Introduction to Comb Filter Effects

DonH50

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Jun 23, 2010
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#1
Comb filter effects are a popular topic of discussion among audiophiles (and many others, of course). However, some audiophiles may not have a good concept of what they look like, why they may (or may not) be bad, and what they can do about them. Or even if they have them and need to do anything about them.

Picture the signal from the speaker as a single sine wave. Two, speakers, two sound waves. If they are in phase, then adding them together simply doubles the amplitude at every point. If you have two speakers playing the same signal at the same level, and are equidistant from them, then you will hear the signal twice as loudly as if a single speaker was playing. Or will you? It’s complicated…

The figure below shows two 1 kHz sine waves generated by two speakers, but one is 3” further away from the listener. The red line is from the closer speaker and the dashed blue line is the signal from the farther speaker. Clearly there is a difference in phase between the two signals, and if we add them together at each point in time (t is relative in this plot), we will not get a doubling in amplitude.

Figure1.png

Remember wavelength is related to frequency and lower frequencies have longer wavelength. If we change the frequency to 100 Hz, there is less phase shift, since the distance is smaller compared to the wavelength. Their sum would still not quite double, but it would be close.

Figure2.png

Take that same 3” difference and look at 2 kHz, and now the waves are nearly 180 degrees shifted in phase. Add them together and they will nearly completely cancel. Ouch!

Figure3.png

So, if there is a path difference between two speakers, and we sum their outputs at the listening position, what we hear depends upon the difference in distance and the frequency. The figure below shows three plots: red is two identical 1 kHz signals summed from two equidistant speakers, dotted blue is the result when one speaker is 3” farther away, and dashed brown is the result with the same 3” difference in distance but at 2 kHz. You can clearly see how the amplitude changes.

Figure4.png

(To be continued...)
 

DonH50

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Jun 23, 2010
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#2
OK, so if we sweep frequency, what happens? At different frequencies the signals will add in phase, out of phase, or someplace in between. In phase, we expect the signal to double, a 6 dB increase. Completely (180 degrees) out of phase, they will cancel –infinity dB). Now we know the amplitude will depend upon both the frequency and difference in distance to the listener. Below shows the response from two identical sources (dark green), the result when one is just 1” further away (blue), and when one is 6” further away (red). You can see the dips in frequency when the difference in distance causes the signals to cancel at the listening position.

Figure5.png

Why “comb filter”? Look at the same plot but using linear (even) frequency steps instead of the logarithmic plot we are used to seeing. Looks like a comb, yes? Whenever the distance is such that the signal from the two speakers is out of phase, then you will get another tine on the comb where the signal cancels and amplitude drops to 0 (-infinite dB).

Figure6.png

Now, the signal is completely out of phase when the distance shifts the signal one-half the period of sine wave. Since sound travels at about 1127’/s (standard conditions), then the first dip or tine will happen at

Frequency of first dip = (difference in distance in feet) / (1127’/s * 1/2)​

So 1” means the first dip (tine) is around 6.8 kHz, and 6” difference means the first dip is around 1.127 kHz. The dips occur at odd multiples (3, 5, 7…) after that, so for 6” we see dips at 1.1 kHz, 3.4 kHz, 5.6 kHz, and so forth. At even multiples the signal is back in phase with the original and does not cancel.

Do you have comb filtering? Well, yes, no matter how well-treated your room might be. There are several things that can cause comb filter effects:

1. Two speakers not equidistant to the listener;
2. Variations in speakers or other components that cause phase differences in the signal path;
3. Reflected signals adding to the main signals;
4. Other path differences that induce delays between the two sources.

The first one is pretty obvious and is what the previous plots have shown. That is one of the argument for hyperaccuracy in placing speakers from the listener. The same thing can happen if, for instance, there is some difference in the AVR, preamp, amplifier, crossover, etc. that causes a phase shift between the left and right speakers (point number two). Number three is coupled with the room treatment debate; reflective surfaces add signals that can cause comb filter effects. Note that studies have shown that if the delay is small, we cannot really isolate the signals, and if the delay is large, we hear them as separate sources. Psychoacoustics is not my field so you’ll have to ask somebody else to explain that.

One of the “other path differences” that happens in the real world is the space between our ears. Not our brains, though that is important, but the distance from one ear to the other. If your ears are 6” apart, then you hear the signal from one speaker the equivalent of 6” “later” than the signal from the other, thus creating your own little comb filter. Toole and many others have discussed this and its impact. It is easily calculated, and/or measured, but there is some debate its impact. Some argue this is a fundamental problem with stereo speakers and there is no way around it; others claim our brain processes the two signals, accounts for the delay, and so we do not really “hear” the dips. When I measure and listen, I can easily hear the comb filter effects from the rear wave of my dipoles (more on that later), and hear how they are gone when I absorb the rear wave, but do not seem to hear dips from a mono signal played from two speakers. I could easily be fooling myself, of course, since I know what I am feeding the speakers and how they are set up. See previous psychoacoustics disclaimer and note I have not kept up in that area (maybe when I retire).

