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In Reply to: RE: For my clarity please, posted by sober1 on May 25, 2015 at 21:06:42
The "rear wave" is a completely natural part of the system.
I look at microphones as the exact analog here. An omnidirectional microphone (like a closed-box loudspeaker, at least in the low frequencies) has a pattern that's the same in every direction; it responds to air pressure. At the other extreme of the first-order patterns is the dipole (with a ribbon microphone, or an open-baffle loudspeaker). This has a "front" and a "back" that are approximately the same sensitivity. When there's a positive velocity at the front, there's the same negative velocity at the back; assuming that positive is "toward the transducer". This actually is a pure velocity machine; there's a directional vector of sound in the air, not a pressure wave. The other significant thing about a velocity driver is that it has a "null plane", perpendicular to the main axis. If you stand to the side of a figure-8 microphone, or to the side of a dipole speaker in free air, there is no excitation at all. You'll be inaudible to the microphone, and you won't hear the speaker.
In between omni and dipole is an infinite range of blended patterns. My favorite microphone pattern is the hypercardioid, which has a slight rear-lobe, and a "null cone" around 120 degrees back. Ideal for a singer with a stage monitor: if you set things up right, the monitor's sound will just sit in the null, and the microphone literally won't hear it.
All these patterns are just blends of a pressure-based (monopole) and a velocity-based (dipole) response. Then the microphone world goes into some special patterns like the EV RE-20, or shotguns, and so on, which add some physical distance (ports, vents) into the mix as well.
So, back to loudspeakers. Cardioid PA subs are quite popular nowadays, because they can keep the stage quiet. For home use - meaning, in a small room - cardioid patterns are interesting because they have the potential to sit nicely in a wide range of real rooms. But the big thing about low frequencies (less than 200Hz, say) in domestic rooms is: the room is small, compared with the wavelength. It's a resonant box full of air, and the loudspeaker moves it. The resonating air in the room is much more important than the free-air response, including the cancelation that happens when the front of the speaker pushes air and it just wraps around to the back.
The resonance that the speaker creates will depend whereabouts in the room you place the speaker, as everyone knows. But it will also hugely depend on the pressure and velocity created by the speaker itself. With a pressure transducer, you only have one dimension to play with: amplitude, or volume. With a velocity transducer, you get *direction* too.
I know there's a science of bass in rooms, that is probably modeled with high precision by some of the sound-reinforcement and acoustic design pros these days. To me it's more of a black art and a place to experiment. But the science is certainly established enough to allow you be fearless and bold and to trust in your own experiences when experimenting outside of monkey-coffin land.
Follow Ups:
Hi Inguz, can you explain the velocity component to me? The speed of sound in a particular location is constant and frequency independent, so how can you have a velocity transducer? That would seem to indicate a change of speed or am I not understanding what your saying.
In Pass's article, if I understand correctly, he claims the speed/velocity is 3 to 1 with his slot configuration. I'm guessing it's similar to where in plumbing and hydraulics you decrease diameter to increase pressure that translates to greater perceived velocity?
"the only thing necessary for evil to succeed is for good men to do nothing"
Google Bernoulli's Principle. Air within a port will have increased velocity compared to within the box. The velocity drops back down when it exits the port. It has no effect on the speed of sound or of a sound wave radiated by the port. Excess port velocity, due to the port area being too small, will cause chuffing noise.
Bernoulli's Principle is for fluid flow, acoustics is not fliud flow so this is the wrong physics. Acoustics is fluid vibration so Bernoulli's Principle is not applicable.
"Bernoulli's Principle is for fluid flow... Acoustics is fluid vibration"
In the context of this discussion... When the front of the woofer cone creates a pressure wave behind the front panel, the wave will disperse into the available volume. If the frequency of the wave is low relative to the area of the slot, the pressure wave will travel through the slot. It will do so in the form of airflow, and all physical rules of pressure and velocity will apply.
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Buy Chinese. Bury freedom.
The air mass within a port behaves in the same fashion as a flowing fluid. I thought I made it clear that the velocity of the air mass in the port has no bearing on the sound waves that the port air mass produces, other than that if the air mass has sufficient velocity its movement will be heard as chuffing.
nt
Why the "hit and run" nt?
Fluid DOES compress.
