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Thread: Ask an Acoustic Engineer (me)

  1. Back To Top    #331
    Wave Shepherd - aka Jazzi Justin Zazzi's Avatar
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    Re: Ask an Acoustic Engineer (me)

    Quote Originally Posted by BigAl205 View Post
    Why do speaker drivers use a voice coil and permanent magnet instead of opposing electromagnets?
    Here is another way to look at this question. If you replace the permanent magnet with an electromagnet, then the polarity of the "permanent" magnetic field (B) would alternate positive and negative in sync with the music signal. This would change the voice coil behavior from push/pull to push/push essentially adding a 2nd harmonic to everything and probably sounding really bad.

    Thank you to Stefan Andrei Chelariu over at Project Ryu for the insight:

    https://projectryu.com/wp/2019/12/14...-loudspeakers/
    Measure with mics, mark with chalk, cut with torch, grind to fit, sand to finish, paint to match.
    Updated Justin tuning sheet (Justin and Erica tuning companion for SMAART and REW)
    Do it for them.

  2. Back To Top    #332

    Re: Ask an Acoustic Engineer (me)

    Assuming you could get an airtight seal. What would the effects of filling an enclosure with a different gas be? Say helium vs air.

    https://www.diyaudio.com/forums/subw...afluoride.html

    Sent from my XT1710-02 using Tapatalk
    Last edited by Jscoyne2; 04-16-2021 at 03:59 AM.

  3. Back To Top    #333
    Wave Shepherd - aka Jazzi Justin Zazzi's Avatar
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    Re: Ask an Acoustic Engineer (me)

    Quote Originally Posted by geolemon View Post
    That's right up my alley - are you taking a psychoacoustics class in person, or is it available online for anyone to take?

    I have done plenty of reading over the decades but now would be a perfect time, with me starting my install with an imaging-heavy install focus.

    Sent from my LM-G820 using Tapatalk
    I'm taking courses as a distance learning student at Penn State's World Campus. I started as a non-degree student since getting admission directly into the graduate school is stupid (need a very high gpa, lots of tests that I don't want to take, etc). After completing five courses as a non-degree student, I used my grades earned as evidence to petition my admission into the graduate program. It's a brilliant concept actually: do well in the program to use as evidence to be accepted into the program.

    https://www.acs.psu.edu/academics/distance/index.aspx

    The psychoacoustics class is ACS 498. I didn't know until right this second that the first two lectures are available for free. Wow, actually there are lots of classes with the first two lectures available for free:

    https://psu.mediaspace.kaltura.com/c...tics/195900263
    Measure with mics, mark with chalk, cut with torch, grind to fit, sand to finish, paint to match.
    Updated Justin tuning sheet (Justin and Erica tuning companion for SMAART and REW)
    Do it for them.

  4. Back To Top    #334

    Re: Ask an Acoustic Engineer (me)

    Awesome - thanks, definitely will check that out.

    I'm unfortunately also familiar with school struggles - I have been through electrical engineering, IT information systems, and nearly completed an MBA on top of it but cut that off at a BS in business.

    Along the way I was working full time for most of it, so I did a bunch of alternative programs for adults - even a first-gen online thing at one point.
    I was 40 before I paid off all those school loans, and I'd paid a bunch in the fly - I didn't borrow it all! And I hear that's worse today.

    Schools suck but sometimes it's the only way. At least these courses seem legitimately valuable.
    Last edited by geolemon; 04-17-2021 at 07:23 PM.

  5. Back To Top    #335
    Wave Shepherd - aka Jazzi Justin Zazzi's Avatar
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    Re: Ask an Acoustic Engineer (me)

    Quote Originally Posted by geolemon View Post
    I was 40 before I paid off all those school loans, and I'd paid a bunch in the fly - I didn't borrow it all! And I hear that's worse today.

    Schools suck but sometimes it's the only way. At least these courses seem legitimately valuable.
    I cannot fully express my raging hatred for how expensive school is. There are not vicious enough words to describe it. It's complete bullshit.

    I wish we could "build a wall" or enforce "separation of" school and profit. If I were king for a day, companies like Pearson would die in a dumpster fire. I would legalize college students literally burning Pearson's buildings to the ground and mark the ashes as legal tender to repay student loans, for a start.

    Even the pace I'm taking (two classes a year) is painful to think about in terms of cost (~$6k/yr) but thankfully my employers have been supportive of reimbursing since it's related to my work. Unfortunately, not everyone has such a chance.
    Measure with mics, mark with chalk, cut with torch, grind to fit, sand to finish, paint to match.
    Updated Justin tuning sheet (Justin and Erica tuning companion for SMAART and REW)
    Do it for them.

