[For your weekend reading pleasure, here is an amusing post from the old blog on a recent acoustics paper.]
There is a marvelous French film from the 1970s called The Tall Blond Man with One Black Shoe, starring Pierre Richard as a hapless concert violinist who unwittingly becomes entangled in an elaborate game of one-up-manship between two rival factions in the French secret service. Specifically, one group -- knowing they are being bugged -- pretends to be meeting an international James Bond-like super secret agent man at the airport, except really, they just randomly pick someone off the flight from Munich and pretend he's a spy.
The person they pick is the violinist, who stands out because he is wearing one black shoe and one brown shoe that particular day (due to a mixup with the hotel concierge in Munich). This being a screwball comedy, wackiness ensues, since the violinist is clumsy and accident-prone, plus is carrying on an affair with his best friend Maurice's rapaciously adventurous wife, Paulette, a harpist by trade (they're all in the same orchestra, natch). Let's just say she likes to play "horsey" -- or fait le cheval -- and the rival faction captures that particular moment on audiotape (per the video clip below).
Anyway, believing him to be a spy, the rival faction springs into action, bugging the violinist's apartment -- including placing a bug in the toilet -- and monitoring his conversations from a white florist's van on the Paris streets. The problem is, they set the volume control too high on the bug in the toilet, which means every time the violinist flushes, it roars through the speakers in the van at very high decibels. This sets up the perfect sight gag: the van driver stops at a light, his window rolled down, and a man in a convertible pulls up beside him. Suddenly there is the loud sound of a toilet flushing from inside the van. Convertible Man stares. The van driver stoically looks straight ahead. And mercifully, the light changes.
All of which is a roundabout way of saying, the sound of one toilet flushing can be very loud indeed. It's even louder if one happens to install a fancy low-flow toilet, common in places like southern California, where water is less plentiful than in, say, my hometown of Seattle. Seattle was the location of this year's Acoustical Society of America (ASA) meeting back in May, and among the highlighted papers presented was one on reducing toilet flushing noise in adjacent offices, by an acoustical consultant from North Carolina named Noral Stewart. It's stuff like this that makes the ASA meetings so much fun.
(This title of this post, by the way, is an allusion to a 1969 "biography" of Thomas Crapper, a 19th century sanitation engineer rumored to have invented the flush toilet. He didn't, although he did take out several patents relating to plumbing. The author of Flushed with Pride was a satirist named Wallace Reyburn, and his "biography" of Crapper was mostly fictional, despite being based on a real person.)
So, Stewart was hired to solve a knotty office problem: overly loud flushing sounds from new low-flow toilets that were carrying over into seven rows of five offices (each row on a different floor), in which the three middle offices were right next to the rest rooms. You really didn't want to be the poor sods who got the middle offices on each of the floors: those shared a wall with both the men's and women's restrooms. Talk about Flush Central.
Preliminary measurements of the sound levels showed many of the flushes were louder than 60 decibels (especially in that middle office), and almost all topped 50 decibels. That's roughly the same level as a typical conversation, which might not seem all that loud, until you realize that every time someone flushes a toilet, those in conversation would be momentarily drowned out by the sound. Noise from ventilation systems, for comparison, is usually around 35 to 45 decibels. (Inside the restroom, sound levels are more like 80 decibels; the loudest Stewart measured was 96 decibels in one of the women's stalls, a good 20 decibels higher than your average toilet's flush should be.)
The recommendation of an initial study was to reduce the flushing sound levels to a more tolerable 40 decibels. How to accomplish this? First, they tried installing an extra wall out of gypsum, filling the cavity with foam, and this brought the noise down to between 40 and 50 decibels. That's because those materials are great at absorbing sound waves.
But there's a catch: the flushing sound has a particularly high frequency -- kind of a hiss, in fact -- and for whatever reason, gypsum isn't as effective when it comes to blocking those particular frequencies.
The solution: use fiberglass in the cavity instead of foam -- or invest in new gypsum panels with a damping agent in the middle to block the frequencies gypsum can't. By doing this, Stewart managed to bring the sound levels down to below 40 decibels. I'm sure the folks in the middle offices were well pleased with these improvements. Nothing kills a business conversation faster than an ill-timed flush -- especially when you're trying to play it cool over speaker phone.
All the adjacent walls were modified in this way, and you'd think Stewart's job would be done. But never underestimate the acoustical complexity of office space (or any space, for that matter). See, while this solved the noise problem for most of the offices, there was one troublesome office right next to the women's restroom where the sound levels of the flushes still topped 40 decibels, despite the modifications to the shared walls. Why? I'll let Stewart explain:
"It was evident that structural flanking was now controlling the remaining sound reaching the offices in this and the other louder cases. That is, the dominant sound in the offices is no longer the sound heard in the rest room coming through the wall. Instead, the sound in the offices is due to direct contact of the toilets and their piping with the walls and the floor. This very efficiently introduces the sound into the structure."
And believe me, this is a case where "efficiency" is not desired. Nor was the ventilation system much help in this respect: for some reason, this office building had a very quiet system, on the order of 30 decibels. That would generally be a good thing, except a louder background noise would help mask the constant sound of flushing toilets seeping into the affected offices. Those are tougher issues to resolve after the fact, which is why Stewart sums up the lessons learned from his case study thusly: "Extra care is required in the building design when [low-flow] toilets are used."
Specifically, you need to make sure the walls are well insulated against the correct frequencies, and that plenty of rubber is used around the toilets and pipes to mute sound waves. Balance that with just the right level of background noise to mask any flushing noise that does seep through, and you'll have far happier office workers. The same goes for designing restaurants; you never want to put tables too near the restrooms, and if you do, make sure the flushing sounds are well insulated to avoid putting diners off their food.
Who knew there could still be complicated science and engineering involved with the lowly toilet? I leave you with more pleasant sounds, however: another classic scene from The Tall Blond Man With One Black Shoe, this time a botched classical performance because Maurice now suspects his wife's infidelity (although still clueless that his best friend is her partner in crime).