Question:

Discussion in 'OT Technology' started by Mycophiles, Feb 21, 2004.

  1. Mycophiles

    Mycophiles OT Supporter

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    My mother is a physics teacher. She gets to play with a bunch of cool little gadgets and such at her job. Yesterday she was telling me about a gadget she has that will detect audio waves and give a readout in volts. She can then convert audio(volt) readings she gets to db.

    Here was her story.

    She had a kid in her class that wanted to find out the db level his car audio system was putting out. They took out the little gadget to his car and did a readout in his trunk. He then complained that it didn't work right because the db reading was nothing near the level it should have been.

    I told my mom this was the reason:

    If he took the readout in the trunk then most likely whatever he was using to detect audio waves was not detecting the full wave from the sub. Say the sub was putting out frequencies between 25 and 40mhz. He might have picked up 35-40mhz frequencies but nothing lower b/c there wasn't enouph room for the entire wave to complete itself within the room he had in his trunk. Think of it like this... when the sub makes a sound it puts out a wave but not in it's full length. It puts out a wave that completes itself, and therefore becomes audible only 5 feet from the sub. *(is this right?) The sub is simultaniously putting out sounds in the frequency range of 25-40mhz. This means that if the mike was put 4 feet from the sub then the higher frequencies were being detected but the lower frequencies were not being detected.

    Ok, now if your following me here, if they had some type of detection device that was detecting the volts from the audio waves it recieved then this meant that the detection device was determining the length of the wave upon detection. Then it calculated the amount of volts it takes to create that wave length. (highter frequencies requiring less power and lower frequencies requireing more power)

    I was telling my mother that his reading were askew most likely because he didn't put the microphone far enouph away from the sub to allow it to detect all the waves the sub was putting out. If he wasn't detecting the lower wave then he was missing half the volt readings on his readout so the db level they get when converting the volts to db would be much lower than what it should be.

    I also understand the device used to detect DB levels in car audio competitions is rediculously expensive. (7k and up)
    I'm wondering if my theory was correct. I really have no idea about this stuff.
     
  2. 04

    04 Guest

    first of all, its "hz" not "mhz" mhz stands for megahertz which is in the millions of cycles per second.

    And no, its not that the device cannot read the entire wave. If there is not enough room for the entire wavelength in your acoustic space's dimensions, the area becomes a pressure field (well when the area's dimensions are less than one quarter of the frequency's wavelength) This also allows for boundry reinforcement, which adds considerable gain to the signal. This is the reason a subwoofer sounds so loud inside the car, but if placed outside, its not nearly as loud.

    Think of it this way, the wavelength of a 40hz wave is 1128ft/sec / 40hz which is 28.2 feet for the entire wavelength. How are you able to hear a 40hz tone with headphones if a wave needs the entire space of 28.2 feet to expand and play?

    And its also very possible that the microphone used has a limited frequency response and does not accurately measure the sound pressure levels of low frequency bass.

    High frequency waveforms of a given amplitude has the same power content as a waveform of a lower frequency with the same amplitude.

    If your mother is a physics teacher, she SHOULD already know all of this, I don't understand why she didnt already tell you this :confused: Does she have a bachelor's in physics?

    Unfortunatly your theory is incorrect. The most likely reason your friends audio system did not read as high as he thought it should could be one of many reasons:

    1. the microphone could have been set to read sound only A weighted (on an A weighted scale, low frequencies are of considerably less power than midrange ones)
    2. The microphone was rather inaccurate
    3. The microphone was overloaded (very likely)
    4. Opening the trunk will reduce the cabin gain by quite a bit
    5. The system was incorectly calibrated
    6. You did not calculate the change from voltage to decibels correctly. Remember, a doubling of voltage is an increase of 6dB so you MUST use 20log(value1/value2)
     
  3. Mycophiles

    Mycophiles OT Supporter

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    "High frequency waveforms of a given amplitude has the same power content as a waveform of a lower frequency with the same amplitude." If power content means what I think it means then I understand.

