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Thread: Ask an Acoustic Engineer (me)

  1. Back To Top    #191

    Re: Ask an Acoustic Engineer (me)

    Quote Originally Posted by jtrosky View Post
    Just to clarify - my question was just based on the fact that I just didn't understand how the GS25 speaker cone could possibly move up to 4mm with it's surround, especially since the GS690 speaker cone only moves a max of 5mm with it's MUCH "bigger" surround.

    Maybe I don't understand the Xmax "spec". I am assuming that an xmax of 4mm means that the speaker cone is capable of moving out by as much as 4mm (and that the "size" of the surround is relative to the amount of movement that is "allowed" by the xmax spec). I just didn't quite understand how the GS25 speaker cone could possibly move out 4mm with it's puny surround while the GS690 speaker cone is only able to move out by 5mm with it's pretty significant surround.

    Hopefully I'm explaining myself properly. :-) The surround on the GS25 just doesn't seem "big" enough to allow 4mm of cone movement being that the surround on the GS690 only allows for 5mm of cone movement.

    The surround on the GS25 is also significantly "smaller" than the surround on any of the other 3"/3.5" speakers I've seen (HAT S3SE, multiple 3" or 3.5" coaxials, etc) - all of which have xmax values less than 4mm.

    Just something that I noticed and thought was strange, that's all. In reality, I can't imagine that the GS25 speaker cone would ever really need to move anywhere near 4mm in real-world usage - it barely seems to move at all, unlike bigger speakers that play lower freqs.
    Quote Originally Posted by jtrosky View Post
    Just to clarify - my question was just based on the fact that I just didn't understand how the GS25 speaker cone could possibly move up to 4mm with it's surround, especially since the GS690 speaker cone only moves a max of 5mm with it's MUCH "bigger" surround.

    Maybe I don't understand the Xmax "spec". I am assuming that an xmax of 4mm means that the speaker cone is capable of moving out by as much as 4mm (and that the "size" of the surround is relative to the amount of movement that is "allowed" by the xmax spec). I just didn't quite understand how the GS25 speaker cone could possibly move out 4mm with it's puny surround while the GS690 speaker cone is only able to move out by 5mm with it's pretty significant surround.

    Hopefully I'm explaining myself properly. :-) The surround on the GS25 just doesn't seem "big" enough to allow 4mm of cone movement being that the surround on the GS690 only allows for 5mm of cone movement.

    The surround on the GS25 is also significantly "smaller" than the surround on any of the other 3"/3.5" speakers I've seen (HAT S3SE, multiple 3" or 3.5" coaxials, etc) - all of which have xmax values less than 4mm.

    Just something that I noticed and thought was strange, that's all. In reality, I can't imagine that the GS25 speaker cone would ever really need to move anywhere near 4mm in real-world usage - it barely seems to move at all, unlike bigger speakers that play lower freqs.
    Yes, I agree. It’s surprising!

    My understanding is that xmax is the maximum linear excursion which is not the maximum length it can travel but the limit for which it can travel linearly.

    I agree, it’s a small surround. And also agree it barely seems to move. I think this is more because it’s not being asked to play low frequencies very loudly.

    I thought this was cool from Andy explaining frequency response graphs and how they relate to cone movement:







    What I think is neat, and correct me if I’m wrong, is that within region one is where xmax is defined - or at least I think it would, as within this region the cone is moving linearly. In this region, where the lower frequencies are, is where you may approach xmax and where the speaker is designed to stay within xmax (the speakers “sweet spot” if you will). Above this region, you are NOT exceeding xmax but rather the cone is being asked to move much quicker (back and forth) and for the given properties of that cone, motor, etc. the cone starts flexing under that stress and “cone breakup” starts to occur.

    You could, however, force the cone breakup to occur much earlier than designed (say in region 1) by sending too much power to it for example.

    Just some neat stuff that as always, Andy describes very well. I thought it was interesting to think about in the context of Xmax.


    Sent from my iPhone using Tapatalk

  2. Back To Top    #192
    Noob rton20s's Avatar
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    Re: Ask an Acoustic Engineer (me)

    I don't really want to jump into Justin's thread, but mauian is correct on Xmax. It is not simply the excursion of the motor. Many manufacturers simply use the overhang method for calculating Xmax (Hc-Hg/2) and call it a day. Some use modified versions of the formula ((Hc-Hg/2)+(Hg/3)) to make their drivers look better, but at least they are open about how they calculate their figure.

    Arguably, the best methodology to determine Xmax (actual linear excursion) is to test/measure the driver with something like a Klippel. Now, the Klippel simply gives you the data and relies on appropriate testing procedures. It is up to the manufacturer (or tester) to determine which data they will use and how they will use it. But the Klippel does allow for measuring non-linearities based on Kms (suspension stiffness), Bl (force factor), Le (Inductance). I won't attempt a deep dive in to how each of the types of nonlinearities might factor in for different driver types. Just know that testing like that done with the Klippel provides a far more accurate representation of Xmax than the overhang (or other calculated) method.

