You are missing the point. Near field gets rid of secondary sources (diffraction). Like I said and know for sure now, you are looking at diffraction more than at resonances. So your exercises with damping are moot I’m afraid. You don’t need the far field response when you’re evaluating these things.
I use a 1/4 inch GRAS Microphone in the box.
Drill a 1/4+ hole push in the mic and seal it up with blue tack.
Drill a 1/4+ hole push in the mic and seal it up with blue tack.
That is feasible. But since you need the resonances that actually get out of the box, preferably measure the cone and the port. Since resonances show big spatial differences in SPL, the mike in box method could miss the one and exaggerate the other.
@Freedom666
Yeah, the little space was an experiment to create a 1090hz resonance chamber, a pipe, to fill in a dip. I don't think it really worked, 1db is not going to get the job done.
And yes, I understand exactly what you mean with a shelving filter, or putting a cap in parallel to bypass the resistor. I will be creating a new crossover, so I will fix everything there, I will likely do it with parallel notch filters though, but I was curious if I could get there with natural acoustics alone.
Yeah, the little space was an experiment to create a 1090hz resonance chamber, a pipe, to fill in a dip. I don't think it really worked, 1db is not going to get the job done.
And yes, I understand exactly what you mean with a shelving filter, or putting a cap in parallel to bypass the resistor. I will be creating a new crossover, so I will fix everything there, I will likely do it with parallel notch filters though, but I was curious if I could get there with natural acoustics alone.
It is hard to have a resonance in the enclosure that does not escape. Don't you think. The MIB largely removes the room from the measurements.
Measure both the impulse inside the box and outside. Even plot both the Waterfall plots. If there are resonances on the exterior waterfall if they also show up inside the enclosure you will have a good head start towards a solution. A impedance sweep is also a good idea.
A funny or should I say odd thing is that when I measure these things is that resonances originating from the enclosure, driver or whatever also show up on the frequency sweep distortion plots. A resonance at 1500Hz will show up as a 3rd harmonic of 500Hz. That resonance at 1500Hz will also show up as a 2nd harmonic of 750Hz.
Thank you for the feedback. Looking into baffle diffraction today. Read a good paper by Jeff Bagby on diffraction. Then ran a diffraction simulation in Vituixcad and looking at the results to see what is correlating and what isn't. I've loaded it into ARTA as a target curve just to visualize.
Red curve is the simulated diffraction, and the maroon is my original enclosure, and light blue is the new enclosure with all the work we've applied. The diffraction chart is modeling the original enclosure. (See chart below for the diffraction curve simulated for the new enclosure.)
The main observation is that we have a bump at 600hz and a dip at 1200hz, and one final bump at 2khz. I'm not sure if the simulation is actually accurate because according to Jeff it should have more and more random bumps and dips as you go up in frequency due to harmonics, which I don't see in this simulation. But, taking it for what it is, it seems that there is a good amount of correlation 600hz-1800hz, so that likely explains the dip at 1khz as you say, good intuition there.
But when I model the diffraction of the test enclosure I built. I get a simulation that predicts a boost at 1000hz not a dip. This diffraction curve should correlate with the blue FR curve.
So I'm not really sure what to think at this point. One would think if it models accurately for one it should for the other as well but I don't observe that. I doesn't seem correct to accept results which agree with my intuition and then reject the very same results when they disagree with my intuition.
Since I'm working within the confines of an existing 3-way tower I don't have any control over its baffle, until I build a new speaker, but I'd rather keep these and just have it work with updated drivers.
The irony here is that my original enclosure which I was complaining about is still flatter than the test enclosure which I built with everyone's feedback.
Should I start over and make another/different test enclosure? How would you change it?
yes you can if you try a waveguide for the mid driver gaining correct efficiency if dimensioned luckily
Diffraction is a very general description of a more complex phenomenon. And a misleading term in this respect. A shape in free space reflects incoming sound. Mostly we examine this by defining every point on the surface of that shape and calculate the (reflected) sound contribution of that point on a certain (measuring) point in free space. We then add up all contributions of all points on that shape surface and voilà, the resulting sound pressure is a fact.
Grasping this, you understand that most apps simplify things to a great extent, when calculating baffle step or ‘diffraction’ contributions to total SPL at a given point in space.
Most of us approach this all with prototyping, a few use BEM (AKABAK) for this. But this all has nothing to do with the problem of driver and enclosure resonances. I assumed that your were investigating the latter and advise to separate these matters when designing a loudspeaker.
Grasping this, you understand that most apps simplify things to a great extent, when calculating baffle step or ‘diffraction’ contributions to total SPL at a given point in space.
Most of us approach this all with prototyping, a few use BEM (AKABAK) for this. But this all has nothing to do with the problem of driver and enclosure resonances. I assumed that your were investigating the latter and advise to separate these matters when designing a loudspeaker.
But how do you know the mike is in the right spot to measure all modal resonances? For a simple rectangular enclosure, you’d have to measure in three corners. And then still, what is the impact of any resonance on a cone or BR port, those being the obvious leaks?
First off the microphone is a pressure sensor, calibrated in Pascals (units of pressure).
Second all those things are baked into the cake. I like to make incremental changes and record the results visually on the inside the enclosure impulse and waterfall plots. I like having printed results to see on the table top. The audible results are difficult to recall from an hour ago, let alone 2 weeks ago. The inside the box measurements are only minimally affected by the room.
