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March 29, 2010 11:20 AM   Subscribe

Possibly NSFW question about the acoustics of peeing.

Background: I'm a male and I pee standing up. Two semesters of university physics is the extent of my knowledge on the subject.

One thing I've noticed during the act is that, above a certain flow rate, the usual pitter patter of the urine falling into the bowl turns into a much deeper, resonant sound. What causes this?
posted by invitapriore to Science & Nature (5 answers total) 2 users marked this as a favorite
While I don't have a physics background, I think it might have to do with the relationship between the force with which the urine is expelled from the penis, the force with which it is hitting the water, and also the reverberations of the louder sounds generated by such force within the bowl, which is a nice shape for an amplifier.

Also, while not totally answering your question, this Yahoo Answers thread is relevant as well. The relevant bit is that gravity and height above the bowl play a role in the sound as well, although that is more about why it sounds louder when guys pee vs girls.
posted by Elminster24 at 12:07 PM on March 29, 2010 [1 favorite]

Here is my shot at this:

A) When rain drops hit the surface of the ocean, larger drops and faster drops tend to make larger bubbles. Holds to reason that the same is true of pee.

B) For small bubbles the resonant acoustic frequency is inversely proportional to the radius. This means faster and larger drops will make larger bubbles and lower frequency noise.

C) Bathrooms tend to be made of acoustically reflective (hard) materials, so there are strong standing waves or modes at low frequencies. So the faster stream will generate larger bubbles that excite the lower order modes.

More than you would care to know about the acoustics of entrained bubbles can be found in "The Acoustic Bubble" by Leighton, especially in Chapter 3.
posted by cjemmott at 12:59 PM on March 29, 2010 [3 favorites]

Some human physiology goes into play along with the physics...
Males have a particular shape at the opening which keeps the stream cohesive in a manner. It has to do with the surface tension. It is difficult to observe, but you may notice that the stream has a look to it that almost seems like a braided rope. That is because the opening is more oval shaped than circular and the surface tension keeps the stream oscillating.
I am sure that the force of the stream and the angle at which the stream enters the water surface has a great deal to do with the sound.
Just please don't ask how I know this.
posted by Drasher at 1:21 PM on March 29, 2010

Play with the faucet in your kitchen for a few minutes, and you'll be able to see it all in action whenever you want..

At the very lowest flow rate the stream of fluid separates into droplets because gravity is pulling the molecules down faster than the faucet lets them out. The size of the droplets is determined by a balance between the force of gravity, aerodynamic properties of the droplet shape, and surface tension.

As the flow rate increases, the point were separation occurs moves down. Eventually, the separation doesn't ever happen, and you have a nice smooth flow of fluid right into the surface below (this hardly ever happens with pee, but it is easy to set up with your faucet).

Increase the flow rate even more, and the fluid becomes turbulent. Also, the fluid stream has enough force to break the surface tension of the liquid pool below. Lots of stuff is happening here. I think the most important is that air is being carried down under the surface by the energetic fluid flow, but quickly bubbling back out.

Somebody who is expert in fluid dynamics might have more insight.
posted by Chuckles at 3:38 PM on March 29, 2010

So I'm taking a Physics course right now, and our class did a lab/demonstration that might correlate to your peeing situation, although it's a pretty different approach from a couple of the other comments.

Basically we all had paint buckets filled with water, PVC pipes, and tuning forks. By sticking the pipe halfway in the water, we created a "closed pipe," i.e. the pipe was cut off by the surface of the water on the bottom but still had an opening on the top. Just like as if you had a straw sitting in a cup of soda.

But anyway, we hit the tuning fork and held it over the opening of the PVC pipe, and we were supposed to move the pipe up and down in the water while doing this, effectively changing the length of the closed pipe.

At a specific length, one that corresponds to the frequency of the tuning fork, the tone from the fork resonates and gets a lot lot louder. I can't really explain TOO well, I don't pay enough attention in that class, but basically what happens is because the wavelength of the frequency proportionately matches the length of the pipe, it resonates.

It has to do with air columns, so it doesn't matter what the pipe is made of or of how big the diameter is, I don't think.

So basically, perhaps the frequency your urine breaking the surface of the toilet water corresponds with the depth of the toilet bowl? Then again, since the sound is coming from the closed end, it might be more accurate to think of it as resonating in an open pipe.

And of course, most toilets aren't a perfect cylinder, but I'm willing to bet some of these properties still apply.

You could probably verify this by taking a set of tuning forks into your nearest restroom stall, or even just singing notes into the toilet.

This might help, but try Googling related terms.

Heavy stuff to think about when you're draining the snake.
posted by hempgranola at 7:38 PM on March 29, 2010

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