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db vs. SPL

Discussion in 'Amps and Cabs [BG]' started by jondog, Aug 14, 2002.


  1. jondog

    jondog

    Mar 14, 2002
    NYC metro area
    Hi, can anyone tell me the difference between db and SPL? At what point, in db or SPL, can you begin to feel (as opposed to hear) bass frequencies? Thanks!
     
  2. BFunk

    BFunk Supporting Member

    Excellent question!!

    dB, (deciBels), is the measure of relative increase in _VOLTAGE_. Think of it as saying the amp increased the voltage by 2x. You mean the same thing when you there is a +3dB gain.

    SPL is sound pressure level. It is the mechanical measure of the change in air pressure due to an acoustic signal. Speakers cause bands of air to compress in waves. This change in pressure is what we measure as SPLs.

    Loudness is the measure of how much sound we hear. The difference between SPL and Loudness is that humans do not hear all frequencies at the same level. In fact the bass frequencies are much harder to hear than guitar frequencies even at the same SPL. As the sound-level increases this difference is less apparent.
     
  3. The deciBel is an electrical term that describes voltage gain. It is actually 20*log (the magnitude of the signal at a particular frequency), but the math's not important. The concept of the deciBel has been applied to more than just voltage gain, though. In fact the unit that SPL is measured in is the deciBel. It's used because, SPL follows the same rules as voltage gain. It needs the logarithmic scale because SPLs span many orders of magnitude. For example 100dB SPL is not 10x the intensity (power) of 10dB SPL, it's 10 to the power of 10 times the intensity. That is a 100 dB SPL sound has 10000000000x as much energy than a 10 dB one. Our perception is that a 10 dB increase in SPL is twice the loudness (generally). This is highly frequency dependent because of the way our ears work. Low frequencies and very high frequencies need to have more energy to seem as loud as midrange frequencies. Our ears are most sensitive to the range from about 1 kHz-4kHz, because that's where all of the consonant sounds (fricatives?) of human speech are centered. Anything lower or higher needs to have more energy to seem as loud, and this becomes extreme towards the extremes of the spectrum. Look up Fletcher/Munson curves for more explanation.
    In short, when it comes to sound levels the SPL is measured in dB!
    Hope this helps and was not too technical, I have a tendency to ramble about this stufff...:D
     
  4. jondog

    jondog

    Mar 14, 2002
    NYC metro area
    Thanks guys. I get the basic difference/similarity now, but I'm still wondering about the point where hearing and feeling overlap. The thump of a bass or kick drum through a good PA is felt as much as heard. That feeling in my gut is what made me want to play bass in the first place. I know that if I touched the cone I would feel it at low power, what SPL is required for people on the dance floor (10 feet from cone?) to literally feel the vibe?
     
  5. geshel

    geshel

    Oct 2, 2001
    Seattle
    It depends on the frequency. I mean, hearing *is* feeling after all, so really they're always feeling it.

    The "thump" you're referring to can probably be noticed when the bass is around 100dB or more, in the band from 50-100Hz.
     
  6. jondog

    jondog

    Mar 14, 2002
    NYC metro area
    Hearing is feeling, very well said! Thanks for the stats Geshel.

    Any thoughts on why the higher frequencies are more heard than felt? I do not feel cymbals in my chest, even when they're really loud, even though I know the air is bouncing off of me. Is it just that our ear drums are a LOT more sensitive to faster vibrations? Is there a resonant frequency for the human body or something like that?
     
  7. You the man Space. What you are talking about is a measure called "Phon". It's a rate of how the human ear percieves sound at varying intensities with varying frequencies.

    [​IMG]

    The chart shows how the human ear reacts to those varying dBa ("a" weighted scale...I'm suprised no one has mentioned this yet) levels.

