Acoustics and reverberation time


Hi everyone,

We often talk about room modes or first reflections but few talk about a major reason for treating a room: reverb time.

Reverberation time is the amount of time an audio signal stays active in a room. An anechoic test chamber has no reverb, since only the source signal can be heard. We often measure reverb time with the measure RT60. That is, how long in time until the signal has decayed by 60 dB. We divide this into multiple bands so we can evaluate room treatment in mid-bass, mid, treble separately, but there is also the question of how smoothly it decays. Signals should decay randomly but smoothly. Peaks indicate an echo. Multiple peaks a slap echo. Too little reverb and you end up with a very dry sounding room, with no ambience.

What does this affect? It is like a TV or computer monitor’s pixel. Imagine the screen having a memory of all the previous pixels, like the screen starts to turn grey or blue based on what you saw a few frames ago. So, it blurs the signal. It also colors the signal. A room with excess mid/treble reverb can make every speaker seem like it lacks bass when the reality is that there is too much mid/treble in the room. A side effect of this is that speakers sound harsh when you turn up the volume. Of course, this is subjective, as you can overload a speaker, but when you are using relatively little power and the sound quality changes, it is often excess treble reverb time.

One curious experiment which will make you a believer in reverb time is to treat bare wooden floors between or behind the speakers with pillows or blankets. Why does this help the mid/treble? Well, reverb time. :)

Perhaps now we can imagine why diffusion works. Instead of being pure absorbers, they scatter the sound. They help maintain the reverb time but prevent these coherent, regular reflections. So when we are looking at room treatment we are attempting a combination of many traits. Controlling early reflections, and maintaining a diffuse, rapidly decaying (but not too rapidly) sound field, in addition to managing room modes.

At the gross level, your room acoustics are tone controls. You are playing with the mid/treble balance, and at the finest levels they are helping to localize sounds and provide an enjoyable playing field for your music.

This should also help you understand somewhat why equalizer solutions, including Digital Signal Processing (DSP) based like automatic room correction or DIRAC, etc. can only work up to a point, and why having good room treatment widens the sweet spot, and makes these tools work over a broader physical area.
erik_squires
For most domestic situations RT60 just doesn't come into play.  The rooms are not large enough.
So when we are looking at room treatment we are attempting a combination of many traits. Controlling early reflections, and maintaining a diffuse, rapidly decaying (but not too rapidly) sound field, in addition to managing room modes.
Very interesting thread thanks....

It is my experience that reaching this equilibrium between, absortion, diffusion, reflection, is the key to passive materials treatment...

But there is also, active treatment i  have work with using interconnected Schumann Generators coupled with resonators of different size, Helmholtz one and others types......

The end results is there, with  no more urgent need to upgrade my Speakers at all....

One clue about a fact not so well known :

Waves sound hate the 4 ceilings corner of the room; i had 5 corners in my irregular room ; remembering the great architect Rudolf Steiner, who replaced regular corners by flowing polihedral surfaces, i just did so to 4 of my 5 corners with great success for peanuts: better imaging and refinement of the high frequencies even a better instrumental timbre ...


The instrumental timbre rendition is for me the key and the basic stone of acoustic perception testing....


My room is a like an adjunct brain connected to my soul and the sound waves are our dual thinking process ...
This was " my imagination" main tool all along my acoustical embeddings explorations....



Merry Christmas....


P.S. my room also is relatively small and dedicated to music only, then no need of electronic correction modes with all my other devices solutions....

@mahgister,

”My room is a like an adjunct brain connected to my soul and the sound waves are our dual thinking process ...
This was " my imagination" main tool all along my acoustical embeddings explorations....” 

This is how I feel. It’s a zen moment when all is right with the room and the music just flows taking you away to your destination and dreams. 
It makes me wonder about Human Room Interaction (HRI), I think I just made that up, but it is a spin-off from Human Computer Interaction (HCI), where a human interacts with the computer. 
@mahgistar I have enjoyed studying the pictures of your 'embeddings' and understand the merit of the techniques you employ. I believe much of it serves it's purpose and does exactly as you say in room treatments. Perhaps if I lived alone or had enough money to build a dedicated listening room I might use some of them myself. Alas, I am married and if I had a gas can sitting behind my listening position and dazzling stones hanging from my ceiling I think soon I would be alone! I respect the quest. I must, however treat my room with more conventional means. Hopefully soon. One guy offered to sell me 12 acoustic panels costing $800 a piece and weighing in at 200 pounds each. That certainly isn't practical either and would cost more than my system!
The end results is there, with no more urgent need to upgrade my Speakers at all....

I mean this sincerely, I often wonder how many audiophiles are on a gear exchange merry-go-round until they get good room acoustics? It is as if the need to spend money, experiment with cables, etc. just stops.
@mahgistar I have enjoyed studying the pictures of your ’embeddings’
Thanks for your kind words....By the way my room is actually crazier than what you can see, it is difficult to gives an image clear of all my new devices with hundred of feet of cables.... 😁
But my only concern all along this incremental set of experiments was sound not esthetic... And i am not the more skillful here probably the less skillful of all....But one of the more creative it seems... 😎


For sure my embeddings controls are cheap, homemade and non esthetically compatible with a wife, especially a good one....

