[00:02] In 1989, the Loma Prieta earthquake&nbsp; shook the central coast of California,&nbsp;&nbsp; collapsing buildings and damaging infrastructure&nbsp; across the Bay Area. Bridges, in particular,&nbsp;&nbsp;
[00:14] suffered extensive damage. In one case, a&nbsp; major section of the eastern span of the&nbsp;&nbsp; Bay Bridge's deck collapsed, falling onto the&nbsp; lower deck like a trapdoor. Sadly, one person&nbsp;&nbsp;
[00:26] died driving off the upper deck. Crews had the&nbsp; bridge repaired within a month, but Caltrans&nbsp;&nbsp; knew that the next earthquake could be worse and&nbsp; started making plans to replace the structure.
[00:38] Knowing that the replacement project would require&nbsp; heavy-duty piles, Caltrans developed a testing&nbsp;&nbsp; program to identify risks and challenges during&nbsp; design and minimize the chance of unanticipated&nbsp;&nbsp;
[00:50] problems cropping up during construction. And&nbsp; they found a pretty big one. In October of 2000,&nbsp;&nbsp; the barge began the pile driving operation,&nbsp; dropping a large hammer to drive the 8-foot&nbsp;&nbsp; (or 2.4 meter) diameter steel pipe deep&nbsp; into the seafloor. Almost immediately,&nbsp;&nbsp;
[01:08] fish began dying in the surrounding area.&nbsp; Biologists involved in the project collected&nbsp;&nbsp; fish and documented injuries to their organs and&nbsp; swim bladders. They weren’t being directly hurt by&nbsp;&nbsp; the hammer itself; it was above the water anyway.&nbsp; The damage was coming from the intense sound.
[01:26] That massive steel pipe rang like a humongous&nbsp; bell on every hammer blow, radiating sound&nbsp;&nbsp; pressure through the San Francisco Bay. It even&nbsp; had serious impacts on aquatic wildlife up to&nbsp;&nbsp;
[01:38] a kilometer away, which was a pretty big deal.&nbsp; Because San Francisco Bay is home to quite a few&nbsp;&nbsp; threatened or endangered species of fish. The&nbsp; problem was that the replacement bridge would&nbsp;&nbsp; need more than 250 of these piles. Caltrans&nbsp; had to figure out how to install them without&nbsp;&nbsp;
[01:55] affecting the wildlife in the process, and the way&nbsp; they did it, I think, is pretty cool. And I even&nbsp;&nbsp; built a model in the garage to show you it works.&nbsp; I’m Grady, and this is Practical Engineering.
[02:16] If you want to know the answer right away, it's&nbsp; bubbles. But I think the most interesting part&nbsp;&nbsp; is why it works in the first place. And this&nbsp; matters. Pile driving isn’t the only thing&nbsp;&nbsp; that creates excessive noise underwater. We&nbsp; do a lot of construction in waterways, oceans,&nbsp;&nbsp;
[02:32] rivers, and bays. We also occasionally&nbsp; have to blow stuff up underwater,&nbsp;&nbsp; like for demolition of structures or safe&nbsp; disposal of old munitions and mines. Any&nbsp;&nbsp; loud work underwater has the potential&nbsp; to disrupt, injure, or even kill aquatic&nbsp;&nbsp;
[02:49] wildlife. The phenomenon we know as sound is&nbsp; just fluctuations in pressure within a medium,&nbsp;&nbsp; whether it’s air or water (or even concrete). We&nbsp; sense those fluctuations mainly through our ears,&nbsp;&nbsp;
[03:01] but pressure fluctuations can do a lot more than&nbsp; just vibrate the thin membranes, tiny bones, and&nbsp;&nbsp; hairs. Barotrauma is the term used to describe the&nbsp; damaging effects of compression and decompression&nbsp;&nbsp;
[03:13] on wildlife. And it really has only been in&nbsp; the past few decades that we’ve really started&nbsp;&nbsp; to apply the science of hydroacoustics to our&nbsp; own activities and try to mitigate the impacts.
