room nodes, EQ, and the goal of acoustic treatment...

[DrWho]

I was reading somewhere else on the klipsch forums and someone mentioned that trying to flatten out your sub's frequency response with an EQ doesn't work when DIPS in the response are room related. The concept was that increasing the output at a certain frequency also increases proportionally the amount of destructive interference and the end result is a higher level of distortion (because the woofer is moving more, and you're still not hearing anything).

Logically, this idea seems sound, but what I don't understand is how I've been in listening environments where this kind of EQ has resulted in flatter both audible AND measured responses. Would it be fair to say that the increase I heard was just the increased distortion faking me out? But how would you explain measurements that explain the audible improvement?

The only explanation I can come up with is to take into account that sound travels and thus it must take time for room nodes to build up. Also, I've heard that tests using pink noise don't excite nodes as much because of their random nature (which is supposed to simulate better what music will sound like, versus a bunch of tones).

Now I'm not trying to justify the fixing of a crappy response with EQ...it just sounds bad, ALWAYS. The reason for bringing this up though is to discuss how "EQ'ing" the room with acoustic panels and all that really affects the sound.

To start off, I think that treating a room to obtain a flat frequency response to be a very wrong approach. To talk in extremes, we all know that listening to music in an anechoic chamber is very unnatural and almost disturbing at times. I'd say this was the extreme of trying to obtain a flat response and the result is that it sounds bad. So wouldn't it make sense that efforts in achieving a flat response is just degrading the sound quality? I know from experience that treating a room makes it sound better, but keeping that in mind, I would say that there is a different goal besides flat response that we should be striving for.

Now for a side thought, I'm going to make up a new term: "Room IQ". I was going to call it room responsiveness or room sensitivity, but I didn't want to confuse terms. While we're at it, there can also be speaker IQ. Anyways, the IQ can be defined as how responsive the room is at various frequencies. Sometimes I think of it as the acceleration of the frequency response. Now I know there are a lot of factors involved with room acoustics and I might be trying to think of a lot of things at once with this term. The main idea I'm trying to get at here is that the frequency response eminating from right in front of the speaker is going to change as that sound travels through the room. Once the "affected response" arrives at the ears, I believe the "affected response" at the ears continues to change over time, thus creating the illusion that different frequencies are more easily created than others. (At least this is my explanation for the "illusion" that I experience quite often)

Let's say you've got two completely different rooms, but both have the exact same measured frequency response using pink noise...let's say both have a totally flat response so now we can compare with an anechoic chamber. Now make the best recording in the world of you hitting your knuckles on your desk. Due to the "Room IQ" I believe that the recording will sound very different in each of the rooms. The reason I believe for this is that you'll initially hear the direct sound eminating from the speakers and then a short time later, you'll be experiencing the effects of the room "flattening the response." Not only that, but the various effects from the room don't all happen at the same time either. You've got first reflection points arriving slightly later, reverberation time, second and third reflection points, destructive interference in 3 planes (each happening at different times), etc etc etc.........The sum of all this I consider the Room's IQ (a measurement of how the frequency response changes over time). So what you end up hearing is that certain frequencies seem to be more easily produced than others, even though the measured response is "flat."

Now all that said, I think the ultimate goal in treating and EQ'ing the room and all that crap is to perhaps make the Room IQ zero (no changes at all in frequency response once the first sonic event has reached the ear until that event has ended). It would follow then that a high IQ would be bad.

The reason I bring all this up is that I've heard rooms "treated" to accomplish a flat response, but it doesn't sound as natural as rooms that when "untreated" (in the same frequency range) have the same measured response. Does this make sense? (cuz this paragraph here is my main "question"). To expand on this, wouldn't treating the room to "fix" a speaker's bad response also be destructive to the sound?

I notice the same thing when comparing speaker designs. A 12" ported woofer that plays flat well below 60Hz sounds better to me than an 8" woofer that also plays flat down to 60Hz. It's almost as if the lower notes (being reproduced by the port in the one) seem wimpy in comparison. Using my new term, it would follow that the Room IQ of the ported speaker would be higher in the 60Hz region. Now I know there's different distortions and all that crap, but let's pretend that they don't exist. The 60Hz from a woofer sounds better to me than 60Hz from a port.

Wow ok, wierd post i know...sorry it got so long, but there is something that I'm hearing and I'm trying to describe it and I wanted to ask some sort of question to see if I've flown off my rocker. Even though I haven't really asked a question, I would greatly welcome anyone tearing apart the logic I've presented. Perhaps we'll all learn something in the process

[jdm56]

I think what you are calling room "IQ" is what is usually just called in-room response. The frequency response variance over time is due to the reverberation characteristics of the room. And true, room resonances due need time to build up. A very dead acoustic will tend to lessen the problem of room boom.

A room node, or cancellation is basically an audio black hole for specific frequencies, and throwing more amplifier power / driver excursion at it yields precious little in return except distortion and speaker or amp damage. You gots to change da room!

