[JohnR]

OK... please clarify what is RC30/20 etc and how it can be measured.

[Jeffrey Hedback]

Hello JohnR,

Room Criteria measures (or sets standards) for background noise in an environment.  I covers from 16Hz to 4,000Hz with a built in trigger (if certain regions exceed the curve) to highlight low end rumble and high end hiss.

RC20 ~ equates to a dBA rating of 30 and RC30 to 40dBA.

It is not easy to measure.  However, the acoustical software ARTA does measure RC.  It requires a calibrated (low-noise) measurement mic like the Earthworks M30.  It requires a low-noise preamp.  I'm sure there are dedicated devices in the HVAC field that measure only these criteria but I do not have a brand.

Better?  It's not something that I generally measure in field.  It is something that I always include in design specs of ground-up spaces.

[JohnR]

OK, thanks, I will look into it more. But I think there may be some diffusion of purpose here. The OP by Nyal says:

These questions and more have come about because of the cool, cheap and accurate measurement products from XTZ Room Analyzer, Room EQ Wizard and Dayton Audio Omnimic. Whist these products allow you to measure your room they do not provide any guidance on how to interpret the results relative to the audiophile situation of two speakers in a room.

I think there may be some conflict here in defining a standard for audiophiles vs defining a standard for acousticians. Is that over-analysing it?

[Jeffrey Hedback]

I'm not sure on your point JohnR.

On one hand, the paper has exposed you to the term RC and if you had/have issues of background noise you could hopefully do more productive research and have more fruitful conversations with specialists to remedy.

On the other hand, I get that there is a certain level of assumed knowledge.  I'd be curious of your reaction to our abstract on page one and if you feel we met the stated goals.

Thank you very much for your responses.

[Nyal Mellor]

Hi all,

Your comments are all very valid. I do agree that the paper is lacking sources and references, and these wold be a useful addition. Something for a later version definitely. The more 'expert' readers (fellow acousticians and other people knowledgeable in this area) should be able to see the links back to many things including a huge body of research on the perceptual effects of reflections (as summarized nicely in Dr. Toole's book). There would definitely be value in adding 'layering' to the paper for people who want to understand the rationale behind each standard. I do not think it would be worth adding these rationales to the paper, since they are covered in other literature.

Our communication style for the paper was based on different people needing different things. I like to use the 'three levels' of listening: following, detaching and fluent.

People in the following stage look for one procedure that works. Success is measured by whether the procedure works and how well they can carry out the procedures. The paper is targeted at people who have the measurement equipment but now need a procedure to follow for how to examine the measurements and what they should look like. This is the audience we have targeted.

In the detaching stage people locate the limits of the procedure and look for rules about when it breaks down. Here they are looking for alternatives, the other 10 ways of examining the measurements and learning when to apply each one.

In the fluent stage it becomes irrelevant to the practitioner whether he/she is following any procedure at all. Their knowledge has become integrated through a thousand thoughts and actions. They understand the desired end effect and simply makes it that way. They are like the Jedi 

Now, if you following this thinking through, you will see that a fluent practitioner would not need a white paper. I agree that in future versions we should make the white paper 'deeper' so that those wanting to learn WHY we state certain things can further their understanding by tracing the arguments and data back through to their root. This would make the paper more useful for those in the 'detaching' stage.

I hope that makes sense!

btw on the RC topic - RC Mark II is the standard adopted by CEDIA in CEB-22. Another suitable measurement would be NCB which seems to have more adoption on the pro side of the world. A quick google search will bring up more than you could ever want to know on NCB or RC Mark II, for example, if you don't fall asleep after reading page 2 of this then you are on the path to becoming a Jedi of noise measurement http://www.cavtocci.com/pdf/tocci.pdf

btw2 personally speaking I think that most of us remain in 'detaching' for 99% of our adult life and that it is a good place to be. Just because someone is fluent does not mean they are a good teacher. Just look at the way Yoda speaks and you'll get my drift!

[Nyal Mellor]

OK, thanks, I will look into it more. But I think there may be some diffusion of purpose here. The OP by Nyal says:

I think there may be some conflict here in defining a standard for audiophiles vs defining a standard for acousticians. Is that over-analysing it?

Hey John - these were intended to be standards for audiophiles. Not acousticians!

[JohnR]

Hey John - these were intended to be standards for audiophiles. Not acousticians!

Cool, thanks 

btw on the RC topic - RC Mark II is the standard adopted by CEDIA in CEB-22. Another suitable measurement would be NCB which seems to have more adoption on the pro side of the world. A quick google search will bring up more than you could ever want to know on NCB or RC Mark II, for example, if you don't fall asleep after reading page 2 of this then you are on the path to becoming a Jedi of noise measurement http://www.cavtocci.com/pdf/tocci.pdf

Thanks for the link, that's everything I ever wanted to know about noise measurement It looks like RC is the simplest to apply, and it could be done easily enough with an RTA (right?).

