You are Thinking About Sound the Wrong Way

Most people tend to think of sound from a speaker as air moving. This leads to the idea that something that will block wind (like thin plastic) will also block sound. The air is moving, but there is no "flow". Instead, think of that movement as shivering - it's vibrating very slightly but not going anywhere.
As with all solids, some sound will reflect off of the thin plastic, but almost all of it will pass right through unobstructed. Think of light hitting a pool of water - some reflects and some goes through so you can see the bottom of the pool.
So a damping material like rockwool performs essentially the same as it would in its raw state, if it is sealed inside a plastic bag.
Sound is energy that travel through and acts upon mediums. Mediums are any solid, liquid or gas (like air) and they vibrate when sound passes through them.
They can also reflect the energy. If the sound is travelling through a medium (like air) and if it meets another medium of different thickness, density and mass some of it will reflect. The new medium (like a wall) will reflect some of the energy and let some pass through.
For example a thick, solid concrete wall will reflect almost all of the energy that hits it, while a thin wall made from lighter, less dense material will only reflect the higher frequencies and let most of the bass frequencies pass through. You can hear this effect easily with your stereo playing in another room.
Damping absorbs energy and converts it to heat. When sound passes through a damping material, some of it is converted to heat and used up in that way. That's how the piece of rockwool I put inside the speaker stopped the standing wave - it absorbed the excess energy from that standing wave and converted it to a very small amount of heat.
Sound is not simple. Most think of it in 2D terms, when it is fully a 4D process. It moves through 3D space with a beginning and end time. It usually starts at a single point, but then expands out in all directions from that point until all of the energy has been converted to heat. This expanding is called propagation and the sound energy is spread thinner and thinner as it happens.
Think of a balloon being inflated and how the rubber gets thinner as the balloon gets bigger. There's still the same amount of rubber there was when the balloon was not blown up, but it's now spread over a much larger area.
For sound to dissipate quickly, something has to speed up the conversion of that sound energy to heat. That's the function of damping, like that rockwool I used.

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[IBuildIt]

Most people tend to think of sound from a speaker as air moving. This leads to the idea that something that will block wind (like thin plastic) will also block sound. The air is moving, but there is no "flow". Instead, think of that movement as shivering - it's vibrating very slightly but not going anywhere.
As with all solids, some sound will reflect off of the thin plastic, but almost all of it will pass right through unobstructed. Think of light hitting a pool of water - some reflects and some goes through so you can see the bottom of the pool.
So a damping material like rockwool performs essentially the same as it would in its raw state, if it is sealed inside a plastic bag.
Sound is energy that travel through and acts upon mediums. Mediums are any solid, liquid or gas (like air) and they vibrate when sound passes through them.
They can also reflect the energy. If the sound is travelling through a medium (like air) and if it meets another medium of different thickness, density and mass some of it will reflect. The new medium (like a wall) will reflect some of the energy and let some pass through.
For example a thick, solid concrete wall will reflect almost all of the energy that hits it, while a thin wall made from lighter, less dense material will only reflect the higher frequencies and let most of the bass frequencies pass through. You can hear this effect easily with your stereo playing in another room.
Damping absorbs energy and converts it to heat. When sound passes through a damping material, some of it is converted to heat and used up in that way. That's how the piece of rockwool I put inside the speaker stopped the standing wave - it absorbed the excess energy from that standing wave and converted it to a very small amount of heat.
Sound is not simple. Most think of it in 2D terms, when it is fully a 4D process. It moves through 3D space with a beginning and end time. It usually starts at a single point, but then expands out in all directions from that point until all of the energy has been converted to heat. This expanding is called propagation and the sound energy is spread thinner and thinner as it happens.
Think of a balloon being inflated and how the rubber gets thinner as the balloon gets bigger. There's still the same amount of rubber there was when the balloon was not blown up, but it's now spread over a much larger area.
For sound to dissipate quickly, something has to speed up the conversion of that sound energy to heat. That's the function of damping, like that rockwool I used.

[franknagel]

Recently a car audio engineer showed me a car speaker design. He told me that it will be sort of revolution in car audio. He said its a ring mode driver. It is a extremely flat full range driver starting around 150Hz and reaching up to 20kHz. I mention this, because it can be seamless integrated in any surface and fully covered with a lot of materials without the need of a grill or opening. The covers almost have almost no effect on the function of the driver. In fact they become part of the driver.

[pierra]

Of course! The best analogy that confirms the argument is seismology, that allowed us to characterise earth layers (size, location, material) with changes of medium (density), by measuring residual energy from the initial energy released by earthquakes and deduce transfer functions.

[bradstone]

More AB comparisons please. These videos have the potential to be timeless.

I'm about to start another build, a big one, and with a Qtc of 0.5 it's already somewhat wide and quite deep but over five times as high as it is deep.

I initially could not tell a difference between the tall and the short box, but I wasn't looking at the screen at the time, and (if it was mentioned) I wasn't aware of how the comparison was being shown.

I now know that even though I can tell a difference, I do not care for that improvement as much as I want/need to be so tall.

Maybe even a SoundCloud or some other lossless way of sharing the comparisons for the quite discerning could be another source of ad revenue?

This is like mythbusters for DIY loudspeaker design.

[floofyprawn]

The way that helps me to more intuitively understand this concept is imagining it being over my ears. I wouldn't expect a plastic bag to have much if any effect on my hearing, while in a more abstract situation I wouldn't be so certain, despite it being the same concept.

