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What are your tips on bass in small rooms?

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  • What are your tips on bass in small rooms?

    In the audio DIY world I've seen lots of advise on designing/building bass enclosures but not much on designing bass for rooms. Subwoofers are a popular bass enclosure but they're typically designed to function up to 80Hz then cross to satellite or main speakers. However, according to Floyd Toole's book Sound Reproduction, bass becomes modal in small rooms well above 80Hz (attached photo from Chapter 6.1 discussing room transition frequencies.)

    I understand commercial speaker designers. They've got to sell to as many people as possible so they cram as many frequencies as possible into their boxes. They even use vented designs to extend bass through resonance. Consequently, their designs ignore the distribution of low frequencies in small rooms.

    My question: what is your design approach to bass frequencies in small rooms?

  • #2
    The only factor I pay attention to in smaller rooms, and larger too for that matter, is cabin gain. Smaller rooms have cabin gain that kicks in at higher frequencies, so the speaker need not have as low a half-space F3 to give the same result as a speaker with a lower F3 in a larger room. Higher F3 can allow using smaller drivers, smaller enclosures and sealed enclosures.
    www.billfitzmaurice.com
    www.billfitzmaurice.info/forum

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    • #3
      Tip #1: Cabin gain. Take room measurements and understand how it relates to F3.

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      • #4
        My room is roughly 12 x 14. The room has substantial boost centered on 45hz. Because of that, all I need is a sealed f3 of about 90 hz or lower. I recently built a speaker with an RS270P-4. It's just about perfect for the room in a 1 cu-ft sealed box. With a larger ported box, I could increase the deep bass output, but I really don't feel like I'm missing much of the 25hz to 35hz range.

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        • #5
          Originally posted by rpb View Post
          My room is roughly 12 x 14. The room has substantial boost centered on 45hz. Because of that, all I need is a sealed f3 of about 90 hz or lower. I recently built a speaker with an RS270P-4. It's just about perfect for the room in a 1 cu-ft sealed box. With a larger ported box, I could increase the deep bass output, but I really don't feel like I'm missing much of the 25hz to 35hz range.
          What do you do about the modal distribution of low frequencies? Are you using wall treatments or optimizing placement for a single listener.

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          • #6
            Two smaller subs in different places

            The rule of thumb for sub placement is wherever it works best with the room acoustically is usually the worst place aesthetically! At least that is always how it works out for me..

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            • #7
              A room is a room is a room. I've not felt the need to alter anything. To me, it sounds well balanced. I've measured the room response with HOLM, and there are some peaks, and dips. There's not much I would try to improve, even if I new how.

              On the other hand, if my intention was to impress visitors, I'd build 3 cu-ft ported boxes for the RS270P, and I'd probably gain 10dB at 20hz. I just don't feel like I need the extra deep bass.

              What would probably be better is something like this. Otherwise, the 45hz range would be excessive.


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              • #8
                According to Floyd Toole, one room is two rooms. Or, one room has two sound fields separated by the transition frequency region visualized in the original post image. Short wavelength high frequencies behave like rays; long wavelength low frequencies interact with boundaries to create valleys and peaks in the room.

                I read a post on this forum a while back that confused me. A pro audio member said they filled a commercial space with subwoofers and complained the bass sounded better outside the building than inside. After working on distributed mode loudspeakers (DML) it finally dawned on me. The bass frequencies in the commercial space were modal. Inside the system you'd hear peaks and valleys depending on where you stood. Outside the system you'd hear the net result of all the peaks and valleys. That's how DML panels work. The physical panel vibrates causing peaks and valleys - modal - then we hear the net product radiate from the panel.

                In DML design using multiple actuators (Multi Actuator Panel) you want to try to locate the actuators where the antinodes build on the panel. At low frequencies below the transition zone, the air in our rooms is a vibrating structure/panel.

