Building ported speaker cabinets

[Scottf]

Is there a ideal enclosure size or do you simply build a cabinet to your liking and then simply tune your port size and depth according to the desired frequency? Also is there an advantage to having a large verses a small port or vice versa or does it not matter as long as it's properly tuned? I wondered if a larger port plays the low frequencies relatively louder than a smaller one? Also anyone have a recommendation for good speaker building websites? Here's a great website for calculating resonant frequency of a enclosure

http://hyperphysics.phy-astr.gsu.edu...cavity.html#c1

[GM]

A driver wants to 'feel' a certain amount of compliance based on its electro-mechanical properties, and for optimum performance has a fairly narrow deviation window of ~one octave on either side of its T/S max flat alignment. This of course doesn't mean you can't 'force' it to perform acceptably in a non-optimal room/loading environment to meet a specific in-room response though.

Vents must be of a minimum cross sectional area for the tuning to ensure it doesn't generate audible noise due to too high a resonant pipe frequency, usually referred to as too high a vent air 'speed', hence larger vents won't play louder, but only have a lower vent 'speed'.

Unfortunately, this applet doesn't takeny end correction into account, so it will tune a cab too low for a given input.

I know there's some good tutorial sites on the net, but I don't save the links. I don't know if it's still around, but there was one called 'speakers 101' that was pretty good.

There's one program to stay away from though IMO, and that's WINISD. It has too many programming errors, though numerous folks have used it to 'design' many speakers that they thought were the 'cat's meow', but that doesn't change the fact that they could have done better with a more accurate program.

Probably the best programs overall are the ones done by this group of hardcore DIYers: http://www.pvconsultants.com/audio/frdgroup.htm

Once you get a better 'handle' on speaker design, then moving up to MJK's Mathcad pipe worksheets is currently where you need to be for best accuracy IMO. http://www.quarter-wave.com/

Note that without using the measured specs of your particular drivers it can be somewhat of a *****hoot, requiring fiddling with cab volume, vents, etc., to get the best performance due to manufacturers not always being the most forthcoming with accurate specs or not bothering to update them when design/manufacturing changes are implemented.

GM

[ckdf]

There is an ideal range for the enclosure size for a given driver. This range can be
defined as a percentage of the driver?s "Vas" volume T/S parameter. However there is
not one fixed range for all drivers. Instead the range depends on the driver?s
Qts value. Drivers with smaller Qts values require smaller Vas percentage values with
a bigger usable range. Vice versa drivers with higher Qts values require higher percentage
values of Vas for your bin size and range is smaller.

In the same way the required box tuning and resulting lower 3 dB frequency can be
expressed relative to the drivers resonance frequency fs. Here it is just the other
way round: For drivers with a low Qts value the lower cutoff and the required box
tuning frequency are at a higer ratio range relative to fs than for drivers with
higher Qts values.

A German speaker builder magazine (Hobby HiFi) recently condensed that into a nice
methodology that allows to very quickly construct bass reflexes using just one diagram
and one formula for the vent calculation. I inquired if they intend to put that on the
web as I find it quite useful and will let you know if they do.

There are numerous programs and books where you can find these tables and calculate
these values.

But be carful:

Most of these assume that you want to build a plain bass reflex bin with a "flat"
response. You cannot use the values they yield to properly tune a bass reflex loaded
horn as in this case you want some "overshoot" in the response curve to compensate
the horn rolloff towards lower frequencies. This means you need to tune to a higher
box tuning and go for bigger bin sizes than for the "plain bass reflex bin" flat
response case.

Another flaw is that most programs simply use the drivers free air radiation T/S
parameters. Unfortunately the real values of y o u r drivers may considerably deviate
from what is specified on datasheets. And those T/S paramters change once you put the
driver on a baffle due to additional air load. They change even more if the driver also
needs to move the air in the vents of a bass reflex. And they change significantly more
if you put the driver into a horn, due to more air load. And any series resistance
like from the amplifier output resistance or from inductors in the low pass change them
again.

[ckdf]

BTW. Scott,

small additional sidenote to the original posting. Not your fault, but the explanation and calculation of a cavity resonance given on the webiste you cite is so horribly wrong in almost every aspect that it could not be much worse!

It should state: Fres = alfa * SQRT ( A / V * L) and to explain the reasons:

Larger opening gives higher resonance since the counterforce of the "spring" formed by the the enclosed air volume is proportional to the square of the area of the opening.

Larger volume gives lower resonance as the counterforce of the "spring" formed by the enclosed air is inverse proportional to the volume.

Longer neck gives lower resonance as the mass of the air in the neck is proportional to the length.

And on top of that you have to include some "end correction" which means that the port appears to be longer than it actually is. This depends on the port geometry and so there is no fixed value.