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Sound Reflection

Discussion in 'Room Acoustics / Isolation / Treatment' started by springforever56, Jun 11, 2015.

  1. springforever56

    springforever56 Active Member

    This doesn't apply to the studio so much in my case because I'm a field recordist but is there a list some where of the amount of sound reflection of surfaces (of all kinds) ? I recorded some heavy equipment today and later I realized that part of the reason the sound was as loud as it was is because the machines were surrounded on three sides by three store brick buildings. The made a horseshoe shape with the equipment at the open side. I thought about it as a large parabolic reflector. I was standing in the building directly across from the equipment so I was at the center of the sound, I think. Then I thought about how much lower the sound would have been if it have been a large open area like a field.
     
  2. DonnyThompson

    DonnyThompson Distinguished Member

    Obviously, the more porous the boundary, the less reflection there is, although that will also depend on the particular frequencies that are hitting that boundary as well. For example... 1" auralex foam will attenuate frequencies from around 1k and up, but it won't do a thing ( or very little) for lower frequencies.

    Brick walls, while perhaps somewhat porous under magnification, are going to reflect sound quite a bit. There might also be some diffusion if the surface is irregular, but the overall reflective properties of the boundary are going to make recording difficult if you are surrounded by them with no other absorbent surfaces resident. And, if you are surrounded on three sides by those hard, highly reflective boundaries, well... I think you've already realized what the results will be.

    But there are a couple different scenarios involved here - if you are standing (and recording) inside those boundaries with the source of the sound that you want to capture, then your mic will pick up a ratio of the direct sound and the reflective sound. If you are standing outside those boundaries, and are recording a sound source within those boundaries, you will get the sound of both the direct source and the reflection as one encompassing sound, but the microphone will pick up this sound as a whole, instead of picking up various ratios of each... but, it will also pick up any other sound sources nearby - or at least close enough to be audible.

    Field recording is its own thing,and I'm not an expert at it, so you might want to wait for others here to weigh in on this... I'm just trying to answer your question regarding that particular situation.

    Here are a few articles you might find useful:

    http://hyperphysics.phy-astr.gsu.edu/hbase/sound/reflec.html

    http://www.physicsclassroom.com/Class/sound/U11L3d.cfm
     
  3. Brien Holcombe

    Brien Holcombe Well-Known Member

    " If you are standing outside those boundaries, and are recording a sound source within those boundaries, you will get the sound of both the direct source and the reflection as one encompassing sound, but the microphone will pick up this sound as a whole, instead of picking up various ratios of each"

    That only depends on where you are at in direct reference to the collective sound wave curve. You will still get the reflections and the direct sound plus the noise in your specific area. True the pressure and velocity might be in phase but the direct and indirect sound are still subject to the surfaces involved. I think they call it reverb, flutter echo, etc.

    The fact that you had hard surfaces...buildings...this is your hard reflector, not parabolic at all, just hard surfaces. Recording in a free field environment should be obviously clear versus with hard boundaries in how the sound waves, pressure, rays, particles, et. al. will behave one completely different from the other. Free field will be lower in sound level and pressure simply because the source, theoretically, is not coming back to you at all...no reflections.

    If you record in a free field often you do not require specified information in the reflective ability of the surfaces around you. You should treat them all the same. A building is hard, a tree is hard, people are reflective, wooden fences will reflect, roads are reflective, etc.

    You should get the Master Handbook of Acoustics by F. Alton Everest, and spend more time recording in the free field with different situations.
     
    kmetal, audiokid and DonnyThompson like this.
  4. DonnyThompson

    DonnyThompson Distinguished Member


    This is the post you should be paying attention to, and not mine. Brien - an acoustics expert - just weighed in for both of us. ;)
     
  5. Brien Holcombe

    Brien Holcombe Well-Known Member

    I am not an Acoustics expert....I simply understand, from standing on the shoulders of giants, some of the things that we refer to. Please, I insist.
     
  6. springforever56

    springforever56 Active Member

    Thanks for the help. I suppose the more you know about your subject and it's surroundings the better. I don't hear the same I used to before I started recording audio, I step out of my car or my apartment and my ears are analyzing.
     
  7. springforever56

    springforever56 Active Member

    I just found that book,
     
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  8. Brien Holcombe

    Brien Holcombe Well-Known Member

    If you did not pay for it....you do not own it and should only use it to satisfy if it will meet your requirements. If it does, then purchase the book as we ALL have.
     
  9. kmetal

    kmetal Kyle P. Gushue Well-Known Member

    I borrowed one from the library for a couple years. I returned it after many reads, it really is fascinating on many levels.

    Fwiw to the op, you may want experiment with how mic patterns interact with your acoustic enviornment. You can learn to use both in a complimentary way. Enjoy!
     
    audiokid likes this.

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