But i was not planning all that in the beginnings, this was the results of 2 years of non planified listening experiments and no money to afford my audio dream with the regular costly known solutions... I succeed to afford my dream at the cost of esthetic for sure... But i also introduce myself practically to acoustic embedding ( mechanical and electrical one also) WITHOUT very straight a priori rules or computers except the very basic rules for sure....


I often wonder how many audiophiles are on a gear exchange merry-go-round until they get good room acoustics? It is as if the need to spend money, experiment with cables, etc. just stops.
Most people dont have a clue about audio embeddings.... And they put their faith in external authorities or in costly engineering but not in their own ears experimental habilities...

I am not a "bat" by the way, but i know how to listen, for me audio and music are simply the educated history of my own listening habit....

And for those who will critic the bias and defective hearing status of old men of my age, i will answer this: audio is not about perceiving some higher range of Hertz frequencies for the sake of them, but about listening to relatively good natural timbre instruments renditions to the room first and the speakers attached to it... No need to be a bat....Only to be attentive listeners, even if we are half-deaf dude, anyway, it is our room and it will be our sound pleasure....

Merry Christmas and good health to all audio friends....
A lot of that stuff no doubt looks pretty batty to most folks. Something I know a little bit about seeing as there's more than a few things in my system that look pretty dubious too.  

One of them that we both have are the Schumann generators. Heard about them before, always seemed crazy, but I know mahgister is for real because he is right on about the Schumann generators. They removed a layer of fog and grain revealing all kinds of fine detail that comes through in a much more natural effortless way now. Pretty amazing for a cheap little $10 circuit board. More is better but even now with 8 we are not talking much money, certainly not for what they do.   

Old school panels and traps are fine for what they are, but no amount of them comes anywhere close to what can be done with some of these hard to understand room tweaks. Synergistic Research HFT are tiny little deals that nobody except maybe Ted Denney understands what they do or how they work, but they do indeed work, and crazy good. But if we don't have the foggiest what is going on with them, but they do in fact work, then what right do we have to dismiss any of this other stuff? None. Not that I can see.   

There doesn't even seem to be any evidence that the traditional acoustic approach is any better. In fact it seems to me that ultimately when it comes down to it the very best results are always obtained not by meters and professional consultants but by trial and error and by ear. For sure the best room I have ever heard is Mike Lavigne's and yes he used everything money can buy but ultimately it was his passion for music, his ears, and his keen eye for detail that got him where he is today.  

It didn't roll out from some UPS truck after a phone call to GIK. He worked at it, and hard, and for years. Just like mahgister is doing.  

Some of the best systems I have heard looked like a mad scientist experiment. Mahgister, not a knock, and you won't believe this, but you do not even hold first place! There was a show one time with this tube amp looked like Tesla's lab exploded and even had a big DANGER! HIGH VOLTAGE! sign on it. People kept their distance- including even me! 

But yours mahgister, boy would I like to hear what you got going there.
Tweaks aside you can get great sound and a great RT60 with plants, natural furnishings, a mix of natural diffraction and of course absorption ( which you can hide behind artwork, etc... )
see my system pictures of Casa Pacifica which achieves almost textbook perfect control room targets for RT60
and has plenty of WAF.
many of your cherished recordings probably exist because somebody cared enough to study acoustics and more...

Studio Six running on an iPad provides a good RT60 tool.
If i dare to give an image of my actual ceiling room all people here will write a petition to include me on a free psychiatric treatment list....

Then trust me i look like a nut in my room...

I dont give a dam, my room was always set to be the tool for my audio dream...

For sure after all these 2 years my hundred of experiments contribute all with inequal value to my actual S.Q. Nobody was there next to me to give me a clue about what to do, except buy this or that, in general with the money i never have...

Sometimes one of my device were disconnected by accident, or by me for an experiment, and the result were the audible clues and values of each one of my experiments...

Classical audio treatment for example cannot replace many devices effects, like Helmholtz resonators, or non-Helmholtz resonators, or stones or crystals installation, or Schumann generators grid, or my golden plate grid etc....How in the world a connected by cable array of resonators can have an audible effect ? Classical acoustic dont deal with nut people like me....But anyway i keep what is working....

For example my last discovery was that ears dont like the empty convex corner of my audio room, but prefer a polyhedral flowing shape.... Is there a classical acoustical book that recommend that? perhaps but it is not orthodox ordinary practice in usual audio thread....

Also i dont have a mathematically measuring rod to objectively evaluate the final value of each of the device i tried or create.... It is my ears who was the judge of which will be the next road to take and which one to cancel....My goal was timbre instrument natural rendition with holographic imaging....i succeed and that was my goal....

Merry Christmas to you millercarbon and to all....





