Do bass traps produce noticeable audible difference? - Page 5

Sorry in my house the bathroom is a room.

and the acoustical energy doesn't behave the same way as it would in a Large Acoustical Space. fact.


there's really nothing more to say if you insist that a guitar amp produces reverb just because the decay "FX" knob is labeled "reverb". it's simply a label.
your guitar amp has a tremolo knob too, yes? tell me about the tremolo in your bathroom.
Edited by localhost127 - 3/1/13 at 4:28am

yet it is reverb. the fact that it doesn't sound like the inside of a church is acknowledged, nay celebrated, in my powt so suggesting itisn't like a different sounjding reverb is hardly saying anything, is it.

No I don't have trem on my amp, as it happens. ANd I am not aware of any room effect that would create tremelo. String players do it with their hands and their bows and the room figures not a single whit into what they do. Nor, for example, am I thinking I have an analog of a room effect when I turn on a chorus pedal or gain pedal. NOne of thoese are room effects so talking aobut how you cannot get them out of a room is like talking about how you cannot get a cadillac out of a sardine cannery. True, but so what?

Going back to your earlier post, moreover, you said "it's merely slang for acoustical reverb." which, as near as I can tell would mean we are using the word reverb as slang for what YOU call reverb. Which is an upside-down use of the word slang. What I think you are trying to say is that the non-local-approved styles of reverb are substitutes for or perhaps imitations of local-approved-reverb. Not a bad starting point, although I'll continue to suggest that it's the nonrandomness of the thing that generates the artistically desireable sound. Look at it this way, if I understand your POV< there's a room size below which the ringing or decay of a room can't properly be called reverb (presumably because RT60 isn't a useful measurement in such rooms.) Presumably, like everything else in real life, there will be a transition between random enough rooms to satisfy the local reverb definition and not random enough rooms. Aren't we arguing about where to draw the line when it's really a wideish gray area? How much does tata matter? I'm not suggesting, and don't know, whether RT60 is a useful measurement in my very oddly shaped room. But to my ears the decay from a clap or snap of the fingers is a (very short) reverb. It's not what I'd call an echo because it's there's too many reflections to pick out the "echo."

Anyway, done. Made my point and although I jsut wasted another few minutes of my life, I really don't care whether anybody disagrees with me because it doesn't matter either to them or to me.

Who am I kidding? I'm fascinated by all this stuff, so I went searching, albeit briefly.. This Acoustics 191 website defines reverberation without reference to randomness of incidence: http://www.acoustics.com/101.asp

"In an enclosed space, when a sound source stops emitting energy, it takes some time for the sound to become inaudible. This prolongation of the sound in the room caused by continued multiple reflections is called reverberation."

Similarly, FWIW, this site http://forum.studiotips.com/viewtopic.php?f=5&t=3316 says

"Keep in mind that only large rooms can have a "real" RT60 - medium and small rooms can't support a random field of reverberations - they are just to small."

What that DOES NOT say to me is that small rooms don't have reverberations, or that the reverberations in a small room aren't reverberations because they don't support a random field of reverberations. It just says those small and medium room "reverberations" aren't a random field. Again, not that they aren't reverberation, just that they aren't random. I can live with that.

AND BTW, FWIW, when I had amps with a tremelo function it was always mis-labeled "vibrato" in the great Fender tradition. The improper label did not convert the function from a volume-based effect (what tremelo is) into a pitch-based function (which is what vibrato is).

I would guess and nearly argue that the really great old studio plate reverbs, like you probably hear on thousands of recordings, likely can come pretty close to recreating the effect of a random incidence reverberant field. Certainly the modern digital convolution reverbs are amazing and gorgeous and once on a recording likely indistinguishable from what microphones would have picked up in the modeled space had the recording been made there. And I would never drop one of those big hall reverbs on the tail of the slow echo that usually accompanies a big David Gilmour guitar solo because it's TOO big and would tend to swallow what he's doing. I might want the long tail, but I likely want a more focused reverberation frequency response. SImilarly, I wouldn't normally put small band Miles in a big orchestral hall (or use a good convolution reverb for such a room on that music) because it would take away the intimacy of the smaller group recordings. Although at least for Miles himself, the big hall convolution reverb might work pretty well because he leaves a lot of space between his notes. But when Coltrane steps up to the mic, I think his flurries of notes might get washed out by the reverberation of the big room.

