Hi Grey! You can see why I haven't had the time to finish the reply to
your email.
As you can also see, I probably did you a favor.......
The problem I have in considering this (assuming it's correct for sake of argument) is that if this is true, this would introduce distortion due to the asymmetry. Seems unlikely to be the case, partly because the loading on a driver is the sum of the load on both sides, one side undergoing positive pressurization while the other side is undergoing negative, simultaneously for each half-cycle with each side providing the same magnitude no matter the direction of movement. I just don't see how asymmetric loading is the issue unless it truly does result in asymmetric movement of the driver diaphragm which would not be desirable.
Dave
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After two more pages of talking, I gues it's time to get back to the real thing.
The parameters of the woofers when they're nude on swings near floor (not entirely the same as in free-air due to more load near boundary, but close enough):
And those with them on the slot loaded OB:
Their fs drop with slightly higher Qts.
But that is not the case at all. The effect you describe is the same as if you were closer to a speaker the same energy is concentrated in a smaller area but that does not have a effect if you are not very close to the slot
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well ........ just deleted at least 15 sentences , realizing that my peanut sized brain can't put it in any practical way ...... so just one thought certainly stays -
PWK's famous line (that PWK , who certainly wasn't humorless bastardo type ) , in which he was saying that world now needs just one proper 5-watt amp - later became complemented with : "and one of my big button badges in every room " ...
PWK's famous line (that PWK , who certainly wasn't humorless bastardo type ) , in which he was saying that world now needs just one proper 5-watt amp - later became complemented with : "and one of my big button badges in every room " ...
from the text
"This subwoofer features a unique concept in passive attenuation".
Not that I pretend to understand marketing speak especially in english
In my incomplete mockup, I'd say pretty good.
Lack of front baffle area makes it much less efficient than expected. And I got a 7~8dB dipole peak problem around 200-250Hz range. (I fixed that with digital EQ for the time being.)
Other than those, it sounds pretty good. Very clean and deep. I heard new sounds in one of my old CD during breif listening last night, some sub-bass noises, quiet but clear. Such LF --- air vibrations -- delivers an illusion that it's pressurizing the room, but of course it's not.
This low level performance fit my current requirement very well. I haven't tried higher SPL, yet, as the rest of the speaker is still under construstion.
from the text
"This subwoofer features a unique concept in passive attenuation".
Not that I pretend to understand marketing speak especially in english
Low pass filter action of the baffle in front of the driver they mean.
Haven't seen it posted here yet (although it has been aluded to) so here's the linky link link to the Emerald Physics thread:
http://www.diyaudio.com/forums/multi-way/148303-emerald-physics.html
Cheers,
Mike
http://www.diyaudio.com/forums/multi-way/148303-emerald-physics.html
Cheers,
Mike
Now how about a little theory. This is being used as a woofer so the frequencies are low. Wave lengths are typically much greater than the driver or slot size. Thus, to first order, in the far field both sides may be considered as simple sources. As Nelson has stated, the volume velocity of the front and back are equal. So what does this tell us about the sound radiated at a distance? For a simple source the sound intensity, I, is proportional to (QxF/r)^2 where Q is the volume velocity of the source, F is frequency and r is the distance form the center of the source. Thus, if the volume velocity is the same, in the far field the sound intensity is the same.
But what about the near field? We don't have simple sources so we can not apply the relationship above in the near field. But we can make a couple of observations. We know that the area of the slot is much less than the total area of the drivers. And we know that the volume velocity is the same front and rear, at lease at low frequency. So perhaps we can get an idea of what is happening buy comparing a 12" woofer to a 5" woofer. If both are radiating a 100 Hz signal into 2Pi space they would generate the same sound intensity in the far field if both have the same volume velocity. But to have the same volume velocity the 5" driver must move with a velocity that is 144/25 = 5.76 times greater than the 12". It should be obvious that when a near field measurement is made the intensity close to the 5" driver will be much higher than that for the 12" driver, a result simply due to the difference in radiating area. If we compare the area of the two drivers to the area of a sphere we see that the 12" driver would be represented by a sphere with R = 3.38" where as the 5" driver would by a sphere with R = 1.41". Referring back to the intensity relationship for a simple source we can see that we should expect the intensity at the surface of a 1.41" spherical source to be much higher than that at the surface of a 3.38" spherical source when they are radiating the a the same frequency with the same volume velocity.
If we measure the intensity of both spherical sources at the same distance, some distance greater that 3.38" the intensity will be the same. However, Nelson indicated that at 1M, and even at 2M he measured greater SPL at the front then rear. There are a couple of issues here that must be considered. Even at 1M there is front to back interaction. At 2M this becomes more uniform. The issues to be considers are the relative delays from front to back and back to front which affect how the sources sum (i.e. the polar response). There is also the relative strength of the sources and how they differer due just to the different path lengths. Even with an equal strength point source dipole, the axial response is dependent on distance as I showed here.
And there is another issue to consider. That is the fact that the initial wave launch form the front and back is very different in shape. While it is true that in the far field, at low frequency, the waves from the front and rear will take on a spherical form, just how far from the sources this occurs will be different for the front and rear. Once the waves achieve a spherical form the SPL will fall as (1/r)^2 and since the volume velocity is the same from the front and rear the SPL should also be the same. Thus, in the far field differences in the front and rear SPL and in the polar response should only be a result of the delay paths. But this will not occur in the near field.
Lastly let's not forget about room interaction. Though Nelson has a large room, this OB is not physically symmetric and not composed of point sources. So even rotating the baffle 180 degrees to measure the rear with the mic position fixed will alter the way the speaker couples to the room and potentially affect the on axis measurement.
