Further sound insulation through bookcases?

[Garald]

As some of you here may remember, what will be my library/piano room (the main room of the house, really) overlooks a fairly noisy two-way road. Newer double-glazing has helped, as has the heat insulation we chose (air + 14.5cm of BioFib Trio (hemp+cotton+linen) + air + 18mm plaster), but it's still not quite what I was hoping for. Now, that wall will have built-in bookcases - really my last line of defense at this point.

 

What should I do in order to maximize how much the bookcases will help with sound reduction?

1. What material to use on the back? (Cork behind wood maybe?)

2. I take the further up the bookcases go, the better, but I can't go flush against the ceiling, as there is moulding (which I am very happy the workers managed to preserve).

 

(Also, does it make sense to add sound-reduction drapes? I will have shutters on the outside. I'm a bit concerned about drapes gathering dust (I have allergies).)

 

I'm not there right now, so this is the best I can do:

 

 

It's the wall with the window here - it's on the double room's long side. The bookcases should go along the entire length. I am a little concerned that the contractor may have put the lights a little too close to the wall, though it is true that I am planning on rather shallow bookcases (about 22 or 23cm on the inside).

 

[saveasteading]

Books will be good. They are dense but also loose which will absorb some sound.

The noise will mostly be through the window. Heavy curtains will help a bit. Get blackout ones and the pores are closed  and one side at least does not gather dust. Heavy is good.

Any sound through the walls will be low frequency from heavy vehicles. The books are good. Anything dense hanging on the wall will help a bit.

Noise gets through gaps, so it is a case of the 'weakest link'

[Garald]

41 minutes ago, saveasteading said:

Any sound through the walls will be low frequency from heavy vehicles. The books are good. Anything dense hanging on the wall will help a bit.

Noise gets through gaps, so it is a case of the 'weakest link'

 

The main source of bother is heavy vehicles (buses mostly) but also motorcycles. One can't hear people talk on the street (where one can from the kitchen, which also got new, supposedly high-grade double-glazing, but where the insulation is much thinner - a few cm of BioFib + some cm of reflective insulation).

So, does backing up the bookcases with cork make sense? Perhaps one continuous sheet? Or should I just get a solid wooden back for the bookcases (as opposed to some thin IKEA-style panel)? Or does it not matter much at all?

[Mike]

From the look of the photos, it looks like the house is of traditional masonry construction, so the walls should already be pretty effective against airborne sound - and I presume that the BioFib is internal wall insulation, which will have boosted that further and broadened the frequency range being damped. In which case I suspect a bookcase would only make a marginal difference.

 

It looks like you could be on the top floor; if so, did you do anything to reduce sound transmission through the ceiling?

 

Windows are likely a weak point unless you specifically chose double glazing units & frames with enhanced sound-resistant specifications, rather than just new double glazing.

 

And, as @saveasteading says, noise also get through gaps.

[saveasteading]

The rumble from heavy traffic is the whole building trembling. Covering the back of the bookcase will make a difference  but not much.

All the furnishings will help, and the weight of the bookcase and other furniture may stiffen the floor and help too. 

A tapestry would possibly be the biggest help....or drapes.

[Garald]

45 minutes ago, Mike said:

It looks like you could be on the top floor; if so, did you do anything to reduce sound transmission through the ceiling?

 

This is actually a first floor. The attic above it also belongs to me - it's home office space and bedrooms. Street noise isn't really an issue there, though the occasional sound of a plane is (there is a nearby airport). Different set of issues.

 

45 minutes ago, Mike said:

 

Windows are likely a weak point unless you specifically chose double glazing units & frames with enhanced sound-resistant specifications, rather than just new double glazing.

 

I did ask the architect to specifically choose double glazing with sound-resistant specifications. I see in her plans that a "coefficient phonique" (decrease in noise?) of at least 35dB was specified; I *think* she got 38dB.

 

As I said, there's a very real difference between noise in the kitchen (where the windows are the same, but where there's much less BioFib) and noise in the library.

