Building a small practice cab

[BryanC]

I am planning on building a small practice cabinet, to handle acoustic practices (No drummer, acoustic guitar, and 1 - 3 singer), something smaller than my 410.

As the box is quite small I am having difficulty fitting the port in.

Is there any reason to not have the port vertical i.e. top ported and opposed to the usual spec of front or back ported.

Also, in WinISD, the 1st port resonance is calculated. What is the significance and what will a resonance of 633Hz have on the sound?

Thanks
Bryan

 

[DukeLeJeune]

You can orient the port however you need to in order to shoe-horn it into a small cab. If the inside end of the port ends up close to a wall, it may drop the tuning frequency a bit lower than your calculations predict. Also, placing an amp on top of the cab over the port can lower the tuning frequency as well. And if the airflow is obstructed, chuffing could set in earlier than it normally would have. If you plan on setting an amp atop the cab, a downfiring port (along with relatively tall feet) might make more sense. What diameter port are you using?

That 633 Hz resonance can add a bit of emphasis at that frequency, sort of like talking through a cardboard tube. Unlikely to be a problem in an electric bass cab, as what's coming off the woofer cone itself is a lot louder.

Flared ports are generally better than straight-sided ports from the standpoint of both airflow and low coloration (I actually use my rear-facing flared ports as additional sources of midrange energy). But they are also expensive. A poor man's substitute is to cut the inside end of the port on a 45 degree angle instead of straight across (the distance to the center of that 45 degree angle is your effective port length). The inner port opening is where chuffing usually starts (the baffle around the outer opening is beneficial from a fluid flow standpoint), and the 45 degree angle cut increases the cross-sectional area right where it's needed the most to delay chuffing. Also, with that angled cut, the port no longer has a strong "cardboard tube" resonance at one frequency, but instead it's spread out a bit, and is lower in amplitude, so it's less likely to add unwanted audible emphasis to one particular note.

 

[Woodstockz]

I am planning on building a small practice cabinet, to handle acoustic practices (No drummer, acoustic guitar, and 1 - 3 singer), something smaller than my 410.

As the box is quite small I am having difficulty fitting the port in.

Is there any reason to not have the port vertical i.e. top ported and opposed to the usual spec of front or back ported.

Also, in WinISD, the 1st port resonance is calculated. What is the significance and what will a resonance of 633Hz have on the sound?

Thanks
Bryan

Which speaker, and what are you cab dimensions?

 

[BryanC]

Which speaker, and what are you cab dimensions?

I will be using a Fane 8-225 driver.

The cabinet will be a 20l (0.822 cu ft) tuned to 65 Hz.
The internal dimensions will be 230 x 290 x 350 mm (9 x 11 1/2 x 13 3/4 in aprox).

I was originally thinking of a slot port but, following Duke's suggestion, I might do a tube port in the bottom with tall feet.

Bryan

 

[astack]

If it's a small cab with small excursion capabilities, I would try cheating the port area smaller, which will allow for a shorter tube. (The formula works such that bigger area = longer length for the same tuning.)

If you're running WinISD, you can model the box, find the max power in the dip area, enter that into the Signal tab and then check the port air speed. I think the rule of thumb is less than 30 m/s, IIRC. It could be ft/s though. The standard port size is a 4" tube which is large and overkill. Probably a 2" or two 1" would be plenty.

The other thing is, tubes are great because they allow you to fine tune more easily after you build the box. Slots or corner ports can't be modified too easily. OTOH, a slot or corner slot port can be shorter than the equivalent tube and tuning, making them easier to fit. But this makes them even harder to get right the first time.

The 65Hz is high for a bass cab. If that driver has a high fs, sometimes you have to cheat that to get a good profile, but otherwise you want it closer to 50 or below. If you do have it that high, a HPF will help you not fart it out as easily.

 

[Triad]

I made a 2x8" with those woofers... I think that woofers smaller than 10" to sound good must be expensive.
My advice would be to use a Fane 10" 300 for reflex or a 10" 200 for sealed cab... you won't regret it.

 

[BryanC]

The 65Hz is high for a bass cab. If that driver has a high fs, sometimes you have to cheat that to get a good profile, but otherwise you want it closer to 50 or below. If you do have it that high, a HPF will help you not fart it out as easily.

I am curious as to why you say 65 Hz is high for a bass cab. There has been a lot of discussion that the fundamental are not that important, but the first few harmonics have most of the energy. Even though it is still good to have some of the fundamental there, I thought I could sacrifice the fundamentals almost completely for the sake of a small grab-and-go practice cab?

My WinISD simulation shows very large cone extension between 40 and 65 Hz, well over Xlim. I thought I might need a HPF, maybe even building it onto the back of the jack plate.

 

[hover]

Rear firing port. Done.

 

[BryanC]

I made a 2x8" with those woofers... I think that woofers smaller than 10" to sound good must be expensive.
My advice would be to use a Fane 10" 300 for reflex or a 10" 200 for sealed cab... you won't regret it.

Thanks for the advice Triad, but I have already bought the 8-225. I have however been curious about sealed cabs and like your suggestion of the 10-200. I might try that next even if the 8-225 is to my liking.

 

[astack]

I am curious as to why you say 65 Hz is high for a bass cab. There has been a lot of discussion that the fundamental are not that important, but the first few harmonics have most of the energy. Even though it is still good to have some of the fundamental there, I thought I could sacrifice the fundamentals almost completely for the sake of a small grab-and-go practice cab?

My WinISD simulation shows very large cone extension between 40 and 65 Hz, well over Xlim. I thought I might need a HPF, maybe even building it onto the back of the jack plate.

Two separate design considerations here and you need to look at two graphs to see them.

The first is the question about the fundamental being important to your sound. The prevailing wisdom here is that it is not, but that's a matter of taste. To design for this, you need a low "enough" f3, which you can read off the transfer function graph.

The other is the excursion limits. Like you said, WinISD shows the speaker blowing up here. That's bad. You can try adding eq in WinISD to see if it can overcome the unloading.

If you look at the max SPL chart, this gives a mix of the TF chart and excursion chart. This can be helpful to see how much output you're loosing relatively due to excursion.

But again, sometimes it works to tune that high to avoid too weird a TF graph. Using a HPF helps balance out the unloading problem.

I wouldn't build it into the cab. It'd be almost $50 (and a few lbs) in parts that *hopefully* match well with the speaker. I'd rather spend that money on a pedal that can do the same and be used with other cabs.