I think it would be very educational to see what you meant by the use of the word echo there. Thanks in advance.

Well put.

Informative

Useful

Can we have more of this, please? Moderators, you know what you should do.

HAve fun,
Frank

Agreed fully. Way to go Amir.

--Ethan

Reading through this again, you have done a fabulous job Amir explaining in plain English what is fairly complex. In particular:

I had seen the Topt option before, but never understood why it's better than T20 or T30. Now I do, so I learned something valuable today. Thank you.

--Ethan

Thanks for the kind words guys. I had fun writing it. Certain amount of satisfaction comes from proving to yourself that you can work through the theory using your own data .

It may be possible to find a big room that provides a more difficult-to-mic situation compared to a great large recording venue for capturing Coltranesque solos. However, I've recorded a wide range of acoustic and electric instruments in halls large and small and I can guarantee that a room large enough to qualify for RT-60 spec can also provide a great venue for recording sax, soprano sax, etc. When faced with a difficult venue where, as you say: "his flurries of notes might get washed out by the reverberation of the big room" I'd just optimize his position on the stage and get his horn closer to the mic. Sax and other horn players I've worked with for decades have never been bashful about putting the mic extremely close to (or inside of) their horn. That positional tweaking will alter the balance on the recording. In terms of unamplified sax in a large venue, I've heard incredible horn tone in big rooms when sitting in a range of audience positions. Empty large halls may be more problematic for the audience, but Coltrane would probably fill the room if we could bring him back for a little test session.
Edited by Syncamorea - 3/2/13 at 5:45pm

P[erhaps I wasn't as clear as I could be. Of course with a cardiod mic and close micing you ought t be able to get a decentsound in almost any room (even my basement) but you're not longer recording any significant sound from the room. Which means that the random reverberations that started my rant are excluded from the recording. So irrelevant, really. but I still would not drop a convolution reverb of a symphonic hall on a Coltrane (or even Sanborn) solo because while you can record them in a big room without capturing the room that's much different from saying you want the room ambience in the recording, which after all was my point and I thought I was even semitransparent about tha, at least for met.

And BTW, after my Eagles rant I had a serendipitous moment. The first Szymzyk Eagles album is On the Border. As It happens, while I could hear that was more two dimensional than the early albums, it is my favorite of theirs. The strength of the songs and performances overall overcomes the fact that the music sounds a little more like plastic than I might wish. Which led me to pull a tune from that album out of my @ss (even though I am pretty sure I never played or sang it before) and it kicked backside at a couple of jams this weekend. Nothing like 40 people dancing and 100 singing along (okay, not if you're performing a vioin concerto, but context matters . . . .) If only I had the royalties for Already Gone . . . .(which is not in fact my favorite tune from the album. That'd be James Dean, but it's not a jam safe song since it has more than 3 chods, was not a big hit, and especially in light of the fact that you're playing with folks who weren't born when the album came out. But that's another story for another day . . .
Edited by JHAz - 3/3/13 at 6:51pm

OK, I'll try this again... Take a look back at my first sentence above:

"It may be possible to find a big room that provides a more difficult-to-mic situation compared to a great large recording venue for capturing Coltranesque solos".

What I was getting at is that a great venue, large enough to qualify for RT-60 can have outstanding acoustics for hearing or recording even Coltrane. Just because it's a large room doesn't mean that it will be an echo or reverb chamber that overwhelms the music. Especially when properly designed, treated and loaded with a full audience.

Ah, I see. Well, I never said you would not apply such reverb to a Coltrane solo, or that any of the hundreds (at least) of recordists, mixers and producers who are better than me would not do so. I said I wouldn't I said why I wouldn't. That's all I intended to say. If I actually implied somethign broader, sorry. I'm not saying your approach lacks artistic validity, any more than I would tell the couple of very fine local guitarists who (IMO) turn their reverb up too high at most gigs are off track artistically. Just not my cuppa.

Too much travelling has gotten in the way of dealing with this last argument which is almost always quoted verbatim from Davis’ text: There are two parts to this: “well-mixed reverberant sound field” and “well beyond critical distance.” Let’s deal with the latter first.