 

 

45 minutes ago, Mike said:

 

And, as @saveasteading says, noise also get through gaps.

 

Well, we worked on those for the heat insulation, but nothing is perfect.

 

17 minutes ago, saveasteading said:

The rumble from heavy traffic is the whole building trembling. Covering the back of the bookcase will make a difference  but not much.

 

True. At the same time, if I could get rid of the motorcycle noise, I could live with the rumble of buses (which go by only every 10 minutes or so, at most).

 

 

17 minutes ago, saveasteading said:

All the furnishings will help, and the weight of the bookcase and other furniture may stiffen the floor and help too. 

A tapestry would possibly be the biggest help....or drapes.

 

Where should the tapestry go? Not behind the bookcase?

At any rate, the recommendation would be what - to get a bookcase with a solid-wood or MDF back panel, rather than a thin panel whose back I would cover with cork? I'm trying to minimize depth (for usage reasons).

[saveasteading]

17 minutes ago, Garald said:

Not behind the bookcase?

Yes the whole wall. A heavy, sealed fabric. A carpet with foam backing, but could look a bit weird, s heavy curtaining. 

I'm not saying this would make enough difference to justify it, as the mc noise will still come through the window.

For sound insulation, the gap in double glazing should be very much wider than for thermal insulation. Something like 30mm I think.

You could try an additional inner glazed panel.

[Garald]

34 minutes ago, saveasteading said:

Yes the whole wall. A heavy, sealed fabric. A carpet with foam backing, but could look a bit weird, s heavy curtaining. 

 

Right, it could look a bit weird, *if it were visible at all*.  Better get something that can be hidden tightly behind build-in bookcases. That's why I was suggesting cork - but then I'm not sure that's actually best. Say I can choose any material I want, as long as it's 1cm thick, uniformly so. What should I get?

 

34 minutes ago, saveasteading said:

I'm not saying this would make enough difference to justify it, as the mc noise will still come through the window.

For sound insulation, the gap in double glazing should be very much wider than for thermal insulation. Something like 30mm I think.

You could try an additional inner glazed panel.

 

I'm not there right now, but this looks reasonably thick.

 

 

(Yes, I was looking at samples for possible drapes...)

 

I will ask for the documents, but I'm positive this is double-glazing that is especially made for soudn protection.

[saveasteading]

8 hours ago, Garald said:

especially made for soudn protection.

Yes that is for sound.

Not great for thermal insulation.

[Garald]

So, in the end, *what* should I put behind the bookcases?

 

If mass were the only thing that mattered, then the best thing (for given thickness, at a reasonable price) would probably be mass-loaded vinyl, no? (Better ask the carpenter if the shelves can take it - unless of course it's pasted on the wall, though it's a pity to do that to a freshly painted wall.)

 

Then it would be rubber, and then wood - and cork would be absolutely terrible. Yet cork is reputed to be a pretty good insulator. So, what gives?

[saveasteading]

22 hours ago, Garald said:

9 minutes ago, Garald said:

So, what gives

 

You need mass and separation. Heavy vinyl is expensive and ugly. 

 

My suggestion remains heavy curtains....blackout probably to seal the fabric. Worth a few dB at most,  so it has to be your decision.

[Garald]

17 minutes ago, saveasteading said:

You need mass and separation. Heavy vinyl is expensive and ugly. 

 

My suggestion remains heavy curtains....blackout probably to seal the fabric. Worth a few dB at most,  so it has to be your decision.

 

Sure, heavy curtains for the windows. But what to to put behind the back panels of built-in bookcases?

(Heavy vinyl isn't nearly as expensive as 2cm-thick solid wood (say 3cm if it is pine), no? What should I expect to pay for mass-loaded vinyl that is 10kg/m²?)

For separation, what would be the solution? Have the shelves be attached to the back panels, not to the walls, and have the back panels be bolted to the wall (for safety) but not otherwise attached?