  6. Back To Top    #336
    Wave Shepherd - aka Jazzi Justin Zazzi's Avatar
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    Re: Ask an Acoustic Engineer (me)

    Quote Originally Posted by DirtyBumOAK510 View Post
    Thread revival!
    I just asked this one on Facebook and I got some answers but not necessarily the facts that I'm hoping Justin and others here can bring.

    So I have heard stuff like "You can't hear distortion in a subwoofer the same you can in a midbass", or that distortion in a subwoofer isn't a big concern due to how we hear those frequencies. I've also heard people say that low distortion subwoofers (such as XBL, Brahmas, C12XL, Ravens etc) suddenly make people get used to what a "low distortion" subwoofer sounds like.

    So how much distortion is audible below 100hz? between 100hz-1000hz? Tweeters?

    Part of where I was getting at is that a lot of amplifiers claim "less than 1% distortion", and we often shoot for .05 or less. With Dacs, these numbers can go into the .001 or lower. I've read on Audio Science Review many articles about the hard numbers of inaudibility for Crosstalk, THD, S/N etc. But how do these apply to subwoofers? Specifically for choosing an amp for a very "low distortion" subwoofer. I've also seen graphs from subwoofer makers that will show at what frequency their subwoofers get localized, which if I remember right was due to distortion where it suddenly becomes localized. So typically we have the rule of thumb that subwoofers become localized above 100hz. S/N ratio of 120db is completely inaudible unless you have "golden ears".

    Where's the data!? What's the number?! What can and can't we hear below 100hz (typical subwoofer duty)
    My teacher in the psychoacoustics class is really intrigued by this question and it seems counter to many of the concepts we have learned in class. She is going to do some research and see what she can find. I'm really stumped too and don't know where this will lead. This one might be very interesting!
    Measure with mics, mark with chalk, cut with torch, grind to fit, sand to finish, paint to match.
    Updated Justin tuning sheet (Justin and Erica tuning companion for SMAART and REW)
    Do it for them.

  7. Back To Top    #337
    Noob Jdunk54nl's Avatar
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    Re: Ask an Acoustic Engineer (me)

    Quote Originally Posted by Justin Zazzi View Post
    My teacher in the psychoacoustics class is really intrigued by this question and it seems counter to many of the concepts we have learned in class. She is going to do some research and see what she can find. I'm really stumped too and don't know where this will lead. This one might be very interesting!
    I can't wait to hear that answer!
    2014 F150 Limited -> Kenwood DDX-9907xr -> Helix DSP.2 -> Alpine PDX-V9 -> SI M25 mki in Valicar Stuttgart Pods, Rear SB17's, Sub SI BM MKV's in MTI BOX. Alpine PDX-F6 -> SI Tm65 mkIV, SI M3 mkI in Valicar Stuttgart Pods

  8. Back To Top    #338

    Re: Ask an Acoustic Engineer (me)

    In my very limited understanding, distortion will typically increase with excursion, and therefore the limited excursion of a given driver in a proper vented alignment would potentially produce less excursion-related distortion than the same driver in a proper sealed alignment.

    Presuming this is true, how might this translate to a passive radiator system, where active driver excursion is limited in the same way that it is in a vented alignment, while passive driver excursion is potentially great? Does passive driver excursion create distortion in the same way and/or to the same extent as that of an active driver?

    I apologize in advance if my question is unclear and/or misguided.
    "That's all, folks!"

  9. Back To Top    #339
    Wave Shepherd - aka Jazzi Justin Zazzi's Avatar
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    Re: Ask an Acoustic Engineer (me)

    That makes sense, good question. You're right that excursion will eventually increase distortion and this will also be true for a passive radiator becasue the suspension stiffness will also have non-linear behavior at larger excursions. So yes a passive radiator will have distortion at higher output levels and this is one reason why you sometimes see it recommended to use two radiators per one driver to keep the surface area higher and the excursion lower for each passive.

    However, ports in a vented box have their own non-linear behavior too. I'm not sure if a port or a passive would have more distortion or under which conditions it might be audible. I want to say the larger surface area of passive radiators will have a better coupling effect with the air and possibly a higher output efficiency, but I'm not sure because I've never heard a rule of thumb about such a thing.

    Interesting question!
    Measure with mics, mark with chalk, cut with torch, grind to fit, sand to finish, paint to match.
    Updated Justin tuning sheet (Justin and Erica tuning companion for SMAART and REW)
    Do it for them.

  10. Back To Top    #340

    Re: Ask an Acoustic Engineer (me)

    Just to add to that-

    Intermodulation distortion is what's being specifically discussed, and that's a first order (directly impacting output as the driver moves) distortion, often described as "the most audible distortion".

