Hi,
Free air can mean two things, the raw driver or the driver
measured on a very large open baffle. The point is the Fs
of the driver is not modified to Fb by a rear enclosure.
A raw driver will peak at a frequency dependent on its
size and then roll-off at 6dB/octave to which you need
further roll-off due the drivers second order roll-off.
You won't see any comb filtering, it doesn't happen.
In a big baffle there is no peak or 6dB/octave roll
-off, just the inherent drivers second order roll-off.
On an open baffle the peak frequency depends on
the baffle size, not driver size, and the baffle loss
is 6dB/octave below that frequency. Open baffles
need good (unusual) driver choices or active EQ.
rgds, sreten.
Free air can mean two things, the raw driver or the driver
measured on a very large open baffle. The point is the Fs
of the driver is not modified to Fb by a rear enclosure.
A raw driver will peak at a frequency dependent on its
size and then roll-off at 6dB/octave to which you need
further roll-off due the drivers second order roll-off.
You won't see any comb filtering, it doesn't happen.
In a big baffle there is no peak or 6dB/octave roll
-off, just the inherent drivers second order roll-off.
On an open baffle the peak frequency depends on
the baffle size, not driver size, and the baffle loss
is 6dB/octave below that frequency. Open baffles
need good (unusual) driver choices or active EQ.
rgds, sreten.
Hi,
Comb filtering simply doesn't happen, unless you have multiple drivers.
And yes you will get it between a driver and its image placed on a table.
The 6dB roll-off is caused by wavelengths being bigger than the driver.
The 2nd order bass roll-off is inherent to all drivers, defined by Fs and Qts.
(Note that with high Q the 2nd order bass roll-off can be a peaking roll-off.)
rgds, sreten.
Comb filtering simply doesn't happen, unless you have multiple drivers.
And yes you will get it between a driver and its image placed on a table.
The 6dB roll-off is caused by wavelengths being bigger than the driver.
The 2nd order bass roll-off is inherent to all drivers, defined by Fs and Qts.
(Note that with high Q the 2nd order bass roll-off can be a peaking roll-off.)
rgds, sreten.
Hi,
Its completely out of phase.
The front pushing is compression, whilst the rear pulling is
decompression, and vice versa for front pulling and rear pushing.
Air does not move for sound waves in the same way water
does not move for waves, its just a transmission medium.
You really shouldn't make statements that disagree with
all known facts, without being sure you know your stuff.
Which you clearly don't. Posing it as a question is an idea.
rgds, sreten.
In a lot of ways it does. I assume you mean by suspending a driver in the air with no baffle.Accordingly shouldn't the radiation of sound from a loudspeaker cancel each other when they are in free air because of opposite phase from front and back?So why is there a sound ? and the measured frequency response result of comb filtering. Thanks
The way sound radiates is very dependent on how large the sound source is vs. how large the wavelength is of the sound. Even with a very large driver like an 18" woofer, a 50hz wave is 6.8m and therefore much larger. The sound then radiates omnidirectionaly from the cone and the front and rear cancel each other to a very significant degree.
At 1500hz, however, the wavelength is .23m. The 18" woofer's diameter is now twice the wavelength of the sound. The wave now propagates much more forward and backward and much less to the sides. Since the front and rear are interacting much less now, you get far less cancellation.
An externally hosted image should be here but it was not working when we last tested it.
This is a completely arbitrarily picked directivity plot, but it shows that in general for almost all cone speakers the sound radiates much more spherically at low frequencies and much more forwardly at higher frequencies. This means that a driver suspended in free air has a lot of cancellation of the front and back waves at low frequencies, and the high frequencies are relatively unaffected.
You are a brave person to question a basic article of faith on the subwoofer forum (but not a "truth" on the full range forum).Accordingly shouldn't the radiation of sound from a loudspeaker cancel each other when they are in free air because of opposite phase from front and back?So why is there a sound ? and the measured frequency response result of comb filtering
Here people believe that when matter and anti-matter meet at the black-hole horizon, they totally annihilate one another, or something like that.
In reality, the stuff the coming out the back heads off in all directions and bounces back in ways that are not feasible to predict - for large baffleboards and for raw drivers alone. That's why drivers mounted on baffleboards work a whole lot better and more importantly, sound a whole lot better than many people realize.... at least down to some not-so-low frequency.
A lot of carefully modelled sub boxes would put better music into your room if somebody removed the back.
Some folks see a textbook illustration of phase cancellation and believe it applies to their room. Doesn't exactly. Even though anybody with a pocket calculator can do the theoretical math, the real world isn't cooperative.
Ben
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This really isn't as complicated as you want it to be Ben.
All sound sources on or in front of a reflective surface will have a diffraction profile. At wavelengths longer than the reflective surface the waves can go around, at wavelengths shorter than the reflective surface the waves will reflect and go forward. At (or near) the baffle step frequency there will be a hump in response and a bit of rippled response above that.
A driver with no box and no baffle has only it's own physical parts to separate the front and back wave, so fairly high frequency waves are allowed to go around the edge and cancel, and most of the bass is lost.
A lot of carefully modeled sub boxes would NOT put better music into your room if you removed the back. Carefully modeled implies decently designed, and if you remove the back of a decently designed subwoofer box the result is going to be drastically different. If it was decent as a boxed sub it will not be any good with the back removed. Boxed and open back require an entirely different design.