I do not want to get into that debate, but can delve into reflections. Reflections add a couple of variable not discussed so far:

1. When a signal hits a boundary and reflects, the reflected wave is inverted from the original wave; and,
2. Unless the boundary is a perfect reflector there will be some attenuation of the reflected wave.

Note that the previous plots assumed no difference in attenuation between the two signals. For the distances in a typical room that is a reasonable assumption; while there may be measurable attenuation due to dispersion and other factors, the difference in sound levels at our ears from two nearly-equidistant speakers is minimal (I assumed negligible). However, walls are rarely perfect reflectors, so there is always a little energy lost. That is why the nulls from boundary reflections are not infinitely deep in the real world, though I have seen 30 to 40 dB or more in practice.
 

DonH50

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Jun 23, 2010
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#3
Consider a single speaker 9’ away from the listener with a wall a couple of feet away. The reflected path is about 12.7’ long, and the signal inverts at the boundary (wall). The resulting frequency response is shown in the next plot. The first null is at 304 Hz and they occur at every (not just odd) frequency multiple upwards (due to the phase inversion from the reflection). Also note that at very low frequencies the response is impacted by the signal interactions, reducing LF response. What this does not show is the impact of the extra delay on the signal. The path difference of 3.7’ is a time delay of about 3.3 ms. See Haas Effect (https://en.wikipedia.org/wiki/Precedence_effect) to see how we might resolve the delay (we typically do not perceive single events 1 ~ 5 ms apart as a separate sound, and up to perhaps 40 ms for more complex signals like music, though arguments have been made about “smearing” of the signal).

Figure7.png

Figure8.png

Adding broadband absorption to reduce the reflected amplitude by half (6 dB) significantly reduces the null depth. This is one of the arguments for absorption in the listening room. Real absorbers are not flat across frequency, of course, so a more realistic plot would show less change at low frequencies and much more at high frequencies, reducing high-frequency ripples to negligible (i.e. small in amplitude and close together so more readily masked by adjacent signals with actual source material).

Figure9.png
 

DonH50

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Jun 23, 2010
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#4
Since I own dipoles, there is one last set of plots worth showing. The next figure shows how the response changes as you move a dipole speaker from the back wall. The frequency of the first null drops, of course, due to the longer path length. Again Hass Effect enters into the audibility since the delay also increases with distance, and the reflection will be somewhat attenuated in the real world.

Figure10.png

How much comb effects matter is subject to psychoacoustics, something outside my field. Please start another thread to discuss that; this is just to present the background and examples. What should be clear is they exist in every system in one form or another, both distance and frequency determine their nature, and that absorption can reduce some of them (desirable or not). Diffusion can also reduce them by decorrelating (scattering) the waves so they do not recombine significantly. Diffusion preserves much of the energy so the room is not as “dead” so many people may prefer that sound.
 

Ron Resnick

Site Co-Owner, Administrator
Jan 25, 2015
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#5
Thank you so much, Don. This tutorial is great!

Not being aware of the comb filter effect I have always preferred not to absorb the rear wave of dipoles, preferring instead to position them about 8' away from my front wall.

Do you absorb as completely as is practicable the rear wave of your dipoles to eliminate the comb filter effect, or do you pull them into your room, away from the front wall, to exactly the distance you calculate minimizes the comb filter effect?
 

DonH50

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Jun 23, 2010
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#6
Thank you so much, Don. This tutorial is great!

Not being aware of the comb filter effect I have always preferred not to absorb the rear wave of dipoles, preferring instead to position them about 8' away from my front wall.

Do you absorb as completely as is practicable the rear wave of your dipoles to eliminate the comb filter effect, or do you pull them into your room, away from the front wall, to exactly the distance you calculate minimizes the comb filter effect?
Thanks Ron.

In a large room I prefer to pull them out at least five or six feet and either do not dampen or use just a little behind them. In my present small'ish cube'ish room everything is fully dampened. I don't mind the "dead"" sound and imaging is great, but it's not for everyone.

