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Thread: Ask an Acoustic Engineer (me)

  1. Back To Top    #21
    Wave Shepherd - aka Jazzi Justin Zazzi's Avatar
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    Re: Ask an Acoustic Engineer (me)

    I haven't tried making an air pump like this, but I think you'll be blasting bass into whatever is at the end of the air pump if you use 30hz. That sounds uncomfortable. Maybe try really low frequencies, like 0.25hz.

    For frequencies less than 1/5 of free-air resonant frequency fs, and for small displacements:

    Xvc,peak = (Vrms * BL / Re) * Cms * (1.414/1000)

    Example:
    BL=14.8 tesla*meter
    Vrms = 2 volts ac rms
    Re = 4.03Ω
    Cms = 0.126 mm/N

    Xvc = (2 * 14.8 / 4.03) * 0.126 * (1.414/1000)
    Xvc = 0.0013 meters
    Xvc = 1.3mm peak, one-way

    This assumes no restrictions from air pressure in the mechanism you make, and it assumes mounted infinite baffle. The actual amount of air pumped will decrease with those realities.

    You can turn this into air displaced per second by adding frequency and surface area of the cone. For an air pump, I think you need to multiply the result by two since a full stroke is peak-to-peak, whereas Xvc is peak one-way.

    If you do something amazing, please share?
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    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    #22

    Re: Ask an Acoustic Engineer (me)

    Hey Justin, thanks for doing this. I asked this in a separate post but would love your thoughts; what are people missing out on when they only use a single USB microphone when using a RTA for tuning? Assuming they follow a good process of starting with each driver, then driver pairs, etc and are moving the microphone to average out measurements? What information is being missed that could be helpful, if any? And what sort of things can be done to overcome these shortcomings? Just wondering what some of the technical aspects are with these sort of measurements.


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  3. Back To Top    #23
    Wave Shepherd - aka Jazzi Justin Zazzi's Avatar
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    Re: Ask an Acoustic Engineer (me)

    Hello mauian, and you're welcome!
    Thank you for the question.

    An RTA that uses pink noise will not capture any time-related information such as phase, impulse response, group delay, waterfall, and so on. Time information adds another layer of complexity to what you're doing, but also another layer of information you can use to your advantage. The thing I use time information for the most is setting time delays and polarity and ensure each speaker is summing together correctly.

    You can use an RTA to see phase interaction near a crossover point but you can only see if two speakers sum together nicely or if they fight eachother. If they fight eachother, you cannot see which one has a phase response you prefer so you don't know which one to try and adjust.

    Time information can be acquired using the acoustic timing reference in REW if you have a USB microphone, however I do not have much experience with that technique. I prefer to use a two-channel sound card with a loop-back cable for the timing reference which seems more reliable. You'll need to use the "measure" button to do a sweep to get time information.

    A relatively new feature in REW is the ability to vector-average measurements to reduce the influence of reflections and background noise. This method requires phase/timing information so you must use the measure/sweep method with some kind of timing reference, and that could be more challenging with a USB mic.

    Something else you miss out on with a simple USB mic is tons of headache trying to connect everything and settings that are initially confusing and you generally not being productive for a while. The learning curve is steeper, but it comes with more potential benefits too. Very double-edged sword kind of thing.
    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    #24

    Re: Ask an Acoustic Engineer (me)

    Quote Originally Posted by Justin Zazzi View Post
    Hello mauian, and you're welcome!
    Thank you for the question.

    An RTA that uses pink noise will not capture any time-related information such as phase, impulse response, group delay, waterfall, and so on. Time information adds another layer of complexity to what you're doing, but also another layer of information you can use to your advantage. The thing I use time information for the most is setting time delays and polarity and ensure each speaker is summing together correctly.

    You can use an RTA to see phase interaction near a crossover point but you can only see if two speakers sum together nicely or if they fight eachother. If they fight eachother, you cannot see which one has a phase response you prefer so you don't know which one to try and adjust.

    Time information can be acquired using the acoustic timing reference in REW if you have a USB microphone, however I do not have much experience with that technique. I prefer to use a two-channel sound card with a loop-back cable for the timing reference which seems more reliable. You'll need to use the "measure" button to do a sweep to get time information.

    A relatively new feature in REW is the ability to vector-average measurements to reduce the influence of reflections and background noise. This method requires phase/timing information so you must use the measure/sweep method with some kind of timing reference, and that could be more challenging with a USB mic.

    Something else you miss out on with a simple USB mic is tons of headache trying to connect everything and settings that are initially confusing and you generally not being productive for a while. The learning curve is steeper, but it comes with more potential benefits too. Very double-edged sword kind of thing.
    I did not expect this sentence... “Something else you miss out on with a simple USB mic is tons of headache.” It really made me LOL. Thanks for that and all the other good info!


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  5. Back To Top    #25

    Re: Ask an Acoustic Engineer (me)

    First off thanks for sharing your knowledge, pretty cool of you. I have a couple questions that aren't near as technical, at least I don't think they are.

    I have a question about sensitivity and xmas. I've heard, well read multiple post online about certain drivers having low excursion but that it was okay due to the high sensitivity, mostly about some midbass drivers. Is there any merit to this? I've always been under the impression that if you had 2 drivers of the same SD but different sensitivity that they would have to move the same amount to produce the same spl at the same frequency and the only difference would be the power used. Just curious if there is something I'm missing because the people that stated this seemed knowledgeable and I can't understand while the xmax wouldn't matter simply because of high sensitivity.

