# Please help with 155Hz null



## GGA (Oct 31, 2006)

Below is a graph of my left speaker showing a major null at 155Hz plus smaller nulls at 123Hz and 88Hz, note the three almost equally spaced. These must be room induced as my right speaker is fairly flat in this range.

I have two GIK 2'x4'x8" square traps and two similar triangular traps. I have tried these in every position I could think of (except the ceiling) and could not affect the null.

The system is in a "great room," 20'x40'x8', with bay windows, alcoves, open spiral staircase, etc., so there is no way to easily predict modes.

Are there an measurements that REW can provide to help narrow down the nature of this problem?


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## Nuance (Nov 2, 2008)

Have you tried moving the speakers and measuring to see what changes, or are the speakers static (cannot be moved, for one reason or another)?


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## GGA (Oct 31, 2006)

Unfortunately the speakers cannot be moved. I have taken measurements over a variety of listening positions and the the null occurs over quite a wide area. Only when one gets within a few feet of the speaker does the null disappear.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Welcome to the Forum, George!



> Are there an measurements that REW can provide to help narrow down the nature of this problem?


 If by “nature” you mean “cause,” the answer is no, REW can’t tell you what it is. I expect that you’d be rather shocked at what you’d see if you extended your readings up to 1-2 kHz. 

That said, it’s not a big a problem as it “appears.” Such sharp nulls are difficult to hear, and since your other speaker is flat, it’s filling in the “blanks,” as it were, audibly-speaking.

Regards,
Wayne


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## GGA (Oct 31, 2006)

Hi Wayne,

Many belated thanks for the welcome. I have read many of your posts and they are always thought provoking. 

You are probably right about the audibility of the null but I sure would like to try and fix it. I didn't think it would be that difficult given the higher frequency of 155Hz should be susceptible to traps. My basic assumtion about the null is that there is a reflection cancelling the signal at the sitting position. If I could trap that then I should reduce the null.

But how to find the reflection? I placed my traps around the room systematically but could not affect the null. This still puzzles me. Perhaps the null is due to multiple strong reflections and my trap only caught one of them?

We can tell by examining the impulse response if we have a side wall reflection and the total distance traveled by that reflection. I was hoping there might some graph that could similarly help with nulls.

I tried windowing at various times in hopes of seeing how the null developed, which might give me some clue as where it was coming from. I tried many times and give below graphs at top 25ms and 27ms (quite a dramatic change over 2ms) and at bottom 50ms and 100ms. These graphs show my left subwoofer and left main.

This made me wonder if an official "reverse decay" graph would be of any use (I have never seen one). We have several graphs that show *decay *over time, but none that show how a signal *builds up*. It seems like it would be useful to see the development of sound in the room over time when compared to frequency.

Anyway if anyone has any suggestions it sure would be appreciated.
George

25ms and 27ms








50ms and 100ms








25ms+27ms


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## laser188139 (Sep 19, 2009)

GGA said:


> ...
> We can tell by examining the impulse response if we have a side wall reflection and the total distance traveled by that reflection. I was hoping there might some graph that could similarly help with nulls. ...


But the symptoms are two sides of the same phenomenon. Generally every null is also a peak at multiples of that frequency. For example, making the simplifying assumption that the null is a direct reflection at some odd half wavelength, then at twice the frequency one sees a full wavelength multiple and the reflection adds instead of subtracting. 

In other words, if you calculate distances from peaks in the impulse curve or ETC curve, you can associate multiple frequencies with each distance, half of which are nulls and half would provoke peaks. 



GGA said:


> ... This made me wonder if an official "reverse decay" graph would be of any use (I have never seen one). We have several graphs that show *decay *over time, but none that show how a signal *builds up*. It seems like it would be useful to see the development of sound in the room over time when compared to frequency. ...


You have something like this in the waterfall and the spectral decay graphs. At the low frequencies, one can sometimes see them build up from slice 0 to slice 1 or 2. But I've seen this only at the lowest frequencies. 



GGA said:


> ... I tried windowing at various times in hopes of seeing how the null developed, which might give me some clue as where it was coming from. I tried many times and give below graphs at top 25ms and 27ms (quite a dramatic change over 2ms) and at bottom 50ms and 100ms. These graphs show my left subwoofer and left main. ...


If you are trying to use the windowing to determine at what distance the offending reflections appear, I think you are going to have to try much smaller window values. The problem, as I'm sure you have already seen, is that at the lower window widths you quickly lose resolution at the low end, which is what interests you. 



GGA said:


> ... But how to find the reflection? I placed my traps around the room systematically but could not affect the null. This still puzzles me. Perhaps the null is due to multiple strong reflections and my trap only caught one of them? ...


