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Steve Johnson
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Posted: Wed Apr 16th, 2008 03:55 am |
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I test on Thursday and am still confussed about this stuff.
- MEEB says "intensity is inversely proportional to the square of the distance from the source" ... the inverse square law. (page 1499)
- It then goes on to say "intensity level changes 3 db with doubling or halving of the power and 6db with the doubling or halving of the distance"... the inverse square law again (page 1502).
So which is it, I or IL? I is watts/cm^2 and IL is decibles... we can't have this both ways.
Additionally, the inverse square law is E=I/d^2 I=intensity, which is power (watts) and this is confirmed on Page 1498 where it states that I=P/A which is the same thing as I=P/D^2, so I am right back to E=I making #1 correct.
I read somwhere that ilumination (E) is comparable to SPL and that SPL and IL are basically the same thing (math wise)... so using that logic that solving for E is the same as IL makes #2 correct.
So, which is it, I or IL? Am I making this too hard? Is it just time to go take the damn test?
Please help.
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icarusburns
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Posted: Thu Apr 17th, 2008 12:29 am |
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Very good question.
. . . add this to the confusion, and we have a realy big acoustical mess. Both from my MEEB book, a doubling of loudness is 10db., and doubling a signal intensity raises the intensity level by 3db. I though that doubling the Number of sources equalled a gain of 3db - taken from an ALS book. So now I am all confused by "loudness" vs. "intensity" - "loudness" acording to MEEB increases by 10db when doubled, and "intensity" increasing by 3db if doubled.
I just can't see the difference between "loudness" and "intensity", and the "sources" part made sense - until I read Steves' number 2 point above.
. . . but this is the way I read it: (IL)
"doubling of power" - 2 chainsaws running at the same power next to each other = the increase of 3db.
(I)
"doubling" of intenisty (not the sam as intensity level) - loudness = increase of 10db
Likewise a doubling of distance decreases IL by 6db.
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Steve Johnson
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Posted: Thu Apr 17th, 2008 01:23 am |
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icarusburns wrote: Very good question.
. . . add this to the confusion, and we have a realy big acoustical mess. Both from my MEEB book, a doubling of loudness is 10db., and doubling a signal intensity raises the intensity level by 3db. I though that doubling the Number of sources equalled a gain of 3db - taken from an ALS book. So now I am all confused by "loudness" vs. "intensity" - "loudness" acording to MEEB increases by 10db when doubled, and "intensity" increasing by 3db if doubled.
I just can't see the difference between "loudness" and "intensity", and the "sources" part made sense - until I read Steves' number 2 point above.
. . . but this is the way I read it: (IL)
"doubling of power" - 2 chainsaws running at the same power next to each other = the increase of 3db.
(I)
"doubling" of intenisty (not the sam as intensity level) - loudness = increase of 10db
Likewise a doubling of distance decreases IL by 6db.
Ah, just great... answer questions with more questions!
I think you are correct. I will share the way I understand using some real world experience:
When in highschool and college I worked for a high-end automative stereo shop building speaker boxes and installing amplifiers. That experience has led me to understand the following:
Power increases Sound Intensity I (I=watts/cm^2 watts=power): If you have a 5 watt stereo and you want to incrase its power by 3db, you have to add another 5 watt amplifier. Just like adding a chainsaw, we just added another power source... we double the source and added 3db to the environment. But 3db isn't audible to most ears, you have to got 5 db to tell a difference. That is why we added HUGE amps to systems, boosting a 5 watt stereo to 500 watts... an increase of power of 100x. So, if 5 watts on top of 5 watts increase db by 3, then 100x that is 100/5db=20 db increments 20x3db= 60db of power TO THE SPEAKER. I is NOT loudness, it is power. This also explains why you can FEEL a car coming before you can HEAR it (hearing is related to IL). Low frequencies (like 25 hz) have HUGE periods/frequencies, that is why you can feel a poorly designed car stereo from a block away... the added power increases the intensity of the wave and makes it travel further. The higher frequencies are also boosted, but their shorter period allows them to resonate inside the car. A good speaker box and speaker forces a long period frequency to resonate INSIDE the car and less outside the car... that gets into sound stage... I digress.
