Page 6 of 7
Posted: Fri Mar 11, 2005 10:00 pm
by audiobass10
Jonathan wrote:"But I do not see how the "atmospheric conditions" can limit the sound waves to a certain maximum amplitude."
The loudest sound level that can exist is determined by atmospheric pressure. A sound wave is compression (pressure increase) and rarefaction (pressure decrease) of air (or the medium you're trying to compress, anyway), so the SPL you can accomplish is limited by the actual pressure of the atmosphere. Don't think of it in terms of pressurizing air in the method of compression, think of it in terms of pressure decrease for a second, it is impossible to create a pressure less than zero (which would be a vacuum), so it limits max pressure change to that of the atmosphere that you are creating sound in. This is why SPL changes drastically with elevation and air density, it is due to the atmosphere that it is in. When you deal with super high SPLs, you are so close to the level of the outside world's atmospheric pressure that the air is reaching it's limits of compression, that is why it is considerably harder to add a db after a certain point. Through logarithmic calculations, 194db is the maximum amount of pressure that you could possibly create in the earth's atmosphere (at sea level). The only way that you can increase SPL beyond this would be to create sound in a pressurized atmosphere, which would be below sea level, underwater, or in special cases such as space shuttles and explosions, which change the property of air due to the gases that they put out, making air more dense.
Well, there you have it. The concrete answer. And the actual number. Good post.
Posted: Fri Mar 11, 2005 10:57 pm
by Jonathan
Oh, and by the way, that 194db is what you could linearly hit in the atmosphere. Theoretically, it would be possible to hit higher, but you would be asymmetric and the power level required to do so approaches infinity. Remember that it applies to the compression example like I stated above, and NOT the rarefaction since you can't get lower pressure than a vacuum, which is what makes it asymmetric, thus making it a shock wave and not a sine wave. You would be essentially pushing the air like a plow, and it would require that you had a speaker (or other object moving air) that is capable of moving faster than the speed of sound, when you move faster than the speed of sound you are overcoming the speed of air molecules in order to create compression, air molecules at that point do not have time to flow around the subwoofer, so compression is created (same thing as the sonic boom from airplanes). The problem is you'd have to have a speaker assembly strong enough to withstand that kind of torture, which is a huge limiting factor.
Posted: Sat Mar 12, 2005 8:40 am
by nist7
Thanks Jonathan, you've cleared this issue up for me.
Now does it not seem that dB measurements are quite inaccurate?
Would it not be more scientific, so to speak, if SPL events were to employ a device that could measure just the intensity of the sound waves in watts per meter squared?
Posted: Sat Mar 12, 2005 9:13 am
by Jonathan
It would be the same thing really. The db scale uses a reference intensity level and is simply a measure of the ratio of one intensity to another. The intensity of sound is proportional to the square of the pressure variation of that sound, for example, intensity level of 100 would be 20db, 10000 would be 40db. The reference intensity level is .000000000001 watts per square meter, while the reference pressure variation is 0.0000002 millibars. Any change past this point can be expressed in decibels since the decibel scale is a ratio of intensity change, and is proportional to the change you would get in watts per square meter. The decibel scale is simply there to give prettier numbers :) and the benefit of a constant intensity change, for example, 1db is percieved as the smallest detectable change, and that applies to 20db or 100db. Here's a chart relating decibels to intensity level:
http://www.sfu.ca/sonic-studio/handbook/Appendix_D.html
Really the reason db measurements are inaccurate is due to the mic, not the scale itself. Humidity, air pressure, etc. take it's toll on a mic and result in somewhat inaccurate readings. You would encounter the same problem if you measured in watts per square meter.
Posted: Sat Mar 12, 2005 9:31 am
by nist7
I see. I should've known, dB is another way of expressing the ratio of the actual intensity to the reference intensity...
So I guess now that SPL competition is not so much who can hit higher dBs as it is who can achieve the maximum dB the soonest.
Posted: Sat Mar 12, 2005 10:39 am
by GlassWolf
At least now everyone seems to be using the same Termlabs mics for events, and they are fairly accurate from mic to mic in that regard, so this takes some of the discrepancy out of the comparisons from event to event.
Posted: Sat Mar 12, 2005 10:49 am
by William from NOLA
Speaking of mics, anyone know how the Linear-X mic compares? A local shop owner has one here and he said it gives a lower number, which could mean that it is more accurate, but I don't know for sure.
Posted: Sat Mar 12, 2005 11:07 am
by GlassWolf
a lower number isn't always better or more accurate. It could just be a less sensitive mic. Just remember SPL is a relative scale, so it'll never be as objective for comparisons as say, quarter mile times or 0-60 acceleration times which are very specific units of measure.
SPL readings on an RTA are better used at a shop to see a base line reading then compare to changes to look for improvements. Once you've gotten all you can out of the system, then it's ready for events, to compete against others in the same environment, on the same measuring equipment.
These inconsistancies between mics, event locations, and so on are one of the reasons SPL competitions are no more objective than SQ events if you state you have a "172.2dB car" you can't really compare that to anything else unless both cars are side by side getting metered.. you may get different results a one event than you will at another.
Posted: Sat Mar 12, 2005 4:30 pm
by 71buickfreak
good stuff people. However, on earth, the SPL limit is considerably louder than 194. How you ask? simple. only 25% of the earth is exposed land. For years now, the Navy has been experimenting with submerged audio systems. capable of reaching well over 225dB 10 miles away. so there you have it. But that is loudest sound ever recorded. Just not recorded on land.
Posted: Sat Mar 12, 2005 8:08 pm
by William from NOLA
When I'm rich, I'll buy a submarine and put some audio on it!