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post #181 of 191 Old 03-12-2013, 10:21 PM
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Originally Posted by GregLee 
Your explanation of this example is incorrect, regardless of the outcome of an experiment with reverberation.
Yeah, I think this point has been lost in the back and forth. I follow your reasoning and it seems intact. Amir's explanation of the particular example is incorrect, even if the impact of late reflections on intelligibility he describes is correct.

Both things can be true at the same time. Thanks for the interesting explanation. Language was never my strong suit. smile.gif

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post #182 of 191 Old 03-13-2013, 12:56 PM
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Originally Posted by audiophilesavant View Post

Do you have a cite to the field experience to which you are referring.
Sure. There was one in the same page you posted the question in! smile.gif http://www.avsforum.com/t/1453370/do-bass-traps-produce-noticeable-audible-difference/150#post_23053784. From Nyal and Jeff's paper referenced within:

"D: Midrange Decay Times T20, T30, T60 between 0.2 and 0.5s"

Unfortunately the other references I have are from presentations by and discussions with major industry experts which are not in public domain. I did some quick looking and found a few references you can access. If you search I am sure you will find more:

Dennis Erskine: http://www.avsforum.com/t/332289/rt60-what-is-a-good-value#post_2993755:
"I try to keep it [RT60] in the range of .35 to .40

Dr. Toole in his excellent book, Sound Reproduction, Loudspeakers and Rooms: http://www.amazon.com/Sound-Reproduction-Acoustics-Psychoacoustics-Loudspeakers/dp/0240520092/ref=sr_1_1?ie=UTF8&qid=1363204226&sr=8-1&keywords=toole+sound+reproduction

”The only reason to measure RT in a small room is to be certain that it is not excessively high (over about 0.5 s) to preserve high speech intelligibility or low (under about 0.2 s) to avoid oppressive “deadness.”

EBU 3276 standard: Listening conditions for the assessment of sound programme material: monophonic and two–channel stereophonic

”The reverberation time is an important characteristic of the reverberation field; it is defined as the time taken for the sound to decay to 60 dB below the initial level. It is usually measured over the range from 5 dB to at least 25 dB below the initial value. The decay time of the measuring instrument and the filters should be shorter than the decay time of the reverberation field. The reverberation time should be measured in the listening room with 1/3rd octave filtering [1] using the listening loudspeakers as the sound sources.

Reverberation time is frequency–dependent. The nominal value, Tm, is the average of the measured reverberation times in the 1/3–octave bands from 200 Hz to 4 kHz.

The nominal reverberation time, Tm, should lie in the range: 0.2 < Tm < 0.4 s”


ITU-R BS.1116-1 standard: METHODS FOR THE SUBJECTIVE ASSESSMENT OF SMALL IMPAIRMENTS IN AUDIO SYSTEMS INCLUDING MULTICHANNEL SOUND SYSTEMS

” 8.2.3.1 Reverberation time
The average value of reverberation, Tm, measured over the frequency range 200 Hz to 4 kHz should be:
Tm = 0,25 (V / V0)^1/3 s where:
V: volume of room
V0 : reference volume of 100 m3”


100 m^3 is ~3,500 cubic feet. For that, you would get 0.25. Since speech intelligibility improves down to 0.1 seconds or so, and BS 1116 is designed for testing such things as speech codecs, they err more on the side of being dead, i.e. lower RT60 times.


Hope this suffices.

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post #183 of 191 Old 03-13-2013, 01:44 PM
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Originally Posted by GregLee View Post

I don't know what one could tell in the presence of reverberation

This is actually pretty easy to test for yourself using just a microphone and a few basic audio tools.
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I am confident that if you operated on recordings to snip away the final consonant portions of "cat" and "cab", so the consonants were entirely absent from the sound, in the absence of reverberation, listeners would be able to distinguish the two correctly.

This is even easier to test. It's obvious that you're interested in this stuff, so I suggest downloading one of the popular freeware audio editors (if you don't have such already) and do some experiments. It's a lot of fun! I can't see how anyone could distinguish cat versus cab if the last part was edited out, though I never actually tested that. However, it's well known that it's difficult to distinguish a violin from a flute if the initial transient is removed. Also, the McGuirk Effect shows that knowing what we think we're hearing is easily compromised:

http://www.youtube.com/watch?v=G-lN8vWm3m0

This not exactly the same as cat versus cab, but it's related.

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post #184 of 191 Old 03-13-2013, 02:47 PM
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Originally Posted by Ethan Winer View Post

It's a lot of fun! I can't see how anyone could distinguish cat versus cab if the last part was edited out, though I never actually tested that.
I'm sure it would be fun for someone with an empirical bent. That's not me, though. Anyhow, the relevant facts are well known.

