[bgewaudio]

Ethan or anyone else, Is the main objective to eradicate 1st order reflections all together, or are we just trying to control them so they don't end up on the wrong side of the room. i.e. (focal points from left speaker staying on left side and focal points from right speaker on right side)?.  Or is this more to do with diffusive properties?

[Ethan Winer]

> Is the main objective to eradicate 1st order reflections all together <

Yes. These reflections confuse imaging both by the echoes arriving early and by some going to the wrong ear. The different arrival times also creates comb filtering (peaks and deep nulls), which to me is at least as damaging.

--Ethan

[bgewaudio]

Are high frequency standing waves just as problematic as low frequency standing waves?  (i.e. will higher frequency standing waves increase amplitude as much as LF waves?).

[Glenn K]

Different problem, but both problems that should be fixed.

Glenn

[Ethan Winer]

> Are high frequency standing waves just as problematic as low frequency standing waves?  (i.e. will higher frequency standing waves increase amplitude as much as LF waves?). <

Yes and yes. Many people don't even realize that standing waves can occur at higher frequencies. But they do, and you can hear them easily. Play a 2 or 3 KHz tone through both speakers and walk quickly from the front of the room toward the rear, and you'll hear the "picket fence" effect (a radio engineering term for the very same phenomenon) as the volume flutters rapidly.

The good news is standing waves at high frequencies are easy to tame with relatively thin absorption.

--Ethan

[bgewaudio]

Wicked answer, I like knowing what all of these effects are called!

Thanks Ethan

[klh]

Hey Ethan... it sounds like you have a fellow New Englander.

[8thnerve]

> Are high frequency standing waves just as problematic as low frequency standing waves?  (i.e. will higher frequency standing waves increase amplitude as much as LF waves?). <

Yes and yes. Many people don't even realize that standing waves can occur at higher frequencies. But they do, and you can hear them easily. Play a 2 or 3 KHz tone through both speakers and walk quickly from the front of the room toward the rear, and you'll hear the "picket fence" effect (a radio engineering term for the very same phenomenon) as the volume flutters rapidly.

Ethan,

What is your explanation for how these high frequency standing waves set up?  Standing waves are caused when a quarter of the wavelength of a frequency matches a physical room dimension.  These standing waves can stack, causing harmonic tones that rise in frequency, but these also reduce in level as they work their way up the spectrum.  So how can you explain the "picket fence" effect, which is indeed there is, as can be proven by your technique or via measurement?

You probably know that I feel the the treatment of first reflections is not effective or desireable, and that I have a different view of how these frequency anomalies form.  I'm not baiting you to argue about this, but would genuinely like to know how you think this measureable effect is caused.

Best,

Nathan Loyer
Eighth Nerve

[Ethan Winer]

Nathan,

> What is your explanation for how these high frequency standing waves set up?  Standing waves are caused when a quarter of the wavelength of a frequency matches a physical room dimension. <

Standing waves occur any time two opposing wavefronts collide. Standing waves do not require resonance, nor must they fit exactly between two opposing surfaces. Some people lump standing waves and room modes together, but they're really very different things. All that's needed to create standing waves is for a sound source to strike a single boundary and reflect back into itself. That is, you can have standing waves outdoors against the brick wall of a building.

> These standing waves can stack, causing harmonic tones that rise in frequency <

I'm not aware of standing waves causing Doppler shift, if that's what you mean.

> So how can you explain the "picket fence" effect, which is indeed there is, as can be proven by your technique or via measurement? <

Yes, it's indeed there. As I always say, empirical evidence trumps theory every time.

