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Thread: Bass frequency response

  1. #1
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    Bass frequency response

    I have been reading some books on acoustics and recording lately and am confused about something. I was reading about how low frequencies need more decibel level than higher frequencies to be heard equally by the human ear. Why is it then that we want loudspeaker and microphone frequency responses to be flat. Wouldn't a flat frequency response sound like the bass is softer than a higher frequency at the same decible level? Does audio gear technology consider the ear's response to low vs high frequencies and have to boost low frequencies to make all pitches sound equal to the listener?

  2. #2
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    What you're referring to is called the "Fletcher-Munson" curve. I suggest you do some digging on it. Here's the deal...

    Over the years us humans have developed a sensitivity in the 2-4K range. We hear those frequencies louder than anything else. We have a strong roll-off after that in both directions.

    One of the weird things about the Fletcher-Munson curve is that our ears sensitivity to various frequencies changes over volume. The louder the sound the "flatter" our hearing. Optimally we hear "flat" at about 90 db.

    If you've ever seen a home or car stereo with a "loudness" button on it then that's what it takes into consideration. It will boost the lows and the highs proportionally to the volume. At least it's supposed too. My experience is that they just boost and that's it. My old Sony car stereo boosted up 'til stupid loud and then one notch more and it turned off completely.

    Have fun and enjoy learning more about how we hear. I'm on that quest myself.

    Peace,

    James

  3. #3
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    Thanks for the reply James. Been reading up on the Fletcher-Munson Curve. Also, I read how sub-woofers are run through filters to create a flatter sounding frequency response during lower decibel outputs. I just thought it seemed strange that we try to design project studios with a flat frequency response, when our ears don't have one.
    You read so much about the importance of bass traps in a studio, but wondered if we needed to have a little extra bass, so that softer bass sounds would sound equal to softer higher frequencies.

  4. #4
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    No problem David!

    You read so much about the importance of bass traps in a studio, but wondered if we needed to have a little extra bass, so that softer bass sounds would sound equal to softer higher frequencies.
    This comment makes me think that you have a misunderstanding of what bass traps actually do. Let's assume a room with perfectly hard, stiff walls. Since all the sound will be contained in this room it's gonna bounce around. When a soundwave bounces off a wall it's gonna come back and cancle itself out at some point in the room. Where this point is depends on the wavelength of the sound and the dimensions of the room.

    With that bit of information saoking in your head think about what different points in the room are gonna be like sonically. You're gonna have big dips and boosts in various parts of the frequency range with each of these peaks/dips being slightly different at each point in the room.

    The biggest problem here is with low frequencies. A nice big 20 db dip at 80 hz at mix position would be a real bad thing for you trying to make mixes that translate to other rooms. To fix that problem we use "bass traps." The best type of bass traps appear to be "panel" absorbers which operate over a range of frequencies. The panel trap will convert acoustical energy into heat energy thus "soaking" up the LF acoustical energy. This prevents the sound from reflecting back and cancelling itself out in the room.

    The net effect is that good bass trapping results in an actual INCREASE of bass response in a room as well as a flattening of bass response.

    Hope that makes sense to ya.

    Peace,

    James

  5. #5
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    That does help, James. I have been thinking about bass traps incorrectly. Makes more sence now. Hadn't thought about the bass trap helping bass response by possibly preventing bass wave cancellation.
    Thank you!

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