    The motor is one culprit so we'll start there [even though PRs don't have them, it's good to get your brain around it].
    Think of what happens at low volumes: The cone starts moving in and out, but not much. The voice coil is staying in the magnetic field, so the motor strength isn't fluctuating - and the speaker will sound clean.

    However as you turn it up, excursion increases, and windings start to leave the magnetic gap at the extremes of excursion. When that happens, the energized coil has no (or rather, less) magnetic field to push against - and so the motor strength is reduced ...but only at the extremes of excursion... causing the cone to slow down at those peaks of excursion, then speed up as it passes through center again, having a "modulating", compressing effect on the sound. It's just one type of distortion.

    So here's two - a "worst case scenario" of a motor that starts to lose motor strength nearly the instant it starts moving (an SPL competition sub could be like this, since they need ALL the BL they can get, to burp one note, AT the port tuning frequency):

    And here's one that's much better - you could crank this to at least 10mm or 15mm in each direction (20-30mm total excursion) before any audible IMD:

    Suspension does the same thing - and that's all a PR has! If you have a small spider, it can only stretch so far, and as it stretches it stiffens up - not unlike a rubber band. That exacerbates the issue in a speaker, because as the motor is petering out at excursion limits, the suspension can be resisting motion a lot, where it doesn't at all as the cone passes back through the center of motion.

    Not all drivers - and not all PRs - are equal in that respect. You've likely seen Klippel or DUMAX drive unit plots - they essentially are measurements to predict how much IMD (intermodulation distortion) will exist at different output levels.

    For motors, this can be mitigated by one of many linear motor technologies out there... Split gap like XBL^2 or the equivalent JL uses on the W7 by drilling out material on the pole piece rather than the top plate - or JBLs wildly complex DDD technology with the old GTi subs. Or even a simple underhung motor makes the BL curve wider and less sloped on each side.

    But suspensions are all that's relevant for a PR. For a passive radiator, you'd want to shop for one that has a huge spider - or something like my Earthquake SLAPS PRs that just have two fat surrounds suspending a disc that has a bolt to affix additional mass to the center.
    To be fair, I have no idea what their Kms curve is - they are just a toy I picked up:

    Here is a suspension that stiffens up quick - like a small, linear spider will:

    And here's a suspension that is maybe like JLs monster progressive-roll spider in the W7, which will allow for lots of excursion before IMD appears (right side chart):

    (There's better, flatter spiders out there, but Google didn't let me find better drive reports)

    There's another distortion tied to ports AND PRs - group delay.

    It's audible but in a different way. Instead of compressing, it's more of a blurring or smearing in the time domain.

    A PR and a vent are kinda the same thing -
    A PR is a mass that's suspended so it can move.
    A vent is a mass of air of a known quantity inside a tube.

    In both cases, resonance caused by the subwoofer acting on the air inside the box, in turn causes sympathetic resonance on the air in the port (making that air mass move in and out), or the PR to move in and out.

    Or - the easier and only slightly less accurate way to think about it is to imagine the cone moving in and out in slow motion...
    Sub cone moves in, that compresses the air in the box, which then pushes against the PR or the air in the vent - causing it to push out.
    And since this is happening VERY fast (30hz is still 30 cycles per second), just as that air is pushed out the port - the sub cone is now moving OUT, creating a suction inside the box, which in turn sucks on the PR cone or air in the vent - pulling it back in, "just in time" for the cone to move back in.

    But I'm sure you can imagine - even AT the tuning frequency, the vent and cone (or PR and cone) are never truly "in synch". It's close enough that you have the combined displacement of the "cone air" plus "vent air" increasing your SPL level, but it's not perfect - it can't be. Every cycle of sub motion will be trailed by some additional time of the vent or PR completing it's cycle - a shift in the time dimension called group delay.

    So another kind of distortion is that caused by this group delay effect. That's something you can't get away from, no matter the passive radiator you use... although tuning lower (meaning: more mass, for a PR - or a longer vent, for a ported design) can help.

    Now - imagine the IMD distortion of a cheap PR on top of the group delay. It's possible to have a clean sub muddied up by a cheap PR.

    It's even one reason vents are more popular than PRs - besides the additional expense of a PR, a vent doesn't have any IMD - it's linear.

    ...well, you could use too small of a vent and cause audible chuffing noise once a given subwoofer excursion is reached, but that's not quite the same thing.
    But - it IS a consideration that has to be accounted for when you go the "ported" route, just like you have to account for IMD with passive radiator designs.

    Have you ever heard the advisement to go with a bigger PR cone than your sub, or to use two PRs per single sub? That basically has everything to do with mitigating IMD, because in both cases you are reducing the PRs excursion, therefore reducing its IMD.

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