This article explains it pretty well:

www.realtraps.com/art_waves.htm

> You probably know that I feel the the treatment of first reflections is not effective or desireable, and that I have a different view of how these frequency anomalies form.  I'm not baiting you to argue about this, but would genuinely like to know how you think this measureable effect is caused. <

I have never felt you were baiting me. There's a similar discussion of the value - or not - of early reflections going on now in this thread at Audioholics:

http://forums.audioholics.com/forums/showthread.php?t=24667

I am firmly in the camp that believes a listening room should not add its own character to the playback. Or at least not the early reflections part that defines the size of the image, and causes you to localize the sound as coming from the loudspeakers. As I mentioned in that other thread:

I've read many interviews with movie mix engineers who confirm that all ambience and directional cues are embedded in the sound tracks. They all seem to agree that direct reflections are not desirable in a room because that just competes with what they're doing.

--Ethan

[8thnerve]

Standing waves occur any time two opposing wavefronts collide. Standing waves do not require resonance, nor must they fit exactly between two opposing surfaces. Some people lump standing waves and room modes together, but they're really very different things. All that's needed to create standing waves is for a sound source to strike a single boundary and reflect back into itself. That is, you can have standing waves outdoors against the brick wall of a building.

And how does absoprtion as you mentioned before fix this?  I think I see what you are getting at, but I'll let you answer instead of speaking for you.

> These standing waves can stack, causing harmonic tones that rise in frequency <

I'm not aware of standing waves causing Doppler shift, if that's what you mean.

No, that's not what I meant.  220 - 440 - 880 - etc.

Yes, it's indeed there. As I always say, empirical evidence trumps theory every time.

Agreed, that's why measurements that show a flattening of the frequency response in a room treated with no absorption lead me to believe there is more to echo and standing waves than is currently known.

I am firmly in the camp that believes a listening room should not add its own character to the playback.

As am I.  I just believe that rolling off the high-frequency response in order to reduce first reflections is not a desireable trade-off, and that there are much more effective ways to reduce standing waves.

Thanks for chatting about this.

-Nathan

[mfsoa]

8th Nerve,
It seems like you are making the assumption that the only way to get a "proper" amount of HF info is by boundary reinforcement? Are speakers designed in this way, to anticipate lively 1st reflection points (dunno)?

Is is true (or how often is it true?) that if the first reflection points are damped, that this will by default result in rolled-off high frequency response?

I thought that speakers were pretty flat in the treble, anechoically, and maybe some damping of the 1st reflection point might result in the benefits that Ethan likes without necessarily over-damping the treble?

Or is it that the lower frequencies are preferentially boundary-reinforced, which in general tips the balance down if the room is overdamped? I think I answered my own question there, and now we come full-circle as to why we need to use bass traps.

I know, highly system dependant etc.

[kfr01]

I am firmly in the camp that believes a listening room should not add its own character to the playback.

As am I.  I just believe that rolling off the high-frequency response in order to reduce first reflections is not a desireable trade-off, and that there are much more effective ways to reduce standing waves.

I am also confused.  I don't want to hear early reflections.  Even if there are "more effective ways to reduce standing waves," isn't the treatment of first reflections also desirable?

Excellent questions, mfsoa.  I think I can answer at least one of them.  Speakers are generally not designed to anticipate lively 1st reflection points.

Additionally, I don't follow the treatment of first reflection points ==> undesirable "rolling off the high-frequency response" logic.

Explanation, 8th Nerve, would be helpful.

[Rob Babcock]

Personally I think most smaller rooms do need to "roll off" the highs slightly.  The long low bass wavelengths can readily pass thru walls whereas the highs don't to such a degree.  My feeling is that small rooms are unnaturally live because of this.

[bpape]

Most speakers don't have consistent upper mid/high frequency response on and off axis.  The wider the dispersion and/or more difference between on and off axis, the more need for side wall early reflection control via absorbtion. 

IMO, boundary reinforcement is a miniscule part of the equation for high frequencies.  As you go lower, it becomes a much larger issue - both good and bad.

Bryan

[Ethan Winer]

Nathan,

> And how does absoprtion as you mentioned before fix this? <

All room acoustic problems are caused by reflections. So absorbing the reflections solves the problems. This is true for excess ambience and reverb, slap echo and flutter echo, "standing" waves, modal ringing - all of it. If a wave reflects off a wall, adding absorption to that wall stops the reflections and solves the problem.