[DrWho]

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On 8/8/2004 9:08:02 PM jdm56 wrote:

I think what you are calling room "IQ" is what is usually just called in-room response. The frequency response variance over time is due to the reverberation characteristics of the room. And true, room resonances due need time to build up. A very dead acoustic will tend to lessen the problem of room boom.

A room node, or cancellation is basically an audio black hole for specific frequencies, and throwing more amplifier power / driver excursion at it yields precious little in return except distortion and speaker or amp damage. You gots to change da room!

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I'm not talking about in room response...i know what that is, lol. I guess I portrayed what I was trying to say backwards. What I'm talking about is how some frequencies seem to be produced easier than others, even though they are the same volume. I started with a sort of explanation of how different variables could affect this and how any "treatment" of the room will have a dramatic effect.

After being away for a while, I was thinking about the room, speaker, and listener interaction and I was thinking that treating the room to reduce nodes and all that is a good thing. To treat the room to compensate for speaker flaws is however not a correct solution, because you're basically using the room's reverberation in an attempt to undo the direct sound that will already arrive first.

[mikebse2a3]

Hello DrWho:

Have you read the book "TIME DELAY SPECTROMETRY" it's an Anthology of the works of Richard C. Heyser on measurement, analysis, and perception.It was put out by the Audio Engineering Society in 1988.I admire Richard Heyser's and Paul Klipsch's works. They were Giants in my book!

Here is my thinking at this time from all I've learned myself and from others works.

(1) What we are doing "recording/playback" is just an illusion a trick if you will or as PWK called it stereophoney.When we record a solo vocalist for example even if you played back the recording on a 2 or 3 channel stereo system in the same room as the recording was made you still have a simulated vocalist created from 2 or 3 speakers located in different locations from the orginal vocalist with a different dispersion pattern than the orginal vocalist in that room.Maybe at best we will be fooled by this "tricking" of our ear/brain system.

(2) The ear/brain has to be the final judge for now because no test can tell us completely what we are going to hear. Richard Heyser was great at tying in some of the measurements methods he was developing(TDS) with the way we perceive sound but there is alot of work to be done to nail down what measurements are going to result in a certain perception of sound.

(3) Basically we have no set standards for recording methods or playback systems.How can we ever expect to achieve great sound from everything when recording equipment and methods as well as playback equipment and playback rooms completely vary from one to the other.It looks almost hopeless that we will ever be able to have recordings that will play on every playback/room system!

(4) The room though necessary(like you said who would want to listen in an anechoic chamber)to me for the most part starts to impair the speakers responce as it was designed.It alters the amplitude and timing of the frequency responce which except for some special circumstance I don't see how they can be corrected but with knowledge of how we hear we can maybe fool the ear/brain into ignoring it or at least liking what we perceive.I see no way we can totally replicate a recording when there are so many uncontrolled variables in this whole process. At best at this time it looks to me like we just as well need to assemble our playback system/room to create that real illusion that connects us with the music thats important to us and except that it simply can't play every recording at this time(Who Knows When If Ever).

(5) I guess my personal preferance is to buy the best equipment I can and treat the room acoustically to do the least damage to what I believe are recordings of the highest quality avaible in the kind of music I chose to listen to. For poorer quality recordings I've attempted to make them listenable by altering there tonal balance through a path not as pure as the one I use for the best recordings which depending on the problems the recordings have this may help but some things just can't be helped.My ultimate goal is to be connected and drawn into the music and to me the recordings and playback rooms are the two weakest links to overcome.Time is to short and I really don't spend much time listening to bad recordings when I've got really good recordings to listen to in this system.Those recordings are better in the car or somewhere their compromises aren't as noticable.

Its late and I've got to go but I would really like to talk about what you had to say when I get a chance.But I do believe the Energy/Time /Frequency as perceived by our ear/brain explaines why you perceive one thing as an improvement or problem even though most measurements may not show any significant difference.We are simply looking in the wrong place for what we are perceiving.

mike

[DrWho]

Well I'll be sure to add that book to my wishlist

I'm well aware of the stereophoney thing and it's kinda funny when you compare the physics between the actual singer and the mic/recording/playback system (don't forget that the mic has it's own pickup pattern and frequency response...the mic is also generally closer than an actual listener would be at a live performance as well). I've been doing a lot of listening in mono lately and it's almost disturbing how different it sounds (often times insanely better...depends on the music). It makes you wonder if there isn't a totally different/better way to be approaching everything (i've often wondered about the concept of having walls that can be manipulated in real time so as to make the room seem acoustically larger or smaller or whatever you want it to sound like).

You mention that recordings and playback rooms are the two weakest links (which I totally agree with)...isn't it annoying that we only have control over one of them? I seriously hate the concept of being a slave to the recording engineer (ironically, im another one of those engineers! eek!)