speaker cab modification

[PAYSON]

I have a SWR 410 cab 96' or 97' w/a 5" port in the back. 23"x23"x18.5". This cab works on the proximity effect. When you're not against a wall (outdoor gig) all the low end goes away. I've modified it by plugging the back port (airtight) & removing the horn & porting the front w/a 3.5" port extending 5" into the cab. Is their anybody who knows if I'm getting the best performance w/ this modification. Should I increase the hole to 4"? Any information would be greatly appreciated!

[g.]

Use the same depth and size as was on the back. IF that's not practical to have those two dimensions the same, use a program such as WinISD (the forumula that describes this is too tough to represent here) to figure alternate depth/dimensions that will work - hopefully without even removing the horn.

 

The important thing is to LEAVE the box tuning the same. The fact that it used to be firing hopefully into a wall or corner is not relevant to the tuning.

[getz out]

Port placement doesn't matter.

 

From the Acme Sound, Ltd. website:

 

Port Placement: Front, Rear, or Side?

 

An Oversimplification

 

(Terry Buddingh mentioned in his generally careful review on the Low B-1 in "Bass Player" magazine, that I had said that "Port placement is irrelevant, because port output is omnidirectional. That is an oversimplification of what I said. For those of your to whom this is a matter of great concern, I have posted here a short discussion of the matter, which is in the owner's manual, and is updated whenever I can think of better words to explain it.)

 

The Merits of Front or Rear Port Placement

 

Bass players seem to enjoy discussing the merits of front or rear placement of ports. Based on their experiences, many players and manufacturers alike are convinced that front placement has distinct advantages over rear placement, or vice-versa. When asked to justify their opinions, these partisans will invariably refer to vague impressions and gut feelings. They will never talk about wave propagation or Helmholtz resonation.

 

Pressure Devices and Their Behaviour

 

In fact, a bass-reflex enclosure, at the frequencies where the port contributes to the systems output, is a pressure device, as opposed to a wave device. So the interaction of the enclosure and the port, and the port's position with respect to the woofer, itself, are unaffected by any directional activity. Direction is utterly unimportant. By way of illustration, examine another pressure-related model, a balloon. As you inflate a balloon, does it expand on one direction only, or in all directions at once?

 

The Omnidirectionality of Low Frequencies

 

Only after a ports output leaves the cabinet, and enters the environment, does it assume the characteristics of a wave, one of which is direction of propagation. Direction does assume importance when speaking of waves. Only, however, at frequencies where the size of the source is large compared to the wavelength under consideration. The highest frequencies at which the port in our Low-B systems contribute to system output have wavelengths of 14 feet or so. The speakers are obviously quite small compared to these wavelengths, and the port output is omnidirectional. Again, port placement is irrelevant to system performance.

 

Putting The Ports Where They Fit!

 

Why does the B-2 have ports in the back? Because thats where they fit. Why does the B-4 have its third port in the front? Same reason.

 

The B-1 system could have been correctly tuned with a shorter, smaller diameter port located in either the front or the back, if location mattered. But it doesnt. The drawback would have been that at higher power levels, excessive port friction would have resulted in diminished power handling capability. This larger diameter, longer port fits when installed on the side of the box. Plain and simple.

 

Tell your friends that the reason your B-1 system sounds so good is that the tremendous innovation of a side-mounted port results in some type of magical quantum-leap in performance. In fact, the truth is much less exciting: a high-quality woofer in a solid and correctly-tuned enclosure. Feel free to let other people lose sleep over where their ports are located. Now you know the truth.

[alexclaber]

Originally posted by PAYSON:

This cab works on the proximity effect. When you're not against a wall (outdoor gig) all the low end goes away.

The reason you're losing low end outside is nothing to do with the port placement, it's because of the lack of boundary reinforcement outdoors. The frequencies coming from the port are very low and thus have such a long wavelength relative to the cab size let alone the port size, that placing the port in the front or back or side or top makes no difference. If I were you I'd put the tweeter back in, seal the front up properly and unplug the rear port.