Of course in real rooms response is unpredictable, but simulators have no trouble at all providing very accurate frequency response predictions of OB speakers in 2 pi space. Some even go further and will allow you to add the back wall and side wall reflections in addition to the floor reflection. It's not at all "bouncing back in ways that are not feasible to predict". Simple first reflections are actually quite easy to predict.
Simple OB design is actually so easy to do I described how to do it without a computer and without any math at all a few years ago. https://sites.google.com/site/amateuraudio/theory/ob-design
It's really not tricky at all but you need to know what you are doing. And here's a quick page on baffle effects, looking at diffraction of both monopole and dipole designs. https://sites.google.com/site/amateuraudio/theory/baffle-effects
There's even a page on boundary effects (primary boundary reflections), and the info on those three pages is pretty much all you need to know to start designing OB.
Note that these were written back in 2009 when I was still very much a novice, but even back then I had more to offer than incomplete and ill conceived thoughts like "a lot of carefully modelled sub boxes would put better music into your room if somebody removed the back" and "the stuff the coming out the back heads off in all directions and bounces back in ways that are not feasible to predict".
No simulator is going to provide a realistic full room prediction, but that really isn't that necessary. If you understand the basics and you have a good starting point it's easy to get a good result with some minor tweaking.
Your main problem with simulators (or one of your main problems with sims) is that they can't predict in room response. That's not what they are for, and it's not required to get a good result if you understand a bit about acoustics and what rooms do to sound.
All sound sources on or in front of a reflective surface will have a diffraction profile. At wavelengths longer than the reflective surface the waves can go around, at wavelengths shorter than the reflective surface the waves will reflect and go forward. At (or near) the baffle step frequency there will be a hump in response and a bit of rippled response above that.
A driver with no box and no baffle has only it's own physical parts to separate the front and back wave, so fairly high frequency waves are allowed to go around the edge and cancel, and most of the bass is lost.
A lot of carefully modeled sub boxes would NOT put better music into your room if you removed the back. Carefully modeled implies decently designed, and if you remove the back of a decently designed subwoofer box the result is going to be drastically different. If it was decent as a boxed sub it will not be any good with the back removed. Boxed and open back require an entirely different design.
Of course in real rooms response is unpredictable, but simulators have no trouble at all providing very accurate frequency response predictions of OB speakers in 2 pi space. Some even go further and will allow you to add the back wall and side wall reflections in addition to the floor reflection. It's not at all "bouncing back in ways that are not feasible to predict". Simple first reflections are actually quite easy to predict.
Simple OB design is actually so easy to do I described how to do it without a computer and without any math at all a few years ago. https://sites.google.com/site/amateuraudio/theory/ob-design
It's really not tricky at all but you need to know what you are doing. And here's a quick page on baffle effects, looking at diffraction of both monopole and dipole designs. https://sites.google.com/site/amateuraudio/theory/baffle-effects
There's even a page on boundary effects (primary boundary reflections), and the info on those three pages is pretty much all you need to know to start designing OB.
Note that these were written back in 2009 when I was still very much a novice, but even back then I had more to offer than incomplete and ill conceived thoughts like "a lot of carefully modelled sub boxes would put better music into your room if somebody removed the back" and "the stuff the coming out the back heads off in all directions and bounces back in ways that are not feasible to predict".
No simulator is going to provide a realistic full room prediction, but that really isn't that necessary. If you understand the basics and you have a good starting point it's easy to get a good result with some minor tweaking.
Your main problem with simulators (or one of your main problems with sims) is that they can't predict in room response. That's not what they are for, and it's not required to get a good result if you understand a bit about acoustics and what rooms do to sound.
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Do you actually believe the things you say? There's some interesting levels of delusion at work here.
Based on the measurements and pictures he's provided to date it would appear that he has a massive, almost surreal amount of room gain in his small probably well sealed and concrete walled senior's apartment. This massive room gain and extremely low spl requirement would account for the fact that he's had no luck with regular designs but reported great results from an antique woofer in a hybrid OB used down to 20 hz and his antique sealed AR1. This (and the clear lack of research into acoustic design) would also explain his often bizarre comments, recommendations and "audiophile" type opinions about how things work, although it doesn't explain why he hasn't removed the back on the AR1 if he thinks it would be such a dramatic improvement. It also doesn't explain why he has an AR1 in the first place, as it uses a moving coil driver and no motional feedback.
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So when a speaker moves the air right in front of it dosnt move? So the air goes thru the driver? I thought near field responses differed from far field and near field was within a wavelength. And water does move with waves, it moves up and down just like air moves back and forth with sound.
Hi,
You think your being clever ? Your not. Sadly you disagree
with the fundamental fact front and rear radiation of a driver
is out of phase. There is no hope for technical delusion.
rgds, sreten.
The front and back side of a driver's output are definitely most certainly out of phase. That's why there's cancellation below the baffle step frequency unless the front and rear output are completely separated by a baffle or box or one side is significantly delayed in time and phase by a port or transmission line. If both sides were in phase there would be no cancellation below the baffle step frequency and really no need for boxes to keep the front and back separated.
There's really not much room for interpretation or discussion on this matter.
There's really not much room for interpretation or discussion on this matter.
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