Note large flat panels radiate little off the sides or top/bottom throughout most of the frequency range (midbass on up) so damping first reflection points on the sidewalls is less important than for conventional speakers.
 

bonzo75

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Feb 26, 2014
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#7
Hi, the apogee grands I heard had diffusers behind them, but what the restorer told me that if you stand a certain distance near field you get a comb filter effect, but at the seating position and behind it was fine. Please note it was a long room, so seating position was around 5 to 6m, and he wouldn't advise those diffusers for a smaller room.
 

dallasjustice

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Apr 12, 2011
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#8
There are so many names for basically the same thing. Some folks call it comb filtering. Some call it Allison effect and others call it Speaker Boundary Inteference Response ("SBIR"). The so called "floor bounce" is another name for the same effect which the floor produces in just about all setups.

To add even more confusion to the topic, every room exhibits this behavior if one is looking at unfiltered SPL sweeps. Fortunately the human ears don't work like a single microphone higher than a certain frequency. So some frequency dependent impulse windowing is needed to get a better (not perfect) idea of what the ear will hear at every frequency. I mention this because sometimes folks look at an unfiltered graph for the first time and think; "oh my, is my room that bad?"

Michael.
 

DaveC

[Industry Expert]
Nov 16, 2014
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#9
Makes you wonder about the emphasis on flat on-axis frequency response, in a typical installation the room mangles it pretty badly.
 

DonH50

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Jun 23, 2010
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#11
Yes. Even if you don't move your head. There is debate over how our brain processes the information and if it matters. I am not really competent to engage in that debate.
 
Apr 3, 2010
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Seattle, WA
#12
Comb filtering is around us all the time in real life. An example is when we listen to loved ones around the home. Whenever there are two paths to your ears, and one is delayed as would be with reflections in your home, there will by definition be comb filtering. Fortunately, there are many reflections in the room and they all combine to fill the holes. In addition each ear hears a different comb filter due to different path lengths to each ear and you see that in reality this is not at all a simple situation.

Fortunately reflections have existed since we moved into caves and the brain has had plenty of time figure out how to evolve and not only ignore it, but use these delayed reflections to good effect. This is why your loved ones don't sound different in different parts of your home. And why you learn to ignore reflections from your seatback. See the top article in this link on audibility of room reflections: http://www.audiosciencereview.com/forum/index.php?articles/&page=5
 
Dec 13, 2010
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#13
DonH50 said:
Number three is coupled with the room treatment debate; reflective surfaces add signals that can cause comb filter effects. Note that studies have shown that if the delay is small, we cannot really isolate the signals, and if the delay is large, we hear them as separate sources. Psychoacoustics is not my field so you’ll have to ask somebody else to explain that.
No explanation at hand, but there is lot of literature relating to echo thresholds, which is the point at which the reflection is perceived as independent sound event. These thresholds are different for different signals, and range from about 1 ms to about 80 ms.

Blauert et al. (2005), “Acoustical communication: The precedence effect”, Proceedings FORUM ACUSTICUM BUDAPEST, OPAKFI Budapest

Damaske 1967/68, “Subjective investigation of sound fields” (in German), Acustica, vol. 19, p.190

Kuhl (1978), „Spaciousness as a component of total room impression“ (In German), Acustica, vol. 40, p.167

Litovsky et al., “The precedence effect”, J. of the Acoustical Society of America 1999, vol. 106, no. 4, pt. 1, p.1633


How much comb effects matter is subject to psychoacoustics, something outside my field.
The amplitude (or modulation depth) of the comb filter depends on the level of the reflection. The lower the reflection level, the lower the amplitude and the coloration decreases. Coloration further depends on whether a single or multiple reflections are present and in the case of multiple reflections, whether these are regularly spaced in time or not. Irregularly spaced reflections result in lower amplitude of the comb filter and the perceived coloration becomes less with increasing number of such reflections.

Quite a lot of information about this can be found in

Salomons, “Coloration and binaural decoloration of sound due to reflections”, PhD Thesis, Delft University, 1995

http://repository.tudelft.nl/view/ir/uuid:7f0331e3-bc1a-4d7f-8d2a-eb5d6cc04fbf/

provides the PDF.


Binaural decoloration is a mechanism a microphone does not have, so a simple in-room frequency response won’t tell you anything about the audibility of those comb filters. As Amir says (good article, hits the nail on the head!), in rooms there are many reflections, irregularly spaced in time, and that’s a good thing, which in general does not require further action.

Klaus
 

jkeny

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Feb 10, 2012
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#14
David Griesinger has studied reflections extensively from the perspective of room intelligibility & envelopment - search for Griesinger & envelopment
 

DonH50

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Jun 23, 2010
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#15
Thanks for the background guys. I used to know (remember) more but decided for this thread I'd leave Haas (precedence effect, at least back when I last looked at it) out of it simply because I don't know enough to personally comment. Calculating the effects are easy; describing what we do with them is outside my area of expertise. Really appreciate the references! - Don

p.s. It looks like I could have saved myself a lot of work by just linking to Amir's article!
 

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