Couple of other comments/questions:

1. Why is RT60 not relevant above 4 khz?

2. On page 24 the paper says "single figure T60s can be higher for  dipoles relative to conventional sources." However, that is the RT60 of the room, i.e. as measured with an omnipole, isn't it? But what most people are going to do is measure with their actual speakers. So if a dipole, then the target for the measured RT60 would be the same - no?

3. Same question about frequency response - is consistency above 4 kHz not important?

4. On low range freq response, why 20 Hz? With regard to the note elsewhere about fundamentals and relaxed decay time below 35 Hz, it seems like this one also should be 35 Hz.

5. Having said that, +/- 5 dB with 1/3 octave smoothing seems like a very lax standard.

[JohnR]

Hey a light just went on  I was looking at ETCs from some measurement files and I was wondering why the AMS doesn't care about "early reflections." It's often said early reflection are bad, and that you have to put absorbers at the first reflection points. However, what I'm reading here is that early reflections are NOT bad, as long as the spectrum of the reflection is the same.

Do I have that right? No need to treat first reflection points, IF your speakers have uniform off axis response?

[youngho]

first reflection points. However, what I'm reading here is that early reflections are NOT bad, as long as the spectrum of the reflection is the same.

Do I have that right? No need to treat first reflection points, IF your speakers have uniform off axis response?

In his book, Toole argues that first sidewall reflections can increase Apparent Source Width and the perception of spaciousness in a pleasing way for the majority of listeners. However, if the speakers have uneven off-axis response, then these reflections will be "colored" relative to the direct response. Also, this increase in ASW will likely be directly related to less specificity of localization that some listeners prefer or require, i.e. pinpoint localization. An important point in the book is that fiberglass does not absorb uniformly across the frequency spectrum, or even the same when sound hits them at different angles or if they are covered with cloth or not! However, other researchers have found that holes in the power response aren't necessarily detrimental, implying that the spectrum of the early reflections don't necessarily need to be the same (or else dips are less readily perceived than peaks, which is already known). This point has been the source of some controversy in large part because different listeners have different preferences.

So, yes, you don't NEED to absorb these early sidewall reflections. You should try it without and see what you prefer.

[poseidonsvoice]

Hey a light just went on  I was looking at ETCs from some measurement files and I was wondering why the AMS doesn't care about "early reflections." It's often said early reflection are bad, and that you have to put absorbers at the first reflection points. However, what I'm reading here is that early reflections are NOT bad, as long as the spectrum of the reflection is the same.

Do I have that right? No need to treat first reflection points, IF your speakers have uniform off axis response?

John,

I think you have that mostly right. The uniformity of the off axis response is key. I have noticed that the more controlled directivity nature the speaker in question is, the less effect room treatments like the GIK 242 (an absorber) makes (I haven't experimented with a diffuser however). There is some improvement but it is certainly not as grandiose as non-CD speakers I have auditioned.

Anand.

[JohnR]

Hi Anand, I think we need to be careful with the terminology - a true monopole also has a uniform off-axis response.

[poseidonsvoice]

Hi Anand, I think we need to be careful with the terminology - a true monopole also has a uniform off-axis response.

You are correct but that is not practice unfortunately. Most of what I have auditioned however does not hold to your statement. I guess the speakers I auditioned that I assumed were monopoles, were not! Back to topic. The CD speakers I have auditioned do not seem to benefit as much as the non-CD speakers with respect to treating 1st reflection points with absorbers.

Anand.

[AJinFLA]

Do I have that right? No need to treat first reflection points, IF your speakers have uniform off axis response?

That is exactly correct John. Which is why 99% feel the need to "treat" their rooms.
If the polar response is a disaster, the brain can no longer adapt and "listen through" the room, because the spectrum of the reflections and sound power no longer track the direct field. The room is now the enemy. Since there is no way to "fix" the polar response of their speakers, listeners must resort to "fixing" the room instead.
And then there is the matter of preference. Do you want a presentation that attempts to emulate real life, (acoustic) music as you would hear it live, with a more diffuse sound (if you are intimately familiar with live music), or a stereo construct, with ultra-precision pan pot spatial effects?
I my experience, the majority prefer the "Hi-Fi" stereo construct pan pot spatial heaven presentation. "Accurate" more so to what might have been recorded/constructed in the studio....in terms of spatial reproduction. I have no problem with that. It's a preference, just like the other alternatives.

cheers,

AJ

[JohnR]

In his book, Toole argues that first sidewall reflections can increase Apparent Source Width and the perception of spaciousness in a pleasing way for the majority of listeners. However, if the speakers have uneven off-axis response, then these reflections will be "colored" relative to the direct response. Also, this increase in ASW will likely be directly related to less specificity of localization that some listeners prefer or require, i.e. pinpoint localization.

Thanks for that, I've read the relevant chapters in the book now.

[Nyal Mellor]

Couple of other comments/questions:

1. Why is RT60 not relevant above 4 khz?

2. On page 24 the paper says "single figure T60s can be higher for  dipoles relative to conventional sources." However, that is the RT60 of the room, i.e. as measured with an omnipole, isn't it? But what most people are going to do is measure with their actual speakers. So if a dipole, then the target for the measured RT60 would be the same - no?

