Hi rythmikaudio, a few questions for you.
You said there is a type of distortion generated by the energy which is kept inside the closed box behind the cone, which is not measurable by normal distortion tests. How can one measure it? Why isnt it measurable with normal methods (i suppose you mean sine sweeps and some fft to identify the distortion components)?
Thanks.
You said there is a type of distortion generated by the energy which is kept inside the closed box behind the cone, which is not measurable by normal distortion tests. How can one measure it? Why isnt it measurable with normal methods (i suppose you mean sine sweeps and some fft to identify the distortion components)?
Thanks.
rythmikaudio said:
Would some physics major please stand up and explain the difference between a standing wave and plain, evenly distributed pressure?
For some related stuff, please everyone read http://www.linkwitzlab.com/thor-design.htm
MaVo said:
It is not a distortion. It is a smeering of sound image and one of the sources of so-called boxy sound. It is difficult to measure it because the waveform will look like a ringing in transient response and one may interpret it as the output from the cone itself. The best way is to put a driver inside and measure the output from the cone. Some may even come up a DSP based reflection cancellation scheme.
Now, what is boxy sound? Boxy sound is a sound of box when one side is open. Try that with cardboard box. That is also one reason people go with OB speakers. OB speakers do not have boxy sound at all.
rythmikaudio said:
Excuse my English. But, I assume OB stands for open baffle. Is it some kind of sealed enclosure, or is it synonymous with dipole? Or synonymous with dipole and all related back wave canceling designs?
If it is, then this is about stored energy in the form of the air pressure acting as a "spring" in a sealed box. But then I don't see what does this have to do with any cardboard box. That is because the frequencies of interest are altogether different. In that case one would fight against "boxy sound" by 1. building a stiff enough enclosure and 2. stuffing it a little.
Thinkcat said:
This may be related. Finite Element Analysis(FEA) is a computer numerical method of solving complex 3-dimensional energy/heat/... transfer problem, anything that related to field equations. The space is divided into a large number of very very small cubes that act as elements. Mark Sheaton has brought up this question before. My answer was: do the programmer who write FEA change their transfer equations between cubes based on frequency? The answer is definitely not. So the so caled pressurization is merely a macroscopic behavior and when you look at the microscopic behavior, it is same as a reflection with finite propagation time except the wavelength is shorter. In other words, it will progration in all directions.
Thinkcat said:
OB is open baffle. a 100hz wave length is 10ft. Assume your speaker is one foot in one dimension, going back and forth is already two feet (or 1/5th of wavelength or 72 degrees phase shift). Wave propagation is not just one trip if the energy is still there. How about two trips, or even more...
Again people mention standing wave. But standing wave is steady state behavior. How does that relate to dynamic behavior that signal is chaning frequency, amplitude all the time? If we focus only on steady state reponse, it is like looking at the macroscopic response, we don't see the detail and is not able to "image" what happen to real time music signal. What if we just have one or two cycle of signals? Do we have a standing wave? Do the signal at the frequency of standing wave and those frequencies next to it behave that much different such that only the standing wave continues to "stay"? No. Their propagation nature is exactly the same. They just keep propagation and radiating, and both of will multiple copies of various delayed sound wave mixed in the enclosure. The only difference is standing wave last longer because those delayed sound wave mention consistent phase relation.
The story of cardboard is just to let you know how open one wall makes the boxy sound more audible. I sure know the standing wave of enclosure is from cone generating acoustic sound into the enclosure, but by the enclosure itself.
Sound damping material is not effective at all in low frequencies. Go google to find low frequency sound damping material. Most honest merchant will tell you there is no good solution.
What you mention sounds like ringing, which should be related to an uneven frequency response and should vanish as soon as one eq's the response flat. I think people with a better understanding than mine would speak of the minimum phase quality of a woofer. It should be visible in a waterfall spectrum image as hills in the graph.
A related type of this response error is associated with relatively higher frequencies, where parts of the box are big enough to be excited by the music, so its a bit strange for me to see this as a problem in the bass range.
Could you please elaborate where i am incorrect?
A related type of this response error is associated with relatively higher frequencies, where parts of the box are big enough to be excited by the music, so its a bit strange for me to see this as a problem in the bass range.
Could you please elaborate where i am incorrect?
MaVo said:
It is not the same as ringing. This is "delayed" (or time displaced) version of singals repeated with smaller and smaller strength. That it causes is maybe +/- 0.2db of frequency that is buried inside the noise. It is all related to "resolution". The correct way to solve this is not via EQ, instead via digital domain and construct a negative version of the "echo" and subtract it. EQ is a short cut. If you really want to talk about EQ, that is another story. Analog EQ, digital EQ, FIR, ....
rythmikaudio said:
Think of sound as electromagnetic wave. It needs AC signals and media to propagate. In other words, you need to have a change in pressure to propagate sound energy. If you have multiple subwoofers (in separate enclosures), try one facing another 3" and gradually pull them further apart, and feed low frequency signals to only one subwoofer and seehow that acoustic energy transmitted outside via free air and causes the driver in another enclosure to move (by placing for hand one the cone). It has very little to do with the pressure level inside the room. It has more to do with the local pressure change around the cone caused by cone movement. Since now we have two separae drivers, one can easily classify one of them as aggressor and the other as victim in a interference study. But inside a single sub, the aggressor is also the victim and makes the separation very difficult.
Since someone mention ringing which is related to Q value of 2nd order filter characteristic (all filtered as constructed with a series of 2nd order filter and zero or one first order filter), we can compare what difference it makes with different Q in sound. With high Q, the image always become larger than the one with low Q. It is a type of imaging smearing effect. Second, standing wave and refection cannot be exactly modelled as regular filtered and corrected with filter. They arise from different causes.
From Laplace transform theory, a delayed signal is multiplied by a term exp^(-D*S), where D is the delay amount, S is the Laplace variable. That term can be expanded into a full Taylor expansion with infinite polynomial sequence. Filter used in PEQ is only 2nd order. It is definitely an approximation at best. I often like to compare the abuse of EQ to the abuse of pain killer medicine. It is purely out of convenience.
rythmikaudio said:
Bud Fried used the descriptive term "time smear" for this kind of distortion.
dave
given the very small box size requirement, and the goal of 15 hz, the only feasible solution i can see is some variation of linkwitz-transform/EQed sealed box (~90 liters, 3 ft^3)
With the high excursions required for this kind of enclosure using 2 drivers push-push for active reactive force cancellation would be highly recommended.
Given the significant budget you have a pair of the Apollo 15" mentioned would be really nice, for a more pedestrian driver i'd suggest CSS SDX15. One could also stuff that box with 6-8 SDX7s for something a bit more radical.
The small box size will not allow you to get the most out of any of these drivers.
dave
With the high excursions required for this kind of enclosure using 2 drivers push-push for active reactive force cancellation would be highly recommended.
Given the significant budget you have a pair of the Apollo 15" mentioned would be really nice, for a more pedestrian driver i'd suggest CSS SDX15. One could also stuff that box with 6-8 SDX7s for something a bit more radical.
The small box size will not allow you to get the most out of any of these drivers.
dave
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