Physics Rant

Hi everyone! This is a little different for the blog, but I started to type it up for social media and realized that it was going to run long, and I wanted to include links and inline images.

First things first, spoiler alert. If you haven’t watched The Expanse seasons one and two, or Lost in Space (2018, Netflix), then either watch them now or make you peace with spoilers.

I watched The Expanse a couple of months ago, and I found it fairly enjoyable. What I really appreciated about it was that other than a few specifically chosen (and mostly acknowledged) violations of known science, the physics was pretty solid. What really made me sit up and take note was a scene where to people are running across a walkway when the giant spaceship’s engines cut out, and everybody goes into zero-G. The protagonists are left floating helplessly a couple of feet above the walkway while people are shooting at them, and without missing a beat one of the main characters clipped a tether, pushed off the other and used conservation of momentum to get his magnetic boots back in range of the deck. It was great! Here’s a clip:

This was an example of The Expanse getting the small stuff right, and that was just great. Now, how about Lost in Space?

Lost in Space tries to sound smart. They throw in enough sciencey sounding stuff and concepts that are close to right that somebody who didn’t know any better might miss it. I can totally forgive that, unless the plot uses incorrect physics as a major point of conflict. One example is in Episode 7 two of the main characters drive their land vehicle into an extraterrestrial tar pit.

Trapped in tar!

They realize that what looked like mud is actually a black liquid tar, and they’re sinking in. That big vehicle they’re in is mostly air, has room for several additional occupants, and the doors and windows seal tight. Somehow this vehicle is heavy enough to sink to the bottom of the tar pit, where they then use a helium balloon to create a vertical tunnel to safety. Hey, an alien tar pit that traps the pair when time is of the essence is a great plot device! Seriously, it makes sense that it would happen and it can be really dramatic. What doesn’t make sense is that they would keep sinking all the way to the bottom. Tar is denser than water, and if you take a regular car and tape it shut, it’ll float pretty well. This thing is so big, and because it came from a space vehicle it would be weight optimized, so unless the air was leaking out and liquid was coming in, it would float like a duck.

1960s Duck

That’s the small stuff. I’m getting worked up about this, so what if I just make it into a bullet point list.

Lost in Space

  1. Gets the scale of space wrong by:
    1. Listing Alpha Centauri as the destination
    2. Taking colonist families there with the intent of reaching the star system in their lifetime
    3. Explicitly stating the colony ship’s top speed as 276,000 miles per hour
    4. Not using any kind of stasis or suspended animation
    5. Neglecting to do the math that at this speed it would take almost 11,000 years to reach the destination
  2. Ignores practical considerations by:
    1. Making the well appointed, overbuilt, stuffed with food and furniture colonist ships capable of reaching orbit on their own by burning liquid fuel
    2. Making these ships able to convert poop to rocket fuel with solar energy, and able to do it much faster if they have better quality poop
    3. Never really worrying about radiation exposure
    4. Being fairly inconsistent with the robot’s capabilities

The Expanse

  1. Gets the scale of space right by:
    1. Including communications delays due to the speed of light
    2. Showing preparation of a colony ship intended to fly for over 100 years to Tau Ceti
    3. Having the development of a revolutionary new engine as the main driving technology for expanding beyond earth orbit
  2. Pays attention to practical considerations by:
    1. Making air/water/nutrient recycling systems a major concern for characters
    2. Making radiation a huge problem to be dealt with
    3. Giving everyone magnetic boots
    4. Has a scene were characters press their helmets together with radios off in order to have a private conversation
    5. Regularly have characters personal effects floating around in zero G until the ship fires its thrusters
    6. Shows blood and liquid in ways that are realistic for the gravity and acceleration in that scene

I’m a big fan of the attention to detail in The Expanse, but I’ve got a few gripes too. Is there a good reason why everyone’s into haircuts with the sides of their heads shaved? Why isn’t anybody fat? Why aren’t drones a much bigger deal? Why did they portray zero gravity sex as something fun and cool until the engines turn on (when you fall forcefully onto the bed, ouch!) instead of something way safer where you basically strap into a sleeping bag with your partner? Was Alex really getting hammered alone while waiting for his crew to go do their thing on Ganymede? None of the stuff with the ‘protomolecule’ bothered me, since it was a major part of the story that this stuff was weird and alien with unknown and incomprehensible powers. For what it’s worth, some of the Lost in Space robot’s ship capabilities felt the same to me. At least if something is weird then everybody should act like it.

Now I want a show that has the attitude of Firefly with the physics of The Expanse and the longevity of Stargate.

Acoustic treatment

A lot of time has passed since my room equalization post in August 2016, and the stereo is now in a different room. Looking back, I think that one of the biggest problems that I had in my attempt at equalization was that I was aiming for the wrong target response, and wasn’t doing enough to actually treat the acoustic problems before equalizing. With the two of us basically living in one room and a bedroom I wouldn’t have been able to really add a lot of acoustic treatment without it being too ugly for Kaitlyn to put up with, but now we have an office in the house where I can make things as ugly as I want. So, here’s the ugly office…

messy_room

There are a few major acoustic problems here.