Critical Distance
Critical distance is the distance from the source at which point the level of the direct sound matches the sum total of reflections. The direct sound is what comes to you directly from the speaker. The reflections are caused by the indirect sound bouncing off all the surfaces. Since reflections on the average have longer paths before they get to us, then their level drops more than the direct sound. At the same time, if we keep moving away from our direct sound, that too drops. At some point the sound pressure level from both of these become equal. That is the critical distance denoted by “Dc” not be confused with direct current which we write as DC.

Well Mixed Reverberant Sound Field
There was an assumption in the above explanation: the level of reflections stayed the same even though we kept moving away from our source. For that to be true, the reflections need to be all around us and not location specific. If that is the case, then we can never get farther from them and hence their level will remain constant. That is what Davis means by “well mixed reverberant field.” Another name for it is diffused field. Another is randomness of reflections, modes, etc. All concepts that I have explained before.

Computing Critical Distance
There are a number of formulas for computing Critical Distance. For now, let’s go with one of its simpler form: 0.03121*SQRT(Q*V/RT60). V is the familiar volume of the space. RT60 you already know and is the reverberation of time of the room. Q is the measure of directivity of the source. If the source is omnidirectional, Q=1 and can be eliminated leading to the more simplified version of this formula without it. Higher values connote a more directional source. For example the human voice has directivity of about 2 as your face somewhat projects the energy away from your face.

Our interest here is speakers which have frequency dependent directivity. While designs vary, in general speakers get directional at high frequencies. Conversely at low frequencies they tend to have broader radiation pattern. Determining the actual directivity of a speaker can be difficult if the manufacturer does not provide it. That said, our interest her as with RT60 is the mid frequencies starting at 500 Hz. At 500 Hz, speakers tend to have pretty wide dispersion. I am going to pick a rough (effective) estimate of 150 degrees horizontal and 100 degrees vertical. That gives us a directivity of 3.7. So while higher than a human voice, it is not hugely different. This is why a human can replace the role of a speaker in some ad-hoc room testing.

For my 21x11x9 dedicated theater, using a Q of 3.7 gets us a critical distance of 3.2 feet. For a human talker with a Q of 2, the distance is a short 2.4 feet. I don’t recall my exact measurement distance for that room but it was in the order of 10 feet. This puts the measurement mic at 3 times the critical distance for the speaker. This may not be “well past Dc” but sure is in the neighborhood since the direct sound is dropping at 6 dB/doubling of distance resulting in 9 dB total drop below the reflected sound. Its contribution to total sound arriving at our ear/mic is pretty low then. If that were not low enough, we have plenty more distance to step back into since our room length is 21 feet (~7X critical distance). That said, this computation may not be correct. See the next section.

Plugging in Ethan’s garage and using a Q of 2 because he was making some of those noises with his mouth, and an estimated RT time of 2 seconds, his critical distance is just 1.9 feet. With a room length of 24 feet, he could go way, way past the critical source if he wanted. So the notion that we have to be “well past Dc” in a classic sense is satisfied here.

Limitations of Critical Distance Computation
So far, we have lived in an idealized space. Real spaces, large or small, don’t work that way. As with the Sabine RT60 equation, as soon as you add any absorption to the room, you lose your diffusivity. Sound hitting those surfaces does not reflect back and even if it did, it would not do so in a flat way with frequency. Put audience in a large concert hall for example and all of a sudden the entire floor area acts like a giant absorber so even though that room is “large” it may have lost its diffusivity/randomness just the same. So technically speaking, rooms big or small if they have absorption in them, they are bound to show error in their Critical Distance computation as well as their RT60. Question then becomes how big this error is in practice.

How about Expert Quotes?
Both Dr. Toole and Davis were quoted as saying we do have a “small room” problem when it comes to these measures. Conclusion was therefore reached that we could never apply these measures to any “small” rooms, including such spaces as Ethan’s garage. Let’s review what they really said so that can see if that was right.