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                • #9
                  What happens inside the room is that boundary reflections combine with direct radiation at various angles of phase, which vary depending on the listening position. Where they're at or near 180 degrees apart you get a null. There are no nulls outside the room because there are no boundaries. It has nothing to do with the Schroeder frequency. This occurs at all frequencies, it's just more noticeable in the longer wavelengths. For reference:
                  https://ethanwiner.com/believe.html
                  www.billfitzmaurice.com
                  www.billfitzmaurice.info/forum

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                  • #10
                    Originally posted by billfitzmaurice View Post
                    What happens inside the room is that boundary reflections combine with direct radiation at various angles of phase, which vary depending on the listening position. Where they're at or near 180 degrees apart you get a null. There are no nulls outside the room because there are no boundaries. It has nothing to do with the Schroeder frequency. This occurs at all frequencies, it's just more noticeable in the longer wavelengths. For reference:
                    https://ethanwiner.com/believe.html
                    I don't know what to say about that page. The author is trying to make a point about audiophile myths so maybe he didn't want to complicate his argument with describing modal behavior of low frequencies. In terms of his overall argument with regard to audio cables it doesn't really matter, the audio cables aren't causing the audible differences. However, the scientific literature describes low frequency behavior in rooms as modal, i.e. vibrations not geometric reflections. It's a consensus view. In addition, engineers solved the problem of low frequencies in small rooms using modal analysis.

                    https://pdfs.semanticscholar.org/00d...e976a1a72e.pdf

                    The air in a room is a structure. It vibrates like a structure when low frequencies are present. That's why modal analysis and solutions work. The pro audio guys heard modal behavior inside the room and the net of all the modes outside the room. I wish I knew then what I know now because I could have told them why it was happening.

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                    • #11
                      You're confusing modes, which are primarily seen as moderate response peaks, and boundary sourced nulls. They're two different phenomena, with boundary reflection sourced nulls being far more audible and listener position dependent.

                      Also, in a large space where pro-sound is employed the most common reason for uneven low frequency response is the power alley.
                      http://www.prosoundweb.com/article/i...e_power_alley/

                      The power alley seldom exists in homes, where room interactions tend to dominate.
                      www.billfitzmaurice.com
                      www.billfitzmaurice.info/forum

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                      • #12
                        Bradley.S - your first post poses a question, but posts #8 and #10 are full of statements, not questions. Rather than telling Bill what Floyd Toole says (which he probably already knows) and explaining to him about air in a room and how it vibrates, why not take advantage of an expert in woofers and pro audio who is directly participating in the thread with you and ask him questions. For example, "Floyd Toole says X and I think Y, Bill what do you think?"

                        Note: I'm not criticizing you, but I want to learn from your thread too and this would seem to be a more effective way to generate a discussion around your topic.

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                        • #13
                          Originally posted by a4eaudio View Post
                          Bradley.S - your first post poses a question, but posts #8 and #10 are full of statements, not questions. Rather than telling Bill what Floyd Toole says (which he probably already knows) and explaining to him about air in a room and how it vibrates, why not take advantage of an expert in woofers and pro audio who is directly participating in the thread with you and ask him questions. For example, "Floyd Toole says X and I think Y, Bill what do you think?"

                          Note: I'm not criticizing you, but I want to learn from your thread too and this would seem to be a more effective way to generate a discussion around your topic.
                          It's ok to make statements when you reference scientific consensus and link to the research. What do you think?

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                          • #14
                            Originally posted by billfitzmaurice View Post
                            You're confusing modes, which are primarily seen as moderate response peaks, and boundary sourced nulls. They're two different phenomena, with boundary reflection sourced nulls being far more audible and listener position dependent.

                            Also, in a large space where pro-sound is employed the most common reason for uneven low frequency response is the power alley.
                            http://www.prosoundweb.com/article/i...e_power_alley/

                            The power alley seldom exists in homes, where room interactions tend to dominate.
                            I should have specified the room was a bar, not an open area or large room.

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                            • #15
                              Here's a Techtonics video that does a good job describing the nature of modal (vibration) behavior in a DML panel. You get cancellation inside the system but you also get a net output outside of the system. Frequencies below the transition region in small rooms will behave similarly to the DML panel. But, in a small room, you're inside the vibrating system so you're listening to the modal behavior not the net output.

                              So, beyond the common bass/subwoofer + enclosure design discussed in this forum you need to address the modal behavior. Harman/JBL figured out how to do it with multiple subs and digital signal processing. In addition, you can add low frequency room treatments. But the first step should be trying to copy whatever it is that Harman/JBL does. I wish I had their DSP algorithms and room measuring protocol.

                              Also, it should be obvious that full range speakers (e.g. towers) designed to reach low frequencies are bunk in small rooms. You can't address modal behavior below transition zone low frequencies in small rooms with two channel main speakers. Instead, you'd want a set of speakers for frequencies above the transition zone and another set(s) for frequencies below the transition zone. And that's probably not easy work to accomplish. Unless you're Harman/JBL. It's easy for them.

                              https://youtu.be/35-ANcQ_fT0?t=114

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