[03:25] You’ve probably heard of sound pressure expressed&nbsp; in decibels. It’s really just a logarithmic scale&nbsp;&nbsp; of convenience thing because meaningful pressures&nbsp; can range across many orders of magnitude. So the&nbsp;&nbsp;
[03:37] decibel system just makes the numbers easier&nbsp; to compare. The equation for a decibel is just&nbsp;&nbsp; 20 times the base 10 logarithmic function of the&nbsp; sound pressure divided by a reference pressure.&nbsp;&nbsp;
[03:50] Sounds complicated, but it just means&nbsp; a 1-decibel increase corresponds to an&nbsp;&nbsp; increase in sound pressure of about 26 percent.&nbsp; The amount of time over which sound pressure is&nbsp;&nbsp;
[04:02] measured also matters. Look at a waveform&nbsp; and you can see there are peaks (both in&nbsp;&nbsp; compression and rarefaction). But that’s only&nbsp; for a split second. So a lot of measurements&nbsp;&nbsp; use a root mean square of the sound pressure&nbsp; over a given time to provide a better estimate.&nbsp;&nbsp;
[04:17] We don’t have to go into the math of that,&nbsp; just think of it as a fancy kind of average. It’s important to point out that, in air, we&nbsp; use 20 micropascals as the reference pressure,&nbsp;&nbsp; which is approximately the limit of human&nbsp; hearing. So that’s 0 decibels. Underwater,&nbsp;&nbsp;
[04:33] we use a reference pressure of 1 micropascal,&nbsp; mainly just for standardization purposes,&nbsp;&nbsp; so just keep in mind that underwater decibels&nbsp; aren’t really equivalent to sound pressures you&nbsp;&nbsp; might have as references in your head like the&nbsp; 75-decibel vacuum cleaner or the 140-decibel jet&nbsp;engine.
[04:53] And really, what you think of as sound&nbsp; has less meaning underwater because our ears and&nbsp;&nbsp; brains are calibrated for the physics of sound&nbsp; in air. The underwater version of “loudness”&nbsp;&nbsp;
[05:05] doesn’t translate well to human perception. But&nbsp; it matters a lot to fish and marine mammals. Sound behaves a lot differently in water&nbsp; than air. Of course, water is denser,&nbsp;&nbsp; and sound moves through it at roughly 4 times&nbsp; the speed it does in air. Sound also carries&nbsp;&nbsp;
[05:22] a lot further in water, and importantly, the&nbsp; acoustic impedance of water is way different&nbsp;&nbsp; than air. Impedance is basically a&nbsp; measure of opposition to sound flow,&nbsp;&nbsp; kind of like resistance in an electrical circuit.&nbsp; It’s a function of the medium’s density and the&nbsp;&nbsp;
[05:39] speed of sound through it. And at a boundary&nbsp; between two media, there are two things that&nbsp;&nbsp; can happen to sound. It can transmit into the new&nbsp; medium or it can reflect back, and the difference&nbsp;&nbsp; in impedance between the two determines how&nbsp; much of each will occur. If impedances match,&nbsp;&nbsp;
[05:57] more sound will transmit through the boundary.&nbsp; If they’re way off, like water and air,&nbsp;&nbsp; most of the sound is reflected. The practical&nbsp; effect of that is a transmission loss between&nbsp;&nbsp; air and water of about 30 decibels. It’s why stuff&nbsp; happening underwater is quiet above the surface,&nbsp;&nbsp;
[06:14] and we can take advantage of impedance&nbsp; mismatch in underwater construction. I built a new acrylic tank for this demo, and&nbsp; I’ve got a new helper in the shop. This is Brady.&nbsp;&nbsp;
[06:28] I figured since half the internet calls me that&nbsp; anyway, we might as well get a Brady in here. He&nbsp;&nbsp; can wave and nod, and he can probably do a lot of&nbsp; other stuff too, but that took me several hours,&nbsp;&nbsp; so he’s just going to bravely hold the hydrophone&nbsp; for now. And on the other end of the tank,&nbsp;&nbsp;
[06:43] I have this bluetooth speaker. It claims it’s&nbsp; underwater rated, so we’ll see if it works out. And here’s the setup; pretty simple. I found a few&nbsp; recordings of hammering and pile driving sounds to&nbsp;&nbsp;
[06:55] play on the speaker. And this is how they come&nbsp; across on the hydrophone, which is connected&nbsp;&nbsp; to a sound recorder.
[07:11] I also did a frequency sweep&nbsp; so we can do a little more scientific comparison.&nbsp;&nbsp;
[07:29] At this point, one of my glue joints on this tank&nbsp; catastrophically failed and flooded my garage with&nbsp;&nbsp; water. I didn’t catch it on camera, but Brady took&nbsp; the brunt of the fall. I got it all fixed up, and now let’s&nbsp; see if we can soften these construction sounds.
[07:47] I have four air stones made for aquariums&nbsp; hooked up to an air pump. When I flip these on,&nbsp;&nbsp; we get a nice curtain of small bubbles between the&nbsp; speaker and the hydrophone. And I’ll record those&nbsp;&nbsp; same sounds again.