Your intuition about "cat"/"cab" is not surprising, because not only is the difference made in the final consonants in conventional English spelling, but also in the phonemic forms of these English words. The phonemic form of a word is a theoretical construct that represents the way a pronunciation is perceived, and here, the pronunciations are perceived by English speakers as differing in the final consonant. But this doesn't make it a true fact about the actual pronunciations as they can be measured by instruments. In this case, it turns out to be more false than true.

It's not that you can't make a distinction at all between the final "t"/"b" if you work at it, but rather that you don't need to. In casual conversation, final stops "b"/"d"/"g" are often said without voicing (a natural tendency that is carried further in German), and those plus "p"/"t"/"k" are often said without a burst of air at the end, a "release", that can carry audible information about the difference among labial/alveolar/velar sounds.

Once these very common phonetic changes are made to "cat"/"cab", there is no way in principle of distinguishing these words by listening to the final consonants, because there is no sound there at all. After the end of the vowel, no air is passing through either the nose or the mouth (since by hypothesis we're considering a pronunciation with "b" devoiced and with no releases at the end), so there is no way to make noise during the articulation of the consonants.

Yet English speakers have no trouble at all "hearing" the "t"/"b" at the end, even though there may be no sound there at all. It's surprising.

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post #185 of 191 Old 03-15-2013, 10:02 AM
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Originally Posted by GregLee View Post

Your explanation of this example is incorrect, regardless of the outcome of an experiment with reverberation. I am confident that if you operated on recordings to snip away the final consonant portions of "cat" and "cab", so the consonants were entirely absent from the sound, in the absence of reverberation, listeners would be able to distinguish the two correctly.
In the absence of reverberations? That is like me saying in a car crash your seat belt can help reduce injury and you disagree by prefacing: “putting aside an accident, the seat belts can’t reduce injury!” biggrin.gif I will take on your test but I really find it surprising that you want to keep arguing about a point that is absent the condition all of us care about: real sounds in real rooms (with reverberations).

Experiment
I recorded the word ”cat” on my laptop and then chopped off the ending consonant as instructed. This is the resulting file. Please take a listen and see if you can still tell it is cat or not: http://www.madronadigital.com/Downloads/RTSpeechTests/foo.wav

Not wanting to wait for you all to vote smile.gif, I ran the test blind by my three family members: my wife, and two college going sons. I asked them what word was being played using headphones with no other hints whatsoever. All three without hesitation said “cat.” I also tested myself sighted and heard “cat.” I then emailed the link to someone and their vote (with previous knowledge of this argument) was that he could not tell. So the score is 4.5 in favor of Greg’s argument, and only 0.5 in favor of mine. Oh boy…. smile.gif

But Greg has a serious problem on his hands because the word I truncated was not cat but cab!!! biggrin.gif Here is the original file before truncation: http://www.madronadigital.com/Downloads/RTSpeechTests/cab.wav

Now the scores are 4.5 in my favor and Greg is left with 0.5. tongue.gif Seriously, it should be abundantly clear that the ending consonant was playing an important role and taking it out seriously damaged our discrimination between the two words – precisely the reason I picked that word pair.

Here is the really fascinating part: with full knowledge of what I just explained, i.e. biasing you to vote the right way, listen to the truncated clip again. I bet if you let yourself hear it, you may still perceive the word incorrectly as “cat”!!! The ambiguity is so strong that your logical mind cannot override it all the time. I created the darn clip and even I am not immune to this: I can easily convince myself either word is being said. There is also a lesson here with respect to reliability of our audio perceptions, issues with sighted testing. But we digress.

It goes without saying that you can try to chop off the word in a different place and the outcome may be different. That fact actually indicates that the test as proposed is faulty and runs against simple rules of linguistics. Namely the fact that the transition from vowel to consonant is smooth and hence there is no clear break point where you can separate the consonant. For this reason, formal research in this area tends to avoid words with formant transition.

Discussion and Further Research
Greg in his last post puts forward that since we are able to understand a speaker even if he truncates the consonants, it must therefore be the case that the rest of the word was a complete predictor of it. While the premise of what he says is true, the conclusion is overly broad and not supported. Our comprehension is better than it should be because our communication channel has fair amount of redundancy if you include the full set of information being conveyed. If I am holding a cat and try to hand it to you while saying “take the ca” without the ending “t,” you still understand it. That is not because the “ca” led you to think I said “cat” but because of visual cue of me offering you the cat. I could even skip the word cat and you would still understand what I am saying.

Sure, if you heard “ca”, you would know I am not saying “dog” so it is helpful that way but it is not a reliable predictor.

We have many such clues from lips moving, to context, to people’s expressions, etc. It is this collective set of hints that let us understand chopped off words. Such hints help us understand speech very well even when as much as 20% is lost/not heard! Unless all of these factors are taken out, you can’t conclude cause and effect as Greg has attempted. The proof point is simply insufficient to make this case.