That's why sound waves can travel through the oceans, to be heard by the supposedly endangered whales.
:)
In a closed vessel, true. That's why we use hydraulic fluid in any number of applications rather than air. But a speaker port isn't a closed vessel.
The principal works in air as well, it hinges on velocity and it is what gives an airplane wing it's lift in air.
The constriction being discussed does not increase the electro-acoustic efficiency or bass output however, although combined with the trapped air volume between the radiator and slot, can act as a low pass filter which could reduce the distortion the cone produces. The air noise Bill mentions is a real factor and turbulence in ports is also why the response of vented boxes can change once above low power.
Ah yes, real life observations of a non-linear world.
This is why I like horns vs. bass reflex. Among other benefits, Their higher efficiency allows them to work without creating thermal issues in the voice coils when kept at normal listening volumes in the home. Another non-linear behavior skirted.
"The principal works in air as well,"
Ummm, he just said that.
And, it's "principle".
The obvious question is; Where does the extra energy to accelerate the sound / air come from? The speed of sound is constant in a consistant environment.
Edits: 05/27/15
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.
Air molecules vibrate back and forth, faster with higher frequency, to produce sound and this is not the same as the constant speeed of sound ~344 m/sec in air. In acoustics when describing the vibration of the air you usually use volume velocity which is a cross-sectional area times the velocity of the vibrating air. So for example a port or driver has a volume velocity that is the circular cross-sectional area times the vibratory velocity (or the back and forth dispacement of the air molecules times the frequency of vibration).
You need to keep velocity of sound and velocity of the vibrating air molecules as two different actions that transmit and produce sound. Remember there is no net flow of air, just very small oscillations back and forth. Once the sound passes, the air molecule is left in the same position/location in the room.
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"
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.
> > > > When there's a positive velocity at the front, there's the same negative velocity at the back; assuming that positive is "toward the transducer". This actually is a pure velocity machine; there's a directional vector of sound in the air, not a pressure wave. < < < <
so my woofers on the baffles are velocity machines? why do you think my ears prefer the 10" Faital and 18" Goldwood together over the Eminence 15" with the 18" GW? possibly too much velocity at too short a distance, or just operator error? is my understanding correct with regards to Briggs statements that when drivers are close enough physically on the baffle they acoustically couple? if that's the case is the parts of the summed output in phase and additive? given the Alpha 15As' and GW-1858 T/S parameters are very close, I assumed the 15 & 18 would be killer which was not the case with my speakers, so my question is, could it be more of a room (cabinet) issue more so than issues with the drivers coupled? maybe something I'll revisit on my next version. < < < <
> > > > If you stand to the side of a figure-8 microphone, or to the side of a dipole speaker in free air, there is no excitation at all. You'll be inaudible to the microphone, and you won't hear the speaker. < < < <
this was one of the first things I noticed setting these up and rearranging them over a couple days period. way cool experience for me as at first I thought I screwed something up.
> > > > My favorite microphone pattern is the hypercardioid, which has a slight rear-lobe, and a "null cone" around 120 degrees back. Ideal for a singer with a stage monitor: if you set things up right, the monitor's sound will just sit in the null, and the microphone literally won't hear it. < < < <
2-parter, is this one of the duties of a sound engineer and enables a vocalist to hear what they sound like during a live performance?
> > > > But the big thing about low frequencies (less than 200Hz, say) in domestic rooms is: the room is small, compared with the wavelength. It's a resonant box full of air, and the loudspeaker moves it. < < < <
so basically my living room becomes the cabinet? In my case, a really leaky vented box as my windows whistle on windy days. the drapes, carpets, etc. become damping materials?
> > > > But the science is certainly established enough to allow you be fearless and bold and to trust in your own experiences when experimenting outside of monkey-coffin land. < < < <
For me it's just as much the journey as the final destination - the milliseconds where reality is temporarily suspended and I'm transported to the venue of the artists I'm digging! I know my skill set is weak, for now, but with trying, stumbling, wrecking stuff and asking for help along the way I will improve my knowledge and skills to enhance my (and others as well I hope) enjoyment of this hobby. Thanks for taking the time my brother!
I know just enough too be dangerous and too dumb to be afraid, ahhhh, my wheelhouse!
"the only thing necessary for evil to succeed is for good men to do nothing"
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