Intensity Level (and Sound Pressure Level) are measured in decibles and are percieved by the listener as "volume" or "loudness" (loudness is actually adjustable on an amplifier by a "gain" switch, or on your head unit by the volume or "loudness" button... it simply increases the power or watts dedicated to a certain range of frequencies like vocals, drums, or guitar... this is also helped by passive and active crossover networks and EQ's. More power from amp and head unit = more intensity from the speaker = more percieved volume or IL). The amount of I is determined by power or watts (we already discussed this) and that is why comparing I to Io (I being the intensity at your location and Io being the threshold of hearing) and multiplying it by 10 log we determine our Sound Intensity, otherwise known as volume or loudness. In other words I makes IL. This is what we HEAR when we put watts/power through a speaker. The inverse square law prooves that doubling the distance makes the IL lower by 6db. We all know that to be true... you walk away from your screaming kid, they get quiter... but they have not altered their power. IL IS loudness or volume. I don't think you can associate I with loudness, just IL.
Moreover, loudness and volume, like brightness is subjective based on your age and surroundings (db comparable to foot lamberts?). Sound Intensity is also similar to light intensity (I guess that means watts is similar to candle power... sound waves are like flux?)
That is how I sepparate I from IL... but THAT is what lead to MY original question about the discrepencies in my study guides as to what goes into the equation.
That may not have helped anyone else, but writting it out helped me. Thanks.
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icarusburns
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Posted: Thu Apr 17th, 2008 03:13 am |
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Steve,
Very good descriptions.
. . . but, where does the "loudness" increasing by 10db, when doubled play into all of this? . . . or does it not, and it's a completely separate issue?
I appologize for putting a stranglehold on your oringinal thread.
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awapniak
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Posted: Thu Apr 17th, 2008 03:38 am |
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icarusburns wrote: Steve,
Very good descriptions.
. . . but, where does the "loudness" increasing by 10db, when doubled play into all of this? . . . or does it not, and it's a completely separate issue?
I appologize for putting a stranglehold on your oringinal thread.
The doubling is percieved.
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Steve Johnson
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Posted: Thu Apr 17th, 2008 11:33 am |
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icarusburns wrote: Steve,
Very good descriptions.
. . . but, where does the "loudness" increasing by 10db, when doubled play into all of this? . . . or does it not, and it's a completely separate issue?
I appologize for putting a stranglehold on your oringinal thread.
Awapniak is right, loudness is percieved. Power and Intensity stay the same, your perception of loudness changes based on your distance from the source, your surroundings (is there a wall between you and the source), and the sensitivity of your ear. Assuming ALL of those variables are consistent from location to location, if you are 15' from a source and your precieved IL is 50 db, and you walk to 30', (double your distnace) then your new percieved IL is now 40 db (10 db less). Now, if while you were walking from 15' to 30' and someone doubled the power of the sound, your db would still drop 10db due to the distnace but then the added power would add BACK 3db for a net loss of 7db (easily perceptible). If they tripled the power during your 15' walk, the net loss would be 1db... or none perceptible.
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icarusburns
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Posted: Thu Apr 17th, 2008 12:50 pm |
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One more time - with feeling:
1. loudness involves the perception as one moves away from, or the source moves - (using the doubing example again): double the distance, the loudness decreases by 10db
2. Changing/ modifying Sound Intesity (I, power, watts) is a function of actually tampering (couldn't think of another word, as I'm typing quckly here) with the source - (again, using the doubling example again): double the source (exact same power output), the ACTUAL (not perceived, as in the loudness scenario) Sound Intensity (not IL) increases by 3db.
3. The Intensity Level (IL, db) is also perceived, but as (I)ntensity (L)evel. As with loudness, this is perceived as well, with no tampering with the source. Moving away double the distance decreases the perception (what the ear hears) by 6db.
