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Simple Low Frequency Sound Generator

Discussion in 'Audio' started by Clemson2019, Feb 19, 2018.

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  1. Hopup


    Jul 5, 2015
    I would say both are not enough for good effects. That frequency is relatively low so you need bigger speaker and better amplifier than in your phones weak one.
  2. Clemson2019


    Feb 19, 2018
    Agreed. So would the schematic that @kellys_eye posted be a sufficient starting point to change according to the output I need?
  3. Hopup


    Jul 5, 2015
    I would just use some small audio module having output of 3-10wrms that and feed the audio signal using the cellphone etc audio source.

    You can download function generator software for phones idk how accurate they are but you probably dont need super accurate adjustment.

    Something like this might be good. This one has only 80hz maximum but it could go higher.
    Last edited: Feb 21, 2018
  4. Audioguru


    Sep 24, 2016
    Why do you think the 3.5mm jack on your phone has enough output power to drive a low impedance loudspeaker? Instead it has low output power to drive higher impedance earphones.

    Are you trying to make the entire scull vibrate back and forth? Maybe you need a small transducer that is designed to vibrate something. They are mounted on a wall or window and cause the wall to vibrate like a huge speaker.

    I looked in Google and found a YouTube video about "128Hz Heart Chakra Alpha Theta Meditation" and the weirdo descriptions about it. The tone is amplitude modulated at lower and lower frequencies and the text mentions the effects on a patient's mind but it did not sync with my mind. Hindu people believe this stuff.
    Here is the video:
  5. AnalogKid


    Jun 10, 2015
    You continue to mix the speaker rating (3 W) with the electrical power of the audio signal driving it (unknown). You can calculate the signal power by measuring the resistance of the disconnected speaker coil (don't trust the label), and then measuring the AC voltage across the speaker when connected. Power = voltage squared, divided by the resistance. This is not a perfectly accurate way to do this because the speaker's impedance changes with the signal frequency applied. But it probably is within 5%.

    Be sure to report what the voltage you are measuring is: True RMS, average, peak, peak-to-peak, whatever. If you are using a meter, tell us the make and model and we can determine it's AC measurement method and accuracy.

    I'm not a huge fan of Class D audio power amplifiers, but they were born for this project. There are many of them on ebay at just about any power level you need, and they will be way more efficient than a standard linear amp. They are small, run cool.

    The circuit in post #4, followed by a single-stage, narrow band, high-Q active bandpass filter such as a twin-T should give an adequate sine wave. Does the signal into the speaker *have* to be a sine wave? If not, things get even more simple.

    Also (side question), doesn't the resonant frequency of <whatever> in the skull change with size?

    Last edited: Feb 21, 2018
  6. Audioguru


    Sep 24, 2016
    I think an empty skull resonates better than a full skull, if you know what I mean.:)
  7. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    No doubt both are responsible. The phone doesn't provide enough audio power to the speaker, and the speaker is inefficiently coupled to the brain cavity.

    Why not? Just remove one of the them from the headband and press it against the forehead. Probably still won't be "loud" enough, but you can expand on the principle.

    In any event, you need to acoustically isolate the sound emitted from the back of the speaker from the sound emitted from the front of the speaker. These two acoustic waves are 180 degrees out of phase with each other and will tend to cancel in open air. You correct this, normally, by placing the speaker in a sealed enclosure, with or without a tuning duct, and with or without internal sound-absorbing material inside the enclosure.

    If a tuning duct is used (bass reflex design), the objective is to reverse the phase of the back-side acoustic wave so that it is emitted in-phase next to the front-side of the speaker and constructively adds to the front-side acoustic wave. A tuned duct is efficient, and ideal for use with a single audio tone, but the required dimensions for resonance may not be suitable for your purposes. Refer to any decent book on loudspeaker enclosure design, written in the previous century from about 1940 or later.

    A less-efficient alternative is to mount the speaker in a sealed box filled with sound-absorbing material. Only the front-side of the speaker is exposed on the outside of the enclosure. The idea is to completely absorb all the sound energy emitted from the back-side of the speaker so there can be no destructive interference between the front-side acoustic wave and the back-side acoustic wave.

    You then need a means to efficiently couple the front of the speaker to the patient's forehead, perhaps with a carefully shaped plastic pipe of suitable diameter to match the speaker on one end and a carefully sculpted opening to closely fit the forehead on the other end. You could use foam padding where the pipe contacts the patient's forehead to ensure a good acoutic seal. If it were my experiment, I would connect the 127 Hz sinusoidal source to a mains-operated audio amplifier. with perhaps as much as 100 watts capability driving a 4Ω speaker load. Include a volume control of course to adjust the speaker sound level.

    If you eventually, through experimentation, find a level and a frequency that "works" you can make measurements of the voltage applied to the loudspeaker and measure the current supplied to it too, thus determining the power required. Using this basic information, derived from actual experimental evidence, you can then decide how to proceed to make your "medical device hand-held and disposable battery operated."

    Don't forget to apply for FDA approval and perhaps patent protection. Or not: sometimes the best approach with a successful new product is to just make and sell a boatload of them until a Pacific Rim competitor copies the product and sells it for one tenth your cost to manufacture.
  8. William Toenjes

    William Toenjes

    Assuming this device will have an ethical use why are you engineering this "backwards"? You could first determine the power you need to have the effect you hope to produce with this frequency that seems to be your only CONSTANT. Then design using this constant instead of any certain speaker or battery. Good luck.
  9. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    @William Toenjes : This is an old thread from 2018, that's about the last time the op was seen on this forum. Thread closed.
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