Tweaks aside you can get great sound and a great RT60 with plants, natural furnishings, a mix of natural diffraction and of course absorption ( which you can hide behind artwork, etc... )
see my system pictures of Casa Pacifica which achieves almost textbook perfect control room targets for RT60
and has plenty of WAF.
many of your cherished recordings probably exist because somebody cared enough to study acoustics and more...
By the way i am sure that you are right Tomic....It is better for WAF also for sure....I dont ask for anyone to take exactly the same road i did.... I only ask people to have faith also in their own creative power....

Merry Christmas to you Tomic....




There doesn't even seem to be any evidence that the traditional acoustic approach is any better.

When the flat earth society takes over your discussion, it's time to pack it in.
Maghister you might get something out of Olson’s book, my dog eared copy came from a library ( retired ) . A lovely book that covers entire rooms shaped as your corners. Merry Christmas to you as well !
MC is right....why would anyone want to implement basic acoustic principles like bass absorption and diffusion when they can alternatively just hang some crystals from the ceiling?  Afterall, what evidence is there that absorbing bass nodes diffusing first reflections actually does anything better?  lol   Seriously folks....you can't make up this stuff that MC says....thank goodness he does it for us and saves us from unnecessary acid trips.  

Look...I'm not dismissing anything that mahgister implements nor the results they may present.  But you don't implement them in lieu of basic acoustic principles....unless you're trying to sell something.  Hmmmm...

Agree with tomic601 here....Erik is offering "sound" advice.
Maghister you might get something out of Olson’s book, my dog eared copy came from a library ( retired ) . A lovely book that covers entire rooms shaped as your corners. Merry Christmas to you as well !
Thanks for the information pal....
But you don’t implement them in lieu of basic acoustic principles....
For sure you are right.... Basic acoustic principles about absorption, reflective surface, diffusive surface or volume, reverberation time all that is basic and useful knowledge.... Contesting that makes no sense....But even with the basic experts are not on all the same boat....Suffice to read a little to know that each acoustician after the basic has his own best road....

Then it is possible also to experiment and try resonators or modified S.G. etc I begin to try other experiments and devices after more orthodox materials passive treatment in my room...I succeed, then anybody who trust himself can....

My point is learn, be creative, and listen....Thats all....It is not to negate the basic.... It is not also necessary to buy costly supposedly indispensable branded name acoustical products....

My corner remedy cost 15 cents....


Sometimes one of my device were disconnected by accident, or by me for an experiment, and the result were the audible clues and values of each one of my experiments...

This I can totally believe, because the same kind of thing has happened to me several times over the years. Even one time had a party where a guy from work thought he would pull a prank like this he thought I would never know, only to watch me spot what he'd done and fix it in like 60 seconds and all by ear. Twice I have screwed up and put things in going the wrong way and people pretend there's no difference but both times I noticed the mistake and fixed it by listening not looking.  

However, I have nowhere near the phenomenal amount of stuff going on as you do mahgister! Muad'Dib!
All of you guys seem to know a lot more about this subject than I do, so let me tell you about my experience and you can ignore it if you want to.  I am on my forth listening room in 40 years with the first two being quite small, the third much bigger and my current one even larger.  Without doubt size matters.  My system has remained about the same over the years, but the soundstage illusion has improved dramatically, wider, deeper and more focused.  My room is U shaped with a brick fireplace and bookshelves separating the U.  My speakers are against the long wall of the U located five feet from the back wall and five feet from the side wall.  The room is 20 feet wide to the center divider of the U with the speakers eight feet apart.  My chair is about 16 feet from the back wall behind the speakers with another 12 feet to the rear wall of the room.  This puts any reflections at least 10 milliseconds behind the directly radiated sound from the speakers.  There seems to be something significant to the 10 millisecond minimum delay but I can't explain it.  My speakers are bi-polar radiators and that increases the reflected sound from back and side walls considerably compared to dynamic box speakers.  I experimented with various homemade flat panel absorbers and diffusers with little success.  I heard about acoustic wave traps or tube traps and made my own that extend floor to ceiling with 12 inch diameter ones in each corner and 9 inch diameter ones spaced every 3 feet against the wall behind my speakers.  What a difference!  The key seems to be that the tube shape increases the absorbing area by a factor of pi.  I wont bore you with anymore but consider this information food for thought.
Awesome, large irregular shaped rooms tend to sound great :-) glad you are getting great sound.  
Magister so right you are, some of what we speak is black art on top of science and because the ear/brain is involved the taste varies widely. No doubt you know of various big buck and ego performance venues needed a fix before or years after opening night.
scottdog, there is a time window of about 3 to 5 ms in which reflections will interfere with our sense of locating the original sound source. That's why getting speakers several feet out into the room and away from walls helps so much. Sound travels roughly 1 foot per millisecond so speakers 3 feet away the reflection travels an extra roughly 5 feet equals 5ms and there's the delay you need for imaging.

There's another longer one for sense of spaciousness but there's more to it than just timing the spectrum has to be right and it needs to be even. A slap echo at 10ms is just a slap echo. A nice broad spectrum smoothed out and dispersed in time 10ms is a nice sense of space. Odd shaped rooms with lots of different surfaces (fireplace, bookshelves) serves as a kind of natural diffuser. In other words, you lucked out.