The Concept of Sound Pressure (SPL)





Introduction: The Concept of Sound Pressure (SPL)

Have you ever been told to turn the volume down on your music?  Have you ever wondered why when something is loud it is said to have more volume?  In this instructable I hope to clear up some major misconceptions about sound, and hopefully help you understand how sound works.

This instructable is more of a how it works as opposed to a how to. Once you understand how it works, it is much easier to learn the how to.

Please note that I am constructing this from a sound engineer's point of view. Most of the content is theory, so please keep that in mind when you read this.

If you haven't read my first instructable "How Sound Works" yet, I suggest you read it first.  You can find it here:

Step 1: What Is Sound Pressure?

Sound Pressure Level is normally abbreviated SPL.  SPL is calculated in decibels or db.  Please note that db is a relative term.  It can refer to almost anything, however, the most well known is SPL.  Most of the time when someone is talking about db, they are probably referring to SPL.

Step 2: The Difference Between Sound Pressure and Volume

The human ear hears pressure, not volume.  The difference between sound pressure and volume is that the closer that to the source of the sound, the more pressure you will hear.  The farther away you get from the source of the sound, the pressure will be less, but the volume will be the same.  A good analogy would be a faucet.  At the top of the faucet where the water comes out, there is more pressure than at the bottom, but the volume of water is the same.  (See picture)

One thing to remember with SPL is that as you turn the pressure up, you generally also increase the volume. Volume does affect sound, even though it does not directly affect how we perceive how loud something is. One of the main characteristics of a professional speaker is that it can produce more pressure with less volume than a consumer speaker can.

Step 3: Why Is It Important?

SPL can also damage the ear.  The human ear starts to degenerate at an SPL of about 85-90 db.  You can buy SPL meters at almost any store that sells professional audio equipment, or radio shack.  Because a standard rock concert has an approximate SPL of 100-130 db, if you are mixing for any event, you should keep a SPL meter handy and try to keep the SPL at or below 80 db.  If you are attending a rock concert, you should make sure to have hearing protection with you.  You can buy discrete hearing protection at almost any department store or store that sells professional audio equipment.

Step 4: The Inverse Square Law

One thing to remember when you are mixing, is that in normal conditions, when you double you distance from the source of the sound, you reduce the sound by 6 db (4X).  So if you are in the back of the room and your speakers are at the front, the pressure at the front of the room is much more at the front than at the back.

Step 5: Conclusion

If you only take one thing away from this instructable, it should be a warning not to ruin anyone else's hearing, and to protect your own.  If our generation continues to attend loud rock concerts, then in 20 years, 10 year old kids will be using hearing aids.

Step 6: Copyright

Please note that I do claim copyright to the information. I did not use any specific sources when compiling this information, all of this is from my personal experience.

You may quote parts of this information for educational purposes. Under no circumstances will you sell this information.

I do not own the copyright to any of the images (except the picture in step #2), however, as far as I have been able to find, I have the right to use them in this instructable.  If there is any question about whether or not I have the right to use these images, please contact me.  I have no intention of stealing anyone's intellectual property.

Use of this information implies that you agree to these copyright terms.

© 2011



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    hi..i wander what is the SPL relation to frequence? at the same given electrical power 60 Hz would have the same pressure of 8000 Hz? in pounds? it doesn't seems by the air coming out of a speaker cabinet.. at this point i ask why the word pressure is there at all..?
    Ears drum hurt at the pressure of water and maybe because of to much acoustic power from a spk.
    but i am so tempted in thinking that the pain of high frequence is more an issue of meccanical vibrations.. what you think?

    Hey there. Good question, and one that is very relevant to audio!

    This is the link you want to check out:

    It explains how frequency (pitch) of a sound relates to volume (the Sound Pressure level measured in dB) and loudness (subjective/something we perceive).

    Basically, a lower-frequency sound needs to have more SPL(volume/power/amplitude) to be perceived as having the same loudnessas a higher-frequency sound. So, for example, if you have a perfect sine wave playing a low C, and then another perfect sine wave playing a C three octaves above that, you'll need to have the lower-frequency (low C) of the two sine waves playing at a higher SPL/dB/amplitude in order for both the sounds to seem equally as loud to the human listening to them.

    For the second part of your question about pressure level... (I looked this up, but I'm glad I did because now I know! =] ) The reason it doesn't seem like "pressure" at all is because it's so much smaller than anything you compare it to for feeling pressure (like pressure on your ears diving underwater, for example). To explain: volume is objectively measured in dB, which is a logarithmic scale based on how much more volume (SPL) a sound has relative to the quietest limit of human hearing. But the basic physical unit for measuring SPL itself is actually: the Pascal. According to the Internet, 1 Pascal = 0.000145038 PSI(pounds per square inch). So yes... it is pressure, the exact same physical phenomenon of having an additional atmosphere's worth of pressure on you (14.6959 pounds per square inch) when you dive 10 meters down underwater - it's just with sound, the differences in pressure are so small we don't actually feel them as pressure, we just hear and perceive those differences as "louder" or "quieter" with the precise sensitivity of our ears.

    For the last thing you mentioned, you can definitely have a high-frequency sound (high-pitched) without it being damaging to the ears (which you probably know - just remember that frequency is pitch, not volume). It's when the volume or loudness is too much that damage occurs. I would be interested to know if damage to our ears is worse based on a combination of amplitude and frequency, or if it's just based on SPL across the frequency spectrum. (If someone knows the answer to this, please post it with a link!) Also can sounds damage our ears if we can't even hear them? I want to know and do everything I can to protect my hearing for life.

    Good info. Thanks

    I estimated that my full pa system can output 134db!

    Just don't destroy anyone's hearing! ;) The thing I don't like about professional audio techs mixing rock concerts is that they seem like they have more pressure than they actually do (there are ways to do that with the EQ), so the amateur techs think that they just have to turn everything up and it will sound professional... little do they know they are destroying everyone's hearing! (That's my little rant and rave... forgive me!) :)

    I do think most concerts are way too loud but it's worse when the gear can't handle it and it sounds terrible!
    I think most music is too loud today. I'd rather ditch my Logitech 40 watt speaker system for a 3 watt a channel tube amp and decent speakers any day!

    I couldn't agree more! :)

    kiteman, relax bro whats wrong with you its just a pic. Nobody looks at the pic anyway we look at the information and as far as I see it. Thegeeke did a great job thanx bro !! :)

    Thanks! I'm glad you enjoyed it! :)

    I do understand where he's coming from... As someone who does AV as a side job, I do fuss a lot with copyrights, and I doubt he would have made a big deal about it if I didn't put the copyright step in. Also, when I first started publishing ibles, I didn't think of specifically stating that I didn't claim copyright to the images, so it made it seem as if I did claim copyright to them. I am grateful to a few ibles members (I think kiteman was one of them) who originally pointed that out. :)

    Thanks again! :)

    Such a big fuss about copyright, one assumes you have permission from the creators of all those images?

    (And that's aside from the fact that copyright is covered by the type of license you choose to publish under.)

    (Fun fact - if you stand next to a Shuttle-scale launch, the sound will kill you before the flames do.)