I think I have a better grasp on this but I fear a question on the exam won't be clear regarding the 10db vs the 6db reduction, as my brain is still a bit fuzzy on those concepts, and they are both perceived.
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Steve Johnson
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Posted: Thu Apr 17th, 2008 01:01 pm |
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icarusburns wrote: One more time - with feeling:
1. loudness involves the perception as one moves away from, or the source moves - (using the doubing example again): double the distance, the loudness decreases by 10db
2. Changing/ modifying Sound Intesity (I, power, watts) is a function of actually tampering (couldn't think of another word, as I'm typing quckly here) with the source - (again, using the doubling example again): double the source (exact same power output), the ACTUAL (not perceived, as in the loudness scenario) Sound Intensity (not IL) increases by 3db.
3. The Intensity Level (IL, db) is also perceived, but as (I)ntensity (L)evel. As with loudness, this is perceived as well, with no tampering with the source. Moving away double the distance decreases the perception (what the ear hears) by 6db.
I think I have a better grasp on this but I fear a question on the exam won't be clear regarding the 10db vs the 6db reduction, as my brain is still a bit fuzzy on those concepts, and they are both perceived.
Yes, I think you've got it (or at a minimum, you and I understand it the same way, correctly or incorrectly  )
I would replace "tampering" with "modifying" or "alterning". Tampering sounds like you are damaging the source. Symantics.
Also, you mentioned 10db... that IS relevatant, because our ear percieves the volume/intensity as "doubled" every 10db. So... if you get 2x as CLOSE to a source (add 6db) and then double the power (3db) you have added 9 db to your perception of loudness... that is 1db away from your ear thinking you doubled the volume.
Is the water muddier now?
I leave for my test in 10 minutes.
Ciao.
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icarusburns
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Posted: Thu Apr 17th, 2008 04:42 pm |
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| Let us know how it went!
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Steve Johnson
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Posted: Thu Apr 17th, 2008 06:01 pm |
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Well, it went. There was NO mention of decibles, intensity... or anything related to sound or light.
Almost ALL HVAC relted except for the random quesitons that you don't know what to do with.
I feel the same way leaving this test as I did all my other passes so we hope and see.
Some of the questions were just odd. There were a lot of LCC questions, some almost word for word from PD.
Like PD and MM, I don't see how "studying" can help you with this test, other than understanding some of the vocabulary and looking at diagrams in MEEB. You really have to have had experience with this stuff to answer their questions.
We will see.
Good luck with yours!
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icarusburns
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Posted: Thu Apr 17th, 2008 06:52 pm |
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. . . I guess this whole thread was all for not as far as your test goes.
Life Cycle topics scare the crap out of me. I really don't understand it completely, except for the basic concept through Graphic Standards. Anything involved with costs of items with relationship to whatever, I'm bad with. I've looked at some of the FTP site stuff, and it really doesn't do me any good.
If I were asked to compare systems and their LCC, I'd be . . .
D O O M E D
Hopefully you did enough to get the pass. I have my test Monday.
Last edited on Thu Apr 17th, 2008 06:53 pm by icarusburns
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newlyARE
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Posted: Sun Apr 20th, 2008 09:39 pm |
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IL Sound Intensity level = decibel
I Intensity of the sound measured in watts/cm2
If you double the distance between the receiver and the source of sound, sound intensity will be decrease to 25% of its original value (one quarter of its original value) this results in a 6dB change in the IL
the Intesity is expressed with logarithmic scales because is a huge number like 10 000 000 000 or 1010
IL=10 log I/I0
W the power at the source is measured in watts
I=W/4TTd2930 TT=pi
the source power is related to the intensity as shown in the formula above and therefore affected by the inverse square law
when you have 2 sources, let's say 2 sources of 30dB (IL) the result is not 60dB but 33dB only increased by 3dB.
I hope this helps! i'm getting confused now 
I & IL are proportional since they are in the same formula if one decreases the other one will too , and the same with W if I decrases w will too
I hope I don't confuse everybody more...
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