[08:11] Here’s a look at the waveforms&nbsp; from the hydrophone with and without the air.&nbsp;&nbsp; Although it’s not a dramatic difference,&nbsp; you can definitely see a difference,&nbsp;&nbsp; especially for the higher pitched hammering&nbsp; sounds toward the end. And here’s a look at&nbsp;&nbsp;
[08:23] the waveforms without and with the air for&nbsp; the frequency sweeps. Even though the sweep&nbsp;&nbsp; should have had a constant sound pressure&nbsp; across the full range of frequencies,&nbsp;&nbsp; the water and demo itself cause pretty serious&nbsp; distortions. You can see a lot of resonance at&nbsp;&nbsp;
[08:39] low frequencies, and a lot of attenuation&nbsp; at high frequencies. That makes it a little&nbsp;&nbsp; hard to gauge the effectiveness of the bubbles.&nbsp; It’s similar to the hammering sounds - not much&nbsp;&nbsp; difference at the lower frequencies, but a pretty&nbsp; substantial reduction at higher frequencies.
[08:55] This is not an ideal setup for one reason:&nbsp; even though there’s a big mismatch in acoustic&nbsp;&nbsp; impedance between air and water, there’s not that&nbsp; much difference between acrylic and water. So,&nbsp;&nbsp;
[09:08] it’s pretty easy for pressure waves to propagate&nbsp; into the acrylic, travel past my bubble curtain,&nbsp;&nbsp; and back into the water on the other side. So&nbsp; I’m not getting the kind of sound reduction,&nbsp;&nbsp; what the pros call attenuation, that&nbsp; you might expect in the real world,&nbsp;&nbsp;
[09:24] for example, by surrounding a pile with&nbsp; a circular ring of air pipes. Thankfully,&nbsp;&nbsp; the researchers studying solutions like this have&nbsp; put a lot more resources into figuring out the&nbsp;&nbsp; right way to do it. The measurements at the Bay&nbsp; Bridge compared fairly well with mine. Attenuation&nbsp;&nbsp;
[09:40] was highest as the higher frequencies. But this is&nbsp; not as simple as just blasting air out of a pipe. These bubble curtain systems require a lot of&nbsp; logistics. Massive compressors or blowers feed&nbsp;&nbsp;
[09:53] air sometimes deep below the surface into complex&nbsp; plumbing assemblies. They usually have filters to&nbsp;&nbsp; remove oil from the air to make sure the water&nbsp; isn’t being contaminated. The system has to sit&nbsp;&nbsp;
[10:05] flush with the bottom to make sure sound can’t&nbsp; travel underneath the bubble curtain. But also,&nbsp;&nbsp; there are currents. Any movement of the&nbsp; water is going to move the bubbles too,&nbsp;&nbsp; potentially creating gaps in the curtain&nbsp; or dispersing it altogether. So it’s often&nbsp;&nbsp;
[10:19] necessary to have multiple levels of plumbing&nbsp; to keep a continuous screen all the way to the&nbsp;&nbsp; surface. If that’s not enough, there are ways to&nbsp; confine the bubbles around a pile or construction&nbsp;&nbsp; activity using an outer casing or even a flexible&nbsp; membrane. But how do you know it actually works?
[10:36] Maybe the most comprehensive engineering guidance&nbsp; on this topic is put out by Caltrans in their&nbsp;&nbsp; manual on the Hydroacoustic Effects of Pile&nbsp; Driving on Fish. Appendix 1 in the report is&nbsp;&nbsp;
[10:48] a nearly 300-page compendium on pile driving&nbsp; sound data. You might not have known this,&nbsp;&nbsp; but we’ve been measuring a lot of pile-driving&nbsp; sounds! If you’re an engineer or environmental&nbsp;&nbsp;
[11:00] scientist trying to get a permit to build&nbsp; something underwater and sound is going&nbsp;&nbsp; to be an issue, this is kind of your bible.&nbsp; It’s got quite a few ways to minimize impacts,&nbsp;&nbsp; including timing work when important species&nbsp; aren’t present, changing designs to reduce&nbsp;&nbsp;
[11:15] underwater work, using vibratory hammers instead&nbsp; of conventional equipment, and bubble curtains&nbsp;&nbsp; that reduce the propagation of underwater&nbsp; sound pressure. Based on all the testing and&nbsp;&nbsp; real-world case studies so far, they suggest you&nbsp; can get about 5 decibels of attenuation this way.