> No, that's not what I meant.  220 - 440 - 880 - etc. <

I still don't know what that means.

> that's why measurements that show a flattening of the frequency response in a room treated with no absorption lead me to believe there is more to echo and standing waves than is currently known. <

If not treated with absorption, then what "shows a flattening?"

The root cause of all frequency response problems is comb filtering. You can reduce comb filtering with diffusion instead of absorption, if that's what you're getting at. But diffusion only "postpones" the problem because it does not reduce excess ambience and reverb, only the comb filtering. And only above 500 Hz (best case for really deep diffusors). You can't practically "diffuse away" low frequency problems and certainly not modal ringing.

--Ethan

[Ethan Winer]

Personally I think most smaller rooms do need to "roll off" the highs slightly.  The long low bass wavelengths can readily pass thru walls whereas the highs don't to such a degree.  My feeling is that small rooms are unnaturally live because of this.

Bingo.

Small-room ambience is almost always bad ambience. It's what causes that boxy (or shrill etc) sound, especially in the front of the room around the loudspeakers. This is why, when someone asks where's the best place to start with only two traps/panels, I suggest the front two corners rather than the rear.

--Ethan

[bgewaudio]

Larger concert halls typically have greater re-verb than the smaller spaces we occupy in our homes.

These larger rooms typically utilize diffusion over absorption because since there is greater re-verb present, 1st orders will not be as problematic because they will reduce in amplitude more quickly than smaller rooms.

The smaller rooms have less re-verb time so 1st orders will be much higher in amplitude when they come in contact with a reflective surface, thus we use absorption to eradicate them so they will be non-existant all together or any reflections that get back out into the room will be so reduced, that echo's won't be so apparent, but then we have the HF roll-off issue?

I think in order to have a euphoric acoustic space, you need to utilize both diffusion and absorption, or abfusion!

[JohnR]

Some people lump standing waves and room modes together, but they're really very different things. All that's needed to create standing waves is for a sound source to strike a single boundary and reflect back into itself. That is, you can have standing waves outdoors against the brick wall of a building.

I never realized that before! (I was going to "correct" you but fortunately thought about it a bit more first )

Still.... it is true though that you can only -- within reason -- reduce first reflections at higher frequencies? At lower frequencies you use traps to reduce modes?

[Ethan Winer]

John,

> I never realized that before! <

This has been the subject of numerous "heated" forum debates over the past few years. One camp considers standing waves and room modes to be the same thing, but that's wrong. Standing waves are waves, and modes are merely a propensity to vibrate if excited. Electrical engineers know that standing waves form in a wire that's not properly terminated. The same impedance mismatch that creates standing waves in a wire is what creates standing waves against a single brick wall outdoors. You can have standing waves, and peaks and nulls, at all frequencies in any room. Not just at resonant frequencies determined by the room's dimensions.

> it is true though that you can only -- within reason -- reduce first reflections at higher frequencies? At lower frequencies you use traps to reduce modes? <

First / early reflections are mainly a mid and high frequency issue. The root problem is comb filtering caused by the same sound arriving at your ears at different times. At some frequencies the fixed delay creates peaks, and at others it creates nulls. As I explained to Nathan, all room acoustic problems are created by reflections, and so it is with both low and high frequencies. Low frequency peaks and nulls are caused by the same comb filtering. But in that case the main culprit is often the rear wall behind you, rather than the first reflection points on the side walls (and floor and ceiling) about halfway between your ears and the speakers.

--Ethan

[bpape]

The other reasons that large spaces us diffusion over absorbtion are:

1.  In a large space you have a TON of people in there - who are excellent absorbers.
2.  In a large space, you have sufficient distance between the diffusors and the listeners for it to be efective.
3.  In a large space, you have sufficient room to use very large diffusors which can be effective to a much lower frequency.

Bryan