3. Same question about frequency response - is consistency above 4 kHz not important?

Hey John

Thanks for your questions.

I'm going to tackle 1 and 3 together....the thinking behind the '4k' limit is really that above that the influence of the room on the frequency response and decay time really tails off. All of the room related things such as use of thin drapes, thin absorbers, etc will have an effect down to 2k or 1k and so be visible within the measured bands we have specified. That's not to say that what is going on above 4k is not important! Its just that the speaker's characteristics, in particular off axis performance, are dominating the response above that frequency and that the room is much less of a factor. At least that's our thinking.

In respect of 2. If you were performing T20/30/60 with a dipole you would expect lower decay times than if measured with an omnidirectional source due the directivity pattern. I think you are saying that the fact that measuring with a dipole would naturally give you a lower number counterbalances the fact that if measured with an omni the number could be larger? I kind of see what you mean but in reality the exact value of T is not critical since so many other factors are at work, that's why we gave such a large range.

I think Jeff is going to jump in on 4&5.

[Nyal Mellor]

That is exactly correct John. Which is why 99% feel the need to "treat" their rooms.
If the polar response is a disaster, the brain can no longer adapt and "listen through" the room, because the spectrum of the reflections and sound power no longer track the direct field. The room is now the enemy. Since there is no way to "fix" the polar response of their speakers, listeners must resort to "fixing" the room instead.

That pretty much says it all right there! Great words

If you put a speaker in a rectangular room and point it at the listener you are getting on axis (0 degree) sound and then the first lateral reflection from that speaker results from a sound wave emitted from the speaker at 45-70 degrees off axis. Most speakers aren't going to have a good spectral matches of these two sounds, hence timbral and spatial distortion result. This might lead one to pointing speakers with variable off axis performance (i.e. 90% of conventional forward firing dome / cones) straight down the room so the angle that the speaker emits the direct sound and the reflected sound is more in balance; they are both different from the 0 degree on axis sound but at least they are consistent.

[Jeffrey Hedback]

Yes John,

Excellent questions.  As Nyal mentioned, we value the opportunity to have such questions raised.

Regarding your question 4, "Why 20Hz for the lower limit of the frequency response and resonance?".  A few reasons lead to that range, first of which is that there is musical content below 35Hz that could excite a room.  Another factor is that we are targeting two fullrange speakers in a room and there is a reasonable expectation that the speaker's range would extend down toward 20Hz. 

Acoustically, depending on room dimensions, such a limit (20Hz) allows the study of the region called the pressure zone and the transition toward the modal zone.  The pressure zone is the range that is lower in frequency than your room's longest dimension (actually the half wavelength of that dimension).  This region's energy rises and falls with the sound pressure of your speakers without room resonances...just another aspect to the interrelation of dimensions, modes, and the experience we hear.

Why do we allow the resonance standards to relax below ~35Hz?  (again, great question John).  On the practical side, it is VERY difficult to acoustically "trap" this region.  But even beyond that, there are studies that show "we" tend to like a bit more energy in this VLF region (Very Low Frequency).  It yields that sensation of being "wrapped" in the bass energy and a sense of fullness.  You may notice that the resonance chart range is widest in this VLF zone.  There are many factors such as speaker design (bass reflex, T-Line, etc...), musical style preferences and even potential structural resonances (or lack there of) that could lead one space toward a tighter or longer decay being optimal in a given situation.

And to Question 5.  A funny way of answering this would be to share Dr. Toole's response to this section. He commented on the +/- 10dB at 1/24th oct smoothing (from 20Hz to 250Hz) part of the standard and said (paraphrase) "that's not much a standard at all is it!".  The real answer is that meeting both factors (the third oct and 1/24th oct smoothed response) is not a simple task.  We'd all like to think in terms of totally flat speaker response graph translating into the room's response.  Sometimes this happens, but often it's quite a wrestling match to not exceed the above.  This is in fact why Dr. Toole developed and endorses his SLM multi-sub approach.  If the average level of the resonance peaks are controlled, the resonance standards are met and the nulls are narrow, you can have great bass from two fullrange speakers.  It's a different experience than SLM.  I really enjoy the LF experience of great fullrange floorstanders.

[Nyal Mellor]

This is in fact why Dr. Toole developed and endorses his SLM multi-sub approach.

I think Jeff means SFM another of those impossible to decipher TLAs (three letter acronyms) that stands for sound field management, a proprietary Harman technology

[Jeffrey Hedback]

Exactly Nyal...thanks...for some reason I had Sound LEVEL Management in my head at that moment.  SFM is correct.

[JohnR]

SFM is an algorithm, but in general the use of multiple subs and EQ can be done by anyone. Is that the approach you mean in general Jeff? So a question then, is the LF standard applying to the room, or the room+speakers+EQ?

Related: having the LF standard from 20 Hz means that you are imposing a performance requirement on the loudspeaker as well as the room, doesn't it?