[11:32] Just like my demo, sounds don’t only travel&nbsp; through the water. They also move through the&nbsp;&nbsp; sea floor and even through the barge on the&nbsp; surface, bypassing the bubbles. 5 decibels&nbsp;&nbsp; doesn’t sound like a big reduction, but you&nbsp; have to remember that it’s a logarithmic scale.&nbsp;&nbsp;
[11:47] A 5 decibel reduction means the actual sound&nbsp; pressure is nearly cut in half. You also have&nbsp;&nbsp; to remember that what we care about most is area.&nbsp; For any loud construction or demolition activity,&nbsp;&nbsp;
[12:00] there’s an invisible ring some distance away that&nbsp; marks the injury threshold level. Since sound&nbsp;&nbsp; pressure decreases with distance, eventually&nbsp; you’re far enough away from the sound that&nbsp;&nbsp; it doesn’t result in injury. So every foot or&nbsp; meter that you can pull that ring back toward&nbsp;&nbsp;
[12:16] the activity through attenuation reduces the&nbsp; impact area proportional to the distance squared,&nbsp;&nbsp; dramatically reducing the area in which fish may&nbsp; sustain injuries. That’s why bubble curtains are&nbsp;&nbsp;
[12:28] used in so many underwater construction projects&nbsp; these days, but that’s not all they’re used for. What’s that old saying? If your only tool&nbsp; is a bubble curtain generation system,&nbsp;&nbsp; every problem starts to look like a loud&nbsp; underwater sound. Something like that. It turns&nbsp;&nbsp;
[12:44] out that bubbles can do a lot more than create an&nbsp; impedance mismatch for sound pressure propagation.&nbsp;&nbsp; For one, they aerate water, which can be useful&nbsp; to prevent algae and other issues with stagnant&nbsp;&nbsp;
[12:57] pools. For two, they create vertical water&nbsp; currents. That can help keep things separated,&nbsp;&nbsp; like trash. You can see it’s a lot harder for&nbsp; me to move this little boat across the barrier&nbsp;&nbsp; Of course, a&nbsp; net or boom or rack can do this too,&nbsp;&nbsp;
[13:14] but those don’t allow boats to pass through. And&nbsp; this doesn’t just work for trash. Bubble curtains&nbsp;&nbsp; have been used to contain oil spills, and they’re&nbsp; often used in underwater construction not just to&nbsp;&nbsp; control sound but turbidity. We really don’t want&nbsp; disturbed sediments clouding up our waterways,&nbsp;&nbsp;
[13:32] again, primarily for environmental reasons, so&nbsp; these can be an important tool when booms aren’t&nbsp;&nbsp; practical. They’ve also been used to control&nbsp; saltwater and keep it from migrating up rivers&nbsp;&nbsp; in tidal areas. And they’ve even been employed&nbsp; to confine herbicides for invasive plants,&nbsp;&nbsp;
[13:48] allowing for fewer chemicals and less&nbsp; non-target damage to nearby flora. I’ll definitely be in trouble with the biology&nbsp; folks if I don’t point out that it’s not just&nbsp;&nbsp; people who use bubbles as a tool. Humpback&nbsp; whales cooperate to create bubble curtains&nbsp;&nbsp;
[14:03] that corral fish to a central point. Then&nbsp; they lunge into the center to gulp them down,&nbsp;&nbsp; And we&nbsp; use bubbles this way on occasion as well, not&nbsp;&nbsp; for fishing but to keep fish out of certain areas,&nbsp; usually to prevent the spread of invasive species.
[14:21] By 2005, the pile driving operation on the east&nbsp; span replacement of the Bay Bridge was complete,&nbsp;&nbsp; Excellence Award by the Federal Highway&nbsp; Administration for all the work they did&nbsp;&nbsp;
[14:35] on minimizing underwater noise impacts&nbsp; on endangered fish species. And the&nbsp;&nbsp; lessons from that project have been applied&nbsp; across the world in the two decades since. You know I love heavy construction. The bigger&nbsp; and louder the machinery, the better. But I think&nbsp;&nbsp;
[14:50] that anything we can do to limit the effect we&nbsp; have on the other things we share this world&nbsp;&nbsp; with is a win, especially when it’s something&nbsp; as clever and creative as blowing bubbles.
[15:03] My first exposure to this topic was reading&nbsp; about how unexploded ordnance is handled in&nbsp;&nbsp; the Baltic Sea. They use bubble curtains to reduce&nbsp; the effects on fish, and I’m curious if any of you&nbsp;&nbsp; know other cool applications in the industry&nbsp; that I didn’t mention here. I’m so fascinated&nbsp;&nbsp;
[15:19] to see and learn more about the differences in&nbsp; techniques and infrastructure across the world.&nbsp;&nbsp; And there’s probably nowhere you see those&nbsp; differences better than in transportation. My friend Mike from the DownieLive channel is&nbsp; on an adventure to travel from the Arctic all&nbsp;&nbsp;
[15:35] the way to Africa, as much as possible, by&nbsp; train. The series is called DownieExpress,&nbsp;&nbsp; and it’s such a cool combination of adventure&nbsp; and infrastructure. So many cool trains,&nbsp;&nbsp;
[15:47] from modern high-speed transit to the most basic&nbsp; coach. I first started watching DownieLive when he&nbsp;&nbsp; was filming this wild quad tandem bike race,&nbsp; and this DownieExpress series just&nbsp;&nbsp; has me hooked. And if you want to watch the&nbsp; whole thing, it’s only available on Nebula.
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