Don Davis’ Sound System Engineering
We were told to read chapter 7 of this book. If we do, we see this explanation:

Our work with the TEF analysis process has confirmed that the simple equations that work so well in “live” rooms should simply not be applied in any form in small dead rooms….In very absorptive rooms, the sound dies away in a few reflections, and the statistical basis of the formulas is weakened. In recent studies done with time energy frequency analysis, typical meeting rooms in hotels have been found in some cases, RT60 = 0.5 s, to develop no reverberant sound field, whereas in others, RT60 = 0.7s, a field barely appears. Our experience with time-energy-frequency measurements causes us to state unequivocally that recording studio control rooms are not proper subjects for use of classic statistical equations.

The emphasis that I have added should already tell you what is wrong with the conclusions drawn. But let’s spell them out:

1. It is absolutely clear that Davis is railing against the simplified formulas, not measurements. I hope you remember the distinction from my last post. Measurements are immune to many of the errors of simple Sabine equations since they measure the actual sound level at our seating position. For example, lack of diffusivity means that we don’t have the same sound level in every place in the room. But we don’t care about every location. We care about where we sit and the mic picks up the sound level there. That measurement does not rely on us having the same reading elsewhere. Nor does it care about the speaker spec. If we change the Q of the speaker, it will be reflected in what the mic “sees” in our location of interest. That is very different than us plugging the wrong Q into the critical distance formula.

2. There is no better proof of #1 than seeing Davis use RT60 *measurement* himself! And what does he use it for? To determine whether a room is reverberant or not. Well, that is precisely the use we have for RT60. The fact that at 0.5 RT60 time the room loses a lot of its diffusivity/randomness is understood and indeed is the reason we want to add additional absorption: to get rid of excess reverberation/late reflections.

3. Davis acknowledges that reverberant field starts to appear at RT of 0.7. I gave a simple example of my room which with carpet and two rows of seating had similar RT. Clearly then it is possible to have residential spaces that cross the threshold into having a reverberant field. Ethan’s garage likely has RT60 times of ~2 seconds. If 0.7 is onset of the reverberant field, then surely 2.0 seconds is way into it being there.

4. Davis is using “frequency blind” RT60. As I showed in my last post, we can look at specific mid-frequencies and eliminate the low frequencies. In doing so, we actually arrive at more accurate metric then he did because it does not vary as much due to changes in location. This is useful in home theater design where we want to know how every seat measures, rather than just one.

5. Davis reserves the strongest words against use of Sabine formula for “recording studio control rooms.” As a rule, folks in that space, especially in the 1980s when he wrote that book, were strong believers in eliminating first reflections. Popular concept such as Live-end, Dead-end (LEDE) for example attempted to eliminate the reflections from the front of the room. If you substantially reduce the power of the first reflections, the rest do die quickly as he states. So it is not surprising that Davis did not find a reverberant field in such rooms. And how did he confirm that? With measurement: “our experience with time-energy-frequency measurements…”

Summarizing, it is abundantly clear that Davis is admonishing us from using simplified formulas to characterize small dead spaces. That does not apply to Ethan’s garage that started this argument. His room is anything but dead and he used his ear which is very different than using a formula. Nothing is being said about small rooms automatically invalidating RT60 measurements since he himself uses this measurement.

Dr. Toole
As with Davis’ text, critical parts of Dr. Toole’s writing were left out which shine a proper light on this topic. From his book, Sound Reproduction:

None of them [sample of rooms examined in the referenced paper] exhibited isotropic distributions at the measurement locations. Strong directional features were associated with early reflections. Small meeting rooms and a videoconferencing room with reverberation times of 0.36–0.4 s, like listening rooms, had anisotropy indices and directional diffusion measures that fell roughly halfway between anechoic and reverberant conditions. Moreover, the values changed with time, with later sound showing increased anisotropy and even changing orientation in the room according to the surfaces that were more reflective (Fig. 5).”

Once again we see that RT60 measurement is not only believed, but actually used to characterize how reverberant the room was – precisely what we use it for. The fact that the room was not isotropic (sound arriving from every location equally as I explained for Critical Distance) did not render RT60 invalid. It would have rendered the formulas less accurate.

Furthermore, as I have explained, it is very true that once you add absorption to get the RT down to recommended level, in this case in the 0.4 range, then your room stops having substantial amount of reverberation/diffused field. But our rooms if they are empty, and Ethan’s garage certainly, don’t start there. Again, my room even with inclusion of carpet and chairs which added substantial amount of absorption due to their large areas still had RT times 0.7 And Ethan's garage much more. These are not not the scenario that Dr. Toole is covering above.

