Internal microphone to determine maximum SPL

[kipman725]

Has anyone experience with microphone in box measurements?

https://www.audioxpress.com/article/measuring-loudspeaker-low-frequency-response

I was thinking with a transducer that remained linear at high pressures such as the MPX2010 series:

https://docs-emea.rs-online.com/webdocs/0ef4/0900766b80ef40f2.pdf

The maximum SPL of capabilities of ported and sealed boxes could be measured without resorting to ground plane measurements.  I'm not sure though what port air velocity saturation would do to the relationship between internal and external pressure?

 

[kipman725]

Seems to work when the port is linear.  InBox is the compensated SPL inside a bass reflex cabinet which has dimensions that should result in valid data <80Hz.  HornResp is the simulation of this sub.  Room compensated was generated by measuring a known subwoofer in the same physical location as the DUT.  This known subwoofer is a sealed box and has previously been measured using close mic spliced with time gated measurements.   

You can see that the response shape for the in box measurement matches that of the compensated response.  This box differs a bit from the Hornresp simulation in that there exists a cavity into which the port and driver fire which may increase SPL outside the low bass.

Next stage I guess is to make a microphone that can cope with those savage in box pressures!

[kipman725]

So finally managed to find a place I can do outdoor testing! its a yard so not reflection free but a lot better than testing indoors.  You can see that the results are matching the in box measurements to around 100Hz.  What I don't get is why this sub is so efficient in the upper bass, the numbers seem impossible? >110dB/1W?

In the Hornresp sim I used a semi inductance model generated from impedance measurements of the driver and simulated as a bass reflex.  The box is a bit different in that the port goes into a cavity with the (isobaric) driver and that there is obviously quite a long acoustic path from one side the of the driver to the other.

 

[kipman725]

Here is the impedance compared with simulation:

[Ricci]

It's hard to say exactly what is going on since there are so many variables. 

What voltage is being applied to the terminals during the measurement? Is the impedance really 15 ohms or greater? Is this 2 8 ohm drivers in series? Are you using a generic mic sensitivity spec or do you have an SPL calibrator? Mic sensitivity specs can be suspect in my experience. 

Pressures inside a cab can get very high especially if testing for system output capability. Finding a mic or sensor and mic pre-amp that can take it won't be easy or cheap most likely. 

Don't forget that despite subs being "omni-directional" they exhibit some directionality at higher frequencies, baffle effects and the apparent acoustic center is often a significant distance in front of the sub. All of this tends to result in measured SPL that is somewhat higher than predicted by simulation. 

[SME]

7 hours ago, Ricci said:

Don't forget that despite subs being "omni-directional" they exhibit some directionality at higher frequencies, baffle effects and the apparent acoustic center is often a significant distance in front of the sub. All of this tends to result in measured SPL that is somewhat higher than predicted by simulation. 

To add to this: Hornresp reports acoustic power output as a normalized value such that if radiation is omni-directional (throughout the solid radiation angle chosen for the simulation, e.g. 2-pi) it matches direct sound SPL that would be measured.  However when there are directivity effects, it's normal SPL to be quite a bit higher than what Hornresp shows, simply because there may be a lot more sound radiating along the ground than say vertically above the sub.

Another potential cause of higher output (albeit over a fairly narrow region) that I don't see mentioned is port resonance.  The Hornresp simulation doesn't seem to show that, but the impedance and measurement data suggest that could be going on here also.

[kipman725]

The drivers are wired in series for 16ohms.  I reach xmax around 60V according to hornresp, within the output voltage limitations of an inuke6000dsp.  By doing this I can run 8 subs off one amp.

The above tests are renomalised to 4V (1W into 16ohm) and 1m.  Actual drive voltage was 5.46V and distance 2m.  microphone was umik-1 which is usb and calibrated, I don't have a calibrator to check but have always got plausible results when using it to develop full range speakers using time gated measurements.  Voltages where measured using a true RMS voltmeter and a constant sine at 50hz at the same drive level as the sweep.

I will have a look at the baffles directivity characteristics.

 

[kipman725]

Thanks for the tip on directivity I'm quite satisfied now that's what I'm seeing in the outdoor measurements:

The gain (~6dB @50Hz) of the 'inbox' and compensated in room measurements over the bass reflex sim in horn resp I have also managed to reproduce by modeling the box as a stepped tapped horn, trying to keep volumes the same and include acoustic path lengths to the front and back of the driver.  This sim is not totally faithfully to the actual geometry but is showing the same mid bass boost.

Also interesting was the power compression tests I did they mainly show the amp running out of steam rather than the sub >70Hz in the long duration sweeps.  So probably better test methodology would be to monitor actual drive voltage during the sweep for the higher level sweeps.