Edited April 7, 2023 by Garald

[Mike]

On 06/04/2023 at 23:55, Garald said:

I did ask the architect to specifically choose double glazing with sound-resistant specifications. I see in her plans that a "coefficient phonique" (decrease in noise?) of at least 35dB was specified; I *think* she got 38dB.

Standard modern 4/16/4 glazing without lamination or toughening gives a reduction of around 31 (-1; -4) dB, so 38dB is a worthwhile reduction (though you'd need the bits in brackets to take into account the frequency range - they are correction factors for speech and traffic respectively, so for traffic noise a reduction of 31-4 = 27dB).

 

FWIW I've just ordered some with a nominal 39dB reduction - actually Rw (C;Ctr) 39 (-2; -6) dB - so my nominal 39dB becomes 33dB (39-6) for city traffic. City traffic - around 80dB - therefore sounds like 47dB (80-33), roughly the normal level of noise generated within a home, and 6dB (approximately 1/3) quieter than 4/16/4 would have been.

 

Unfortunately I've not done any calculations for drapes.

 

 

Edited April 7, 2023 by Mike

[Mike]

1 hour ago, Garald said:

cork would be absolutely terrible. Yet cork is reputed to be a pretty good insulator. So, what gives?

Cork is a good insulator for impact sound - for example between floors - but for airborne sound you normally need weight. Ideally heavy materials of several different weights to broaden the frequency response.

[Garald]

6 hours ago, Mike said:

Cork is a good insulator for impact sound - for example between floors - but for airborne sound you normally need weight.

 

Well, by the time street noise gets through layers of brick and 18cm of insulation, it is no longer airborne, no? We are trying to protect the inside from outside noise, not the other way around. (We are using weight to insulate the ceiling of the laundry closet, for instance.)

 

6 hours ago, Mike said:

Ideally heavy materials of several different weights to broaden the frequency response.

 

Meaning of several different densities? Then hiding mass-loaded vinyl behind the back panel of a bookcase sounds like a good idea, no? (Of course what really should have been done is put it behind the wall panel, no?)

[Mike]

2 hours ago, Garald said:

Well, by the time street noise gets through layers of brick and 18cm of insulation, it is no longer airborne, no?

Impact sound is when there is a direct strike on the wall - a football, door shutting, switch being switched - so you're not dealing with that. If the structure of the building is vibrating as a result of impact or airborne sound, you need physical separation of the interior of the room from the exterior - a room within a room set on dampened springs, but that's not at all viable for a regular domestic room. Airborne sound therefore the only thing you can easily reduce. And I'm sceptical that any treatment behind the bookcase would make a difference because, as you say, that's been substantially reduced by the structure already.

 

2 hours ago, Garald said:

9 hours ago, Mike said:

Ideally heavy materials of several different weights to broaden the frequency response.

 

Meaning of several different densities

Yes, you can buy sound-reducing drapes that have multiple layers of fabric of different densities. They may make a worthwhile difference since the window area seems the most likely problem area and the most readily treatable.

[RichardL]

Suggestion - a slip of paper,  go around all the window seals and double check they are all making contact with the wall, slide the paper between the window and wall looking for loose/easy spots.

 

It only takes a small gap and the sound will leak in.

[Garald]

2 minutes ago, Mike said:

Impact sound is when there is a direct strike on the wall - a football, door shutting, switch being switched - so you're not dealing with that. If the structure of the building is vibrating as a result of impact or airborne sound, you need physical separation of the interior of the room from the exterior - a room within a room set on dampened springs, but that's not at all viable for a regular domestic room. Airborne sound therefore the only thing you can easily reduce. And I'm sceptical that any treatment behind the bookcase would make a difference because, as you say, that's been substantially reduced by the structure already.

 

I see. But would it then not make a difference whether I attach shelves directly to the wall or whether I have made-to-measure bookshelves that are just attached to the wall (loosely?) at a couple of points?

