Reasons not to build a midrange ported box?

[oublie]

Hi,

Why wouldn't I build a ported midrange box?

e.g a midrange with an fs around 100hz with t/s parameters specific to ported design. What i'm thinking is extending the bass down to around 60hz in a ported enclosure?

If i model it in winisd i get a 4l box with a 2inch port 12inches long fed with 10watts for 94db flat to 65hz and port velocity of 10m/s at 60 hz.

Of course i could always make the box bigger a bit like a midrange llt but how does the port resonance come into play and how does it affect the mid range?

[doug20]

So you are really saying you want to use a mid range driver as the woofer in a ported speaker design?

[Loren42]

Originally Posted by oublie

Hi,

Why wouldn't I build a ported midrange box?

e.g a midrange with an fs around 100hz with t/s parameters specific to ported design. What i'm thinking is extending the bass down to around 60hz in a ported enclosure?

If i model it in winisd i get a 4l box with a 2inch port 12inches long fed with 10watts for 94db flat to 65hz and port velocity of 10m/s at 60 hz.

Of course i could always make the box bigger a bit like a midrange llt but how does the port resonance come into play and how does it affect the mid range?

The reasons depend mostly on what you want to do with a woofer.

Other considerations would be intended crossover frequency, the polar response of woofer and mid about that frequency, distortion of the midrange at the frequency, size of the midrange cone also determines how low you can go, baffle diffraction, and a host of other things to consider.

Lastly, look at the group delay and phase plots of the ported midrange. You will need to see how those align with the woofer at the intended crossover frequency.

Personally, I see no reason to run a mid down that low. While the frequency response curves may show a fine frequency response, there are other factors like group delay, phase, and distortion to be concerned with.

A general rule of thumb is to not cross over a driver any lower than one octave above its Fs (resonance frequency). If the Fs of the mid is 50 Hz, then 100 Hz would be the lowest you can expect to crossover.

A better number is two octaves, so a 50 Hz mid should not be crossed over lower than 200 Hz.

When selecting crossover points you need to keep in mind where the crossover takes place in the spectrum and what types of instruments and voice it may intersect. Your ears are most sensitive to those crossover points and phase and group delay anomalies are easier to pick up when they are dividing a band where there is something of interest. Generally, for a 3-way system a lower crossover point of 300 to 500 Hz is typical and for good reasons.

Most mids are in a sealed box. The box size is smaller, the frequency response is more predictable, and more importantly, the phase and group delays are much more linear. Ports disrupt the phase and group delay and placing those disruptions higher up in the midband makes them more pronounced to the ear.

Also, although the software models the ideal frequency response of the driver and looks interesting, the actual response of the speaker may be something else. Check to see if the software is modeling the driver strictly on its Thiele/Small parameters or is it actually using the manufacture's published frequency response (acoustic data). Theory and actual response are two different things.

[woody]

I seem to rember about 25 years ago Focal had a lot of drivers for the
diy market. They had a couple of designs that did call for a ported midrange.

[LineArray]

Originally Posted by oublie

Hi,

Why wouldn't I build a ported midrange box?

To me there would be one reason:

There is sound radiated unwanted from the port, even above fb.

If uncolored midrange is the goal, this can be a major drawback.

An "aperiodic" midrange enclosure with a mainly resistive port
(e.g. to provide higher mechanical damping or for shaping polar
dispersion) would be far more accepatable for me.

Best Regards

[oublie]

Originally Posted by Loren42

The reasons depend mostly on what you want to do with a woofer.

...
Theory and actual response are two different things.

Thanks Loren,

That all makes sense, closed with a linkwitz transform to 60 or 80 hz things look a lot cleaner than ported. All ported would allow is higher spl but with worse sound quality.

As for crossing over 1 or 2 octaves above fs is this really necessary I have 10 of these drivers running open baffle at the moment with 9db gain around fs useable output is to about 80hz or so which suits for now and due to the number of drivers excursion is kept to a very acceptable level - this is just a prototype.

[oublie]

Originally Posted by LineArray

To me there would be one reason:

There is sound radiated unwanted from the port, even above fb.

If uncolored midrange is the goal, this can be a major drawback.

An "aperiodic" midrange enclosure with a mainly resistive port
(e.g. to provide higher mechanical damping or for shaping polar
dispersion) would be far more accepatable for me.

Best Regards

I might have a play around with one in an overstuffed box I might grab some 4 inch piping and a U bend. If i model for a 4 litre box and a 47 inch "port" then stuff the port I'm assuming i lose port gain to a certain extent but keep a certain amount of the low frequency sound?

A ported box like this gives a 70hz tuning frequency but is there any actual benefit over a closed box since the stuffing will be attenuating the low freqs to a certain extent?

[Loren42]

Originally Posted by oublie

Thanks Loren,

As for crossing over 1 or 2 octaves above fs is this really necessary...

I can't really comment because I don't know what midrange driver you are using, what the rest of the drivers are, how they are configured, and enclosure design.

As far as how they seem, well, using your ears is pretty subjective and you also have the room, speaker placement, and source material that are part of the system.

The only real way to quantitize what you have is with a good testing program, but that is another ball of wax.

[Baldin]

Not saying it is good or bad, just pointing out that Wilson Audio is using ported mid driver for some of their designs (Watt, Sophia, Sasha ..... )

[LineArray]

Originally Posted by oublie

I might have a play around with one in an overstuffed box I might grab some 4 inch piping and a U bend. If i model for a 4 litre box and a 47 inch "port" then stuff the port I'm assuming i lose port gain to a certain extent but keep a certain amount of the low frequency sound?

A ported box like this gives a 70hz tuning frequency but is there any actual benefit over a closed box since the stuffing will be attenuating the low freqs to a certain extent?

In some cases a higher mechanical damping is advantageous, especially
if the elecromechanical damping is reduced by a passive crossover.

Depending on shape of the cabinet e.g. a rear resistive port can serve
to form a kardioid like dispersion, which can make the loudspeaker more
directional (and directivity less frequency dependent.)

I would not rely on port gain for midrange for the same reasons Loren42
pointed out before and the addtitional sound radiation problem (port resonances)
which is not as acceptable in the midrange like in lower bass regions

... why do you need port gain, Qts of driver too low ?