JBL E140-8 cab build

[ThisBass]

I know WinISD has it's issues, but the graph and my measurement are a pretty good match. It's port prediction was 17% to long.

I think the error does raise with the number of ports that are used for the cab.
Long time ago and with my own cab (and two roundish ports such as yours) the predicted port length by WinISD was about 50% too long.
The error might be a little bit down at smaller percentage numbers once only one port was used for the cab.

 

[ThisBass]

The equation stated is for adjusting the port output to sync up to the driver output in a modeling program.
The port will apply more pressure to the mic compared to the driver at the same drive level. Simply Vd difference between the two.

In first instance Vd has got (almost always) nearly nothing to with SPL.
If any, most of the time Vd might tell something about the amount of reactive acoustical power which does not contribute to SPL.

Both sources loudspeaker and ports are very narrow aligned each other on the baffle of the cab. In the bandwidth of interest up to ~150Hz there is full acoustical coupling between the sources, with just the same outcome as if there was only one source present on the baffle.

 

[6thplanet]

In first instance Vd has got (almost always) nearly nothing to with SPL.
If any, most of the time Vd might tell something about the amount of reactive acoustical power which does not contribute to SPL.

Both sources loudspeaker and ports are very narrow aligned each other on the baffle of the cab. In the bandwidth of interest up to ~150Hz there is full acoustical coupling between the sources, with just the same outcome as if there was only one source present on the baffle.

Far field, sure. Not near field. See my measured response.

 

[6thplanet]

Also speaking of Vd in near field measurements.

My WinISD prediction was with two ports.

 

[ThisBass]

Another cab design consideration for the E140.

Gainable acoustical power (SPL) for the very essential lowend bandwidth ~50..100 Hz

white - 2.8 cu.ft tuned to Fb 55 Hz
yellow - 4.2 cu.ft tuned to Fb 40 Hz

Once there was a high steep HPF present that was set to ~45 Hz cut off, the smaller chamber and higher tuning Fb helps pretty much to improve the gainable acoustical power (within driver Xmax margins) at the very essential lowend bandwidth.

 

[Alex Xander]

In first instance Vd has got (almost always) nearly nothing to with SPL.
If any, most of the time Vd might tell something about the amount of reactive acoustical power which does not contribute to SPL.

Both sources loudspeaker and ports are very narrow aligned each other on the baffle of the cab. In the bandwidth of interest up to ~150Hz there is full acoustical coupling between the sources, with just the same outcome as if there was only one source present on the baffle.

6th Planet is using Keele’s nearfield technique for measuring low frequency response. It’s my preferred method too, despite living in a farm with no shortage of open space. It’s detailed here Measuring Loudspeaker Low-Frequency Response
There’s also a link to Keele’s original paper on that page.

 

[ThisBass]

Also speaking of Vd in near field measurements.

My WinISD prediction was with two ports.

In the very nearfield to a source any microphone does noticeable emphasize lowend response. I'd suggest ~1 Meter of distance to the source.

It's not valid to draw SPL directly from Vd.
Any 15" cone may generate plenty of sound pressure level at 500 Hz while the cone is hardly moving (rather smallish Vd numbers).
While at 50Hz the same cone may move lots of air (plenty of Vd) while the generated sound pressure will be weak, unless there was support by a Helmholtz resonator (bass reflex) that helps to improve lowend response (and efficiency).

 

[ThisBass]

6th Planet is using Keele’s nearfield technique for measuring low frequency response. It’s my preferred method too, despite living in a farm with no shortage of open space. It’s detailed here Measuring Loudspeaker Low-Frequency Response
There’s also a link to Keele’s original paper on that page.

Yet there was still need to have appropriate calibrated microphone systems that are suited for nearfield measurements.
Without them, it doesn't work properly nothing at all.

 

[agedhorse]

Very near field measurements have additional inaccuracies, as the radiated energy can vary across the cone and dust cap. This is also frequency dependent.

when the measures and predicted response disagrees, there is something wrong with one or the other.

