The baffling questions thread.

[Chrisbee]

It has troubled me for years that weak assumptions are made about enclosures and ports.

Some light-hearted musings:

If one stuffs an enclosure with shelf braces to stiffen it then how does the entire enclosure "know" it still has a total-volume Helmholtz resonance?

Let's look at ports: If you bring the port flush with the baffle (as usual)... then how does that effect the tuning resonance of the port and its own pipe resonance?
Isn't the port pipe tune lowered by baffle reinforcement? Does the baffle reinforcement vanish if you pull the port further out of the box?

How does a port with trumpet flares both ends know which notes to play? Do the computer design programmes take double flaring into account?

Why do you never see ports external to the enclosure? They put all sorts of elbows and U-bends in their ports to squash long pipes into a small space. The poor port hasn't a clue how long it is meant to be when it can't tell its "arse from its elbow"! Why not make a feature of ports and run them outside the box like modern architecture where the "plumbing" and airconditioning is all on display?

Why not bring the ports out of the sub box and use them to lift a speaker box off the floor on its own reflex port speaker stand? Makes sense because you then have more room inside the sub box on the floor.

There used to be special doughnut flare castings to insert into K&N filters to aid airflow on 'hot' engines. These things had a special, cam-shaped cross-section ring which claimed to allow air to flow freely from well beyond 90 degrees to the carb inlet axis. Why don't we see these on ports?

Why don't we see big sponge-rubber doughnuts on the hidden inner end of the port?
There is often fierce competition for clean air flow with enclosure walls and damping material here.

Why do you never see larger telescopic ports so the user can change sub tune without plugging multiple, smaller pipes?

Why don't they fit very large flares around IB manifold outlets if they really cared about unwanted noise from rapid air movement?

[Darren]

Why don't they fit very large flares around IB manifold outlets if they really cared about unwanted noise from rapid air movement?

I got this one!

Because of the large opening the air velocity isn't high enough to create any chuffing.

[Chrisbee]

Only because Thomas told you that on the Cult Forum!

I remain unconvinced until I build an undersized, minimum-opening manifold.

The proof of the pudding probably lies in fitting smaller and smaller diaphragms over the manifold opening until SQ goes south for the winter.

[Darren]

Nope, I experienced this one myself. No Thomas in the equation here I have a minimum-opening manifold... 14.5 X 14.5 and the air doesn't flow fast enough or have enough volume for the opening to create any noise whatsoever.

I've placed my head in the opening to get a feel for the flow and the velocity is minimal although I'm not sure I didn't scramble a few cells. I don't recommend this without hearing protection!

[Chrisbee]

Darren wrote:

Nope, I experienced this one myself. No Thomas in the equation here I have a minimum-opening manifold... 14.5 X 14.5 and the air doesn't flow fast enough or have enough volume for the opening to create any noise whatsoever.

I've placed my head in the opening to get a feel for the flow and the velocity is minimal although I'm not sure I didn't scramble a few cells. I don't recommend this without hearing protection!

That's actually very interesting to hear actual first hand evidence! I presume you had some cone movement going on? Despite the discussion of the Cult I still wanted there to be considerable air movement. There just had to be with 4 x 15" drivers flapping back and forth by well over half and inch with only one hole for the air to oscillate through. Perhaps my imagination got a bit carried away?

I know what you mean about SPLs close up to an IB too. Even spread out in my array the drivers can produce considerable discomfort at close quarters. It always seems worse in the enclosure because the higher frequencies are stripped away. According to recent Swedish industrial noise research there is vastly reduced (if any) risk of damage from LF compared with HF.

I was once within a few metres of a squealing lathe working on a large stainless steel component. Even wearing approved headphone ear defenders the real sense of acoustic pressure in the air was simply awful up to around eight-ten metres! The operator was standing at 1 metre without any hearing protection and refused to wear any. By coincidence I had my trusty SPL meter with me at the time and it read "only" 110dB(A) @ 1 metre. I imagine the complex squealing sound must have had a very wide frequency spectrum going well up into the ultrasonic. Extremely dangeous to the hearing I would presume even over very short exposures.

I have regularly seen 120dB(C) in the bass from my IB but have never really felt the need for ear protection. My stomach and chest seem to dislike the IB far more than my ears ever do! Storms with very high wind speeds have a similar effect on me! It must be all those free radical infrasonics.

[Darren]

I thought there would be more velocity as well with that opening and 4 15" drivers. I had them moving pretty good, remember these things have 24mm of one way XMAX or more than 2"... I probably had them moving 1.25" with the lower frequency tests and while the SPL was huge, the velocity of the air out of the hole was minimal. You can feel the breeze but it isn't enough to cause any noise at the perimeter. The tactile feel was massive but the air velocity was spread out enough that it wasn't much at all. More impressive is higher frequencies like 40Hz at high db. While the cone movement is significantly less the tactile feel is through the roof.

[Chrisbee]

Great stuff!

There has to be a way to sell this head banging in the manifold as an alternative therapy. It would help pay for the upgrades!

[Darren]

It does make for a good massage but you can't really see straight.

[Chrisbee]

That could well be your USP!

Grow your hair long, get a white coat and you could retire at 40.