 

Just a data point - the kitchen, where insulation is thinner but the masonry is the same, is noticeably noisier. So, clearly quite a lot of sound gets through the masonry (and half the insulation, say). 

7 minutes ago, Mike said:

Yes, you can buy sound-reducing drapes that have multiple layers of fabric of different densities. They may make a worthwhile difference since the window area seems the most likely problem area and the most readily treatable.

 

I'll get sound-reducing drapes- hopefully they will help even when not drawn, since presumably much of the noise gets in through the edges, no?

[saveasteading]

There is only so much you can do with your fixed circumstances. The  advice is all good. Pick and mix whst suits you.  Don't obsess about the book-case. Settle for some improvement anc accept that there will still be noise.

Good luck, and please report back when it is done.

[SteamyTea]

When all else fails, turn to the literature.

How to build a sonic crystal

 

1 March 2011

 

What you’ll need: poles or tubes, a saw, measuring tape, a drill, a hammer, nails, two planks of wood (one as a base plate and another as a top plate)

1. Collect poles or tubes (almost any type of pole can be used – metal, plastic or wood, hollow or solid) and decide on the range of frequencies you want to block. The wider the tubes, the broader the range of frequencies the crystal can stop. The pole separation also affects the frequencies you can block so you may want to calculate this now:

Pole separation (a) = speed of sound in air /(2*frequency).

2. Cut the poles into pieces of equal length. To be most effective, the poles should be a long as possible. The crystal should be at least five poles deep to be effective.

3. Mark out a grid on your base plate to show the position of the poles by using the pole separation equation given above.

For example if you want to attenuate low frequencies around 440Hz (this is the frequency of the note A above middle C in the musical scale) you should use cylinders separated by a distance of 39cm and the diameter of each cylinder will need to be 26cm. This structure will be very large!

4. Attach the poles to the base plate. If you’re using solid poles, the simplest option is to nail them to the plate from behind. Depending on the poles you’re using, you may also want to use glue or screws.

5. If the poles are unstable, add a top plate to keep the structure rigid.

6. The best way to test the crystal is with a sound at a single frequency. To create a test tone, you can use a tone generator (like this one available free online) to create a sound file with a specific frequency and duration. Upload this tone to your cellphone.

7. Position your cellphone on one side of the crystal in the centre of the array, roughly 30 cm away from the poles. Play the tone and listen for it on the opposite side of the crystal. You’ll know your structure is effective if the sound is muted.

 

 

 

 

Sculpted sound

By Philip Ball

23 March 2002

 

PUBLIC sculpture is all very impressive to look at, but what use is it? It just sits there providing a perch for pigeons and a backdrop for tourists’ photos. But a few years ago, Francisco Meseguer of the Institute of Materials Science in Madrid found an unexpected answer. He discovered that a minimalist sculpture on display in Madrid can block out sound. Erect such barriers throughout a noisy city or along the sides of a motorway and the tumultuous modern world might not only become a quieter place, but a more attractive one too.

The structure he stumbled across is called a sonic crystal because it scatters sound waves from a periodic array of “atoms”—anything from glass spheres to metal rods. Unlike conventional, solid sound barriers, sonic crystals are mostly empty space. And they’re a lot easier on the eye, too.

Now Meseguer and his colleagues have shown that these structures can do a lot more than simply block sound. Sonic crystals can shape and manipulate sound as if it were putty, reflecting, bending and filtering it in unexpected ways. They might even transform unpleasant environmental noise into something far more soothing.

And the Spanish team hopes to make similar structures for controlling other kinds of unwanted vibrations. In the grandest and, by their own admission, the craziest of such schemes, they think it might be possible to build gigantic sonic crystals that dampen seismic waves and protect buildings from earthquake damage.

Meseguer first stumbled across sonic crystals while he was working on tiny structures called photonic crystals. Developed in the 1980s, these can be used to manipulate light, and are now being turned into devices such as lenses and fibres for all kinds of optical communications technology (New Scientist, 12 June 1999, p 36).