If we wanted to reach into literature, we could find countless references (including some peer reviewed ones) where RT60 is used. Here is an example referenced by Dr. Toole in his book: “Acoustical Measurements in Some Canadian Homes,” Canadian Acoustics, Bradley, J.S. (1986):

The mean measured receiving room reverberation times and a one standard deviation range are given in Figure 7. In all 1/3 octave bands the mean reverberation time was close to 0.4 seconds with a standard deviation of just under 0.1 seconds.

The above was the mean and deviation of 602 homes surveyed in Canada. Clearly then RT60 is being used to characterize “home” and “small” listening spaces.

BTW times have changed relative to when that study came out. Trend these days is toward a lot more hard surfaces including the key elimination of carpets in favor of hardwood floors. If your room falls in that category, then it is likely to be very “live” with RT60s confirming the same. Here is another personal example in the form of our living room:



RT60 is a whopping 1.7 seconds at mid frequencies. Sadly it sounds that way too. Speech intelligibility is very poor. Get past 6 or 7 feet and it starts to get hard to understand someone over say the background level of TV playing. And forget about understanding them from adjacent room. The room is very bare as this is our vacation house and we have not put much furnishing in it. If we did, we would have this type of experience outlined in Dr. Toole’s book:



Once again we see RT60 measurements being used by the very experts who were portrayed as telling us to not use them. Notice how adding furnishings which act as absorbers, the RT60 time nicely tracked with them. If RT60 generates meaningless data, it should have shown random changes. It did not. More absorption = Smaller RT60 times, precisely what the Sabine formula predicted. Mind you, the changes may not be proportional to exact specs for those “absorbers” but it did track them generally. We don’t need more precision than that.

What does it mean to you
There is little need to measure the critical distance in home listening spaces. There is however usefulness in using RT60 to determine if your room is too live or too dead. You have seen examples of this from the experts quoted and if it is good enough for them, it certainly is good enough for the rest of us .

Now, if you don’t have measurement tools that is OK. You can do as Ethan did in his garage and use your ears and convenient noise source. Have someone stand where your speakers are and clap their hands. If you hear a long “reverb trail” , then your room is too live and needs general absorption. That can come in the form of furnishings such as carpets, drapes, etc. or purpose built absorbers. It matters not where you put them since the aim is to reduce general reverberations, not specific early/strong reflections. Note that you don’t want to clap yourself as that would mean the speaker is in the same spot as you which is not how it is in real life.

So as you see, no matter which way we approach the topic, the results are the same: RT60 measurement works and is a useful tool to estimate the amount of late reflections in the room. While the formula version can generate less accurate results (in both small and large rooms), it too can be useful if for example you have not yet built your room and want to estimate how much total absorption you should have. Dr. Toole gives an example of this in his book. Hence the comment: "In the meantime, the simple Sabine formula provides estimates that are adequate for our purposes in small listening rooms."

Wow Amir, that's an amazingly compelling explanation, presented so logically I can't imagine anyone could refute it. Right off the bat this popped out, with the emphasis on "dead" rather than "small":

Now, let's look back at my original post about this:

So why did it take 100 additional posts just to reach the same conclusion?

Thanks Amir, you totally nailed it.

--Ethan

I was going to post this a few posts back. Well maybe more like about a hundred posts back. Anyway, I have a feeling my room is too dead. I have Linacoustic on the walls up to a height of 47" and the back wall is completely covered with Linacoustic from floor to ceiling. I also have a tiny room (about 9' x 12.5'). I have put in 4 DIY bass traps that are 4"x24"x24" at the 4 corners on the floor.

Here is my Topt as measured by REW with the front L and R speakers:

the book-report is a nice and all, but upon reading your commentary i don't think you quite grasped what the concept of "critical-distance" even is ...


i would love to be in any position in your room that is considered at Dc or past Dc (let alone "3.2ft from your speaker" !!!), but sadly it simply doesn't exist (contrary to your wild claims above).

as for the rest, well .. it's clear that if you fundamentally misunderstand what "critical-distance" is and what it represents then the rest is assumed misunderstood as well. explains the difficulty of the discussion of acoustics (not "ball-parking").
Edited by localhost127 - 3/5/13 at 5:32pm

Are you ****** kidding me? Please don't feed the beast, many members are here to stroke their own ego through writing what they have learned, read and experienced with the satisfaction of simply posting. While others are will...don't feed it.

