The pdf shows Fs to be 58Hz to 70Hz and the graph shows the phase angle to be 30 degrees to 35 degreees at those frequencies, therefore, the statement, "Typical behavior of a loudspeaker is as shown below (0° below fs, -90° at fs, -180° above fs)" can not be valid.

I think pdf and phase graph are not related... The amount of free answers I can get from them is exhausted though, I think.

 

In summary, all your comments seem to prove that my idea of having a bass speaker with zero or low phase shift is flawed.

Thanks

 

Sounds good, but what does it mean?

pressure is low compared to velocity

To make sound waves we need compression and decompression of air from the sound source not just moving air.

If we were to slowly press a small flat object (like a pencil eraser) on a drop of water on a pane of glass the water would mainly just move (angles different from the direction of force,velocity with low pressure, like high current with low out-of-phase voltage in a impedance mismatch) around our object instead transferring most of the force directly to the glass but if we use larger (flat) objects to press the drop, less fluid moves away from the surface at angles different from the direction of force (velocity with pressure, like in-phase current and voltage transferring real power in a impedance match) as the fluid in the center of the object can't move away faster than the force pushing it forward.

 

@12 Much of what you said is also explained on this website: http://lenardaudio.com/education/05_speakers_3.html

Thanks

 

To make sound waves we need compression and decompression of air from the sound source not just moving air.

If we were to slowly press a small flat object (like a pencil eraser) on a drop of water on a pane of glass the water would mainly just move (angles different from the direction of force,velocity with low pressure, like high current with low out-of-phase voltage in a impedance mismatch) around our object instead transferring the most of the force directly to the glass but we use larger (flat) objects to press the drop, less fluid moves away from the surface at angles different from the direction of force (velocity with pressure, like in-phase current and voltage transferring real power in a impedance match) as the fluid in the center of the object can't move away faster than the force pushing it forward.

I still don't find this any clearer, sorry.
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Praondevou,


you were interested in an explanation of radiation efficiency. It is all to do with velocity (mismatch). The greater the velocity mismatch the lower the transmission efficiency.

This works as follows.
Consider any sort of wave travelling in any sort of medium. As long as the medium properties remain homogeneous the wave is transmitted from one point to the next with 100% efficiency.
If however there is an abrupt change of property at a boundary or change of medium only a portion of the wave passes through the boundary. This is measured by the ratio of the velocities in the respective media and the ordinary laws of refraction and reflection are observed.

Now the velocity of sound in air is about 340 m/sc.
Consider a speaker cone moving 10mm each way, a total displacement of .02m.

At 20 Hz this corresponds to .02 x 20 = 0.4 m/sc
At 200 Hz this corresponds to .02 x 200 = 4 m/sc
At 2000 Hz this corresponds to .02 x 2000 = 40 m/sc

This is an enormous mismatch in velocity which explains why speaker efficiencies are so low, and falls with lower frequency.

If you are interested we can explore this further to develop some simple maths to model the inverse square curve of the response. This is complicated by the fact that the speaker system is an electromechanical device.

 

I still don't find this any clearer, sorry.

This is the source document I used for information about (very) near field effects and ranges.
http://www.toyo.co.jp/file/pdf/mft/ebook/ebook_2_sound_and_vibration.pdf

 

That link, thanks.

It's taking an inordinate time to download, but the contents pages doesn't seem to deal with loudspeakers?

To make sound waves we need compression and decompression of air from the sound source not just moving air.

Bernoulli's equation tells us that air suffers decompression as soon as it moves, and the faster it moves the greater the decompression.

edit
You also need to be careful when reading about 'pressure' in relation to sound waves. Sound generation represents the addition of pressure energy to a medium. Analyses are often conducted in terms of the additional pressure involved. This is often called 'overpressure'. So when considering gas or other laws you need to know which pressure you are dealing with.

 

That link, thanks.

It's taking an inordinate time to download, but the contents pages doesn't seem to deal with loudspeakers?



Bernoulli's equation tells us that air suffers decompression as soon as it moves, and the faster it moves the greater the decompression.

It's not speaker centric but the theory applies to them.
...
Sure, you also have to be careful to use Bernoulli's principle in the right context.

http://en.wikipedia.org/wiki/Bernou..._principle_in_common_classroom_demonstrations
http://www-stud.rbi.informatik.uni-frankfurt.de/~plass/MIS/mis6.html

 

The background for this is my all-time favorite (mostly) thought-experiment about why active noise cancellation is so damned difficult to implement. Even for a small space.