    So what does that mean? Pretty easy, but requires a little thinking. If we look at the 10phon level chart, it tells us that at 10dBa, 1000Hz the Percieved sound intensity is the same as 42dBa, 100Hz. So you'd need a gain of nearly 32dBa at 100Hz to equal the apparent loudness of a noise at 1000Hz. This is why bass players need more power (well...ego has a little to do w/ it prolly also :D ). Then if we follow the chart on the 100phon line, we notice that the ear gets pretty flat in it's repsponse. Just showing that it varies in relation to actual SPL to Phon. And you can also see that the most sensitive portion of human hearing is in a range of roughly 1-4kHz (good call SG), because that freq chart is also a logrithmic scale.

    If you want, there's more info (got the chart from them) Here

    Asking the question of when you begin to feel bass depends more on the frequency rather than the Sound Pressure Level (dBa). Sounds like it'd be an interesting study...but I don't have that info lol.
     
  8. geshel

    geshel

    Oct 2, 2001
    Seattle
    It's more related to the physics of vibration in general. It takes more force to shake something quickly than it does to shake it slowly. This is because when the thing reaches the limit in one direction or another, you have to stop it and turn it around in a shorter amount of time (and it's also going faster to start with).

    Doing this takes more force (force = mass * acceleration). With sound, this force is manifested as the pressure that exists at the peaks of the sound wave.

    Since what we interpret as "loudness" is based on this pressure, for a higher frequency sound to have the same loudness (and hence, pressure), it must have a much smaller peak-to-peak vibration. This is (mostly) why tweeters don't need to move nearly as far as woofers to produce the same SPL.

    Hmm. . .this didn't quite go where I thought it would. Gott a think about it a little. . . :)
     
  9. jondog

    jondog

    Mar 14, 2002
    NYC metro area
    I appreciate your efforts :)

    It was making sense to me up until the tweeter part . . .

    What about deaf people? They would have to feel instead of hear. They would feel the same frequencies in their chest that I do, but would they notice a cymbal solo?
     
  10. geshel

    geshel

    Oct 2, 2001
    Seattle
    The tweeter part: if a high-frequency vibration of the same amplitide (distance moved) takes a lot more force (pressure), if you lower the amplitude until the pressure matches that from a low-frequency vibration, the amplitude will be much smaller.

    I think though in reality it comes down to just what you're able to feel. A kick drum hit at 100 dB moves a lot of air, and the air comes at you in a pulse that's bigger than your body, so it hits you more or less all at once. A cymbal crash at the same SPL doesn't move as much air (because the air has to move faster), and the wavelength is smaller than you are, so parts of it are "pushing" on you while other parts are "pulling" on you, so the net effect is that you might feel it on the surface of your skin, but it doesn't convey much total force.
     
  11. awesome

    awesome

    Aug 14, 2002
    Belgium
    SPL is sound pressure level and is expressed in pascale, dynes/cm² or microbar. some examples: 0.00002 Pa is threshold of hearing (we don't hear anything below this). 100-200 Pa is threshold of PAIN
    dB isn't an electrical term and has nothing to do with voltage like stated before, db is a powerratio. "dB = 10 log(power2/power1)". This are relative dB's.
    With absolute dB's your initial power is fixed.
    The formula for dB SPL is "20 log (final power/0.00002Pa)". (Here it's 20log because of ohm's law)
    For Wattage: dBm references to milliWatt
    For voltage: dBu references to 0.775V (=1mW over 600 Ohms)(used for pro mixers) or dBV references to 1V (used for consumer stuff)
    For digital: dBFS references to max digital level (Full Scale)
    SPL: dBA, dBC and dBD = dBSPL with filters that match the equal loudness contours (phon lines)

    This is probably more than you asked for, but I wanted to correct the mistakes the others made.

    greetz,
    Awesome
     
  12. jondog

    jondog

    Mar 14, 2002
    NYC metro area
    Wow, I thought I knew what was going on. :eek:

    So, if the right unit for SPL is Pa, at what point do you start to "feel" the band. You're obviously feeling them at 100 if that's pain, what's the bottom of the feel range?
     
  13. LJW

    LJW

    Feb 8, 2002
    northeastern pa.
     
