pair of cabinets) to make bass-tuned cabinets worthwhile.

While most guitar speakers don't have much available below 100Hz, a few speakers used for guitar actually have resonant frequencies down around 50Hz and the power handling to exploit it. The Eminence Delta ProA will usually be found in the Pro Audio section of the Eminence website, but makes a very good guitar speaker, has a resonant frequency around 52Hz and handles 200+W RMS with the proper box.
#5
Quote by Phil Starr
...[ ].... The only tuned cabs are ported cabs which increase the bass output at or around the resonant frequency of the speaker....[ ]...
That's not really what happens. A "bass reflex" enclosure actually diminishes the resonant frequency peak and creates smaller peaks both above and below the primary resonance. The trick with ported enclosures is that they are much more efficient than sealed, or, "acoustic suspension" enclosures. While the numbers seem low in relation to transferring electrical wattage into acoustic wattage, a bass reflex cabinet converts 3% to a suspension cabinet's meager 1%!

For reference, horn loaded designs can convert 30% of electrical watts into acoustic watts. Which is why you see horn loading sometimes even on the woofers of PA systems.


Quote by Phil Starr
That's usually around 50-70Hz, the lowest note of a guitar is 80Hz and there is almost no output of fundamental from pickups on an electric guitar which would need to be in the middle of the string to pick up the fundamental well. Because it only makes a difference where it doesn't matter and makes design and construction more complex ported cabs aren't really what you want for a guitar. The only people who use them for instruments are bass players and keyboard players.
Well, not entirely true either. Modern octave generator pedals can produce 40 Hz easily from the E-6 fundamental. For an even quirkier application, but at a slightly higher fundamental, octave pedals are great for use with a capoed guitar, to restore lost bass.

Quote by Phil Starr
Sealed cabs give the possibility of slightly more bass output than open backed cabs and you can tweak a sealed cab a little to give a pleasing bass hump which will give an extra punch to the bass but most of the sound of a guitar cab is about the driver you choose, the cab really only affects the bass.
Tuning sealed enclosures correctly (?), is a math heavy annoyance involving a known mass stuck to the speaker cone and a bunch of meters and frequency generators. It's definitely not for the feint of heart, nor weak of resolve. YMMV
Last edited by Captaincranky at Jan 27, 2017,
#6
Quote by dspellman
...[ ]....While most guitar speakers don't have much available below 100Hz, a few speakers used for guitar actually have resonant frequencies down around 50Hz and the power handling to exploit it. The Eminence Delta ProA will usually be found in the Pro Audio section of the Eminence website, but makes a very good guitar speaker, has a resonant frequency around 52Hz and handles 200+W RMS with the proper box.
Once upon a time, Eminence had software available to assist with cabinet design. IIRC, it was either free, or free with speaker purchase.

I agree you can utilize response clear all the way down to 40 Hz from a standard guitar, even if it's only to fill in for an absent bass player, using an octave pedal.

Here's the Eminence homepage: https://www.eminence.com/

And the link for "US Speaker" http://www.usspeaker.com/eminence%20index-1.htm They're a full line dealer of Eminence, and other major brands.
Last edited by Captaincranky at Jan 27, 2017,
#7
Quote by Captaincranky
That's not really what happens. A "bass reflex" enclosure actually diminishes the resonant frequency peak and creates smaller peaks both above and below the primary resonance. The trick with ported enclosures is that they are much more efficient than sealed, or, "acoustic suspension" enclosures. While the numbers seem low in relation to transferring electrical wattage into acoustic wattage, a bass reflex cabinet converts 3% to a suspension cabinet's meager 1%!

For reference, horn loaded designs can convert 30% of electrical watts into acoustic watts. Which is why you see horn loading sometimes even on the woofers of PA systems.


Well, not entirely true either. Modern octave generator pedals can produce 40 Hz easily from the E-6 fundamental. For an even quirkier application, but at a slightly higher fundamental, octave pedals are great for use with a capoed guitar, to restore lost bass.

Tuning sealed enclosures correctly (?), is a math heavy annoyance involving a known mass stuck to the speaker cone and a bunch of meters and frequency generators. It's definitely not for the feint of heart, nor weak of resolve. YMMV


Without wanting to start a long complicated argument or detailed breakdown of the maths involved you've got this a little confused and I suspect some of your information is probably a misreading of some old textbooks. The efficiency gain only happens in the area where the port is in action, above that there is no rise in efficiency. The increase in output is typically 3dB across the bottom octave but will vary depending upon the exact characteristics of the driver and the size of the box, to comment further you have to do the maths or at least allow the computer to do it for you.

For hi fi use there is a need to modify the drivers to work well in a sealed cab which to simplify is because putting a speaker in a sealed cab raises it's resonant frequency. To allow for this lower resonant frequency speakers are needed to achieve full range reproduction of bass frequencies. Ultimately this involves using heavy cones and long throw drivers which are less efficient. For the most part this isn't the case for pro audio, or not to the same extent.

Yes of course you could use an octaver to turn 80Hz into 40Hz and make your guitar effectively into a bass. It's usually used to thicken up the sound higher up though and if you did want to use your guitar as a bass then you'd have to use a bass capable cab or risk losing a lot of drivers.

