• Big errors in your thinking - David Aiken 16:18:50 05/23/11 (3)
  In Reply to: RE: Second thickness of gypsum posted by Richard P on May 23, 2011 at 10:42:21

  "how is it that, for example, a 60Hz fundamental with a wavelength of nearly 19 feet can be "reflected" by anything (rigid or not) until the fundamental's period is complete?"
  
  All frequencies are reflected, whether the walls are rigid or not. As far as bass performance in a room goes, 3 factors are important:
  
  1-when a room dimension is exactly the wavelength of the tone or an exact multiple of that, you get modal behaviour and that means fixed zones of reinforcement and cancellation along that dimension of the room.
  
  2-stiffness of the wall effects how effectively the wall reflects the sound. A very stiff wall reflects bass frequencies more effectively than a flexible wall. More effective reflection also strengthens both the reinforcement and cancellation that occurs at modal frequencies so the behaviour noted in 1 above is more apparent in a room with stiff walls than in a room with flexible walls. Flexible walls reflect less at bass frequencies than do rigid walls, and that reduces the severity of modal behaviour.
  
  3-you've got 3 axial dimensions in a rectangular room, more in some other shaped rooms. If 2 room axes have modes at the same frequency, the reinforcement and cancellation occurring when a mode is excited can be reinforced even more.
  
  At non-modal bass frequencies, the reflections don't "overlap" precisely and therefore don't result in the reinforcement and cancellation that occurs at modal frequencies.
  
  Modes have a bandwidth and it tends to be narrow in frequency. At low frequencies there are few modes per octave so each mode tends to "stand out", to be quite obvious. As the frequencies of the modes increase there are increasingly more modes per octave and while their bandwidths remain narrow, the greater number of modes per octave starts to cause the bandwidths to overlap and that starts to smooth out the modal behaviour which is why modal problems occur only at bass frequencies. Looked at over the whole audible range, the problem at bass frequencies is not that you find modal frequencies in that range but rather that you don't have anywhere near as many of them occurring as you have at higher frequencies. Modal behaviour can't be avoided in an enclosed space but the more modal frequencies you have per octave and the more they overlap, the less of a problem they are. The problem at bass frequencies is that there aren't enough of them to smooth the room response.
  
  Reflection also has nothing to do with directionality in this regard. It's the reinforcement and cancellation which occur at fixed points in the room which are the issue. Strong reflections can influence our perception of the direction from which a sound is arriving but that's less related to frequency and more related to strength of the reflection.
  
  If your basement listening room was open to the larger cellar area, Ie if it had a permanently open entryway or you listened with the door open, you would have the 3 axial dimensions of the room generating modal frequencies plus you'd also get some more modal frequencies generated by the dimension of the axis running between the walls opposing each other through the opening, plus the coupled space of the cellar outside the listening room would also influence low frequency behaviour in the room.
  
  
  
  David Aiken
  
  

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  • RE: Big errors in your thinking - Scholl 05:07:18 07/02/11 (0)
    In Reply to: RE: Big errors in your thinking posted by David Aiken on May 23, 2011 at 16:18:50

    Interesting discusion...I find in my house the room with best bass sound has a concrete slab floor and drywall walls and ceiling. The second best has wood floor and drywall walls and ceiling. The worst is the room in the basement which is drywall on concrete floor but inside of concrete walls and an armstrong ceiling.
    
    Scott
    

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  • RE: Big errors in your thinking - Richard P 10:01:39 05/26/11 (1)
    In Reply to: RE: Big errors in your thinking posted by David Aiken on May 23, 2011 at 16:18:50

    Thanks. While I find the title of your subject line unnecessarily harsh, I do think that the term "reflection" could lead to a misunderstanding in regard to discussions of low frequency standing waves, modes, nulls etc.
    
    Unfortunately, soundproofing considerations, the overall climate here in the northeast, and practical building consideration dictate rigid walls in my case. While it may be interesting to have rubber or paper walls, it would be more practical to know how much additional reinforcement of LF modal peaks and nulls occurs for every additional layer of 5/8" drywall (for example): 3dB, 6dB, 20dB?? I'm guessing it must be frequency dependent as well.
    
    While every material has it's own unique sonic characteristics, it strikes me that the tradeoff made between rigidity (and the apparently abhorrent LF performance that comes with it) and structure-borne resonances, rattles and extraneous environmental noises that accompany rooms with lesser soundproofing to be an equitable trade.
    
    
    

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    • RE: Big errors in your thinking - David Aiken 14:37:46 05/29/11 (0)
      In Reply to: RE: Big errors in your thinking posted by Richard P on May 26, 2011 at 10:01:39

      Didn't mean the subject title to be harsh.
      
      I haven't seen any data for how much reinforcement of modal behaviour occurs with additional stiffening of a wall, but I also don't know how one would quantify such shifts in the degree of reinforcement and, as well as being frequency dependent, it may also be dependent on room size as well. Those 2 factors are linked in a critical way in that room dimensions determine modal frequencies but I think that room size will have some independent effects.
      
      And yes, there's the trade off you mention, but numbers probably wouldn't help you find it. I suspect there's probably a fair bit of individual variation in preferences for where the 'sweet spot' in that trade off lies and that, even for the same person, you might find different preferences if the room were a living room in which they could not install visible acoustic treatments vs a dedicated listening room in which they could.
      
      In some way or other we all face compromises in optimising our listening environments. Some people get to build their room and get some degree of control over factors like size or wall stiffness but, as you point out, things such as climatic considerations can limit the degree of control over a particular factor, while others have to accept an existing room in a house. Some can install visible acoustic treatment while others can't. Some have freedom to place speakers and listening position where they will while others don't and so on. I don't think very many people ever get a totally free choice on everything affecting their listening environment. My advice is not to obsess about trying to achieve theoretical perfection on every detail of room construction, especially since setup of speaker and listening positions in the room is going to critically affect modal behaviour anyway. My view is that you get best results by accepting the limitations and compromises inherent in whatever room you have and working with them and the room in order to find what works for you in that room, then fine tuning things until you get the best you can. I don't think anyone can provide a nice, neat, step by step set of instructions that will deliver a result that will satisfy everyone, not only because of the huge range of variation in our rooms but also because we aren't all looking for the same result.
      
      If you understand what playing with a particular parameter in a room does, then you can often experiment with that parameter a bit more easily and you can develop ideas of how you can control the variation in that parameter in order to get things closer to what you want. Since the same aspect of the sound you hear is often influenced by more than one parameter, you can also often work out alternative strategies when playing with a particular parameter isn't possible.
      
      I think that reading a book like Everest's "Master Handbook of Acoustics" which devotes a lot of attention to not only the factors that affect sound behaviour in rooms but also to aspects of room construction and how they influence those factors might be very useful to you. The other book I recommend, Floyd Toole's "Sound Reproduction", doesn't address physical acoustics to the same degree and doesn't touch on room construction issues. It's also worth reading but judging from your concerns I think you'll get more of what you're interested in with Everest's book.
      
      
      David Aiken
      
      

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