 

If you can get hold of a signal generator you can test this out. Play a 100Hz sine wave through your amp and then sweep the frequency downwards whilst holding your hand near the port and stop sweeping once you hit the port's (actually the port/cab system's) resonant frequency (you'll feel the msot air moving from the port at that point). Now step a few feet away from the cab and walk around the cab in a perfect circle - you'll notice that the volume of that sine wave stays constant.

 

And now going somewhat off-topic...

 

The way the port works is analogous to a mass going up and down on a spring. The mass of air within the port is analogous to the mass, the area of the port is analogous to the size of the spring as you look down at it (for a round port the diameter is analogous to the spring diameter) and the volume of the cab is analogous to the softness of the spring. So basically the smaller a port is and the larger the cab is, the shorter it can be to attain an equally low frequency. However the more bass the cab puts out, the larger the port needs to be to avoid chuffing (air noises and friction) and thus the longer it has to be, which thus reduces the cab volume and therefore in turn demands a longer port length.

 

But you didn't need to know that...

 

Alex

[Edendude]

I'm willing to bet that moving the port to the front will have no audible effect on the cab's performance outdoors.

 

More power and more speaker cone area is about the only cure for the dreaded negative effects of the outdoor stage gig for a bass player.

 

When I play outdoors I add an extra 300 watt head and a second 2x10 cab, and even then it only sounds about the same volume as when I use a single head and one cab in an indoor club setting.

 

I think you'd be wise to put the horn back and unseal the rear port.

[g.]

getz76,

 

Yeah, I've read that many times. Some people still aren't satisfied, they ARE convinced they hear less with some placements, so maybe there is something wrong with the scenarios. Also, people seem to be affected by the air pressure one can feel coming out of a port, psychologically they like to have it "pointed" though long waveforms really are tough to truly point...

 

Being dependent on a specific placement/distance from a wall is a separate issue actually, but I do recall that one can actually cause cancellations in some circumstances, this fuzzy memory coming from some touring subs that made use of boundary reinforcement. 1/4 waveform usage may have its downside...

 

I could look further into this, but I have sparse reason to - all my stuff ports forward or semi-downward into quasi-horn.

 

So I just tell Payson the right way to port.

 

ED,

 

Yep. Outdoors PA subwoofing requires at least double the power and subwoofers. It seems. No "buildup", no boundary reinforcement from walls (+3 dB max) or corners (+ 6dB max) for the very lowest freq's and diminishing as wavelength becomes shorter... Asking a bass rig to be impressive outdoors actually makes less sense than expecting PA reinforcement to do the job. Most rigs don't have enough power or the right cabs to raise the freqs that boundary reinforcement affects the most.

[Whappo]

"What hump?" -- Marty Feldman in YOUNG FRANKENSTEIN

Although it was not your intention, this thread reminds me of another quote from the same movie. "Oon't shit...to the lumber yard!" Inspector Kemp

 

My partner & I use this quote to each other when we need to modify something in our cabinets. (It helps to laugh at set backs) Although there are rules in speaker & sound design that one should keep in mind, breaking the rules to experiment with new possibilities can at times lead to new revelations.

 

Mark

AccuGroove.com

[PAYSON]

Wow, lots of cool info. I,m aware you need more power outdoors. My question is since the size of the port dosen,t matter, how far should it extend into the cabinet. & if I go w/a 4" port how far should that extend. I have a/b the cab w/ back port open then plugged the port. The band members agree, sounds better more focused, more consistent from gig to gig. Better air push. Dosen't rattle the snare drum quite as much. Since I'm a drummer as well(someone who hangs out w/ musicians) the less snare rattling is good.

[g.]

Hey Whappo,

 

The marketplace would seem to prove that one can never REALLY get free lunch. But it sure helps to think out of the box when building a box.