One of the simplest forms of photonic crystal is simply a block of silica with a regular array of holes drilled in it. The difference in refractive index between the silica and the air in each hole means that light is scattered at the boundary where they meet. Make the distance between the holes about the same as the wavelength of light shining on it and the scattered photons interfere destructively. Simply put, the crystal blocks light in that range of wavelengths. It has a “photonic band gap”.

Nowadays, photonic crystals are often made by allowing microscopic glass beads to settle out of a suspension into organised lattices called colloidal crystals. Structures like these form in nature too. The iridescent appearance of opal, for instance, comes from the light-scattering properties of the tiny silica spheres that make up its lattice.

For photonic crystals with band gaps in the visible region of the spectrum, the holes or spheres must be a few hundred nanometres apart. But in 1995, while Meseguer was chatting with acoustics expert Jaime Llinares of the Polytechnic University of Valencia (UPV), they realised that if these structures were scaled up to centimetre dimensions—corresponding to the wavelength of sound—they might be able to create an acoustic analogue of a photonic crystal. Inside their imaginary sonic crystal, the scientists reasoned, sound waves should bounce off the “atoms” in such a way that the waves interfere destructively, cancelling out the oscillations in the air.

Llinares suspected that some form of sonic crystal might already exist. He remembered that on the UPV campus there was a sculpture by the Spanish minimalist artist Eusebio Sempere, made from an array of vertical metal bars of various lengths, of about the right thickness and spacing, like a set of surreal organ pipes. Could this block sounds?

To find out, they placed a loudspeaker on one side of the sculpture and a microphone on the other. The speaker broadcast white noise through the sculpture, but when they measured the intensity of the sound on the other side, there was no sign of a band gap.

The problem, they decided, was that the sculpture was made of hollow metal cylinders which resonated like organ pipes. Since some of the pipes were very short—about 10 centimetres, the same as the separation between pipes—they were resonating at frequencies inside the expected acoustic band gap, and this vibration masked the gap.

Fortunately, Sempere had constructed a similar, but larger bar sculpture, on display at the Juan March Foundation in Madrid. In this outdoor sculpture the pipes are up to three metres long. The two researchers set off with their loudspeaker and mic to try again. This time they found that the sculpture actually blocked out sound.

It wasn’t a perfect “crystal”, however, so the acoustic band gap was rather leaky. Worse still, the measurements were muddied by noise reflected from nearby buildings. To improve their data, the researchers decided to make their own minimalist sculptures by hanging cylinders of stainless steel or wood from a frame. This created a regular forest of bars that wouldn’t look out of place in any modern art gallery.

In 1998, instead of entering it for the Turner Prize, they mounted their crystal in an echo-free acoustic chamber and began experiments to measure how sound travelled through it. Their data revealed that the structure strongly suppressed sound waves in the audible range, at frequencies between 1400 and 1700 hertz. At last they had clear evidence of a sonic crystal.

Just months ago, the team also revealed how sonic crystals could be used not only to block sound but also to manipulate it (Physical Review Letters, vol 88, p 023902). At frequencies below the acoustic gap, sonic crystals are transparent to sound. But they don’t let sound waves pass unscathed. Just as light is bent by refraction when it passes from air into glass, so sound waves are bent when they pass into a sonic crystal.

The researchers realised that they could use this to create a lens that focuses sound. The lens they built is a convex array of cylinders (see Diagram). Put a sound source on one side and the lens focuses the sound waves on the far side. The focus is rather blurred, however, partly because the surface of the lens is quite rough—you can’t make a smoothly convex surface from a small number of cylinders, the structure is just too “grainy”. It’s like trying to make a lens from a handful of atoms.

 

 

 

They also built an acoustic analogue of another common optical device: a Fabry-Pérot interferometer. This is made from a stack of thin films that create interference patterns when light reflects off each layer. The acoustic version is simply a “slab” of sonic crystal, with rows of cylinders hanging parallel to the flat faces of the slab.

