The duration of resonance will be longer but the amplitude will be less. The more massive car with stiffer springs will simply have less translation in response to a road bump compared to a lighter car with weaker springs, even if resonant frequency is the same. This is no different than my beam example and can be quickly proven in closed form.
The point you are making is that it takes LONGER to damp the vibrations from a more massive MKC system with a given damper. The point I and others are making is that the duration may be longer buy the amplitude is lower. Amplitude translates directly to spl.
If we are returning to the issue of cabinet resonance I repeat that the relevant measurements show that neither Q nor peak level drop when cabinet resonances are shifted up with thicker cabinet walls. (Those papers you promised to read.)
Why don't we revisit the issue when you have some contrary evidence?
David S.
I don't know if this is still an issue in this thread, but I saw that my name occurred a few pages back so I Thought I should make a post here.
I must first confess that I have not read the thread completely so I might have misunderstood the question, but still...
Here we go:
As I understand it, there is a debate regarding the relation between the pressure inside a loudspeaker box, and the sound pressure at a distance, outside the box.
The relation between the two is a tilt of -12 dB/oct, ie there is a bass lift inside the box. This is true both for closed boxes and bassreflex boxes.
Here is why:
The loudspeaker, seen as a point source emits a sound pressure given by the formula p=Q*rho0*f/(2*r), where Q is the volume flow out of the point source, rho0 is the density of air, f is frequency and r is the distance to the point source. Thus pressure outside the box i ~f, or pressure is ~ the derivative of volume flow. (This is alternately seen as a frequency dependent radiation resistance.)
The box, on the other hand gets the same volume flow pumped out of it. The box acts as an acoustic capacitance, ie it stores the air (="charge") that is pumped into it, The amount of air is ~ the integral of the flow, or ~1/f
So, box pressure is ~1/f, outside pressure ~f. The ratio is (1/f) / f = 1/f^2.
Since intensity is ~pressure squared, the energy ratio would be 1/f^4.
I don't know if this was at all requested, but anyway here it is.
I must first confess that I have not read the thread completely so I might have misunderstood the question, but still...
Here we go:
As I understand it, there is a debate regarding the relation between the pressure inside a loudspeaker box, and the sound pressure at a distance, outside the box.
The relation between the two is a tilt of -12 dB/oct, ie there is a bass lift inside the box. This is true both for closed boxes and bassreflex boxes.
Here is why:
The loudspeaker, seen as a point source emits a sound pressure given by the formula p=Q*rho0*f/(2*r), where Q is the volume flow out of the point source, rho0 is the density of air, f is frequency and r is the distance to the point source. Thus pressure outside the box i ~f, or pressure is ~ the derivative of volume flow. (This is alternately seen as a frequency dependent radiation resistance.)
The box, on the other hand gets the same volume flow pumped out of it. The box acts as an acoustic capacitance, ie it stores the air (="charge") that is pumped into it, The amount of air is ~ the integral of the flow, or ~1/f
So, box pressure is ~1/f, outside pressure ~f. The ratio is (1/f) / f = 1/f^2.
Since intensity is ~pressure squared, the energy ratio would be 1/f^4.
I don't know if this was at all requested, but anyway here it is.
Hello,
You state that pneumatic coupling is 33% or 10db down from the mechanical coupling of the driver to the speaker.
Just for grins let’s give this thing the smell test.
I opened WinISP and got some data for a JBL2225 in a sealed box:
X-max is 0.20 is inches
Box volume is 1928 inches
Speaker cone area is 139.5 in square
Change in box volume at X-max is 28 inches cube
Neglecting adiabatic heating and using Boyle’s Law the change in internal box pressure is ~ 0.2166 PSIG. Or ~ 55 pounds of force over the area of the, 20.2 inch x 12.62 inch, back of the enclosure. That is enough force to deflect the rear panel. The other 5 sides of the enclosure will be deflected in proportion to their area.
Now for the feigner experiment, take the back off of the speaker. The sealed box pneumatic pressure and coupling should be gone. By the stated 33% notion 67% of the box noise should remain. I do not believe this to be true.
Open baffle dipoles are reported to be box noise free. I believe this is due to lack of pneumatic coupling.
Where does this 33% number come from?
DT
All just for fun!
Consider this:
All the energy available ia caused by the cone moving back & forth. Conservation says that as a result we have an equal and opposite reaction (ie the basket gets the same energy (absolute value)) With a rigidly coupled driver that is transmitted to the box.