  14. awesome

    awesome

    Aug 14, 2002
    Belgium

    You're talking about SPL's now, 100Pa is about 140dB SPL (and 20 Pa =120dB)

    I don't really know at how much SPL's you start feeling them (it also depends on the program material), I guess something like 100dB A-weighted.

    But the distance is also very important. Ever heard of the "inverse square law"? This law states that for every doubling in distance, you lose 6dB. This means when you have 100dB SPL at 1m, you only have 94 at 2m (theoreticaly). (So if you like to feel the band you'd better be at first row.)

    (Just some basic stuff you should know as a sound engineer, not as a bass player)

    greetz,
    Awesome

    (note to Taylor: Very, very nice bass)
     
  15. While we're correcting mistakes.........;)

    As an aside, the inverse square law often doesn't work very well indoors. Boundary loading and room acoustics tend to mask a lot of the effects of the law. Proper PA hang and calibration can further lessen SPL losses at distance. I'm not going to get into all of that, though.
     
  16. awesome

    awesome

    Aug 14, 2002
    Belgium
    Mark,

    About the "electrical term"-thing: you're a more knowledgeable man than I am.

    About the" dBu-dBv"-thing: I made a little mistake there

    About the inverse square law: I was talking theoretically
    This law can only be used theoretically because it states that:
    1)you need "free field condititions" (which does not exists in real life)
    2)you need a "one-point source" (which seldom happens)


    This is off-topic but do you know if a bass amp should be twice the RMS-power of the cab? (they do this for PA, but I have never seen this for bass rigs)

    I hope I didn't make any mistakes this time (I'm still doubting about that "you're more knowledgeable"-thing ;) )

    Awesome
     
  17. awesome

    awesome

    Aug 14, 2002
    Belgium
    Mark,

    About the "electrical term"-thing: you're a more knowledgeable man than I am.

    About the" dBu-dBv"-thing: I made a little mistake there

    About the inverse square law: I was talking theoretically
    This law can only be used theoretically because it states that:
    1)you need "free field condititions" (which does not exists in real life)
    2)you need a "one-point source" (which seldom happens)


    This is off-topic but do you know if a bass amp should be twice the RMS-power of the cab? (they do this for PA, but I have never seen this for bass rigs)

    I hope I didn't make any mistakes this time (I'm still doubting about that "you're more knowledgeable"-thing ;) )

    Awesome
     
  18. Awesome:
    Sorry, my post wasn't meant to belittle you in any way. All those terms are pretty hard to keep track of, sometimes I still have to look at textbooks to remember which is which. You seem to be quite knowledgeable yourself. The Inverse-square law actually works fairly well outdoors. Especially on big wide open fields like most concert sites!

    About the power thing: There's not really any rule of thumb that absolutely has to be followed for bass cabs or PA. I think that under normal use an amp with rated power twice the RMS power rating of the cab would work just fine because in the real world, the average power delivered by the amp is nowhere near its max power. If you're really pushing the amp on a long term basis (i.e. if the average power put out by the amp is greater than the RMS handling of the cab), you might run into problems of course. Amps with power anywhere between slightly less than the cab's rated power to twice the rated power should be fine.

    Again, I apologize if I offended you in any way, that wasn't my intention.
    cheers!
     
  19. jondog

    jondog

    Mar 14, 2002
    NYC metro area
    Well nobody offended me and you are all more knowledgable than me! And you've answered my question! Feel at around 100 db SPL, pain at around 140+. A weighted to make all frequencies sound equally loud. One meter, but remember the inverse square law and whether you're outside or not.
    :D
    Thank You!
     
  20. geshel

    geshel

    Oct 2, 2001
    Seattle
    jondog, don't forget the most important dB SPL: hearing damage! It depends on the exposure time, but over 90dB can damage your ears (it also depends on frequency, it seems that the damage scale is like the phon scale but upside-down: the more sensetive frequencies are the more damaging).

    120 dB for just about any length of time will cause damage. Also, the pain threshold is frequency-dependent: a 110dB klaxon horn (like the fire alarm where I work - ouch! :mad: ) in the 1kHz area is painfully loud, whereas 110dB of 50Hz probably is not.