My idea was that it should have been possible to keep pressure near a speaker at a constant level, e.g. at 20cm distance. Not impossible for a frequency up to 100Hz. But the first obstacle is the one I just learned: The phase shift of the speaker has to be taken into account....

I would really like to know what effort (electronics/programming) they put into a system like this: http://mashable.com/2012/05/26/noise-canceling-tech/

@NSASPOOK

This ebook is great!

 

The background for this is my all-time favorite (mostly) thought-experiment about why active noise cancellation is so damned difficult to implement. Even for a small space.!

Good! I was beginning to wonder whether you were just now starting to explore acoustics or you were getting a little crazy.

In the usual context, listening to music, phase difference is irrelevant as long as both speakers are the same. I was really starting to wonder why you would even try to get a zero phase shift. Thanks for explaining.

 

or you were getting a little crazy.

Sometimes I ask myself the same thing..

I first had this idea when I was 15 I think, it just looked so good on a (two-dimensional) sheet of paper. Then, one day I read about the Sennheiser noise-canceling headphones...

But somehow development stopped there - with headphones.

Eliminating sound when it can come from virtually every direction, it seems indeed to be horribly difficult.

 

I think I can help to understand your questions.

1.) Speaker phase shift. Voltage across the inductor that makes the speaker operate has a +90 degree phase shift if the voice coil has no resistance and only reactance. This scenario (ideal inductor) doesn't exist, making the real voltage leads always less than 90 degrees of the current. The current however is what drives the speaker, and will be nearly instantaneous. There are speakers that operate solely on voltage and the attractance of opposite charges meaning no phase shift. They are called electrostatic speakers. But they will never produce the lows of a subwoofer cost effectively. I mean a small tower speaker is over a grand, and to produce one that is capable of reproducing less than 100Hz frequencies could edge close to 30 grand and need it's own power station to supply the electricity for it.

2) Why a small speaker cannot produce lows like a sub woofer. Sound is observed through pressure differences, not just pressure. Atmospheric pressure changes daily, but you never hear it because the pressure is able to establish equilibrium over time. The same thing applies to low frequency sound. Since the wave length and period are so long for lower frequencies, pressure has more time to equalize than for higher frequencies. Higher frequencies don't even require a box for the speaker to separate the pressure difference from the back to the front of the speaker. This is because the wavelength is shorter than the distance from the center of the cone's front to the center of the rear of the cone. If you take that same speaker and try to produce lower frequency sound, it will not do it. The pressure difference is negated quicker than it's created. This causes a need for a box that establishes pressure differences inside and outside the box. The distance between the center of the front cone and the rear cone is infinite for a sealed speaker box.

This doesn't completely solve the issue. The pressure difference can be negated by atmospheric elasticity (compress-ability of air) as well, an equilibrium is created and no more sound. Remember how I stated that sound was pressure differences. Well the derivative of the sound wave determines how fast a pressure wave is changing. And how much it will move either a microphone diaphragm or the ear drum in your ear. Either way, this means that for lower frequencies with the same magnitude, there will always be a shallower rate of increase, meaning slower movement of the diaphragm or ear drum. This means less energy transfer to the diaphram or ear drum. The only way to increase this rate of change for a given frequency is to increase the amplitude/magnitude of the sound wave. And the instantaneous rate of change becomes steeper. This means more movement of the eardrum or diaphragm. Moving more air with a larger cone does this.

The other problem you face here is that inductors oppose changes in current flow by storing energy in a magnetic field. This means that the magnetic field will not be as strong as those for higher frequency signals at the same magnitude of the signal. Ergo the voice coil and cone will move less with lower frequencies than for higher frequencies, introducing the need for more current and a larger coil. As a result, larger speakers, with larger heat dissipating coils, are used to reproduce these lower frequencies more efficiently.

 

Sigh -- another newbie thread resurrection from the dead.
Welcome to the forum micmore80.
But you do realize you are reviving a thread that is nearly 3 years old(?).

 

No one answered the questions correctly, and I thought they needed answerring. Sorry to bother you with this thread's resurrection. I knew it was old, but was looking for something along the same lines and came across it. It just seemed odd that no one took the definition of sound into account and that's a big hole in understanding this topic.