First, it’s a square room. Any reflection or reverberation that goes from the front to the back wall also happens from side to side, so the shape is fundamentally bad for even bass reproductions. This is a 12x12x8 foot room, so since the speed of sound is 1125 feet per second you’d expect the first resonance to happen at about 47 hz. The math behind that is 1125 feet per second/12 feet = 93.75 cycles per second, and the first resonance happens with the pressure fluctuation (zero velocity) on the wall, and velocity fluctuation in the center of the room, so a half wave, so the resonance happens at half of 93.75.

Here’s a raw measurement of one of the speakers playing before I’ve done anything to help:

response

So, what’s wrong? Obviously there’s a huge peak in response a little lower than that predicted 47hz, but there’s something else going on too. As I said, my first attempt at correction in the old place was aiming at the wrong target response. When I applied the correction filters it sounded weak in the bass and I didn’t understand why. It turns out that there’s something called the Harman Target Curve that describes the frequency response that sounds most natural. You’d think that it would be perfectly flat, but when a sound system is set up that way it sounds weak in the bass and too bright in the highs. The preferred shape is a bass boost starting below ~150hz, flat response up to 1-2khz, then gradually falling response above that.

It turns out that the high frequencies were pretty good, and when you line up the bass according to the desired curve, that big peak at 47 isn’t even the worst problem.

Target response

In the new office, this is something I can hear. The bass between ~60 and ~200 is what gives bass that tactile feeling, where you can feel the drums, and that’s what not coming through like it used to, or like I know it could. The problem is that you can’t just boost frequencies that are too low. They’re too low because things are physically cancelling out or are driving modes that have dead spots at the listening position, and increasing the drive level doesn’t help much and adds distortion, cone excursion, and power usage without much benefit.

In order to address some of this, I looked at SBIR, speaker boundary interference response, and found that I should have a pretty big cancellation around 90hz because my chair is 30″ from the rear wall. It’s really hard to absorb frequencies below 500hz, and most of the acoustic foam panels that you can buy do very little at these frequencies. You need something that goes deeper, a lot deeper. I looked at a page of a bunch of absorption coefficients, and played with a porous absorber calculator to see what it would really take to be able to address these frequencies and kill the reflected signal and get my bass back. I settled on [Roxul Safe’n’Sound](https://www.homedepot.com/p/ROCKWOOL-Safe-n-Sound-3-in-x-15-1-4-in-x-47-in-Soundproofing-Stone-Wool-Insulation-1-Bag-RXSS31525/202531875) which was cheap, relatively non-hazardous, essentially fireproof, and capable of absorbing very low frequencies. In order to hold it in place at a distance from the wall, I used some Ikea Ivar shelves that I already had available. This produced a 6″ thick absorber with 6″ of air space between it and the wall.

Here’s a graph showing the effect of the absorber.

frequency_plot

The smoothing is a different scale to make things easier to see as averages, but the main takeaway is that the green line is about 6db higher at 100hz than the purple line. That means that by adding these sound absorbing panels behind my chair the bass notes at 100hz got twice as loud. There were other benefits too. The overall response got a little more even (see 500hz), and the clarity of the sound was improved because I no longer hear the reflected sound behind me delayed by ~4ms. Any early reflections happening before 6 milliseconds will blur the apparent location of the sound source; our brain/ear averages those out. Reverberations happening between 6 and ~50ms are integrated into the perceived sound as loudness, but the first occurrence of the sound is what is used by our brain to determine location in space. This is called the precedence effect, I hope I’ve explained it right. This is why speakers and chairs really need to be at *least* three feet from any walls or major reflecting surfaces for optimal stereo sound, and why most households just don’t get to have that type of sound reproduction.

This is what it looks like on a graph; this is the “Energy time curve“or ETC graph. The graphs shown above show magnitude of sound with respect to frequency, but these show magnitude of sound with respect to time. A perfect source would look like one big sharp spike with nothing after it. A real room shows a series of peaks afterward, the reflections.

IMG_9482

Ideally you’d want nothing sticking up within 6ms of that first peak, and nothing sticking up above the -20 mark at all, and everything steadily decaying. I’ve circled that first peak at a little after four milliseconds, that’s my reflection from the back wall. Here’s another ETC graph showing what the absorber was able to do.

IMG_9483

That first peak is totally gone, and many of the later peaks are lower in level. I suspect that the two large peaks that are till hanging out at around 8 and 10ms are reflections from the floor and ceiling. I think that the other peaks that went away were from sounds that made a round trip from the back wall to the front and back. I can hear a difference, and the stereo image now sounds clearer, with a more realistic presentation of the performers and instruments occupying specific points in space. Bass is fuller and more even, and things like plucked double bass sound more realistic. That 47hz resonance is still there, but it’s very easy to use an equalizer to knock just that one down a little bit without robbing the rest of the bass.

So, what’s next? There’s still a big dip in response from 100-300hz, and 100 could be louder still. My next step will be to add more absorbing material into the corners of the rooms to make bass traps to absorb some of that resonant energy. This isn’t a high priority since we are making preparations for Baby Carrow to join us soon, but I may do it by the end of the summer since it won’t cost very much or take very long. In the longer term, I may purchase some software to further process the sound. I had a free trial of one package that made a very noticeable and favorable improvement, but it’s really not a priority right now and if I wait then the price might come down or a competitor could come up with something better.