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Thread: Small Room Acoustics

  1. #11
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    Hi John


    Nice post and even if I haven’t read it that closely, IŽll bet IŽll agree with most of it … except this:

    Quote Originally Posted by jhbrandt View Post
    If your speaker face is 1.22 meters or 4 feet from a boundary the delay will be 3.5 mS. This is half the length of a sound wave at 141 Hz., therefore full trapping is required at this frequency to eliminate the destructive reflection. NOW, push the speaker closer to the boundary so that it is half that distance or 61 cm / 2 feet. The resulting reflection will cause a delay (compared to the source) of 1.7 mS, corresponding to the half-wavelength of 282 Hz.
    Assuming we talk about the reflection from the front wall (0 degree incident angle relative to wall) and receiver right in front of the speaker (if not; we need exact Cartesian coordinate positions relative to boundaries in order to calculate the path length for the reflection(s) and direct sound, in order to figure out the difference in path length between them):

    If the distance from speaker face to wall (behind the speaker) is 1,22 meters, the path difference between reflection and direct, is 2,44 meters thus the delay will be about 7 ms and thus the first cancellation frequency (only taking the front wall into consideration) will be around 70 Hz.


    Sincerely Jens Eklund

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    Quote Originally Posted by Jens Eklund View Post
    Hi John


    Nice post and even if I haven’t read it that closely, IŽll bet IŽll agree with most of it … except this:



    Assuming we talk about the reflection from the front wall (0 degree incident angle relative to wall) and receiver right in front of the speaker (if not; we need exact Cartesian coordinate positions relative to boundaries in order to calculate the path length for the reflection(s) and direct sound, in order to figure out the difference in path length between them):

    If the distance from speaker face to wall (behind the speaker) is 1,22 meters, the path difference between reflection and direct, is 2,44 meters thus the delay will be about 7 ms and thus the first cancellation frequency (only taking the front wall into consideration) will be around 70 Hz.


    Sincerely Jens Eklund
    Jens,

    LOL! It's not a matter of agreement! There is no disagreement in math unless it's wrong!. And yes, in my 'quick explanation' I did indeed forget to add the distances. Therefore, the actual, direct (0 degree incident angle relative to the wall behind the speaker) reflection at 4 feet or 1.22 meters will be delayed by 7.1 milliseconds which corresponds to half the length of 70.5 Hz which will be the lowest frequency cancellation caused by this bounce.

    The point in my example is still valid showing the shift upwards in frequency as the sound source nears a boundary. The higher frequency reflection is easier to treat.

    Cheers,
    John

    PS. More to come soon. I had been away since we were having flooding problems in our area. But I'm back home now with good internet again.
    John H. Brandt Acoustic Designs - ABOUT US - OUR WORK - RESOURCES "Twenty thousand dollars worth of Snap-On tools does not make you a Professional Diesel Mechanic"

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    SBIR calculations can be a brain twister indeed, and often subject for confusion (thinking of the big SBIR-thread on Gearslutz, I actually found the excel sheet I put together very useful).

    Sorry to hear about the flooding problems, nothing serious I hope.

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    Hi John,

    Thanks a lot for this thread. I have a question regarding mic sound reflection filters.
    Do you believe they can improve recording (vocals) in an untreated room - as I have heard views both agreeing and disagreeing, and which would you recommend?
    Many thanks,
    Robert

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    Robert,

    It is important to eliminate destructive reflections into the microphone when recording, however panels placed behind a cardioid pattern microphone will do little for room reflections. If the recording is important, the room should be treated so that the frequency response at the mic is uniform and uncolored as it is very difficult to remove the room coloration without affecting the original sound of the voice or instrument. Although today's software IS very good and filters can do the job, that's not the way a good record is made.
    Panels around the vocalist CAN improve the recording in a TREATED ROOM. They can HELP in an otherwise untreated room, but small panels that are mounted on the mic stand are, In my Opinion, far too small to have much of an effect - especially in an Untreated room.