It may be but you are better off to look at the decay times as that is the major problem in smaller rooms. Also you want to view the ETC for early reflections.
If you would like, save the REW file, zip it and post it here (if you can't you can email me though my company website) and I can take a look through my own REW. Also when you test you want to do one speaker at a time.

More about understanding decay times.
http://gikacoustics.com/understanding-decay-times/

The heart of the matter... technically, and a great way to end another clearly understandable, relevant post.

Too bad your detractors are unable to respond in kind, even the "professionals" who decrie the poke/stabs and grandstanding, even as they support the statements many of those posts contain.

"You can tell a man by the company he keeps."

Have fun,
Frank

Describe your bass traps in greater detail.

This is what I would expect with a large area of thin broad band absorbers combined with thick broadband absorbers designed to extend bass absorption to lower frequencies.
- remove a lot of the Linacoustic from the side walls, and treat the front wall with some of it
- replace some of it with diffusers on side walls
- replace/alter the DIY traps so they only absorb bass frequencies and don't augment other treatments.
- add waterfall measurements to your Topt vs frequency, so you see individual bass resonances and so can insure your DIY traps are effective at those frequencies.

The curve shape is not bad, it's just too low, and the measurement doesn't extend far enough at the low end, nor with enough resolution to see individual modes.

Have fun,
Frank

Just make sure that you are still covering the early reflection points. You could also use a slat design over them to retain/scatter upper frequencies.

Agreed. Just not in the early reflection points.

Are you talking about something tuned? Yes that can work, but opens up a whole new understanding of treating the room. You could use something like FRK on the front and or a slat design over top of them.
Edited by myfipie - 3/6/13 at 11:28am

Thanks, Glenn. I will take a look at ETC and decay time and I may just take you up on that offer to look at the REW files. At this time I'm trying to get the best location for my sub so I'm concentrating on the low end. I've done some ETC/ string method but never got to a final result.

They are 4" thick x 24" x 24". They are made using OC703 (2" thick doubled up). They have a 1"x2" on the back side for spacing although at this time they are just leaning in the corners. They are wrapped with speaker cloth.

Lots to digest here. I need to do some homework and more measuring!

I sure will try! Thanks.

I don't think your decay times are too short given the small size of your room. But your overall treatment is less than ideal. Absorption on the side walls should be at the specific reflection points. It should be centered vertically at ear height, and extend at least a foot above and below, if not two feet. Absorption is very useful on the rear wall, but it should be very thick - at least four inches. If this were my room I'd have ten (or even more) corner bass traps.

--Ethan

I'm getting some good advice here today. So, you're saying I should have 4" on the back wall. Would that be the entire wall or just the reflection points. I'm assuming that a 4" panel, as I described previously with an air gap (say 2") would do the trick?

Remind me to send you a bill. [kidding!]

I'd do the entire wall, or perhaps some in the center and some in the corners. Sound spreads outward from speakers, so by the time it hits the rear wall it pretty much covers all of the wall. Further, you especially want thick absorption in the rear corners, which you wouldn't have if you did only the center area behind your head.

--Ethan

I would go with a 4" gap if possible. Even better make them 6" with the 4" gap. Also you will need more then just one. Honestly though the ones in the corners are going to have the largest impact on the low end.

so does this imply you are in full agreeance with him? is there a critical-distance in your living room? do you agree with his claim that: "the notion that we have to be “well past Dc” in a classic sense is satisfied here." ?

the silence is deafening...

it's comical to see the onslaught of copy-paste google searches to distract from previously made erroneous claims (and fundamental misunderstandings of the terms), only to once again fall into the same trap - this time with his claim regarding critical-distance "3.2ft" from his speaker. you cannot make this stuff up. mickey mouse acoustics.

sorry, but i'll side with Toole on this one.

he still can't even provide us with the "conditions for reverberant sound-field", as asked since page 1 of this very thread. the best he could do was quote dragonfyr.

Edited by localhost127 - 3/7/13 at 5:31am

it appears you and amirm don't quite agree on using absorbers at side wall reflection points.