Note also that when someone teaches linguistics at school, the focus is on elementary research devoid of real world conditions we face. Very clean voices, recorded well and listened to in quiet, non-reverberant spaces are used for example. Such is not the case for us. Dialog in movies for example is rarely without background music, noises, special effects, etc. Likewise, if you listen in multi-function spaces, noises from everyday living and adjacent spaces help reduce our effective signal noise ratio. Now add to it accents, not being a native speaker, getting older with less than perfect hearing, etc. and you see how what we face is a degraded speech comprehension environment. This is why it is important to design our listening spaces such that they don’t add to the problem.

Going back to our topic of interest, room reverberations are a form of “self-noise” as they raise the residual sound level in the room, and therefore, degrade any component of speech which plays at levels that are similar or lower than it. As I post before, research shows that the most damaging aspect here is with regards to consonants. Vowels in contrast tend to not suffer nearly as much. This factor so important that a specific metric was created for it called ALcons (Articulation Loss of Consonants), created by research that Peutz performed in 1970s as summarized in this excellent AES paper, What You Specify Is What You Get (part 1):

”With Articulation Loss of Consonants (ALcons) only the wrongly understood consonants are counted. Peutz found that for speech in rooms the vowels are much easier understood than consonants and hence the loss of consonants are the deciding factor in speech intelligibility. ALcons is expressed in %. Under perfect conditions (speech direct on headphone) a combination of a very good speaker and a very good listener will have an ALcons of 2.5%. In excellent room acoustical conditions they can have on top of that 5% ALcons or less. An extra 5% loss is still considered as good and another 5% extra loss is still considered fair and sufficient for most messages. The initial 2.5% is considered the zero correction or proficiency factor. Which target to set for a certain situation depends on the proficiency (to be expected) of the talker and the listener.”

Computing %ALcons for my theater example which had an RT of 0.7 is 9% which is outside of the recommended range of <7%. Recall that our recommendation for RT60 was to not exceed 0.5. We exceeded that in my theater and the result is higher potential for loss for consonant comprehension.

For grins, I computed the %ALcons for Ethan’s garage and got a whopping 26%!!! eek.gif It probably makes for great pipe organ music but don’t try to have a kid’s play in there smile.gif.

Here is a very nice visualization of the effect of noise and reverberation on speech by Professor Boothroyd, of SDSU based on a model created from listening tests:

i-MrZXTSj-L.png

There are a number of graphs there. One is for intelligibility of “CVC” constructs which stands for consonant-vowel-consonant words – like my example of cat and cab. The other two are for word recognition in easy and difficult sentences which is yet another vector/metric of comprehension (remember my example of “take the cab” or “take the cat:”).

The top left quadrant is a quiet room with reasonable reverberation of 0.5. The Quadrant on the right keeps the room very quiet but ups the reverberations to around what my living room is at around 1.5. There we notice a sharp drop in comprehension of words even in easy sentences. Indeed the result is pretty similar to bottom left quadrant where we keep the reverberation low but raise the noise floor from 35 db to 60 db. Put another way, upping the RT60 time resulting in a whopping loss of 25 dB in our signal to noise ratio!

Of course all hell breaks loose when we combine noise with high reverberation times in the right bottom quadrant. Note again that we have the equiv. of “noise” in our movie content with respect to other background sounds. You can try to sit closer to the source to compensate as the graph represents but that won’t change the noise level since our “noise” can be in the content.

Bringing the lesson home, listen to this “convolution reverb” of my cat-cab example. If you don’t know what that phrase means, convolution reverb is a process by which we profile a room, in this case an “empty living room” using an impulse response and then by multiplying (convolving) that with any audio clip, we can make it as if it were in the room we profiled. The accuracy is not perfect and depends on how good the impulse response is but is close enough for this exercise. Here is the file with that reverb added to it:

http://www.madronadigital.com/Downloads/RTSpeechTests/cab-cat-repeated-Reverb.wav

You likely hear mix of “cat” and “cab”, right? If not, listen again.

Guess what? I only said “cab” in that clip! Indeed it is the word “cab” in my last example simply concatenated four times:

http://www.madronadigital.com/Downloads/RTSpeechTests/cab-cat-repeated.wav

Listen to how much clearer it is without the high reverberations of that empty room applied to it. Once more with full knowledge listen to the reverb version and I bet you can still hear “cat” in it! This strange brain we have smile.gif.

What does it mean to you
There is uncanny resemblance between this argument and the last. As with the concept of reverberations in the room, what is being quoted from textbooks is not wrong. It simply is misapplied to the topic at hand. How do we know that? Well, we confirm our understanding by actually performing the test Greg suggested. If the results agree, then we know we understood the science well. It didn't in this case. It is also critical that we triangulate our knowledge by reading more than one text to make sure we are not applying what we have learned to a broader situation as occurred here.