    Second quick question because I suck at searching, can't find relevant info. I've read before that everytime you double woofers you lower distortion but I can't remember by how much was thinking 6 or 12dbs. Have 4 6.5 subs that I intend to use, a pair up front and a pair in the back both in manifold configurations. Just wondering if they will be up to par distortion wise with the rest of my system as I will probably lowpass around 150hz at 24db.

  6. Back To Top    #26
    Wave Shepherd - aka Jazzi Justin Zazzi's Avatar
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    Re: Ask an Acoustic Engineer (me)

    Quote Originally Posted by Sandman View Post
    First off thanks for sharing your knowledge, pretty cool of you. I have a couple questions that aren't near as technical, at least I don't think they are.
    Hi there Sandman, and welcome to the forums!

    Quote Originally Posted by Sandman View Post
    I have a question about sensitivity and xmas. I've heard, well read multiple post online about certain drivers having low excursion but that it was okay due to the high sensitivity, mostly about some midbass drivers. Is there any merit to this? I've always been under the impression that if you had 2 drivers of the same SD but different sensitivity that they would have to move the same amount to produce the same spl at the same frequency and the only difference would be the power used. Just curious if there is something I'm missing because the people that stated this seemed knowledgeable and I can't understand while the xmax wouldn't matter simply because of high sensitivity.
    This is a tough one to answer without some context. Can you show where you found these claims so we can read the conversation too? There are a few ideas tangled together and I want to make sure I understand the context.

    Quote Originally Posted by Sandman View Post
    Second quick question because I suck at searching, can't find relevant info. I've read before that everytime you double woofers you lower distortion but I can't remember by how much was thinking 6 or 12dbs. Have 4 6.5 subs that I intend to use, a pair up front and a pair in the back both in manifold configurations. Just wondering if they will be up to par distortion wise with the rest of my system as I will probably lowpass around 150hz at 24db.
    I haven't heard that rule of thumb yet. It might work if you add more woofers and then play the same overall volume level as before. This way each woofer is moving a little less so each woofer will have a little less non-linear distortion since it is staying closer to the rest position. I don't think the amount of distortion reduction can be estimated simply like you suggest since there are so many variables involved.

    Are you maybe confusing the other rule of thumb where doubling cone area and also doubling power can result in +6dB acoustic? This one would also be easier if you share where you read this claim so we can read it too.

    Distortion in a subwoofer is usually caused be making the cone move too far. Cone movement increases with loudness and also with lower frequencies. If you install those subs and you think there is too much distortion, try adding a high-pass filter at 25hz, 30hz, or 35hz to limit the excursion at the lowest frequencies and then they can play the other frequencies a little louder while still behaving. This is also called a sub-sonic or infra-sonic filter and many amplifiers have them built-in.
    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    #27

    Re: Ask an Acoustic Engineer (me)

    99% its a dumb question but would you hear a difference if you were listening to a speaker that was spinning? Spinning as in you were still seeing the front of the speaker at all times.

  8. Back To Top    #28

    Re: Ask an Acoustic Engineer (me)

    I wonder if you could help me with this question...

    https://www.caraudiojunkies.com/show...asound-Whisper

    ...seeing as I haven't made any practical progress yet, due to work

    Thanks in advance

  9. Back To Top    #29

    Re: Ask an Acoustic Engineer (me)

    Quote Originally Posted by VX220 View Post
    I wonder if you could help me with this question...

    https://www.caraudiojunkies.com/show...asound-Whisper

    ...seeing as I haven't made any practical progress yet, due to work

    Thanks in advance
    You could rout some baffles with maybe a 10mm depth out of 18mm wood, oversize the outside and them cut some 9mm ply or mdf to a ring that is 1mm smaller to allow for grill cloth and make a press fit grill to suit (if trimming the pillars in vinyl allow 2.5mm) if trim painting the 1mm is good, spray the baffle area with paint forst so it colours it, then mask off the recess and trim paint the rest of the pillar, remove masking, fit speaker, fit trimmed grill

    i would round the inner edges if you can do so safely and put a small round on them, however you are dealing with tiny work pieces, I think I’d cut the outer edge of the grill nearly all the way through leaving 0.5mm of material, then cut the inner all the way through and then while still attached to the bigger piece of material use that as a handle to cut a small radius on the inside, Then cut the outer edge with a Stanley knife and remove any paper thin excess with sandpaper

    get a grill cloth that matches the interior and a vinyl or trim paint that matches also
    Last edited by dumdum; 04-01-2020 at 04:32 AM.

  10. Back To Top    #30
    Wave Shepherd - aka Jazzi Justin Zazzi's Avatar
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    Re: Ask an Acoustic Engineer (me)

    Quote Originally Posted by Jscoyne2 View Post
    99% its a dumb question but would you hear a difference if you were listening to a speaker that was spinning? Spinning as in you were still seeing the front of the speaker at all times.
    I like this question!

    I don't think there would be any difference if the speaker is spinning at a slow speed like 1 revolution per second, and if you are directly on-axis. If you were off axis by quite a bit then you might (maybe?) hear a kind of warbling in the higher frequencies where cone breakup occurs. I think this would be true since cone breakup modes are not always symmetric with respect to rotation.

    If you were listening to a horn then you would certainly hear a difference if you're off-axis since horns are usually designed with a very specific vertical and horizontal off-axis response that are different. For example horns for live sound can have a wide horizontal response and a narrow vertical response.
    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.

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