How Sound Works (In Rooms)

Hi, I’m John Calder of Acoustic Geometry.
Let’s talk about Acoustics, which is basically how sound works in rooms.
It may seem complicated, so let’s make it simpler.
Most rooms have flat walls and flat ceilings and sound bounces off of these.
So how does that affect the sound? I’ll use these two Nerf guns to demonstrate.
I’ve got this one aimed so this disk goes directly to the ear.
That represents direct sound. I’ve got this one aimed so that disk bounces
off the wall and it represents reflected sound. I’ll shoot them both at the same time.
Reflected sound arrives at our ears later than direct sound, even though it started
out at the same time, because it’s traveling farther.
And this wall is only one flat surface. There are at least 6 in the average room and
that’s a lot of reflected sound. But why is reflected sound bad?
I’ll demonstrate using these two identical patterns.
The blue pattern represents direct sound waves. The red pattern represents reflected sound
waves. They start out together, but when I move the
red one backwards, like a delayed sound reflection, it creates destructive interference patterns
which changes the original sound wave. Here’s the problem.
Original sound waves are distorted by strong later-arriving reflections.
Also, sound travels really fast. About 1130 feet per second.
A sound wave will bounce back and forth between these two walls about 60 times in one second.
Sound travels so fast it fills a room almost instantly.
This is only one bounce angle, every room has thousands.
How can we make our rooms sound better? Remember our Nerf guns?
I’ll shoot these at the same time, again representing a sound wave bouncing off a wall.
Both discs bounce together in the same direction, which means the reflected sound is at full
strength. Now let’s use the first of our two acoustical
tools, an absorber, to reduce the strength of sound bounces.
To a sound wave, an absorber looks a little like a hole in the wall, so some of the energy
doesn’t come back. An absorber works by reducing the strength
of reflected sound that would otherwise cause more destructive interference.
But if we use only absorbers in a room it makes it sound dull and unnatural.
Historically, humans don’t like overly absorbent rooms.
So, let’s use the second of our two acoustical tools, the curved surface diffusor.
It also reduces the strength of sound bounces. A diffusor works by scattering the sound reflections
in different directions, smoothing out destructive interferences throughout the room.
Room acoustics are greatly improved using a combination of absorption and diffusion. It’s all about reducing those flat-surface reflections.
Use a combination of absorbers and diffusors and your room will sound a lot more natural.
Thanks for watching.

100 thoughts on “How Sound Works (In Rooms)

  1. What about a "room" without walls, like outdoors, why is there still an echo? And why do we hear a clear specific sound exactly like the original just delayed a bit, if the sound is in fact bouncing off millions of surfaces at all different distances? I would only expect such a clear echo if there was one wall where each point of that wall was the same distance from the created sound, and only if I'm standing (i.e. my ears are) at the same place where that sound was created. So why do we hear such clear echoes anywhere we go?

  2. Every theory in mankind history is just a justified asumption, it works,but it does not mean that its the right answer.

  3. Does sound die? Someone said to me that people are getting more insane because of the chaos generated by honking vehicles and speakers. The more the sound will increase, the more people will get mad, because noise affect our mental system… Someone please clarify. Thanks

  4. Do drapes, sheers, couches, carpeting, etc. help improve the sound at least – without going to the expense of buying absorbers and diffusers and then figuring out if and where they will fit and then enduring the lessened room space?

  5. Nice video. You know of course your dummy is gay, right? Not that there`s anything wrong with that

  6. Just bookmarked under 'Interiors'. Our Decorator schooled us in these issues, decades ago. But people think I'm crazy when I start talking about acoustics. Now, I can just show them a video.

  7. greatly explained
    but when lot of you guys putting a reverb plug-in , what's the point of using a absorber

  8. I'm watching all these sound engineering videos as if I'm about to make a podcast, but I have nothing worth to say…

  9. What material can you use it a diffuser? and is it best that the diffuser is hollow i.e. its not packed with sound absorption material?

  10. I get the idea but it looks like your diffusers and absorbers are made from the same material, wouldn't the diffuser still act as an absorber even though it's curved?

  11. Great video, there's some really helpful information in this video for anyone interested in building recording rooms.


  12. Direct is good, reflected is bad? That is not only fundamentally incorrect but a gross oversimplification. I know you aim to simplify but the fact is, even your voice, in making this misleading statement, is being supplemented by reflections in a highly beneficial way. We never, ever hear solely direct sound, and if we did we’d find it very disorienting. What matters, and it can be quantified as well as controlled, is the ratio of direct to reflected sound, the times of those reflections and the relative tone of those reflections, which we call reverberation. Real acoustical engineering designs for those three basic elements of reverberation to provide a sound field that is most conducive to the human mind’s expectations, which are very different for speech and music.

  13. Sound travels 4.3 times faster in water , your body is 80% water, you hear through your whole body , which synthesises sound , sort your body out first, olive oil & vinegar in ears does the trick

  14. As a poor man's solution, I hung corduroy fabric on parts of the walls using small diameter wood poles, hooks and brackets – like wall tapestry hangings. It improved the sound immensely. Thanks for the great video.

  15. Good explanation. I think our brain accepts the totally distorted sound of a stereo in a room because it 'knows' it is in a room, but that's psysiological.
    I once stood in a room completely covered in sound dampening foam. I clapped my hands so hard it hurt but all I heard was a dull thud, like two tennis balls colliding. Unreal.

  16. thanks so much for this video. as a hobbyist trying to make improvements to my recording space this video definitely changed my ideas on how to go about it. fuggit. add some absorbent and diffusive pieces and then move stuff around til' you like how it sounds? use a few built-for-the-purpose items and then the indigenous contents of the room?
    aside from acoustic panels, etc , think i'm going to put up some shelves with old paperbacks? maybe add an overstuffed chair? how bout stuffed animals? worth a try anyway.

  17. you could use that clip of the wood clappers for an 80's inspired music video…reminds me of some of back when MTV used to play music videos. Kinda funny.

  18. well I too studied for many years the acoustical problem and if what this channel says is the absolute truth then Bose would be out of business because that's what made them the empire they have become today _Direct Reflecting Technology, their flagship the 901

  19. This is what I figured after playing instruments for so long. Simply having a fan set to low messes with sound frequencies too. It's interesting how it causes them to sound. Thanks for the video!

  20. Mr. Calder seems like, AND IS, such a dedicated, industrious (20th centuryisque) genius who simply loves his craft.
    wish youtube would have more of THESE instead of the over animated 'fake' exuberant ones whose charade is all about fame, name and being up on the game.

    youtube, please have more of the likes of Acoustic Geometry, the Engineering Guy , Grand Illusions and the likes.

  21. Interesting and somewhat helpful. You should have mentioned Standing Waves and Room Modes and their effect on the sound. I find that my irregular shaped room is best for music reproduction. You do want some reflected sound to make it more natural. Also avoid "exact" distances between speakers and boundaries. The old equilateral triangle position is another fallacy for good sound. Just place your speakers where they sound best, but not the middle of the room. Fuhgeddaboudit!

  22. 3:18 – "More natural" – That's funny to hear.
    Sound is supposed to bounce. It must be living inside of a box that isn't natural.

  23. Fascinating and informative! We are currently in the middle of recording at our home studio and trying to get our sounds right and this puts a wonderful visual on WHY things happen the way they do.

  24. This is what bugs me most:
    fArther = fUrther
    Eleven hundred and thirthy = One Thousand one hundred and thirty

    Why do Americans make everything sound stupider with their slang

  25. You're saying all this about flat walls causing bad sound, but you're in a room with flat walls and the sound on the video is perfect

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