Now lets look at the energy from the cone. 1/2 goes towards making sound outside the box, 1/2 goes inside the box.
So (-1 unit into the box via mechanical coupling) = (1/2 unit into the box airspace + 1/2 unit into the room airspace). the energy into the room space can be ignored.
So at most 1/3 of the energy coupled into the box comes from airspace.
Further. Air is not that effective a coupler. Some energy gets dissipated as heat. If there is a vent significant energy is routed outside the box.
dave.
All the energy available ia caused by the cone moving back & forth. Conservation says that as a result we have an equal and opposite reaction (ie the basket gets the same energy (absolute value)) With a rigidly coupled driver that is transmitted to the box.
Now lets look at the energy from the cone. 1/2 goes towards making sound outside the box, 1/2 goes inside the box.
So (-1 unit into the box via mechanical coupling) = (1/2 unit into the box airspace + 1/2 unit into the room airspace). the energy into the room space can be ignored.
So at most 1/3 of the energy coupled into the box comes from airspace.
Further. Air is not that effective a coupler. Some energy gets dissipated as heat. If there is a vent significant energy is routed outside the box.
dave.
Not in my experience. One of the "boxiest" speakers i've ever heard was Silver Iris on an OB. An OB panel is as likely to resonate as a box panel (all things equal -- which they never are).
What they don't have is time smeared, heavily filtered sound coming back thru the cone. All the time smeared sound comes from around the baffle.
dave
You can frame the equation in terms of whichever variable you wish, but my point stands.
Its not like I'm way out on a limb here. Harwood says essentially the same thing I have been in his introduction. I don't think there is any question that what I have said is correct between resonances. The only point of contention seems to be what happens at resonance. I have had time to only read the first bit of the harwood paper, and my phone that I typically am forced to do any forum browing on obviously doesn't have any useful simulation tools. Even an old mathcad worksheet I probably have saved somewhere should be adequate to investigate the behavior of a simple mkc model.
So can we agree that thicker more massive walls push first resonance higher, that transmitted sound is reduced below first resonance as a function of stiffness, and that transmitted sound is reduced above first resonance as a function primarily of mass except for at resonances?
The area to investigate is thus just what happens at resonance. However, even if amplitude is not reduced, which I find doubtful but have yet proven to myself, I don't see how a panel that has lower transmitted sound at all frequencies other than resonance and at worst equal transmission at resonances can be anything other than a superior solution? I'll first continue reading and do the sims when I have time and am on a capable device, and then we can address that question.
Hello,
The confusion continues!
In Newton’s world there are equal and opposite forces, stationary objects and objects that move; impulse and reaction, equal and opposite work is not required.
After surgery I do a lot of pushups. I push on the floor, the floor pushes back. My body weight moves the concrete floor does not. E=0.5*MV^2. The energy goes into the object in motion not the floor or for that matter the well braced heavy panel the driver mounts to.
In the realm of speakers the equal and opposite work is in on opposite sides of the speaker cone.
I maintain that air pressure is a key factor to panel resonance and speaker design if not the key factor.
Perhaps accelerometers attached to the enclosure panels with the back attached and removed would tell the story. (Agreed? It is motion that causes sound? Or is it the falling tree in the woods?)
DT
All just for fun!
But it does. The ratio of the 2 masses is just so great as to get VERY, VERY SMALL movement of the concrete floor (and whatever is under it).
dave
For a lark i did a ballpark calculation. Assume you weigh 100 kg and your wrist to chest distance is 50 cm and that the centre of mass is such that pivoting around your toes, half is supported by your toes and half by your arms. Further, that the concrete floor is rigidly attached to the bedrock, and that we freeze ALL activity on the rest of the earth (ie the other 100 kilo guy on the other side of the earth doing push-ups) and ignore any "squishyness" of the earth, then your push-up moves the earth ~4*10-23 m = 0.00000000000000000000004 m
The size of a hydrogen atom is 2.4*10-10 m =
0.00000000024 m
dave
My physics may be a bit rusty but isn't it the momentum (i.e. m*v) that is conserved?
That is m*v for the box and cone is equal in magnitude but with opposite sign. This would mean that the cone gets far more of the energy (since the cone is typically lighter 😉 ).
Sadly that's not really true, I reckon the baffle can be pretty audible at times, and if it's full range with big woofers it moves a lot!
Hello,
Besides living in a world where I measure things by the length of the King’s foot you and I are having “frame of reference” issues.
