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Speaker Parameters, Pt. 3

DIY cabinet design.

Last month, we discussed how various speaker parameters relate to designing guitar cabinets with modeling software (specifically Eminence Design Software, designed by Harris Technologies). We examined a cone displacement graph, and are now ready to practice designing a cabinet using custom amplitude response.

For the purpose of our discussion, the speaker I’m modeling is a 12” with a Qts parameter suitable for closed, vented or open back designs. The first model I’m showing is an open back design. I can simulate an open back cab using an infinite baffle design – something like a sealed enclosure design, 10,000 cu. ft. Of course, that won’t provide an optimal size, but it is useful for determining output and response shape below 200Hz (fig 1).

Figure 1
Open back design: 10,000 cu. ft., F3=110Hz

Figure 2
Closed box design:
red line- 1 cu. ft., F3=117Hz
orange line- 2 cu. ft., F3=112Hz
yellow line- 20 cu. ft., F3=110Hz

The next model is the same speaker in a closed back cab. You’ll notice that the size of the cabinet does not have a very big impact on the guitar range (fig 2). This is due to the stiffness of a guitar speaker. The amount of backpressure in the cabinet is dominated by the stiffness of the speaker. This suggests that cab size is not as crucial compared to a vented enclosure. Of course, any subtle change may still be significant in terms of tone. The software will suggest a box size, if needed (fig 3). I can choose from high output, which emphasizes high SPL and minimal cone excursion; high fidelity, which emphasizes a maximally flat response; or I can enter a desired F3, or specified cut off frequency. I can also specify the desired box size and use the software to calculate F3. Note that for any of these options, the software will either adjust or give an error message if the calculated size of the box is too small for the speaker.

Next, I’ll model the speaker in a vented cab (fig 4). This is where it can get tricky!

What the software predicts as optimal may be abusive to the speaker. Remember, we’re dealing with lightweight, full paper cones and very little Xmax. We may want some distortion, but we don’t want mechanical abuse. The software’s only mechanical concern is Xmax. There is no consideration for cone response or the nuances associated with typical guitar cones. The lightweight, full paper cones are used to achieve efficiency, detail, and harmonic content.

It seems that a lot of players today are using drop tunings or seven-string guitars that demand tons of low-end response, and sometimes these goals are unrealistic for a conventional guitar speaker. What looks good on paper may sound like crap and potentially destroy your speaker. I can’t stress enough how important it is to include trial and error. Some of this will be very obvious by simply knowing which frequencies the guitar produces.

For example, if I want the software to model a high fidelity enclosure, the suggested tuning is 58Hz. I’m pretty sure that’s going to be too low for my guitar application, and very abusive on the speaker. My other concern with a vented enclosure for guitar is where to tune versus what I want to hear, and what’s safe for the speaker. This is where it becomes an art; you can only decide through trial and error. One way you can determine what you like is by varying the vent sizes—however, when comparing multiple speakers, your opinion on an optimal tuning frequency will likely change with each speaker.

Figure 3
Closed box design:
red line- high output, 0.86 cu. ft., F3=113Hz
orange line- high fidelity, 1.94 cu. ft., F3=105Hz
yellow line- desired F3, 1.06 cu. ft., F3=110Hz

Figure 4
Vented box design:
red line- high output, 1.68 cu. ft., Fb=90Hz, F3=78Hz
orange line- high fidelity, 4.78 cu. ft., Fb=59Hz, F3=51Hz
yellow line- desired F3, 0.8 cu. ft., Fb=119Hz, F3=107Hz

A speaker with a larger voice coil diameter, denser cone and/or more Xmax will have a more defined low end and handle lower tuning frequencies better than a low power speaker with fast break-up and higher efficiency. In terms of what is safe for the speaker, it is not difficult to hear what is abusive as opposed to what is musical. For a vented cab design, the software will be useful to help you find the appropriate cabinet volume for realistic tuning. It can also help you determine how to vent an existing cabinet for desired tuning.

In summary, if you want to use software as a tool for designing an optimal guitar cabinet, you have to consider your past experiences, tests through trial and error, and use good judgment. Remember, designing guitar cabs is as much an art as it is a science. Software is useful, but there is no substitute for trial and error and lots of experimentation.

I certainly welcome your comments or any experiences you’ve had in designing the ultimate guitar cab. Please feel free to email me.

Anthony “Big Tony” Lucas
is a guitarist and Senior Lab Technician at Eminence Speaker LLC, where he specializes in guitar-speaker design and customer support. Big Tony has been with Eminence for over 10 years and is responsible for many well-known guitar speaker designs.