JBL E140-8 cab build

[Alex Xander]

The non-linearities are quite obvious.

A sincerely genuine question - do you consider the non-linearities material given the goal here is the measurement of a DIY bass cab to confirm a correlation with the designers initial intent?

 

[6thplanet]

For merging a port and speaker in a DIY situation, it's absolutely fine.

 

[ThisBass]

@ThisBass Not sure why your not grasping this. Vd is a given volume displaced. When you place a mic a 1/4" from the dust cap of a speaker and play a sine sweep, the mic sees this as an SPL reading. With nothing changed but moving the mic to the port, that same volume displaced by the speaker is now radiated through a much smaller Sd. This effectively raises the Vd the mic sees, which it reports as a rise in SPL.

In first instance any Vd (displaced volume) has got nothing to do with pressure gradient such as it is mentioned by @agedhorse in post #58

If any, then for a given magnitude of radiation impedance it was THEN possible to predict how much of Vd may be necessary to generate a desired magnitude of SPL.
Anyway, once a cone shall generate SPL at the lows, most part of Vd is nothing but reactive acoustical power that does not contribute to (generated) SPL nothing at all.

Different consideration for ports. Vd inside the port does NOT generate (nothing at all) any SPL close/narrow to the ports. Neither in the very near field nor in far field to the port.
Personally I do often recognize that the principles of how a port really works is hardly understood even by many engineers.
Explained in rough words, many explanations about how port works, and these you can find plenty of them on many internet/youtube platforms, are almost always telling half truth at best. Many of them are totally misleading the point.

edit:
the comments made in the vid are more suitable for lots of confusions about ports rather than good explanation how a Helmholtz resonator (respectively any port) really works.
6:00

 

[agedhorse]

A sincerely genuine question - do you consider the non-linearities material given the goal here is the measurement of a DIY bass cab to confirm a correlation with the designers initial intent?

Yes, because we know that there are uncertainties but not how they combine because we do not know their relative phase. This is the reactive part that @ThisBass was getting at.

We know for certain that there is phase shift because the port Acts the same as a narrow bandpass filter, so summing the two signals must include the phase difference.

A 1M composite measurement takes care of the complex math summing “automatically”.

Exactly the same thing occurs when summing component electrical filter signals, the complex (real + imaginary) math is difficult (for me anyway) to do efficiently and accurately, which is why composite measurements are made to validate the assumptions. Trust me, it’s a lot easier being wrong than right with this math.

 

[ThisBass]

Port and speaker close and 4" away.
View attachment 4938120
As you can see the port is louder, hence the need for SPL adjustment for proper splicing.

The sound out of port does always reach max SPL when the cone movement is down at its minimum.
That's the basic principle about how the Helmholtz resonator works.

It makes me very wondering why the measurement chart shows max SPL for the port at ~45 Hz while the cone does simultaneously reach its minimum of excursion at ~35 Hz?
Personally I'd expect better matching charts in this field.

edit,
if I was in your shoes I'd try to consider the following mindset:
might it happen that sound out of the port does cause noticeable interference to the measurement of direct sound at the cone?
Keep in mind, the sound that is coming out off the port is maxed at Fb while simultaneously direct sound coming off of the cone is down at its minimum.

 

[fdeck]

My write-up has an explanation of how ports work. It basically solves for free and driven resonators coupled within the same box.

But like most DIY'ers, I'm not equipped to do commercial quality measurements. Maybe I'll try an outdoor measurement, when it's not 32 degrees F, raining, and dark out.

Meanwhile, I have my own method, which has worked awfully darn well. I understand and trust the basic electromechanical model and driver datasheet, so I don't need to prove or disprove the laws of physics. Then I make what limited measurements I can, to confirm my parameter choices, but I don't need to make absolute SPL measurements. A near-field measurement will confirm the port tuning frequency and allow me to make adjustments as needed. I can also confirm some other parameters of my design model with an impedance curve.

I'm not trying to squeak the last 1/2 dB of performance out of a speaker. For one thing, that's impossible to model unless you know what the input waveform looks like. I've done some modeling of crude simulated bass waveforms, and they're informative.

 

[Alex Xander]

My write-up has an explanation of how ports work. It basically solves for free and driven resonators coupled within the same box.

But like most DIY'ers, I'm not equipped to do commercial quality measurements. Maybe I'll try an outdoor measurement, when it's not 32 degrees F, raining, and dark out.