Fabry-Pérot devices are commonly used in microwave technology as filters. But Jose Sánchez-Dehesa, a physicist at the University of Madrid who worked on the project, admits that it is not yet obvious where the team’s sonic analogues of lenses and filters might be used, because they are so large.

The basic sonic crystal, however, which blocks sound waves within a tunable frequency band, might find all sorts of uses. Imagine a barrier made from crystals that are designed to have all the aesthetic qualities of a Sempere sculpture but which cuts out traffic noise on the far side.

“Sound-deadening barriers could be an interesting application of our findings,” says Sánchez-Dehesa. They would be more expensive than regular barriers, because they’re more elaborate, but he argues that in residential areas they would be a definite improvement on “ugly concrete panels”. His colleague Juan Vicente Sánchez-Pérez at UPV is now planning to patent a prototype sound barrier based on an array of cylinders.

Not everyone is convinced that sonic crystals offer any real advantages over conventional materials. Victor Krylov, an acoustics specialist at Loughborough University, believes that the sound barriers currently used along motorways and railways are at least as effective, and cheaper too.

Certainly, the need to space the cylinders at a distance of about one wavelength is a drawback, because it means that sound-proofing structures have to be big and thick. In 2000, however, Ping Sheng and his colleagues at Hong Kong University in Kowloon showed that there might be a way around this. They made sonic crystals from a cubic array of lead balls just one centimetre across, each ball coated with silicone rubber and glued into the array with epoxy resin. It was a kind of giant sonic opal. This structure displayed acoustic band gaps for sounds with wavelengths of around 1 metre and 25 centimetres—wavelengths that are hundreds of times as large as the spacing of the sound scatterers. They attributed this unexpected effect to the rubber-coated balls resonating at specific frequencies—they vibrate like heavy masses attached to springs.

Sheng’s colleague Che Ting Chan says the Hong Kong team are now making sonic crystals that absorb sound. They believe sonic crystals like these could be used to block sonar signals at sea, for instance. Make submarine hulls from this stuff and they would be invisible to sonar from ships or other subs.

Alternatively, say Peter Matic and Narendra Batra from the Naval Research Laboratory in Washington DC, the acoustic gap might be useful for filtering out particular frequencies generated by heavy machinery. They aim to design multifunctional materials which can be stiff and tough as well as providing acoustic shielding. Build them into a ship’s hull and you might be able to silence the sounds of the vessel’s engine, making it harder for submarines to detect.

Sánchez-Dehesa thinks it might even be possible to design barriers that change the quality of the sound as it passes through. Just as photonic crystals can filter the wavelength of light passing through, it might be possible to design a sonic crystal so that objectionable noise passing through it becomes more tolerable—even pleasant. “My goal is to look for a panel based on a sonic crystal that could transform bad sound on one side, like traffic noise, to good sound on the opposite side, like the sound of the trees or ocean waves.”

If that sounds daring, it is nothing compared to another idea they are toying with. Since these materials work for light and sound waves, it should be possible to build a structure that can block or transform other kinds of wave—including seismic waves in the ground. Just as you can make a slab of material impermeable to light by perforating it with a lattice of tiny holes, so you might make the Earth’s crust impenetrable for seismic waves by drilling an array of huge holes in it. Surround a city with such a lattice, and you could shield it from earthquakes. “If seismic shielding could achieve an attenuation of two points on the Richter scale [a hundredfold reduction in energy], it would be great,” Meseguer enthuses.

To test the feasibility of the idea, in 1999 Meseguer and his colleagues drilled two vertical lattices—one triangular and the other honeycomb-shaped—in a bed of marble in a local quarry. Each hole in these lattices was 6 centimetres across, 160 centimetres deep and separated from its neighbours by 14 centimetres. Then they created vibrations by dropping a steel ball bearing onto the quarry floor, and used sensors to measure how the vibrations passed through the lattices of holes. Both lattices significantly damped down the vibrations.