You are a little confused about tuning a cab too. Adding the mass to a cone and taking readings is about deriving some of the Thiele/Small characteristics, which the manufacturers provide anyway on their spec sheets. The mathematics of speaker modelling is moderately complex but there is plenty of software out there to do it for you. Win ISD is a piece of freeware which does a good job and is widely used, Eminence Designer uses the same maths as do all the others with slight variations. Frankly if you know enough to carry out measurements using the added mass method you wouldn't be reading this.

You missed the bit about the positioning of the pickups, there is relatively little fundamental in the output from any guitar or bass with the pups only a few inches from the bridge, again this is down to basic physics which can be mathematically modelled.

So I stick by the advice to the OP that there is little point in getting involved in designing and building reflex cabs for guitar. If you are using an octaver on anything below the A string then be careful, look to use a bass cab or something specialist.
#8
Quote by Phil Starr
....[ ].....You missed the bit about the positioning of the pickups, there is relatively little fundamental in the output from any guitar or bass with the pups only a few inches from the bridge, again this is down to basic physics which can be mathematically modelled.
Just a wild guess from someone you apparently think is badly misinformed, but I expect the maximum fundamental would occur at the 12th fret, the point of maximum string excursion. But like I said, just a wild guess.

Quote by Phil Starr
So I stick by the advice to the OP that there is little point in getting involved in designing and building reflex cabs for guitar. If you are using an octaver on anything below the A string then be careful, look to use a bass cab or something specialist.
I was envisioning using the guitar as a bass in a practice or home recording, not so much trying to blow walls and windows out with the guitar cabs in a pro performance environment

And no, I don't know how to do do the math involving tuning acoustic suspension, which is why, if you check below in my post to dspellman, I provided the links to a place where such software can be obtained.

I use the lower octave multiplier in a home setting, to recover bass perceived as being lost when a capo is in use.

As for the rest of you're post, you're doing a bit of preaching to the choir. If I were builing a 4 x 12, rest assured it would be open back, with triangular cleats to maintain its shape and strength when being moved.

If you'll take a moment to go to this address: http://math.armstrong.edu/faculty/hollis/plem/plempaper.pdf Topic 8 (ignore all the leading math), shows what I was attempting to explain about the port actually dropping output at the speaker's resonant frequency and redistributing the peaks to either side of it.

The document being a PDF, I couldn't simply copy the image address, and post that here..
Last edited by Captaincranky at Jan 30, 2017,
#9
Captaincranky sorry, I'm a grumpy old bugger at times. I used to design and build speakers for a living and then went on to teach science and on a bad day the teacher in me can't help themselves.

So anyway yes you are spot on, you do get maximum fundamental at the 12th fret, somewhere I've got a link to a program that calculates the harmonic content of the pickups output, you can slide the pickup into different positions and it will display the output.

Because I still design speakers I'm very concerned about designing in reliability and practicality for people who play live in bands where the conditions are often at the limits of the speakers capabilities. Using an octaver at home isn't going to cause any damage unless you turn everything up, in which case it's your ears that are likely to get damaged before your speakers.

Thanks for the link, though I've read the original papers by Thiele and Small, I've gor Beranek on my shelves somewhere.

Just in case people don't get the maths I'll try and explain in words

Speakers like guitar strings have a resonant frequency below this the output falls. The trick in a ported or reflex cab is that it acts like an organ pipe and has it's own resonant frequency. If you tune the cab to the speakers resonant frequency it will resonate and pump out that note very loudly. Every action has an equal and opposite reaction so this pushes back on the speaker cone and damps the movement of the speaker. If you get everything right you can get double the output at this frequency and raise the bass output by 3dB. In practice both speaker and port start to resonate a little above and below the central resonance so this effect is spread out over roughly an octave and if you get it right will give you some extra bass in the bottom octave. Above that the air in the port won't move and the cab behaves in exactly the same way a sealed box does.

All this shows up if you measure the impedance of the speaker and the peaks you see in all the graphs are usually from an impedance plot, so yes you are right about the peaks either side but it's a little more complex than that, and probably beyond what the OP wanted. I've simplified this too quite a lot.

If anyone wants a bit more I've written something for UG, comes with a health warning though, I'm pretty geeky https://www.ultimate-guitar.com/lessons/the_guide_to/choosing_speakers_to_drive_your_cabs.html
#10
Phil Starr

This whole ported enclosure discussion brings up a story from my feckless late 20's. One of our local Hi-Fi dealers, (forget the store name), was an authorized JBL dealer. Well way back when, "JBL" wasn't spelled in Chinese characters. Anyway, after JBL released the home version of their 4310 monitors, the "Century 100", every jitterbug in the surrounding counties would be at the store demanding a demo. The service tech/ salesman at the store, "Chatlie", would be running around the store practically tearing his hair out and mumbling to himself, "here we go again, nasty peak in the bass". I guess JBL didn't get the ports designed to his taste.

We all know what was really wrong, don't we? Too much damned woofer in a too little box.
Last edited by Captaincranky at Jan 31, 2017,