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In Reply to: RE: if I may guess posted by MJ King on May 27, 2015 at 10:31:32
I think you guys are confusing the speed of sound, how fast a sound moves through air, to the speed of vibration of the air molecules making up the sound.
and this is a law of physics(?) that allows me to differentiate a trumpet from a sax from a stand up bass? Speed of sound is what ~ 622MPH or so, so the speed of sound is a constant without regard to whatever action produced it, correct? the variable in sound reproduction then is the excitation of air molecules (vibrating but not being pushed in any straight-line direction)? so varying the sizes of baffles almost could be thought of as air dampeners?
I apologize for my lack of knowledge here, but these last couple of threads have been enlightening for me! thanks!
"the only thing necessary for evil to succeed is for good men to do nothing"
Follow Ups:
Sober1,
Read pages 3 through 5 of Dr. Eric Heller's "Why You Hear What You Hear".
Also, "net-study" isn't going to get you anywhere other than confused and misled. Read real books on the pertinent topics. In addition to the afore-mentioned book, get Harry Olson's " Music, Physics and Engineering", Everest's "Master Handbook of Acoustics", and Dickason's "The Loudspeaker Design Cookbook". Those will be a good start.
:)
Think of it as a large chorus on a stage and you are sitting in the audience. There are bass singers, tenors, and altos all singing the same words at the same time. The sound of the chorus arrives at your seat at the same time so that you can understand the words of the song. The frequencies of the notes are all different, the bass is low frequency and the alto is high frequency. But is all arrives at the same time thanks to the constant speed of sound in air.
In an OB speaker the plus wave from the front of the driver and the minus wave from the rear of the driver also travel from the speaker to your listening position at the speed of sound. If there was no baffle (the driver's cone can be considered a small baffle) the plus and minus waves would arrive at the same time and cancel, no sound. The baffle adds an extra distance that the minus wave has to travel and the size determines at what frequency the cancellation will gradually cease to occur. Below this frequency the plus and minus waves add destructively and the output rolls off at 6 dB/octave, above this frequency the sound is almost the same as from an infinite baffle.
For an OB design the size and shape of the baffle, the driver's T/S parameters, and the crossover / EQ all need to be considered to determine over what frequency range the speaker will perform. It is all predictable using math and physics. Do the engineering up front to avoid a disappointment after you invest time, money, and effort building an OB / dipole speaker system that sucks.
Do the engineering up front to avoid a disappointment after you invest time, money, and effort building an OB / dipole speaker system that sucks.
First off, thank you Martin for your thoughtful and instructive responses!
After looking over your baby baffles, did I read the distance measurements were taking a 1 meter?
I presently don't own the tools necessary for your level of design process nor the knowledge, as of yet, to utilize them. On the plus side, I'm having a freaking ball learning. My current PizzaBoards sound good to me in my home with my music. Like most things though.......if One is Good, Two is Better!
Life is not a journey to the grave with the intention of arriving safely in one pretty and well-preserved piece, but to slide across the finish line broadside, thoroughly used up, worn out, leaking oil, and shouting GERONIMO!
The distance is 1 m but the input is not normalized to 1 watt (Omnimic does not monitor the signal going into the speaker), so the value of SPL in the curve is a function of the volume knob on my preamp.
If you want an example of a small OB speaker system using a passive crossover look at the following link.
http://www.quarter-wave.com/Project12/Project12.html
The design simulation is shown in a linked pdf file and the in room SPL measurement correlates very well. This is really a very small OB speaker system that produces strong bass down to just below 50 Hz.
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