    And in car audio, chances are you will have no idea HOW a manufacturer determined the Xmax (if provided) for their drivers. There are a few that provide that information, but they are certainly the exception, not the rule. There have also been several instances where manufacturers who were "caught" with exaggerated Xmax figures when independent Klippel testing was performed.
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  3. Back To Top    #193
    Noob cueball981's Avatar
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    Re: Ask an Acoustic Engineer (me)

    Quote Originally Posted by geolemon View Post
    I would as well - and I'd also advocate Bnlcmbcar to do some searching some of the DIY audio forums that have home theater sections.

    From the diagrams that you posted, I'd be concerned as you are (especially in a car) that you'd narrow the image, because you are pulling some of the L and R content into the center - which, most troublingly - brings some of the L content to your R side, if you are in the driver's seat. Conversely, for a passenger, brings some of the R content to the L of them. Eek.

    What I'd like to see in a center channel for a car, would be "(L+R)-(L-R)". That way, it's only bringing the content from both speakers to the center, and even if there's a little content that's on both channels but simply louder on one than the other, it still mitigates that by attenuating that sound at the center channel. I'd believe that would at least help keep the stage width - hopefully as wide as with just a stereo pair.

    But I think really, in a car, the KISS rule applies... there's already glass, and absorbant upholstery, and plastic, all pointing different directions - you could have ONE speaker in a car and end up with a nightmare of multiple pathlength distances, direct and reflected (with each of those having a 180 degree shift - plus pathlength difference offset!) creating anything but a flat response as it arrives at your ears.

    So I subscribe to the "the fewer speakers the better" theory for car audio. There's exceptions - for example, three way components where you actually aim the mid and tweeter - can provide better imaging. But my default recommendation is simplicity over complexity, for those pathlength reasons.

    I'm really interested in this myself, but in no way for creating a center channel (IMO, there's already enough direct and reflected pathlength sounds wreaking havoc on image-killing phase interactions as they all ultimately arrive at the listening position)...
    I want to make a "L-(L+R)" channel and a "R-(L+R)" channel to add some rear fill (something I otherwise also don't believe in, for those same image-killing phase interaction reasons) plus some additional delay, so I'm going to be researching this soon myself. My DSP will only help with delay and passband. I'd be interested if you find any good threads.
    I have to agree with your simplicity statement. The best sounding systems I've had are the ones where I've only used a 2-way front stage with 1 sub. I'm currently running 3.5" coaxial in the upper corners of the dash, which I call a pseudo 3-way, and my staging and center imaging are fantastic!

  4. Back To Top    #194
    Noob cueball981's Avatar
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    Re: Ask an Acoustic Engineer (me)

    I'm one of those who is using just my truck's stock electrical without any issues. No "big 3", no upgraded alternator (truck comes with 220amp stock), and no capacitors...just one big 1/0 gauge power wire off the battery and a very solid short (<6 inches) ground at the amp ramp. I'm running 2 Alpine PDX-V9s and a PXE-850S DSP without any problems whatsoever. I'll be honest, I agree with JZazzi...it's better to install your equipment first, then see if anything needs upgrading. You may end up being surprised that there's nothing more you need to upgrade. Now you have money to invest in other areas of your system like sound damping!

  5. Back To Top    #195

    Re: Ask an Acoustic Engineer (me)

    Is it possible to build a sealed box that would act like both small and large boxes using resonators inside? Think ported boxes inside a totally sealed box.

  6. Back To Top    #196
    Wave Shepherd - aka Jazzi Justin Zazzi's Avatar
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    Re: Ask an Acoustic Engineer (me)

    /r/maybemaybemaybe
    Measure with mics, mark with chalk, cut with torch, grind to fit, sand to finish, paint to match.
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  7. Back To Top    #197

    Re: Ask an Acoustic Engineer (me)

    Quote Originally Posted by ckirocz28 View Post
    Is it possible to build a sealed box that would act like both small and large boxes using resonators inside? Think ported boxes inside a totally sealed box.
    Like a bandpass?
    Click image for larger version. 

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  8. Back To Top    #198

    Re: Ask an Acoustic Engineer (me)

    I'd like to educate myself more on the idea of absorption and materials and how they're rated. Perhaps how acoustic absorption works in a car.

    I was reading about the Magic Bus awhile back and while he is.. eccentric..he was talking about having the acoustic energy or decay in a room be equal across the spectrum. I don't recall exactly what it was.

    Im looking for reading material that doesn't take a degree to understand. Really anything on absorption that anyone here would recommend.

  9. Back To Top    #199

    Re: Ask an Acoustic Engineer (me)

    Quote Originally Posted by ckirocz28 View Post
    Is it possible to build a sealed box that would act like both small and large boxes using resonators inside? Think ported boxes inside a totally sealed box.
    Quote Originally Posted by Jscoyne2 View Post
    I'd like to educate myself more on the idea of absorption and materials and how they're rated. Perhaps how acoustic absorption works in a car.

    I was reading about the Magic Bus awhile back and while he is.. eccentric..he was talking about having the acoustic energy or decay in a room be equal across the spectrum. I don't recall exactly what it was.

