At what frequency does bass cancellation become an issue?

Discussion in 'Archived Threads 2001-2004' started by Martin Rendall, Jan 16, 2002.

  1. Martin Rendall

    Martin Rendall Screenwriter

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    As the subject asks, what is the typical frequency where bass cancellation (super crests, troughs) from multiple sources becomes a real issue for music?



    The consensus seems to be that bass management is a really great thing for this, and running a single sub is better than multiple subs placed around the room (the exception being multiple subs stacked).



    This seems to be less of an issue for higher frequencies, since people don't discuss it much here, so I assume there's a threshold where it becomes an issue. What would it be?



    I expect that room dynamics will affect the answer, but, if you were to generalize...



    Thanks,



    Martin.
     
  2. John Garcia

    John Garcia Executive Producer

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    Low frequency waves are quite long, as large as 8-10 FEET, which makes them much more susceptible to cancellation. This is also greatly affected by the size & shape of the room and the location of the sub relative to the walls. I would say this is more likely to happen below 50Hz, just to take a stab, as I think it's kind of hard to say at what point it really starts without a spectrum analyzer. I'd be curious to hear if anyone has done any readings.
     
  3. Bob McElfresh

    Bob McElfresh Producer

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    There are several issues & theories that are kicked around:
    PEAKS: Rather than a "trough", the most common problem is too much bass at some frequencies caused by wall enhancement/reflections.
    For a given wall, X, the wall reflect frequency F most efficiently by the following formula (with X measured in feet):
    F = 565/X
    So a 8.5 foot wall will reflect:
    F = 565/8.5 = 66 hz
    But you also have to worry about F x 2, F x 3, ... up to about 300 hz. So your 8.5 foot will reflect all the following frequencies better than others:
    66, 133, 199, 266
    This alone is not bad. But you need to do the numbers for all 6 surfaces (dont forget the floor->ceiling dimension for all walls). This will give you a mass of numbers.
    You put these numbers in a spreadsheet and sort by frequency. If 2 of the numbers are within about 10% of each other - a good guess you will have too much enhancement around this frequency.
    (Note: this analysis assumes smooth, un-broken walls all around and is more of an issue for dedicated HT rooms. Our living rooms have broken-up walls with doors/windows/fireplaces etc, which make this analysis a bit useless.)
    VALLEYS: One thing that helps reduce both the peaks and valleys is to NOT put the sub in a corner. The length-analysis tends to make the assumption that the sound has the full-length of the walls to reflect from. If you move the sub a few feet along a wall, it breaks the wall up into 2 sub-sections.
    This is why for music systems, putting the sub 2/5 the length along the longest wall is often prefered. It reduces the maximum peak in favor of more complex, but less intense bass. And reducing the peaks also tends to reduce the valleys (or break them up some more).
    Because of all the issues, it is almost easier to use a RTA or SPL meter and run 10, 20, 30, ... hz tones and read the values and discover where your room has too much enhancement.
    But to find the valleys, you need to repeat the test but move the SPL meter to the alternate seating locations and compare the results to the ones obtained at the primary location. (So yes, it becomes a big 3-D map of your room).
    Does this help?
     
  4. Martin Rendall

    Martin Rendall Screenwriter

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    Yes, it does, to some degree. It suggests that my question may have been based on a flawed assumption. You suggest that most of the "peaks and valleys" are due to reflection from walls. My question assumed that most would be due to having multiple bass sources, and them interacting with each other, directly. I'm assuming from your reply that their interraction indirectly (via. walls and other surfaces) is the principle cause of the super crests and super troughs. I must admit, I'm still not convinced they don't interract directly, substantially. I remember learning how to do that calculation in highschool... [​IMG]
    Martin.
     
  5. Kevin C Brown

    Kevin C Brown Producer

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    Martin- OK, so you can get peaks and valleys with two sources at various points in a room, and at different points in the room depending on what the freq is.. (I'm guessing this goes back to 2 subs placed near the mains... [​IMG] )
    I kind of view it like tossing 2 stones, at some distance apart, in water, and then watching how the waves interact when they hit each other. But that's just one freq too!
    Remember that an EQ can do almost nothing with the valleys. It can only attenuate peaks.
     
  6. Martin Rendall

    Martin Rendall Screenwriter

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    Kevin,



    Good guess, and almost correct. I'm not interested in a sub by each main, but I am wondering if I could improve bass performance by getting an external crossover for my mains. It all comes down to whether my mains could be crossed over below 90Hz, and not introduce more problems.



    That is, my mains are Paradigm Studio 100's, which should easily produce bass down to 50Hz or below, without difficulty. I'd like to crossover below 90Hz to the sub, and set the receiver's bass management to use mains only for LFE/Bass.



    This would theoretically have the side effect of filling a hole with my LFE, due to LFE going as high as 120Hz, while my sub is rated to 100Hz.



    BTW: I understand why it's not cut and dry - but if there's a frequency cutoff below 90Hz where the problems are minimal, I'd love to know.

    Martin.
     
  7. Ergin Guney

    Ergin Guney Agent

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    Martin,

    Two subwoofers placed in a room will, of course, interact. As an experiment, anyone can try putting two subwoofers side by side, with the phase of one inverted in relation to the other, and watch how virtually no low bass is produced due to the cancellation effect. Of course, that's an extreme example (not to mention a senseless one) but the subwoofers would interact under normal conditions in more subtle and indirect ways too. The reflection-based standing waves explained in Bob's calculations are due to the interaction of bass sound waves going one way with those that are reflecting from the wall and coming back the other way. With two subwoofers you have sound waves similarly going in opposite directions, but this time because of being emitted by two subwoofers in opposing positions rather than being reflected from a wall. It's not difficult to understand that some of the same mechanisms will be at work.

    As for your original question as to the frequency at which bass cancellation would start becoming an issue, perhaps a better translation of the question would ask "what's the biggest size of a bass null point (or reinforced peak) that you can tolerate?" Because, cancellation will happen at every frequency actually. For instance, the 66 Hz room mode that came up in Bob's sample calculation will actually have repeating instances all the way up to 6600 Hz, 13,200 Hz, 19,800 Hz, etc. (They're all multiples of 66.) But the difference will be that the size of each such standing wave in the room will be on the order of the wavelength of the frequency or half of it or so. In other words, a 6600 Hz standing wave (if it happens at all; see next paragraph) will occupy a diameter of only one or two inches. Meanwhile a null point at 120 Hz, for example, will have a size of around four to eight feet.

    Of course, another reason such high-frequency standing waves are not really produced in real rooms to begin with is that things like bookcases and framed pictures (and even the paint on the wall) spoil the perfect reflection of such high frequencies, and room geometry and wall finishes are hardly perfect enough to reflect these high frequencies precisely enough to cause persistent standing waves at those frequencies anyway.

    Getting back to the "size of the null spot" as a key to answering your question, we can do a calculation based on the highest frequency produced by your subwoofer to reach a "best case" number. Since most people don't use their subwoofers for any sounds above 100 or 120 Hz at most, we can take the 120 Hz example I've already used above. Since the null spot at this frequency will be at least four to five feet in "diameter", it means that the smallest null spot ever likely to be produced by the sounds coming from your subwoofer will be easily enough to engulf a single-person seating position (maybe even two people side by side). This, to me, means that there is no frequency produced by a subwoofer at which the bass cancellation effect will be ignorable.
     
  8. Martin Rendall

    Martin Rendall Screenwriter

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    The replies have been outstanding, so far! Obviously, some time has been put into them - very much appreciated!

    Martin.
     

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