Originally Posted by noah katz
The height of the water at any point on the surface is analogous to pressure.
However at very low freq the overall water level will rise and fall more or less uniformly throughout the tub and there will be no discernible waves.
The water level/pressure changes, but I would submit that there is no wave, so how can they reflect and constructively/destructively interfere?
Once again, pressure is pressure, whether it be SPL or barometric; if that's too "silly" for you to comprehend, so be it.
Water waves are irrelevant to the conversation. Your illustration is one of displacing the water with a solid object and your premise is that just because you plunge the solid object into, then pull it out of the water more slowly that you aren't creating reflections at the tubs walls. Failing to mention that the water is in a water tight container is convenient as well. What happens to your analogy when you unstop the bathtub drain? Just as with the "pressure is pressure" generalization (my masseuse applies pressure but that doesn't mean it relates to the discussion), these generalizations do nothing to prove the premise.
Bathtubs, spaceships and syringes, everything but a subwoofer in a room.
Here is what evokes the 'silliness' comment, and maybe you can address it directly without taking me out to sea or into outer space:
I wanted to look more realistically at the syringe illustration that's "a little out of scale" and bring it a little more into scale. I took the volume, boundary area and air loss area of my room and drew it as a syringe, roughly to scale. The 'plunger' is the equivalent Sd to a 4 X 15 sub, which is what I used to create the window open/shut graph and the opening, far from being even close to the hole in a needle in the syringe example, is equivalent to the openings in my room:
Of course, I've worked at a drawing table for some 35 years as part of my job description and hobbies, so I didn't need to draw it for my own benefit. I was able to picture the actual scale quite like the drawing shows, thus my 'silly' comments.
The theory that the cone causes an ambient atmospheric change simply by reducing the frequency of of it's motion is... well, silly and as far as 'ignoring the calculations' goes, feel free to use the scaled drawing to make new ones that might actually be relevant. I don't need calculations for this one. It's a no-brainer.
In a perfectly air tight chamber, cone motion would cause a fluctuation in ambient atmospheric pressure at all frequencies. That does not mean that there would not be reflections or at some 'magic frequency' between 3-120 Hz reflections cease to exist. Do sound waves behave differently in an airtight enclosure? Dunno, I'll leave that to you because, although it might be interesting, it is irrelevant information.
Sound waves move through the air at the speed of sound through air. There is a point source where the sound wave begins and ends. There will be reflections. Those reflections will be either constructive, destructive or unity and everything in between. They will set up standing waves between parallel surfaces if the wavelength is twice the distance between the parallel surfaces. They will excite sympathetic resonances. At some room dimension-dependent frequency a room will no longer have the ability to support a standing wave (this does not mean the room exhibits no modal behavior below that point). As frequency decreases below that point, more and more of the rooms surfaces will cause only constructive reflections.
This is where I'm coming from, based on my own experiences and measurements. I would appreciate actual data or evidence that adds to or refutes any of the following:
Air losses (increase or decrease) do not change room gain. I've documented this with accurate-to-4 Hz measurements. I've done measurement comparisons with the doorway open vs sealed, with a window open vs sealed and with all 3 openings open vs sealed. All results in the so-called PVG region were either unchanged or the opposite of what would be predicted if the pressure pot theory were correct.
Separate placement of multiple subs can cause cancellation in the so-called PVG region. I've documented this with accurate-to-4 Hz measurements, many times. If there are no reflections of sound waves, if the pressure pot theory is correct, this result would be at least worth examination.
The SPL varies by varying the position of the microphone in the room. If there are no reflections, this also would at least be worth examination.
If the pressure pot theory is correct, how exactly does the room gain increase as frequency decreases? In nearly every sub I've built, I model to limit excursion with enclosure size and signal chain roll off. This means that driver excursion at 5 Hz is approximately the same as it is at 10 Hz, yet from 10 Hz to 5 Hz there is a room gain increase of +13dB.