    Im looking for reading material that doesn't take a degree to understand. Really anything on absorption that anyone here would recommend.
    At first glance, I was thinking you meant "box stuffing" before my brain kicked in - and since my brain is wired for car audio rather than real life, before I realized what you really meant, I said "Yes, box stuffing IS sort of like what @ckirocz28 is talking about!"

    It's funny you raise this topic though, here.

    In my house, I'm currently looking for sound absorbing materials, simply because I have a finished basement with a drop ceiling, and the cold air return goes over the middle of the space down there... so any sound, TV, stereo, video games, anything - only has a thin sheet metal cold air return preventing the sound from being piped up to the first floor via the cold air return. So in my case, I'm looking for a material that will BLOCK sound. And in the home space, there are sound-absorbing boards, sound-absorbing acoustic foam (like anechoic chambers or recording studios), and I've recently been told by a professional commercial-space drywall buddy of mine to look into mineral wool - sold pretty cheaply in the insulation aisles of home centers.
    Those materials are designed to absorb sound, so it doesn't pass through them.

    (box stuffing does that also - but now makes me wonder why we use polyfill rather than mineral wool, since I believe mineral wool has better properties)

    On the other hand, if my cold air return was buzzing and vibrating every time my furnace kicked on, that would be annoying sound caused by the panels themselves. Same if my basement stereo system caused panels to buzz every time certain songs were played loud. in that case, I'd be looking for damping materials - like Dynamat or Second Skin (or whatever the kids use these days) to stop the panels from vibrating. You'll notice if you close a cheap car's door, it makes a higher pitched "empty can" sound than the dull "thud" of closing a luxury car door. That's mainly the effect of damping materials lowering the resonance of the doors. If I apply that material to those panels that vibrate - I'll (hopefully!) stop them from vibrating and resonating.

    And a third type of acoustic absorption might actually be what you are talking about... In a car we've got a terrible combination of hard, reflective glass and plastic surfaces - and plush, soft, foam-filled seats. The glass causes some reflections, and the seats cause some of the sound to be absorbed. Both, in combination wreak havoc on our actual frequency response at our seating position. I hope you aren't asking for a simple (or even complex) way to predict these effects in any accurate way?

  10. Back To Top    #200

    Re: Ask an Acoustic Engineer (me)

    Quote Originally Posted by ckirocz28 View Post
    Is it possible to build a sealed box that would act like both small and large boxes using resonators inside? Think ported boxes inside a totally sealed box.
    I think like Holmz said - a bandpass box would get you close, although it's more the speaker between two boxes.
    What you are asking is "what would the effect be, if you built a big sealed box with a little vented box just sitting inside it - no speaker attached to the vented box, just a box with a port in it" right?
    Let's think through it to see if we can get close-
    I'd believe sure enough the sub in the sealed box would cause pressure fluctuations in the sealed box... which would in turn act on the air in the vent, which would move in resonance with whatever frequency it was tuned to given that box... and that would follow the same rules as vented boxes, as far as the vent resonating in phase or out of phase with the cone...
    That WOULD have the excursion limiting effects on cone motion (I believe) above the tuning frequency, since the port air motion in this case would purely be adding to or decreasing the effective pressure in the enclosure - same as a vented box, but not [significantly, anyway] contributing to the sound outside the box, since the vented box is trapped inside the sealed box.
    Also similar to a vented box, I would think that below the vent tuning frequency, the vent-effect collapses, and now you'd re-capture the air inside the vented box, for use by the sealed box.
    Same goes if you used a passive radiator, if my thinking above is all correct.

    So let's assume we stuck a 1 cu.ft. empty box with a vent (or passive radiator) in it tuned to 40hz (but no sub or sub hole) inside of a sealed box that was large enough to give us a 2 cu.ft. sealed volume even with that vented box inside it...
    if the air inside box one is Vb1 and the air inside box two is Vb2, I'd predict (but I don't know for a fact):
    • Below 35-40hz, the sub would behave as if it were in a 3 cu.ft. box. So if you modeled up a 3 cu.ft. box, and looked at 40hz and down - that's what you'd get.
    • Right near 40hz, the sub and vent are as close to in-resonance as it gets - which would in this case cause the vent to add pressure inside the sealed box causing a sharp reduction in excursion, causing a dip in frequency response.
    • Above 40hz, the sub and vent go progressively out-of-phase, so that destructive interference would fade away, until you get to whatever frequency the vent wouldn't have any impact any more... above that frequency, we're behaving like we're in a 2 cu.ft. sealed box... which at those upper frequencies is going to be pretty close to the 3 cu.ft. response anyway.


    So - overall, I think if you modeled your sub in a 3 cu.ft. box and imagined a decent dip in the frequency response around 40hz (I'm guessing about 6dB), and then rising back up to the -0dB level again progressively by about an octave higher (1 octave is also a guess) as you move further up the frequency spectrum.

    But that's just me trying to think through it - I can't say I know that is what would happen. I don't believe there's any software that models this, to know for sure.

    Do you have a need for a sealed box with a dip in the response? I could only see this as being really beneficial for subs stuffed in sealed boxes that are too small and have peaky responses to iron out - but in those cases of course you don't have room to stuff a vented box inside, or you'd have enough air space to begin with.

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