Scanspeak Illuminator 12MU/8731T-00 4" Midrange

Scanspeak Illuminator 12MU/8731T-00 4" Midrange

In this blog post I test the Scanspeak Illuminator 12MU/8731T-00, 4" Midrange.

The driver is mounted in a sealed 5 liter enclosure. The baffle has a very shallow waveguide to help time alignment with the 1159 tweeter waveguide. 

Impedance Sweep

The impedance sweep shows a small blip at 1.5kHz. The inductive rise is kept well in check on this driver. 

Frequency Response 

I started by measuring the raw frequency response. The response is relatively flat with the exception of a small peak at 1.5kHz. The driver has a nice roll off before the mechanical breakup at 10kHz.

This matches closely to the ScanSpeak's own published data shown below. Looking at the published data shows that the driver becomes directional starting at 1kHz gradually narrowing to 3kHz. We see directivity narrow quite quickly beyond 3kHz. 

I then measured the off-axis performance on the shallow waveguide which is displayed below. We see good pattern control down to about 600Hz roughly. Power response is more even across the midrange. 

I then rigged up a mild baffle step compensation to continue the testing. 


The response with the BSC is shown in black below.


Burst Decay 

Burst decay is devoid of anything significant across the midrange. The first breakup at 10kHz is extremely high for a driver in this size category. Even the breakup at 10kHz causes a dip in the response which is an ideal scenario. We don't see stored energy on our burst decay plot until around 12kHz. Could this be the perfect fullrange driver? 

Cumulative Spectral Decay

  • Mild stored energy at 1.5kHz
  • Mild breakup at 12kHz. 



Harmonic Distortion 

I then tested harmonic distortion starting at 75dB SPL at 1m shown below. 

Increasing to 85dB is shown below. 

Increasing to 95dB is shown below. 

Intermodulation Distortion 

I first tested my test environment noise to confirm adequate signal to noise ratio which is shown below. 

I first tested at 75dB test SPL which is shown below. IMD is -64dB in the lower midrange improving to -70dB at 3kHz. 

Increasing the test SPL to 85dB is shown below. If I was designing a crossover for this driver I would try to put a dip in the response at 1.5kHz just to see that minor rise in IMD disappear. It's interesting to note that we don't see anything in this region with the harmonic distortion test. 

Increasing SPL to 95dB is shown below. 

Subjective Listening Impression 

Vocals project into the room with authority and presence. There is literally nothing to spot wrong with the sound character of this driver. 

The Scanspeak Illuminator 12MU/8731T exceeds my performance target for audiophile sound quality.


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