In any event I also plan to build a model of this slot loaded OB configuration and intend to make a full set of measurements, both near and far field and full polar plots. I don't expect to see any significant divergence from a dipole response at low frequency. May take a while though.
But what about the near field? We don't have simple sources so we can not apply the relationship above in the near field. But we can make a couple of observations. We know that the area of the slot is much less than the total area of the drivers. And we know that the volume velocity is the same front and rear, at lease at low frequency. So perhaps we can get an idea of what is happening buy comparing a 12" woofer to a 5" woofer. If both are radiating a 100 Hz signal into 2Pi space they would generate the same sound intensity in the far field if both have the same volume velocity. But to have the same volume velocity the 5" driver must move with a velocity that is 144/25 = 5.76 times greater than the 12". It should be obvious that when a near field measurement is made the intensity close to the 5" driver will be much higher than that for the 12" driver, a result simply due to the difference in radiating area. If we compare the area of the two drivers to the area of a sphere we see that the 12" driver would be represented by a sphere with R = 3.38" where as the 5" driver would by a sphere with R = 1.41". Referring back to the intensity relationship for a simple source we can see that we should expect the intensity at the surface of a 1.41" spherical source to be much higher than that at the surface of a 3.38" spherical source when they are radiating the a the same frequency with the same volume velocity.
If we measure the intensity of both spherical sources at the same distance, some distance greater that 3.38" the intensity will be the same. However, Nelson indicated that at 1M, and even at 2M he measured greater SPL at the front then rear. There are a couple of issues here that must be considered. Even at 1M there is front to back interaction. At 2M this becomes more uniform. The issues to be considers are the relative delays from front to back and back to front which affect how the sources sum (i.e. the polar response). There is also the relative strength of the sources and how they differer due just to the different path lengths. Even with an equal strength point source dipole, the axial response is dependent on distance as I showed here.
And there is another issue to consider. That is the fact that the initial wave launch form the front and back is very different in shape. While it is true that in the far field, at low frequency, the waves from the front and rear will take on a spherical form, just how far from the sources this occurs will be different for the front and rear. Once the waves achieve a spherical form the SPL will fall as (1/r)^2 and since the volume velocity is the same from the front and rear the SPL should also be the same. Thus, in the far field differences in the front and rear SPL and in the polar response should only be a result of the delay paths. But this will not occur in the near field.
Lastly let's not forget about room interaction. Though Nelson has a large room, this OB is not physically symmetric and not composed of point sources. So even rotating the baffle 180 degrees to measure the rear with the mic position fixed will alter the way the speaker couples to the room and potentially affect the on axis measurement.
In any event I also plan to build a model of this slot loaded OB configuration and intend to make a full set of measurements, both near and far field and full polar plots. I don't expect to see any significant divergence from a dipole response at low frequency. May take a while though.
I had not followed that thread but looking at it quickly one thing I did not see mentioned is that IF the aperture acts as an LP filter it will impart some delay to the response emanating from the port which will affect the way the front and rear sum. The same applies to NP's OB. I mention in my previous post that these delays must be considered.
Hi John, salas, all
On could demonstrate the slot has no effect on low frequency loading by building a two sealed back versions with the same size baffle one using the slot loading and the other with conventional mounting. The difference between the two is described here in post #130.
Salas wrote “Some modern stadium line arrays use it in different guises.”
Interestingly the move where sound quality is desired is away from line arrays altogether and to full range point sources. These can provide a greater SPL AND intelligibility AND sound quality. These use multiple drivers in and acoustic low pass filters connected to a single horn.
Examine the DIY approach to that kind of single acoustic source system shown here;
http://www.diyaudio.com/forums/multi-way/195955-unity-horn-budget-drivers-active-x-over.html
and the stadium sound installations shown here;
Danley Sound Labs, Inc. | Facebook
One of the more recent installations is Lambeau (Packer) field in Greenbay where even the players can hear the difference..
Rodgers Nice to be back at Lambeau - YouTube
Best,
Tom
On could demonstrate the slot has no effect on low frequency loading by building a two sealed back versions with the same size baffle one using the slot loading and the other with conventional mounting. The difference between the two is described here in post #130.
Salas wrote “Some modern stadium line arrays use it in different guises.”
Interestingly the move where sound quality is desired is away from line arrays altogether and to full range point sources. These can provide a greater SPL AND intelligibility AND sound quality. These use multiple drivers in and acoustic low pass filters connected to a single horn.
Examine the DIY approach to that kind of single acoustic source system shown here;
http://www.diyaudio.com/forums/multi-way/195955-unity-horn-budget-drivers-active-x-over.html
and the stadium sound installations shown here;
Danley Sound Labs, Inc. | Facebook
One of the more recent installations is Lambeau (Packer) field in Greenbay where even the players can hear the difference..
Rodgers Nice to be back at Lambeau - YouTube
Best,
Tom
I was a little sloppy in that post, although I did say originally that I was only talking about what happens at the mouth of the slot and not anything at all about far field or room effects.
I agree that in the far field the size of the source doesn't matter. However, my analysis shows that the energy in the slot mouth is (ratio of area)^2 greater than the energy from the rear of the driver. The asymmetry is real and so is the difference in SPL (at low frequencies) between the slot and the rear, but I should probably stop there without predicting that the measured SPL is higher than from the same driver in free space given the same input power.
Jeremy
The same argument could be used to suggest that for a ported box the port area should be minimized because the radiated power at the port mouth would increase as (ratio area)^2. So a smaller port would appear more efficient. But once the wave propagates into free space there is no difference. In fact, resistive losses in the smaller port would, if anything, reduce the free field energy.
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