Space vs. bass

June 28, 2022
 by Paul McGowan

Sometimes I get lucky.

Our loudspeaker guru, Chris Brunhaver knows more about the art and science of loudspeaker design than anyone I have ever known (and I’ve known quite a few).

He recently answered a poster’s question about bigger vs. smaller woofers in the same size cabinet. His answer is so well said that it bears repeating here in this post.

“Well, it comes down to “space versus bass” like a lot of speaker design decisions. It might seems counterintuitive but, all things being equal, using a bigger driver in a given enclosure actually gives you LESS bass extension. Larger drivers will give you more output capability (all other things being equal) but require a larger enclosure.

The amount of air displacement required for a given output level quadruples every octave you go down. For instance, to play 20 Hz at the same volume as 40 Hz for a given cone size, the drive needs to move 4 times the amount.

If you can double the excursion of a driver for a given distortion level, you can gain 6 dB of output at low frequencies (where displacement is the limiting factor). There is an IEC standard around the level of distortion for this rating (measuring the limits around the motor force, suspension compliance and inductance vs excursion). If you can optimize these curves (to give you more excursion for a given distortion), you aren’t adding distortion or creating issues.

It’s not so much about low mass or a strong motor – those are related to sensitivity, not excursion and linearity versus stroke, it’s about how linear these parameters are with excursion.

For what it’s worth, your use of transient response in this context is probably better described as “large-signal behavior” or dynamic linearity or something like that. I know that you are referring to reproducing big transients but the term doesn’t mean that in this context.

Here’s a little poster on the causes of these distortions and how speakers are optimized for greater excursion. We used these techniques in our woofers too.

Of course, it would be even better to have a larger driver with a higher level of excursion but the resulting enclosure size would need to be proportionally larger to the cone area difference.

Thanks, Chris.

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21 comments on “Space vs. bass”

  1. When I see (hear) how little amplification is needed in active speakers for a similar amount and quality of bass, compared to the amp power and damping factor needed in passive designs, I’d be interested why not at least in bass, an active design is favored? The need to tweak around with different amps then would still be possible in the mids and top region.

    Is it lacking coherence that would be an issue if no fully active or passive design is used?

    1. This is why subwoofers are almost exclusively active. Most “passive” subwoofers are actually “active” in real-life use, because of the use of a crossover upstream of the amplifier (this is how I run my homemade “passive” subwoofer).

      It’s been decades since I’ve seen a sub used with a passive crossover (after the power amp), mostly because the inductors required to get a steep 45 Hz low-pass are silly expensive compared to a miniDSP.

      The weird alternative is a passive line-level crossover ahead of the power amp, such as those by Harrison Labs.

  2. Paul, you say “Chris Brunhaver knows more about the art and science
    of loudspeaker design than anyone I have ever known”…are you sure?
    This can’t be right.
    Surely our esteemed ‘CtA’ knows more than CB…I mean he likes to infer that he does.
    Besides, CB doesn’t have a Klippel NFS to measure loudspeakers
    with & so how could he possibly know what he’s talking about? 😉
    (Love your brilliant mind Chris!)

    I am always amazed at how much clean & deep bass comes out of a single 45lb bass ported DeVore Fidelity loudspeaker cabinet with a single 10 cardboard cone in a 18′ x 13′ x 9.5′ room
    …now there’s some genius!

  3. One look at that linked poster told me to not only be in awe, but to completely trust those who design and manufacture speakers. Now I can have another Twinkie.

  4. What are some of your favorite track for hearing low freq? I personally like The Eagles Live – When Hell Freezes Over…Hotel California. Practically the opening with the kick drum. What other tracks can you recommend.

    1. Try ‘Golden Heart’ from Mark Knopfler’s album of the same name. It’s not as obvious as on ‘Hotel California’ from HFO, which also gets a big thumbs up from me, and your system or room might not like it.

  5. I have to take issue with “bigger driver in a given enclosure actually gives you LESS bass”. Experience tells me otherwise. I am only a hobbyist at building speakers, however I have built enough speakers to hear more bass with larger drivers in a bass reflex speaker design. Maybe in acoustic design speakers but in bass reflex design with correct port size, larger drivers dig deeper and provide more output giving much better presences every time. Under sizing an enclosure and/or not providing enough port area is another matter. Just my two cents worth.

    1. We’re talking extension and not output and it is very seldom that the same motor coil suspension compliance etc. is used on multiple driver sizes in a DIY so you are isolating just the cone area and cone mass contribution to the T/S parameters. You can enter mock speaker parameters into a box modeling program to just adjust the Sd and Vas etc.

      Yes, you can make a larger driver play in a smaller box by upping the mass, BL and using a stiffer suspension, but that’s not “all things being equal” and is shifting other parameters around (when the discussion was about cone size specifically).

  6. Dear Fat Rat,

    How do you know how much “clean & deep bass comes out of a single 45lb bass ported DeVore Fidelity loudspeaker cabinet with a single 10 cardboard cone”?

    Are you taking precise acoustic measurements? Or is it just your subjective impression?

    I have observed that people often think that what they are listening to is lower in frequency than it really is.

    20Hz is, in actuality, a lot lower in sound and tone than people often think it is.

    I am skeptical that a 10″ cone in a small-ish box is producing significant sound pressure level near 20Hz.

    Best wishes,

    Ron

    1. Hi there RonRes,
      What do these three brands of loudspeakers:-
      Peter Snell
      Audio Note
      DeVore Fidelity
      have in common?

      You left out, “in a 18′ x 13′ x 9.5′ room.”
      Coming from a home-audio retail sales background I’ve
      never been that concerned with “precise acoustic
      measurements” per se.
      My audio experience centres around my hearing &
      feeling the ultra-low bass permeate through my body.
      I have made no mention of 20Hz.
      I hope that this helps you by way of explanation.
      Cheers,
      Martin.

      ps. both PS Audio – FR30’s & DeVore Fidelity – O/93’s are rated down to 28Hz.

  7. I find this article interesting.
    I had a Kustom ported cabinet with three 15” JBLs. I was told it was for bass and I bought it from Dave Poling, a bass man.
    I went against the grain and played guitar thru it. I powered it with a Univox U-1086.
    It was crispy clear and Its midrange crunch would blow a toupee off. Even tuning and noodling unleashed a wave of dopamine and endorphins from the beast.
    My theory is it was like a big block V-8. It did not have to work to move air.

    1. Yes Tim we all have our own opinions but I believe also there ain’t no substitute for cubes. Bigger woofs move less and are working less than smaller drivers. I have never been disappointed with larger woofers. Paul’s infinity’s have many speakers reason being they are not working as hard move less and have less distortion.

  8. I like sealed acoustic suspension high excursion single or multiple smaller woofers 8″ or smaller designed to use the boundaries of the room for bass reinforcement. A larger subwoofer with a smaller bass coupler works great too. Also small high quality main speakers on quality speaker stands with quality subwoofers correctly placed in the room also do the job quite well. Woofer sizes and their performance really depend on the room and where they are being placed in the room.

    Speakers also perform way differently in a very large room versus crammed in a small room that cancels out frequency response and output. The room can be friend or enemy based on the design of the speaker. With electronics the designer doesn’t have to design it with much consideration to room size. It’s hard to make a speaker that works equally as well in all rooms.

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