With respect to what to do in your room, nothing has changed: if you have difficulty understanding speech in movies for example, one factor may be an overly “live” room. Measure RT60 and look at mid-frequencies. If it is above our target range of 0.2 to 0.5, you need to put additional absorption in the room. Such absorption can be ordinary furnishings such as a carpet on the floor or dedicated products. Location of the absorber is not critical but if given a chance, combine with control of other acoustic issues such as floor bounce (i.e. put carpet on the floor). Otherwise, you may wind up with too much absorption when the job is done.

Note that there are other causes for speech intelligibility such as poorly recorded content, too little early reflections, poorly designed center speakers, and hearing loss. If your room is already in that target RT range, especially if it is at the lower end of that scale, then the problem is one of these.

Edit: someone please teach me how to spell right. biggrin.gif

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post #186 of 191 Old 03-15-2013, 10:50 AM
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Originally Posted by amirm View Post

Experiment
I recorded the word ”cat” on my laptop and then chopped off the ending consonant as instructed. This is the resulting file. Please take a listen and see if you can still tell it is cat or not: http://www.madronadigital.com/Downloads/RTSpeechTests/foo.wav
I couldn't tell, trying to answer this exact question. However, if asked what word it sounded like, I'd have said "cap".

Reading on and listening to your cab.wav, I wondered if you might have clipped too much off the end of cab.wav to get foo.wav, removing the last part of the vowel as well as the final consonant. In cab.wav, I could hear a falling pitch toward the end of the vowel which seemed to be missing in foo.wav.

Actually, as I recall, what I said earlier was that I was confident people would be able to hear the difference between "cat" and "cab" with the final consonants removed. That is not exactly what your experiment is testing. And such experiments are usually done putting the test words in a frame that is more like what could occur in natural speech.

It's great that you're putting this all to the test.

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post #187 of 191 Old 03-15-2013, 12:16 PM
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Originally Posted by GregLee View Post

if asked what word it sounded like, I'd have said "cap".

rofl3dbig.gif
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It's great that you're putting this all to the test.

Yes! I wish more people would test the stuff they argue about rather than rattle on for weeks. biggrin.gif

(I don't mean you Greg, but rather others who hold strong beliefs that could be disproved in two minutes with a proper test.)

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post #188 of 191 Old 03-15-2013, 04:09 PM
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Did amir just become an overnight expert in yet another field? I'll never understand why someone would speak as if from a position if authority after a night of Googling.

But its an interesting topic. I'll listen to the clips at work later.

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post #189 of 191 Old 03-15-2013, 04:25 PM
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why is the default position whenever amir does a 'body of work', as in his last post for example, to criticise and or demean? There seems to always be attributed to it some sort of ulterior motive.

I mean I don't know the guy at all, maybe he DOES only do it to 'win an argument' because he is pathologically incapable of being in error, but maybe, just maybe, he has an interest and is willing to investigate?

Agree or disagree, view it rationally or with bias as a result of personal feelings toward him, but thank god that we move away from armchair theorising. ****,. we lambast the audiofool who will argue for twenty pages about how dbts or their results in no way show that for most (amps/people/situations) amps are not that different, we urge them to shut the fvck up and simply do the damned test. Then they might have an inkling of what is being discussed.

Yet, when amir does it (gets off his arse and tests something, which most of us don't do) we become snide.
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post #190 of 191 Old 03-15-2013, 10:07 PM
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Originally Posted by terry j View Post

why is the default position whenever amir does a 'body of work', as in his last post for example, to criticise and or demean? There seems to always be attributed to it some sort of ulterior motive.

terry, it's called "tone." There's a way to say things, and a way not to say them. The transmission of facts and message may be essentially the same, but the reception can be drastically different. I have tried to make this point to people like dragonfyr and localhost as well, who have gobs of knowledge and experience to share, if only others were willing to listen.
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I mean I don't know the guy at all, maybe he DOES only do it to 'win an argument' because he is pathologically incapable of being in error, but maybe, just maybe, he has an interest and is willing to investigate?
The two are not mutually exclusive. I have no doubt that Amir likes to help people, and it is commendable the effort he will put into researching a topic in order to "share" information. It is also no doubt that his "sharing" often becomes "preaching" and quite defensive if anyone questions a single word he has written.

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post #191 of 191 Old 03-16-2013, 04:18 PM
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thanks bigus..you got me at a disadvantage as I don't know your name, unless bigus is your given name and your surname is the same as on The Life of Brian? biggrin.gif

Tone is a good word, esp in an audio forum no? And as we all know, one persons tone is another's horrid noisy mess haha.

Anyway, I am certain you have the point I was making, all too often lingering upsets or past disputes colour the reception. When that occurs the baby is often thrown out with the water.

It worsens when the pack mentality kicks in tho.

cheers
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