    No, she doesn't have a bachelors degree in physics.

    Another question: Why do indoor subs require less power than car audio subs? Does is have to do with the room? Why do subs require more wattage than components? (in a general sense)
     
  4. 04

    04 Guest

    That means if you want say 110dB at 62.5hz and 110dB at 1000hz, both will require say 200 watts of power. If the speaker has a flat frequency response at both 62.5hz and 1000hz, it will take 200 watts to get 110dB from each. However, your speaker's cone will have to move 4 times more per halving of frequency at a given spl. So if the woofer has to move 3 inches peak to peak at 62.5hz with 200 watts, it will move .011 inches peak to peak with 200 watts at 1000hz.

    Home subwoofers require MORE power than car audio subwoofers all things equal. It may seem that car audio subwoofers need much more power because they use tiny boxes, and because of this, they need a lot of power to get the same output as a large home enclosure. But as far as loud bass is concerned, a small car like a hatchback is the best place to get ultra high levels of bass.
     
  5. Dobis P.R.

    Dobis P.R. Guest

    Why is your mom teaching physics without a degree?

    In car audio competitions the microphone is usually placed on the dash.

    SPL meters have limits. i.e. mine doesn't go above 128db and isnt very accurate in the lower octaves.

    People usually give their subwoofers more wattage than components because of two reasons. Subs are frequently less efficient. They want to play their bass at levels louder than the rest of the range.
     
  6. Mycophiles

    Mycophiles OT Supporter

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    She minored in PHysics and got her major in Biology. Then her masters. She's taught physics and chemistry for 25 years. She must have thought I was speaking strictly from a car-audio perspective.

    ps. You CAN'T teach without at least an associate's degree (you can be a substitute w/ an associates).


    I understand what you said 04. Here's another question. When you hear a sub out of tune. How do you tune it? Is it strictly done with the box or what? I heard a system today while I was studying at starbucks that had me floored. These people no doubt were on the other side of a busy street and putting out SERIOUS sound.

    It had to have been a professional job and def. more than one sub but what struck me as awesome was the great sound coming from the sub at such distance. (I dont' think my sub can even be heard from 100 feet much less the 100 yards these guys were away)

    Also, when building a box for a sub how much difference are things like nails and such going to make in the overall tuneing of the sub? How can I get better tuning from my sub? Take the port out? What should I start thinking of doing here? I'm imaging building a new box. I had an enclosed box at one time and it sounded exactly like those guys on the street. VERY VERY loud and playing ALL the tones very well with great responsiveness. With the port it's a different beast. It sounds good but on the low notes I feel it more than hear it. That leads me to think it could be a tuning issue. Is there anything I can do about it?
     
  7. On_One

    On_One Guest

    My guess is the meter was a weighed or an industrial meter/mic that maxs out at 120.
     
  8. 04

    04 Guest

    I don't understand what you mean by hearing a sub out of tune? Are you talking about a misaligned woofer enclosure?

    If you are talking about the vent or helmholtz resonator, then the tuning frequency is determined by the surface area of the vent opening and the length of the vent. It is COMPLETELY independent of the driver. A 3 cubic foot box for example tuned to 25hz will always be that way regardless of the speaker you have in it.

    I would guess that the car you heard across the street was misaligned, and designed to peak excessively at a specific frequency. Thats why it sounded so loud. So in effect, it was probably poorly tuned.

    The nails and construction materials are important up to a point. It isnt going to matter for instance if you use liquid nails or wood glue to glue the enclosure together.

    As for your system setup, as far as high output is concerned, you would want to go with a vented enclosure. My guess as to why the vented enclosure did not go very low in frequency is because you tuned the vent much too high in frequency, and it unloaded when you played such low frequencies. I would next time try tuning to around 25hz or so if you like the really low frequency bass, because it seems most car audio guys tune their subwoofers quite high in frequency, often times 40hz or higher.
     

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