Newton’s third law is stated from an internal “frame of reference” where there are equal and opposite forces and equal energies in motion, two guys on ice skates. No I do not play hockey. From an internal frame two interacting objects have equal energies. The other way of viewing that is that the energy of one of the items is limited to 50% of the total system
The real world that we live in is an external “frame of reference”. In the real world or external “frame of reference” there are fixed nonmovable objects and objects that move.
Here is the example; since we are speaking of an electric motor of sorts (driver) let’s look at an electric motor. There are equal and opposite electromechanical forces at work. From an internal view the forces rotate opposite each other. From the external view one part is free to rotate the other is bolted to a steel frame and concrete base and not moving. There is no one in China pushing back. System efficiencies are in excess of 50% for the rotating center part, this can only happen in the external frame. Rule of thumb is 1000 watts input and 750 watts work or one horsepower out.
Speaking of drivers the frame is fixed to the box in the external frame and half the energy is not consumed by the box due to mechanical coupling.
DT
All just for fun!
I'll agree to all of that. If we had a resonance free cabinet then making the walls more massive (or more rigid) would be a total cure. Same as with TL in architecture (Transmission Loss, as in-through a building's walls), making the walls more massive increases the loss.
The problem is all about the resonances. Every study that measures the performance of cabinets shows that cabinet transmission is high at the resonances. Reducing transmission between the resonances (or above and below) doesn't do us any good. We hear the sound transmission where it is high and it is exacerbated by the resonance dragging the energy out in time. We don't need to reduce the mean level of energy through the walls, we need to reduce the peaks.
The problem with manipulating mass or stiffness is that it doesn't make any difference at resonance. By definition a resonance means that mass reactance and stiffness reactance are exactly equal (opposite in phase) and thus cancel, leaving only the resistive losses inherent in the system.
If we want to reduce cabinet transmission we need to increase the losses.
David S.
Regarding acoustical vs. mechanical coupling I'll repeat my experience at KEF.
We developed decoupling sytems that floated the woofer on soft mounts to reduce the mechanical coupling of reaction force to the cabinet. It considerably reduced cabinet vibration, although the ultimate reduction is limited by the air pressure force stll being in effect. My recollection was that we got about 10 to 15dB less cabinet vibration, so clearly the mechanical transmission is the more significant factor.
The cabinet may be relatively massive compared to the cone but it is not infinite in mass and it also has considerably greater area than the cone, so cabinet radiation is significant.
David S.
We developed decoupling sytems that floated the woofer on soft mounts to reduce the mechanical coupling of reaction force to the cabinet. It considerably reduced cabinet vibration, although the ultimate reduction is limited by the air pressure force stll being in effect. My recollection was that we got about 10 to 15dB less cabinet vibration, so clearly the mechanical transmission is the more significant factor.
The cabinet may be relatively massive compared to the cone but it is not infinite in mass and it also has considerably greater area than the cone, so cabinet radiation is significant.
David S.
IMO the ideal cabinet is acoustically and mechanically inert. It doesn't flex and sound does not pass throgh it. In the real world no such thing exists. Different strategies have been tried successfully. AR used 3/4" plywood, KLH 3/4" heavy duty marine plywood. 3/4" MDF seems to be popular now. Wharfedale used an inner and outer enclosure separated by a layer of sand in the 1960s. That should have worked very well. I might try two layers of 1/2" plywood separated by a layer of Quiet Rock. This engineered product is similar to sheet rock but is designed to absorb sound limiting transmission. It seems more practical than sand. Internal cross bracing is often used too. Usually speakers that are well constructed are very heavy for their physical size.
You can clad your speaker in whatever you like. It can be unfinished, painted, contact papered, laminated plastic, or expensive veneers. Expensive speakers ususlly use fine veneers that are finished like fine furniture. For some reason people are very fussy about this yet when you look at used speakers they are invariably marred by scratches, gouges, chips corners, water stains from drinking glasses set on them without coasters, cat pee stains, and all other manner of damage. So what's the point of such added non functional expense that jacks up the initial selling price??
You can clad your speaker in whatever you like. It can be unfinished, painted, contact papered, laminated plastic, or expensive veneers. Expensive speakers ususlly use fine veneers that are finished like fine furniture. For some reason people are very fussy about this yet when you look at used speakers they are invariably marred by scratches, gouges, chips corners, water stains from drinking glasses set on them without coasters, cat pee stains, and all other manner of damage. So what's the point of such added non functional expense that jacks up the initial selling price??
- Status
- Not open for further replies.