Yes, it is possible that two reflections, not at an odd half wavelength, could combine in phase to create a sharp null. 

At one point, I actually tried enumerating every peak in the ETC curve, converting these to distances, calculating the 1/2, 2/2, 3/2 wave multiples of these, to see which distances might correspond to nulls I saw. Unfortunately, because there are so many reflections, this exercise was ultimately unsatisfying. It did confirm for me that one of my nulls was from a ceiling reflection. And I do see a null similar to yours which is a reflection from a front speaker off the fireplace at the opposite wall. But, otherwise, I was left with many questions that I expect will be answered only by placing treatments in various places and measuring the changes. 

When you wrote, though, that this null appears at many positions around the room, that sounds to me like reflections I have seen from one of my front speakers located near a corner. With the corner placement, there is a natural null from a half wave reflection out of the corner, which will be seen over a wide area of the room. You can calculate what frequency has a 1/4 wavelength distance matching the distance from speaker to corner, and see if that matches ~155Hz.

Have fun experimenting,
Bill


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## planetnine (Mar 3, 2009)

You could try the empirical method of tracking it down. Set the frequency generator to "sine wave" (pure tone) and click the box to track the cursor. Play the tone and turn it up so you can hear it, drag the cursor exactly onto the null and listen to REW change the tone to this frequency.

Then take a wander around your room and listen to any changes in the perceived intensity of the tone. If it's a cancellation product, you'll notice a 20 dB or so difference in level between some different zones in your room. If it's a product of your speakers, the difference will not be so noticeable.

Have you tried inputting your room dimensions into a spreadsheet such as Modesv2p or RealTrap's ModeCalc? sometimes this will correlate your measurement with a known room mode and you can then treat your room with an appropriate trap in a useful place to help defeat these reflections. If your room is not a simple recangle, you need to input distances between any large sets of parallel surfaces, and maybe look across several "guestimate" tabulations to notice the culprit.

>


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## glaufman (Nov 25, 2007)

There is a feature in REW that can at least help you narrow down the possibilities.
Take your original plot, generate a waterfall, and post it, making sure the scales are the same as your original, and the freq axis is in LOG mode. 
From that we can help tell if it's modal. If it is, then you start looking on that route. If it's not, then it's probably a simple reflection, and your investigation takes a different turn.

EDIT: I misspoke a little. When there's a peak in the FR, it's easy to determine if it looks modal on the waterfall. Not quite as much so with a null. But, any null at one position caused by a mode should have a peak at another position. If you can find that peak, and run a plot there, generate a waterfall, etc... look particularly near walls/corners for this peak, where I wouild consider any spot where that frequency comes above the average SPL reading by a view dB (say 3 or more) to be likely to be enlightening.


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## GGA (Oct 31, 2006)

Many thanks for the replies. The null at 155Hz is very deep and so I am assuming the cancellation wave is very frequency precise and it must be a standing wave. If I knew when the cancellation wave was first arriving or how it was behaving, I'm thinking it could help me track down its pathway and eliminate some possibilities, thereby assisting me in the placement of the trap.

My 25ms/27ms graph shows the beginning of cancellations at 110Hz and 190Hz, which are transitory, as the steady state does not show them, so these are nothing to worry about. The 50ms graph shows the beginning of the 155Hz null, so it looks like the cancellation wave is taking some time to develop in my 20'x40' irregularly shaped room, so it could be bouncing around quite a bit. At 50ms at least I know it is not the long dimension of the room causing the problem. 

Are these assumptions correct? Would not a "reverse" waterfall showing the development of the sound over time rather than its decay be useful? Trying to compare individual time slices by changing the measurement window is very cumbersome.

Thanks,
George

Here is the waterfall graph.


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## planetnine (Mar 3, 2009)

Did you listen to this frequency in your room? 

For 40*20*8' there are a couple of tangential modes both stacked at about this freqency, preceded by coinciding axial modes from _all three room dimensions_ just before it at a little over 140Hz. The fact that one of your room dimensions is double the other is going to give you a potentially problem room and a very lumpy room response. 

Modes stack-up, combine where they occur at the same frequency. The significant cause of these is large pairs of parallel surface areas at certain distances (ie wall to wall, floor to ceiling). If 5% of the area of a wall is taken up by bays, then that will reduce the reflection mode by about that (and add a few different ones to the mix), but in that case 95% of the conditions for that mode will still exist. Your 40' and 20' dimensions are going to work together to give some rotten combinations, and the 8'height is 1/5 and 2/5 of those dimensions too, respectively, so the room will ring like a bell at some frequencies.