However, to protect against real seismic waves, the holes would have to be hundreds of metres across and at least a kilometre deep. “It is clear that such a proposal is not feasible yet,” confesses Sánchez-Dehesa. “But you can protect an isolated building.”

Used in this more modest way, underground “ring fences” of seismic crystals could make labs or buildings immune to vibrations such as those from passing trucks. This could be very useful, since some high-precision instruments—scanning probe microscopes for studying structure at the atomic scale, for example—are very sensitive to disturbance. Meseguer has already started looking at the practicalities of such a system. And who knows, one day these giant structures could become art in their own right.

 

A sacred bird’s voice seems to be trapped in the very stone of the Mayan Pyramid of Kukulkan at Chichén Itzá, surrounded by the jungles of the Yucatán Peninsula in Mexico. Stand at the foot of one of the stairways that climb the outer walls, clap your hands and a chirp rings back at you from the stone surface.

According to David Lubman, an acoustic consultant based in California, this is the sound of the quetzal bird, the spirit of the Maya incarnated in their plumed serpent god Quetzalcóatl. Lubman claims that it takes only a little imagination to hear in the curious echo from Kukulkan’s staircases the sound of the quetzal as it would have been heard by a Maya in the Peruvian cloud forest.

This, says Jose Sánchez-Dehesa, may be an example of Mayan acoustic engineering. Architects have designed buildings since ancient times to generate and exploit particular acoustic effects, such as the whispering gallery of St Paul’s Cathedral in London. But this is engineering of a particularly sophisticated kind: for the Kukulkan staircases, with their periodic array of scattering surfaces, have many of the characteristic features of a sonic crystal.

Could the Mayans really have known the secret of sonic crystals as long as 900 years ago? “I think the answer is yes,” says Francisco Meseguer. Sánchez-Dehesa agrees: “We can say that the Maya are the first people to harvest sound by making a sonic crystal.” And he hopes to prove it with a series of experiments at the pyramid. We may soon know whether the quetzal bird sings at Chichén Itzá by chance or design.

[Garald]

57 minutes ago, SteamyTea said:

When all else fails, turn to the literature

According to David Lubman, an acoustic consultant based in California, this is the sound of the quetzal bird, the spirit of the Maya incarnated in their plumed serpent god Quetzalcóatl. Lubman claims that it takes only a little imagination to hear in the curious echo from Kukulkan’s staircases the sound of the quetzal as it would have been heard by a Maya in the Peruvian cloud forest.

 

The *Peruvian* cloud forest?

 

More seriously, while I can't put this kind of "sonic crystal" in my library's windows, maybe something like this could be done for heat pumps. They are getting quieter and quieter, though.

[SteamyTea]

4 minutes ago, Garald said:

The *Peruvian* cloud forest?

Is that similar to Columbian Marching Powder.

[Garald]

2 minutes ago, SteamyTea said:

Is that similar to Columbian Marching Powder.

 

There *is* such a thing as Peruvian cloud forest, and some farmers in that general region do grow coca (partly for legal consumption within the country - tea and chewing coca - and partly for ill-advised, illegal sales to cartels - some Colombian - which of course get the lion's share of the profits). Hard to do that without hacking part of the forest first, though. Same goes for cash crops that are made into completely legal drug foods (coffee, cocoa) - there's more and less responsible agriculture, but of course there's no such thing as zero-impact.

 

That's off-topic, though. More to the point, there are no Mayas in the Peruvian cloud forest, unless they went there by plane, as tourists.

 

 

 

 

 

[Garald]

Not to beat a dead horse (an activity at cross-purposes with noise reduction), but don't we need:

 

- mass (books)

- decoupling (bookcase that is only loosely attached to walls, and that only at a few points)

- absorption (provided by something like, well, cork, or rubber, or acoustic foam, between the bookcase and the wall)?

 

 

[saveasteading]

12 hours ago, Garald said:

Not to beat a dead horse

You are I think. I have nothing to add. If you need a quiet room you have the wrong room.

Prob my last comment on this. "I'm out".