Meanwhile, I have my own method, which has worked awfully darn well. I understand and trust the basic electromechanical model and driver datasheet, so I don't need to prove or disprove the laws of physics. Then I make what limited measurements I can, to confirm my parameter choices, but I don't need to make absolute SPL measurements. A near-field measurement will confirm the port tuning frequency and allow me to make adjustments as needed. I can also confirm some other parameters of my design model with an impedance curve.

I'm not trying to squeak the last 1/2 dB of performance out of a speaker. For one thing, that's impossible to model unless you know what the input waveform looks like. I've done some modelling of crude simulated bass waveforms, and they're informative.

And that's why I posed the question about whether the non-linearities agedhorse mentioned in nearfield measurements are material in the context of the DIYer

Yes, because we know that there are uncertainties but not how they combine because we do not know their relative phase. This is the reactive part that @ThisBass was getting at.

We know for certain that there is phase shift because the port Acts the same as a narrow bandpass filter, so summing the two signals must include the phase difference.

A 1M composite measurement takes care of the complex math summing “automatically”.

Exactly the same thing occurs when summing component electrical filter signals, the complex (real + imaginary) math is difficult (for me anyway) to do efficiently and accurately, which is why composite measurements are made to validate the assumptions. Trust me, it’s a lot easier being wrong than right with this math.

And if we were measuring at 1m, there would no doubt be many a punter telling us about the pitfalls of working around reflective surfaces when we don't have the benefit of an anechoic chamber, or in the case of some people, even a wide open space free from houses, fences, vehicles etc (hence some of us using nearfield measurements). Collecting phase data is no harder than SPL with consumer electronics, and while I agree that the maths is hard when summing multiple signals, especially when incorporating phase, it is not impossible.

What method of measurement would you suggest is best for those of us without access to professional and commercial systems? If all my cabinets had ports in the front and I owned a backhoe, I would dig a hole with the baffle flush to the ground firing up and hang a microphone 1m from the ground. Bill Fitzmaurice suggested this decades ago, but it requires a lot of shovel work or earthmoving equipment, and a wife more understanding than mine when it comes to digging random holes in the ground!

 

[byacey]

My write-up has an explanation of how ports work. It basically solves for free and driven resonators coupled within the same box.

But like most DIY'ers, I'm not equipped to do commercial quality measurements. Maybe I'll try an outdoor measurement, when it's not 32 degrees F, raining, and dark out.

Meanwhile, I have my own method, which has worked awfully darn well. I understand and trust the basic electromechanical model and driver datasheet, so I don't need to prove or disprove the laws of physics. Then I make what limited measurements I can, to confirm my parameter choices, but I don't need to make absolute SPL measurements. A near-field measurement will confirm the port tuning frequency and allow me to make adjustments as needed. I can also confirm some other parameters of my design model with an impedance curve.

I'm not trying to squeak the last 1/2 dB of performance out of a speaker. For one thing, that's impossible to model unless you know what the input waveform looks like. I've done some modeling of crude simulated bass waveforms, and they're informative.

I've got the hayfield and some measurement gear, but the problem is waiting for a day calm enough so the wind doesn't skew the readings. You have to have everything on standby and ready to go in order to take advantage of a dead calm day. I lucked out a few times when there was an approaching storm and it wad dead calm for an hour or so.

 

[Mike Caldwell]

The sound out of port does always reach max SPL when the cone movement is down at its minimum.
That's the basic principle about how the Helmholtz resonator works.

And with a tone generator that's the down and dirty way to check the cabinet tuning.

 

[fdeck]

I think if I was working with unknown drivers, the first thing I'd rig up would be a way to measure the Thiele-Small parameters. That would give me numbers that I could plug into the model, for the basic response curve. Also, measuring the port tuning by any of the basic methods.

I'm not sure how I'd measure Xmax. I wonder if the coil inductance would begin to change dramatically when the cone is pushed beyond Xmax.

Now you've got me thinking about setting up a measurement. I've got a spare driver and box that I'm not using.

 

[byacey]

I think if I was working with unknown drivers, the first thing I'd rig up would be a way to measure the Thiele-Small parameters. That would give me numbers that I could plug into the model, for the basic response curve. Also, measuring the port tuning by any of the basic methods.

I'm not sure how I'd measure Xmax. I wonder if the coil inductance would begin to change dramatically when the cone is pushed beyond Xmax.

Now you've got me thinking about setting up a measurement. I've got a spare driver and box that I'm not using.