Set your sine-wave generator to 155Hz (and 141Hz) and go take a wander -you'll be surprised.

>


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## glaufman (Nov 25, 2007)

GGA said:


> Are these assumptions correct? Would not a "reverse" waterfall showing the development of the sound over time rather than its decay be useful? Trying to compare individual time slices by changing the measurement window is very cumbersome.


Not necessarily. You can look at how things change by looking at individual slice on the waterfall. This should be better than gating the windows.


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## GGA (Oct 31, 2006)

Thanks again for the help. I will study the waterfall in more detail with a new outlook.

I am going to experiment with the signal generator set at 155Hz and see what I will see.

Below is a graph of my room. Sorry I did not post this sooner. There is a room mode at 25Hz of around 8dB, but that is a discussion for another time. Otherwise the response is fairly smooth. The RT60 is very flat at .3. The two hand drawn rectangles 133" apart are my speakers.

Anyone have any thoughts for the most likely pathway for that 155Hz null? I have tried my 2'x4' traps all across the front, the "den" at the left and the alcove at the right, but not the 8'3" ceiling.


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## laser188139 (Sep 19, 2009)

The difficulty in isolating the null, as always, is that it is not obvious what kind of null it is. Is it a simple reflection? Is it a modal response? Is it a combination of multiple reflections? There are several possibilities. 

You might follow the suggestion Nathan made, but using REW. If it is a modal response, it will vary with listening position. Depending on which dimensions contribute to the mode, it should vary (or stay reasonably constant) when the mic is moved left/right, forward/back, or up/down. Unfortunately, even this simple statement becomes complicated in the real world as the variation is gradual, not always sharp, and in your room you have multiple walls. But you could try moving the mic left and right in one or two foot steps and see what changes. Move it forward and backward and see what changes. Move it up and down and see what changes. A dimension along which it does not change actually tells you more, as it suggests that that dimension does not contribute to the mode. 

Assuming your 20' and 40' dimensions are correct, I played with Nathan's other suggestion, that your null might be a combination of several tangential modes. Many of these possibilities in the 155Hz range include the vertical dimension. If this really is your issue, you might see confirmation if you try the mic at 1/4 the ceiling height, 1/3, 1/2, etc. 

If you've not already tried it, you should look at using REW's RTA (Spectrum) function for this. You could set up a custom band limited pink noise test tone in a range that includes 155Hz, verify that the spectrum shows a null at 155Hz at your primary listening positioni, then move the microphone around and observe the differences in the curve. It is a little less time consuming than doing full sweeps at each measurement position, but is not quite as detailed in its measurements as it accumulates the data in buckets. You can use the Mode selection to choose RTA buckets down to 1/48 octave. As it is doing averaging, you have to wait for the data to stabilize or reset the averages after moving the microphone.

Good luck,
Bill


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## GGA (Oct 31, 2006)

Thanks for the good suggestions. Initially I did a number of measurements moving the mic front to back and left to right without much change. I never did up and down measurements. I think I will do another set of complete measurements as I have a better idea what to look for now.

Just to complete the picture, here is a graph of my right speaker with sub. The only eq is on the sub with -3.4dB at 44.5Hz for a small bump. I don't know if there is any point to mess with the 20-25Hz range (there is a nearly identical bump at 25Hz on the left sub). The sub uses a 2nd order 60Hz low pass and the main a 2nd order 80Hz high pass.









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And just for fun here are two sets of graphs for that left speaker. I need to teach my ears to only listen to the 45-50ms window. 

Null at 40ms, flat at 45ms









Still flat at 50ms, slight null at 60ms, permanent full null at 70ms


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## glaufman (Nov 25, 2007)

Is there a reason you're not generating a waterfall?


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## GGA (Oct 31, 2006)

I did post a waterrfall of the left speaker at 500ms in #9. I've looked at waterfalls at other gatings from 50ms to 200ms and nothing struck me as odd, but I don't know what I should be looking for.

I'd be glad to post any other waterfalls you'd like to see.


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## glaufman (Nov 25, 2007)

Sorry, I must have missed it. I'll go back and take a look.


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## glaufman (Nov 25, 2007)

Ok I see it now. The perspective makes it especially difficult to see what's going on. Perhaps a Spectral Decay plot would be easier here. For this one, start with the same 45-105dB scale, but then slide it up so the peak in the response is close to the top of the plot, to maximize the amount of decay shown.

That's certainly a complicated floor plan. Could you highlight on it exactly where you've tried placing those traps? You could check with Bryan in Acoustics, but I think those traps will function best 8" from a surface...


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