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Thread: Improving low frequency sound

  1. #1

    Improving low frequency sound

    Hi everyone,

    I am building a synthesizer with the Teensy 4.0 and the audio library. So far this has been delightfully accessible and I am liking the results. I ordered a MAX98357A Class D Amp to test the sound with an old bluetooth speaker which had defective bluetooth functions. For the higher frequencies this sound really nice, however I would like to have richer, fatter bass sounds. I would love to be able to replicate sounds like the Moog Bass!

    Right now using a sine wave with at full amplitude and having the MAX98357A at full amplification you can barely hear frequencies below ~120Hz, the only audible sound is the speaker rattling on my desk.
    Now I'm not really sure how to proceed.
    I ordered some MCP6002 op amps just to tinker, and maybe amplify the lower parts for another speaker just for the bass parts, but since the lower frequencies need so much more drive I'm not even sure if the 5V amps will suffice, or if 12V are needed. I'd like to stay with 5V to power the system over USB.

    One more thing: The MAX98357 does not have single ended output, though I'd like to use the Line-In from my guitar amp. I thought about using the MCP6002 to make a differential amplifier, which converts the signal to single ended for me to use. Is this a viable approach? Do you have any suggestions how I can improve my lower frequency sounds? Do I need more power, better parts?

    Cheers

  2. #2
    Senior Member PaulS's Avatar
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    I'm afraid the only way to improve the lower frequency sounds, is to use a bigger speaker. You are talking about a bluetooth speaker so I'm guessing the speaker itsself is probably 2"-3" in diameter? Such a small speaker can not move a lot of air required for a decent bass. Depending on the cabinet, such a speaker has a roll-off of -12dB/oct below 100-150Hz.
    Now, you could electronically compensate for this roll-off by adding 12dB/oct gain at low frequencies but the amount of power rapidly goes up to huge numbers of Watts [12db means 2^4=8 times the power - every 3dB is double the power]. Even if you could supply these watts, which the MAX98357A can't, then you would still need a speaker that can cope with the large cone-excursions that these Watts will cause.

    So, as said, only a larger speaker will really bring you fat bass...

    Regards,
    Paul

  3. #3
    Yeah the speaker is about 2". Maybe for this project I'll stick to converting the differential output to single-ended then and stick to my normal standing amp, although that's not made for very low frequencies either.
    Any things I have to look out for, when looking for speakers if I ever decide to invest in them? Most I have looked at only specify a resonant frequency, but no real frequency response.

  4. #4
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    Consider either headphones (low bass with low power) or a subwoofer (which will need size and power).

  5. #5
    Senior Member PaulS's Avatar
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    Quote Originally Posted by i2sflew View Post
    Any things I have to look out for, when looking for speakers if I ever decide to invest in them?
    Depends on what you need them for but since you mentioned synthesizer and guitar, those speakers do not necessarily need to be high-end loudspeakers. In your case I would look for high-sensitivity loudspeakers in the 95-100dB @1W range - saves you money on power amps.
    This one is still on my wish list to experiment with: Fane Sovereign 12-250 TC. Costs about 80, not too bad.

    Paul

  6. #6
    Senior Member PaulStoffregen's Avatar
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    Are there some techniques (probably at one time patented) which improve the apparent bass response of small speakers?

    I know almost nothing about how this is actually done, but this is the one I've seen mentioned.

    https://en.wikipedia.org/wiki/Missing_fundamental

  7. #7
    @Paul sounds affordable, but still 80€ is not an investment I'd make any day. Something to consider for a future synthesizer project, which might have more oomph!

    @jonr I'm thinking about adding a Line-Out which would solve both problems. I could drive headphones, and if I need more bass I'd just connect it to my standard amp, which solves the power problem for me.