[okcyurwin]

When a neighbor notices the noise coming from your listening space.
When you hear the family shouting in the other room.
Yep, sound is an energy that travels through solid stuff👍🏼👍🏼

[ppdan]

If I am not mistaken it doesn't really go through the plastic bag, but is "reproduced" on the other side.
It's like a transducer only in this case you are using sound to produce sound.

[danielbal]

I think many of us had the wrong idea because of people saying things like, " boy, that subwoofer moves a lot of air" or "bigger speakers move more air".

[jrkorman]

John, for completeness. Did you try a plastic bag filled with air?

[somebodyx]

Most tell tale comparison would be: Stuff the cabinet with absorbing material and run a full audio band impedance sweep. Then bag the same material and put it back inside at the same places. Run an impedance sweep again. Compare.

[paulhirst]

I cannot believe that you went to the trouble to do this. For your safety you really should have a haz mat suit on when handling that stuff. Plastic bags are nasty. 🙃

[RobertCookcx]

Sounds like you tested this around 40 Hz. The length of a 40 Hz wave is about 28 ft (8.6 m). The wave pressure the bag "sees" is essentially isobaric with respect to it's surface. The wavelength is too long for a phase offset to cause any practical motion. I like how you dive into the physics of your tests, like you are challenging monther nature to give up her secrets!

[Pgr-ptep]

If anything, the bag's pliability will help add to the polyfill's affect on slowing down the speed of sound even more. I.E making the apparent enclosure volume even larger. In the upper kHz frequencies however the plastic films do attenuate/reflect significantly.

[vmoutsop]

My understanding is that the energy created by the sound is what moves the air as well as the movement of the objects (hands). The bag and rock wool will absorb the energy.

[nathandaniels]

But the plastic bag is having the opposite problem most people were concerned about, which you show. The intent of insulation is to absorb reflected sound that would normally bounce off the internal walls of the cabinet, correct? You did show in your video that the plastic bag actually reflected certain frequencies….frequencies that would normally be absorbed by the insulation.

When you held the plastic bag close to your face, the timbre of the recording changed. Moreover, when you covered the mic with the empty bag, you commented that the mic picked up more sound, which would seem to indicate some sound was being reflected around inside the bag, causing a boundary reinforcement.

Am I missing something here? Are you saying the frequencies being reflected by the plastic are not crucial frequencies for the insulation to absorb? Are you saying the overall reflected sound is so low that it’s negligible? Both? Or something else?

[whtube]

Very well explained. Thank you very much 😊👍

[RogierYou]

Seal it in a silk or high threat count cotton bag?

[jsaurman]

All you would have to do to disprove the naysayers is to take a speaker and cover it with a blue plastic tarp then crank it up. It will neither rattle/crackle the blue plastic tarp or really block much of the sound. You'll lose some of the high end, but not all of it.

[FOH]

A few observations.
- The mic/empty bag test; ... perhaps the surface area grabbed more wave energy, coupling that air mass more ideally.

- For decades concert PAs have had to fire through covers, banners, huge signage.
Certainly not ideal from a FOH perspective. Encountered often on budget festival type events with a title sponsor.

- Its generally accepted that covering bass trapping in 6mil sheeting for example, performs like a low pass filter.
It varies, but it returns energy above a midpoint ... it's often employed to retain liveliness, when adding proper effective LF control via many bass traps.

Most rooms need all the LF absorption possible, yet doing so without preserving vital liveliness may not be as pleasant.


- NSYNC came thru our venue, RCA Dome, in the early 2000's. This was the biggest tour production in the world.
No expense spared touring.
But for "cool" visuals ... the PA fired thru an external grill!
I couldn't believe it.
Pop Odyssey Tour

Big point source PA, each cabinet faced with an exterior shiny metal grill ... for aesthetics ... a tour that big.
That should never happen!

fwiw, everyone got their hands on that tour ... "bill to band", "bill to band", "bill to band" ... I saw it.

Every element of that tour was as over the top as you can imagine. Circus like.

The "steel" arrives a week earlier. Cranes handle and build the stage structure, from a constant flow of flatbeds negotiating our King-Kong door airlock.

Over a few days, they build every trick feature a concert could have.

As an opening misdirection, they appear on an elevated platform midfield (unknown to concertgoers, the act was rolled into position via road cases, right thru the center of the crowd!).

In addition to cabling to fly five wide across the length of the stadium ... trick hidden features everywhere. Conveyor belt thrust catwalks out into the crowd. Five futuristic mechanical bulls appear ... with every lighting, pyrotechnics effect possible, platforms rising and falling.

Every effect ... smoke, fire, light, platform, pyro, laser, the main video display dropped down into an example!

Akin to a Vegas permanent residence show ... that level of complexity.

We parked 20 to 30 motor coaches, INDOORS for that show. A car hauler of exotic cars, sport bikes for toys while they're in town. Additional haulers with 60 golf carts ... 60, one band, one night ... two hours!
Excess, layers of it.

Like I said, EVERYONE had their hooks in while the getting was good.

Someone had the bright idea to fire the finest PA cabinets in the world thru a set piece, an ornamental grill.

John, your video reminded me of those times.

[TheVTRainMan]

I was just going to ask about foam. When i was younger I made a car subwoofer and lined it with foam ("egg-crate" foam to be exact). Then later moved it to my home theater system. It was actually a 12" home theater woofer to begin with. It sounded great... that's all I remembered. So I did the same to my floor standing 3-way speakers and thought it improved the sound as well. Probably not... maybe. But, in any case, look forward to that next. Always learn something new . In fact, I am realizing how little I knew to begin with. :)