    For absorption panels to work properly in an otherwise untreated room, you will need to place them AROUND the vocalist & microphone so that they; 1. absorb the initial sound waves from the vocalist and reduce the sound available to reflect off of surfaces IN the room. and 2. absorb any reflections that 'get through' and keep them from entering the microphone at interference levels (higher than -20 dB relative to the source sound).

    Panels will do next to nothing for frequencies below 250 Hz. The frequency range of 80 - 300 Hz is the most problematic for voice / vocal recording... That's why trapping is so very important for a vocal booth.

    Cheers,
    John
    John H. Brandt Acoustic Designs - ABOUT US - OUR WORK - RESOURCES "Twenty thousand dollars worth of Snap-On tools does not make you a Professional Diesel Mechanic"

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    Quote Originally Posted by jhbrandt View Post
    Robert,

    It is important to eliminate destructive reflections into the microphone when recording, however panels placed behind a cardioid pattern microphone will do little for room reflections. If the recording is important, the room should be treated so that the frequency response at the mic is uniform and uncolored as it is very difficult to remove the room coloration without affecting the original sound of the voice or instrument. Although today's software IS very good and filters can do the job, that's not the way a good record is made.
    Panels around the vocalist CAN improve the recording in a TREATED ROOM. They can HELP in an otherwise untreated room, but small panels that are mounted on the mic stand are, In my Opinion, far too small to have much of an effect - especially in an Untreated room.

    For absorption panels to work properly in an otherwise untreated room, you will need to place them AROUND the vocalist & microphone so that they; 1. absorb the initial sound waves from the vocalist and reduce the sound available to reflect off of surfaces IN the room. and 2. absorb any reflections that 'get through' and keep them from entering the microphone at interference levels (higher than -20 dB relative to the source sound).

    Panels will do next to nothing for frequencies below 250 Hz. The frequency range of 80 - 300 Hz is the most problematic for voice / vocal recording... That's why trapping is so very important for a vocal booth.

    Cheers,
    John
    That is something that I always found in theory to be ineffective with the 'reflection filters'. It does nothing for the room itself. Yes, it will help a bit with some reflections to a degree, but the big issues are not addressed. For one user to say it works, does not mean that it will effectively work well in another users environment. Correct me if I am wrong, but as little as 1 foot of length or width of a room can make a drastic change in the modes that cause issues right?

    Seems that in any situation where at least basic room treatment (the good kind) isn't applied, it is just guessing whether a 'reflection filter' will be useful. Not to say that it will not help, but It just seems to me that money would be well spent in more effective areas.

    It seems to me that $200 in bass traps, and a few blankets would do much more than just one of these devices.

    I do not have experience with these devices so I am just speculating here.
    PC Win7-64-24G i7-4790k/Cubase 9 Pro 64-bit/2-Steinberg UR824's/ADAM A7x/Event TR8/SS Trigger Plat Deluxe/Melodyne 4 Studio/Other things that don't mean anything if a client shows up not knowing what it wants.

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    I've read over and over that thing about how the listening position should be at 38% (or whatever) the length of the room. Can you comment at all on that, and specifically how it relates to your points about the distance from monitors to front wall?

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    Thanks for all the responses. I'm trying to piece together the best advice and suggestions.
    John, I understand what you say about the direction the mic is facing. How would view using a SR Filter, to cover the rear and side of a mic, with blankets put up as a wall behind the singer, in the direction that the mic faces?

  9. #19
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    I know you asked John, but blankets will help some. Obviously not the best choice, but you gotta deal with what you resources you have. Some get away with hanging blankets over boom stands or pvc pipe frames around singer and get decent results. The basic idea is to hinder the reflections of the room. You will still have low end issues because the blankets will not absorb them, but your sound will be a bit more focused.
    PC Win7-64-24G i7-4790k/Cubase 9 Pro 64-bit/2-Steinberg UR824's/ADAM A7x/Event TR8/SS Trigger Plat Deluxe/Melodyne 4 Studio/Other things that don't mean anything if a client shows up not knowing what it wants.

  10. #20
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    Appreciate the response.

    So, if blankets were coupled with a high-end reflection filter..?

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