To my way of thinking the pressures in the enclosure push back on the driver cone and the sound comes out the front. This could be long and complicated.
In this thread we are talking sealed mid-range speakers. That being the case I am not too concerned about the Qtc or the contribution of the sealed air volume to the suspension of the driver. A couple octaves above the speaker resonance it is largely about cone mass rather than the added air suspension of the sealed enclosure.
I prefer larger sealed mid-range speakers filled with fluffy stuffing to absorb reflections.
Thanks DT
I observed much of this using a GRAS 40BD microphone in the box.
Your enclosure volume is in the ballpark for your driver. The response probably is not far off from what is expected, either. However, the shape may contribute to response anomalies. You may consider making the walls smaller, and deeper, and perhaps non parallel (tapered), as well as stuff it more, and probably that is as good as you are going to get. You could make it a tad smaller volume. Good luck.
I am sorry, they are not. Study modal behavior and resulting SPL changes and you’ll understand it’s important exactly where you put the microphone.
Yes, if I do place the microphone in different locations in the enclosure I do get slightly variable results. I am not going to put an array of microphones in the enclosure. There is even a 6 inch long Listen microphone in the cage that I have used.
The big however is that I do get consistent reliable measurements with my calibrated GRAS microphone in the enclosure.
As mentioned previously, there are no standing room modes to color the measurements.
I know of at least one major driver manufacturer that uses a inverted tweeter facing into the enclosure to make MIB measurements. Hint for those that want to do this diy style at home. The tweeter resonate frequency may be a bit problematic.
Thanks DT
Well, this was a slip of the pen. Simple rectangular forms only need one corner of course, symmetry reasons.
Keele proved already that measuring at max 0,11xcone radius brings reliable and exact measurements of the isolated cone output. You could of course place the microphone near to the cone from the inside to get a view on the enclosure modes that work on the cone. But don’t forget a cone is quite transparent to sound and listening room modes do actually come through from out to in.
Hello All,
Not so much as to convince @markbakk, that will be difficult, that is okay.
Think of the room as a larger enclosure with walls, windows, doors, with furniture, drapes carpets, you know my living room. On the other side of the mid-range cone the speaker is sealed in a enclosure.
Place identical calibrated lab grade microphones in the room and in the mid-range enclosure. Keep in mind that microphones are calibrated in Pascals. (metric units of pressure)
Is there more sound from the room entering the mid-range enclosure through the Mid-driver cone or or sound from inside the the mid enclosure entering the room?
There is better than +40dB (SPL) between inside and outside the enclosure. That makes the pressure (SPL) outside the enclosure less than 1% of the pressure (SPL) inside. That is 150+dB's inside. In the analysis of things that makes the influence of room modes inside the box unmeasurable.
Thanks DT
Be careful at home with your microphone inside the box, 150+dB's may destroy it.
Thanks DT
Yes and no.
I bought a pair of identical 1/2 inch mics from AP / GRAS, years later they still measure within a dB or two of each other.
More recently I bought a used 1/4 inch GRAS microphone and sent it off to Denmark for repair and calibration.
The point is that the microphones are precision lab grade.
I do have a piston calibrator on the bench that I use frequently.
If the mic is calibrated, the preamp(s) are calibrated and the audio analyzer is calibrated then you can take the results to court.
Do I trust my mics, yes. Do I trust a $20 Berhinger microphone, no.
For home diy home use, calibration is not strictly required.
Thanks DT
I don't question your measurements inside an enclosure and want to stress that what you are doing is fine. The only objective here to me was discussing this: what enclosure modal patterns (air resonating) are problematic? Those who are noticeable on the outside of the enclosure.
Since I've long come to the conclusion that even the strongest standing sound waves (resonances) inside an enclosure have a hard time bringing enclosure walls to the point that these produce troublesome sound output on the outside (well, apart from cardboard enclosures), I focus on the resonances that emerge through openings in the enclosure. Being cones and BR ports mainly. And those are perfectly observed by close range measurements on the outside, while measuring inside leads only to guessing what's happening outside. Because you don't take the transfer function of the cone or port, whether or not agitated by the modal pattern, into the equation.
I like pictures. So to address what I meant: the above plot, taken at 1cm (outside the box) from the cone of a FaitalPro 12PR320 in an enclosure, clearly shows resonance issues. Just under 200Hz, around 400Hz and likely just above 1k, two or three internal modes brake the cone movement so badly that output drops 5dB.
In the impedance plot the resonances show up too. This is just dynamic air pressure hampering the cone to move. I'll leave the readers to their conclusions.
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I like pictures too.
The attached plots are from the 8 inch Purifi driver in a 1 FT^3 cube shape sealed enclosure. With a little fine tuning of the stuffing you can control the reflections in a normally difficult shape enclosure.
Thanks DT
The attached plots are from the 8 inch Purifi driver in a 1 FT^3 cube shape sealed enclosure. With a little fine tuning of the stuffing you can control the reflections in a normally difficult shape enclosure.
Thanks DT
The small wiggle at ~ 310Hz is the resonance of the spider, i measure that as well with my 8" PTT8.0X08-NAB2 (alu cones), the "magnitude" varies a bit with Rsense values (0.135 vs 50 Ohm)
PS i assume the response was measured at close distance.
PS i assume the response was measured at close distance.