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Room Considerations During Subwoofer Design

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  • Room Considerations During Subwoofer Design

    Question one - Do people consider the room during subwoofer design?
    I'm thinking about the frequency response/box tuning. My thinking is that if I can get the same frequency response using a smaller, higher tuned box, because of the room/cabin gain, there are a number of advantages. Among them less material costs, less finishing time, smaller and lighter (I'm not as young as I used to be), easier to build, higher WAF. Moving the Fb up by 3dB could reduce the box size by a 1/3 of the volume which is a substantial amount especially if using multiples.

    Which leads to my second question - what's the best way to measure the room impact on the response?
    My first thought is to baseline a sealed subwoofer using nearfield measurements, it's too cold and wet to be outside for a true farfield measurement. Then put it the room where the subwoofers will be used and measure it again. Make multiple measurements using different positions for both the subwoofer and the microphone. Apply smoothing, normalize the response curves, average them and then compare to the baseline. Might have to massage the data a little bit before the smoothing to remove any large room reflections.

    Does this seem reasonable? Any fatal flaws in my plan?

  • #2
    Yes, room gain can add dB where your sealed box rolls off. Depending on the room you can get reinforcing of certain frequencies. Jeff Bagby has a room gain and room pressure calculator that I have used in the past.

    Using a mic you can measure the frequency response at your listening position for room plus woofer. Move the woofer around to see how the response changes at your listening position.
    Use a near field measurement for the woofer only if you want to check you box build. Not sure why you want to do a bunch of other measurements.
    John H

    Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

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    • #3
      Isn't that what REW software is all about? I haven't used it, but taking the room into account I think is its forte.

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      • #4
        Originally posted by jhollander View Post
        Yes, room gain can add dB where your sealed box rolls off. Depending on the room you can get reinforcing of certain frequencies. Jeff Bagby has a room gain and room pressure calculator that I have used in the past.
        That's exactly what I'm trying to figure out and the best way to determine what it is. The car guys design flat to around 60Hz and use cabin gain to bring natural roll-off back to flat for the bottom couple of octaves. I want to do the same thing, but for my room. I want to determine what the room is doing vs what the subwoofer is doing so I can take that into account when I'm doing my final design. Jeff's calculator isn't going to work, the room has multiple openings and a stepped width.

        Originally posted by jhollander View Post

        Using a mic you can measure the frequency response at your listening position for room plus woofer. Move the woofer around to see how the response changes at your listening position.
        Use a near field measurement for the woofer only if you want to check you box build. Not sure why you want to do a bunch of other measurements.
        I'd be doing multiple measurements to try and figure out what the room is doing. I'm doing subwoofers for the room, not an individual seat.
        As an aside, if you're trying to determine the best location for a single subwoofer with a single MLP put the subwoofer at the MLP and move the microphone around.

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        • #5
          You could just put one in the room, and tweak the adjustment knobs till you like the sound!

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          • #6
            Originally posted by devnull View Post
            ... I'd be doing multiple measurements to try and figure out what the room is doing. ...
            In effect, you want to plot the room's acoustic decay rate vs. frequency. REW will provide a sonogram option that can tell you an awful lot.
            Click image for larger version

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            This is my 22x15x8' room. It should have a minimum resonance at ~25 Hz due to the 22' length. And there are little peaks at 25 Hz and 38 Hz (15'). The interesting one is the peak at 14-15 Hz.

            My room has a 5' doorway. That gives it a 1/4 wave resonance that shows up at half the predicted low peak. BTW, this is also a reasonable goal; 30dB of decay in >300 msec. with minimal variation with frequency. The biggest issues here are linear distortion, not decay rates.

            HAve fun,
            Frank

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            • #7
              Clueless here. What do the colors indicate?

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              • #8
                The plot shows decay time as a function of frequency. Colors are SPL level at that frequency and decay time, red near 90dB, blue near 60 dB.

                Note that 0 decay is not at the bottom, but rather along the ridge of peak SPL, as one would expect.

                Frank

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                • #9
                  To the OP question: I typically don’t consider the room when designing a sub and here’s why: my room changes. And my subs don’t always stay in my room. Or in my house for that matter. I figure that I should design a sub to be as flat and play as low as possible, and adjust as necessary for a room. Most mid or high end receivers have at least some rudimentary EQ that I can use to flatten out peaks if I want to.

                  To the second question: REW.

                  In the past, I didn’t take cabin gain in to consideration when designing subs for cars - mostly because I didn’t know how to properly model them. A little while back, I got tired of trying to shoe-horn 8’s in to my truck and gave in to the slim-sub movement. After modeling about 30 subs, I found one I liked. I was very surprised to discover that while the anechoic response looked absolutely wretched as it mimicked the silhouette of the Grand Teton, factoring in cabin gain gave a ridiculously flat response in to the low 20’s. Intrigued, I built the box, threw it under the backseat, and whuddyaknow? Great low end without the usual over-emphasized bottom octave or so. Not saying I don’t like loads of bass, but I was pleasantly surprised to see that cabin gain could accurately be accounted for.

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                  • #10
                    Originally posted by Blenton View Post
                    To the OP question: I typically don’t consider the room when designing a sub and here’s why: my room changes. And my subs don’t always stay in my room. Or in my house for that matter. I figure that I should design a sub to be as flat and play as low as possible, and adjust as necessary for a room. Most mid or high end receivers have at least some rudimentary EQ that I can use to flatten out peaks if I want to.

                    To the second question: REW.

                    In the past, I didn’t take cabin gain in to consideration when designing subs for cars - mostly because I didn’t know how to properly model them. A little while back, I got tired of trying to shoe-horn 8’s in to my truck and gave in to the slim-sub movement. After modeling about 30 subs, I found one I liked. I was very surprised to discover that while the anechoic response looked absolutely wretched as it mimicked the silhouette of the Grand Teton, factoring in cabin gain gave a ridiculously flat response in to the low 20’s. Intrigued, I built the box, threw it under the backseat, and whuddyaknow? Great low end without the usual over-emphasized bottom octave or so. Not saying I don’t like loads of bass, but I was pleasantly surprised to see that cabin gain could accurately be accounted for.
                    Yup, in the past I've done my designs as flat as possible. In this case I was going to try to design smaller boxes for ease of assembly for a permanent install in my attic. That idea has been kicked to the curb, after crawling around up there to take some measurements I decided it's just too much of a pain in the a**.

                    I've never take my builds out of the house except for the occasional measure during build. A quick phone call to my brother-in-law gets me portable prosound speakers, subwoofers, power cables, snakes and a diesel genny. They do a lot of outdoor events where he works so a small setup is 8 JBL EON615 speakers, 4 JBL618S subwoofers and a 12KW Onan genny.

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