 

[6thplanet]

I really don't know what else to say about this.
My measurements are a proven way to get the frequency response.
My measurements and the prediction pretty much match.
No it's not exactly optimum, but who's the authority as to what optimum really is. I'll adjust some things an see what happens. Thanks for all the suggestions.

 

[ThisBass]

No it's not exactly optimum, but who's the authority as to what optimum really is.

I think the authority for optimum is in your own hands.
The things we can tell, its nothing but a helping hand that may be suited to make your personal desired optimum true.

 

[Alex Xander]

Very near field measurements have additional inaccuracies, as the radiated energy can vary across the cone and dust cap. This is also frequency dependent.

Low frequencies would be less affected by the phenomena you describe, wouldn't they? I'm just supposing here, but like @6thplanet, I have used the near field technique with reasonable success with subwoofers/bass cabs.

One correction, for the sake of clarifying @ThisBass 's earlier argument, the correction for this technique is based on driver diameter (and port diameter), not displacement. Vd is not the parameter used, but Sd.

 

[Alex Xander]

Yet there was still need to have appropriate calibrated microphone systems that are suited for nearfield measurements.
Without them, it doesn't work properly nothing at all.

What do you call "an appropriate calibrated microphone system"?

Dayton offers measurement microphones calibrated by the serial number at prices well within the reach of the average DIYer, that integrate very simply into Room EQ Wizard, which is free. Am I missing something?

 

[6thplanet]

Low frequencies would be less affected by the phenomena you describe, wouldn't they? I'm just supposing here, but like @6thplanet, I have used the near field technique with reasonable success with subwoofers/bass cabs.

One correction, for the sake of clarifying @ThisBass 's earlier argument, the correction for this technique is based on driver diameter (and port diameter), not displacement. Vd is not the parameter used, but Sd.

Yes, the driver stays pistonic at low frequencies, upper is where things can vary.

Yes, the equation uses diameters, as i stated. The difference in SPL between the two is because the Vd difference, hence the need for the SPL adjustment to splice them together. It's just easier to aligne the tails of each for basically the same result. This is because both the port and speaker roll off at the same rate.

 

[agedhorse]

Low frequencies would be less affected by the phenomena you describe, wouldn't they? I'm just supposing here, but like @6thplanet, I have used the near field technique with reasonable success with subwoofers/bass cabs.

There are several sources of inaccuracies, generally more pronounced at higher frequencies, but not entirely.

Because the shape of the radiating surface varies across a cone (dust cap and also curvilinear cones) there will be differences, and also standing waves across the cone-dust cap structure will result in small differences that vary with frequency depending on reflections back from the surround.

Also, reflections from the interior surfaces of the enclosure through the paper will be summed with the directly radiated sound, this is far from uniform as there is almost none where the motor blocks reflected sound.

This is why near field measurements may be a valuable investigative tool but should always be corroborated by another method, just like modeling predictions need to be confirmed by measurements.

 

[ThisBass]

The difference in SPL between the two is because the Vd difference, ...

Both the cone area dimension and the frequency does determine radiation impedance.
Once the radiation impedance is known then its possible to predict SPL for a given magnitude of Vd.

To the contrary port consideration.
The port area has got absolutely nothing to do with radiation impedance such as is for a (pistonic) cone.
Neither does Vd inside of the port have got anything to do with generated SPL (such as is for a cone).

 

[6thplanet]

@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.

 

[agedhorse]

One component of placing a mic 1/4” from the surface is non-linearity due to pressure gradients. This can be confirmed by making another reading 4” away and seeing if the numbers corrected for distance agree. This too will vary by frequency since the air is not a perfectly linear elastic medium.

This is one of several reasons 1M is commonly used as a measurement distance. It’s a good compromise.

 

[6thplanet]

Port and speaker close and 4" away.

As you can see the port is louder, hence the need for SPL adjustment for proper splicing.

 

[agedhorse]

The non-linearities are quite obvious.