I think the Xmax would be limited more so by the compliance restrictions rather than the coil excursion out of the gap. I'm not sure how you could measure the inductance in a dynamic condition.

 

[fdeck]

I think the Xmax would be limited more so by the compliance restrictions rather than the coil excursion out of the gap. I'm not sure how you could measure the inductance in a dynamic condition.

Sounds like a challenge.

Actually I was thinking of just pushing the cone by a given distance while watching a display of the inductance, but probably easier said than done.

 

[byacey]

Sounds like a challenge.

Actually I was thinking of just pushing the cone by a given distance while watching a display of the inductance, but probably easier said than done.

You could do that, but it would have to be held absolutely stationary to get a proper reading. The other issue is guessing when you are beginning to stress the limits of the compliance.

 

[agedhorse]

I think the Xmax would be limited more so by the compliance restrictions rather than the coil excursion out of the gap. I'm not sure how you could measure the inductance in a dynamic condition.

Both, which is why THD threshold of ~10% is now more commonly used. There are ways to alter the performance of the motor to change how this non-linearity is accomplished, the rate of change of flux at the fringes of the field. Another set of trade-offs.

This is why composite measurements are so useful, all of the variables, both defined and undefined, asre accounted for.

 

[agedhorse]

And that's why I posed the question about whether the non-linearities agedhorse mentioned in nearfield measurements are material in the context of the DIYer



And if we were measuring at 1m, there would no doubt be many a punter telling us about the pitfalls of working around reflective surfaces when we don't have the benefit of an anechoic chamber, or in the case of some people, even a wide open space free from houses, fences, vehicles etc (hence some of us using nearfield measurements). Collecting phase data is no harder than SPL with consumer electronics, and while I agree that the maths is hard when summing multiple signals, especially when incorporating phase, it is not impossible.

What method of measurement would you suggest is best for those of us without access to professional and commercial systems? If all my cabinets had ports in the front and I owned a backhoe, I would dig a hole with the baffle flush to the ground firing up and hang a microphone 1m from the ground. Bill Fitzmaurice suggested this decades ago, but it requires a lot of shovel work or earthmoving equipment, and a wife more understanding than mine when it comes to digging random holes in the ground!

This is why I have said before that inaccurate measurements are easy, accurate measurements are difficult. Pick your errors, accept them and embrace them rather than postulate around them or ignore them.

 

[Arjank]

As far as low-end measurements go I only do nearfield with sealed enclosures when necessary, this works pretty well. For ported cabs and the like I just make sure I have a good set of TSP's (e.g. measured with DATS) and solid simulation software (e.g. AJhorn which is really accurate). The predictions made by this are more accurate then half-baked measurements.
Anything above approx. 275hz is what I can measure accurately at home with MLS software.

 

[6thplanet]

Even with the stated nonlinearities, I proven near field measurements can be viable in post #34. ...with WinISD no less.

 

[ThisBass]

Even with the stated nonlinearities, I proven near field measurements can be viable in post #34. ...with WinISD no less.

At some point an inaccurate measurement might also validate an inaccurate modelling software tool.

post #16
measured Fb via DATS impedance measurement - 41.4 Hz

post #34
SPL measurements of cone and port

The peak center of port SPL hits with good accuracy the measured Fb 41.4 Hz
while the valley of cone SPL does not.

The measured valley of cone SPL is settled at 33..34 Hz (way below Fb)

max SPL port ~77 dB
min SPL cone ~60 dB

As I already told above, at Fb (where the cone movement does reach its minimum of movement) the cone SPL does contribute ~1dB (give or take) to the total Sound.

For coherent signals:
77db + 60dB = 78.1 dB

The issue with the measurement is.
The valley for cone SPL is measured about -8 Hz too low.
This means that cone SPL at Fb is measured about +18dB too high.

Both the SPL of port and SPL of cone are summed up to get the total sound.
At Fb the chart of total sound predicts 80dB

As the measurement of cone SPL is obviously inaccurate and delivers too high SPL magnitude for the cone at Fb, the predicted total sound at Fb will be about +2dB to high.
Probably the total SPL at Fb will be more close to 78dB (in real practice) rather than "measured" magnitude that was equal to 80dB as shown in the chart for total SPL.

 

[6thplanet]

Will try to figure out why valley of speaker measurement is off.

 

[Arjank]

Here's a pretty decent article on this matter
Measuring Loudspeaker Low-Frequency Response