  8. #8
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    There's a rule of thumb for speakers - you can't have all three of these qualities, but at most two of them:

    1) small speaker enclosure
    2) high power efficiency
    3) good bass entension

    So the usual answer is fairly inefficient speakers with medium sized ported enclosure, gets you down to maybe 40Hz if lucky. With efficient
    speakers you tend to need several cubic feet of enclosure for good bass (this approach has fallen out of favour due to the room required).
    A tuned port or ports helps bring the bass response down by adding a resonance just where the natural response would be dipping.

    If you build a speaker from scratch you need to design the enclosure and ports to match the bass driver parameters to have a
    hope of flat frequency response. There are many software packages to help with this.

    My advice when shopping for speakers is avoid any that don't have a frequency response graph in the specifications, and
    look at the response with a critical eye.

  9. #9
    Quote Originally Posted by PaulStoffregen View Post
    Are there some techniques (probably at one time patented) which improve the apparent bass response of small speakers?

    I know almost nothing about how this is actually done, but this is the one I've seen mentioned.

    https://en.wikipedia.org/wiki/Missing_fundamental
    Never heard about that, that's a freaky concept! But I'd also like to have the pure sine waves come out in the end and get rid of those pesky overtones.

    Quote Originally Posted by MarkT View Post
    If you build a speaker from scratch you need to design the enclosure and ports to match the bass driver parameters to have a
    hope of flat frequency response.
    Oh so for proper bass I need to match the sorroundings as well? Sounds almost as complex as RF Design!

  10. #10
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by i2sflew View Post
    Oh so for proper bass I need to match the sorroundings as well?
    Surrounding room, not usually. But surrounding the speaker with an enclosure, yes, absolutely. Even the very best woofers give poor bass performance without an enclosure around them.

  11. #11
    Senior Member PaulStoffregen's Avatar
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    The other thing which has a huge effect, but takes a lot of extra space and is worthless for non-bass sound, is a "folded horn" structure.

    Lots of info can be found online by searching for "folded horn subwoofer". Here's one random image from google search to quickly show the general idea.

    Click image for larger version. 

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    Some of my Burning Man friends have built about 20 huge 16 foot long speakers of this design to run with a 12kW sound system, powered by a large diesel generator. I can say from 1st hand experience it gives pretty incredible bass!

    How well it might help a little 2 inch speaker, I have no idea.

  12. #12
    Quote Originally Posted by PaulStoffregen View Post
    Some of my Burning Man friends have built about 20 huge 16 foot long speakers of this design to run with a 12kW sound system, powered by a large diesel generator. I can say from 1st hand experience it gives pretty incredible bass!

    How well it might help a little 2 inch speaker, I have no idea.
    Haha well I think we have gone from one extreme to the other now. I guess I'd like to settle in the middle somewhere. I want to build a synthesizer some time, with a full range of keys and about the same dimensions of regular stage pianos, so pretty much the regular kind. (No space for diesel generators! )

    Those should be alright to be equipped with two speakers, that can do some damage bass-wise. Probably not able to be powered by as little as 5V, 2A I guess though

  13. #13
    Senior Member PaulStoffregen's Avatar
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    For a smaller scale example, search for "Bose wave radio". Lots of info online about its amazing design which makes the most of 2 relatively small speakers. Some seriously good sound design there!

    I have personally heard it too, and it sounds quite amazing in an ordinary room, even if it's nowhere near the scale of thousands of people at a rave in the desert.

  14. #14
    Thanks, some very nice info in this thread. I'll see if I can make it work, and at least get some better low freq response than this old bluetooth speaker.

  15. #15
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    Quote Originally Posted by i2sflew View Post
    Oh so for proper bass I need to match the sorroundings as well? Sounds almost as complex as RF Design!
    The speaker driver has to match the speaker enclosure, as they are both resonant systems and couple to each other.

    The surroundings are another thing entirely - typical rooms trap lower frequencies so there is a rise at low frequencies due to this,
    most noticable further from the speaker - if these resonances of the room are a problem, acoustic treatment of the room is the
    way to fix it. Carpets and curtains and soft furnishings do a pretty good job of taming room effects in a domestic environment.

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