This blog post features a test where I mounted four 4" (10cm) midrange drivers into the E-250 Front Horn. I then compared this configuration to a single driver mounted in small pencil tower speaker. I did this so that I could compare distortion measurements among other things.
The driver used is the THIEL 10F/8424G02 4" Fiberglass Midrange.
Below is a picture of the four drivers mounted to the E-250 front horn. I had to make a 3mm thick adapter plate to mount the drivers. This spaced the drivers away enough to allow clearance for the surrounds.
The picture below shows the four drivers peaking through. This horn is my prototype horn and I've done very little finish work to the horn.
I began my testing by getting a full set of measurements on a single driver mounted in a small transmission line pencil tower. You can see the pencil tower speaker in the background in the image below.
I then conducted a full set of measurements on the E-250 Horn with the same mic distance (1 meter).
Below is a picture of my test setup (excuse the mess). The horn sits on a bass cabinet simply to get it to a proper test height. The bass cabinet then sits on my custom built rotary table used to get off-axis measurements.
Measurements
Below are the measurements that I acquired. I set the test at 80dB SPL for the pencil tower. I then tested the horn with without changing the volume so that the input voltage would be matched between the two tests. I also wire the four drivers in series-parallel so that the nominal impedance would be the same as a single driver.
Frequency Response
Below is the measured frequency response in ARTA for the single driver
Below is the measured distortion (THD black, F3 is brown). As you can see the distortion at 80dB is around 1.5% THD from 150Hz - 8kHz. F3 varies quite a bit and is generally around 0.2%.
Below is the burst decay plot that shows excellent decay characteristics with no major resonance. I can see a very subtle diaphragm breakup mode at 18kHz which is excellent for a 10cm midrange.
Below is the Jet Sonogram plot which is similar to the burst decay however the frequency response has been normalized. This is more useful to use for this test since the frequency response is so drastically different between the two configurations. No gating was used for this result. The Jet Sonogram shows how much resonance and reflections will arrive at the ear after the initial impulse of sound from the driver.
Below is the off-axis coloured polar map for pencil tower. This map shows that the driver has very wide horizontal coverage until 5kHz where it quickly starts to beam. Above 5kHz it basically stays at a steady 80 degree coverage window up to 20kHz. One thing to note is that this driver has very little diaphragm breakup modes. If the driver did have significant breakup then it would show up as anomalies in the off-axis. This is a very well behaved driver.
E250 Front Horn Measurements
Below is the frequency response for the E-250.
Below shows a direct comparison in the frequency response between the horn and the single driver pencil tower. This comparison is with input voltage matched between the two configurations. As you can see there is a 25dB increase in sensitivity from 250Hz - 2kHz (3 octaves). Above 2kHz the multiple drivers provides about a 9dB increase up to 17kHz.
Below is the horn distortion measurement at the same input voltage as the single driver. As you can see the THD% distortion is at around 2% at 250Hz but drops below 1% starting at 1kHz. F3 is below 0.2% from 250Hz -4kHz.
Adjusting the volume down 15dB to match the SPL of the single driver yields the following measured distortion. For reference this is 80dB SPL at 1 meter which represents average listening levels. %THD drops to below 0.5% from 400Hz - 2kHz. F3 distortion drops to 0.1% from 500Hz - 1.5kHz.
Below is the burst decay for the horn.
Below is the Jet Sonogram results for the horn. Keep in mind that the frequency response has been normalized (made flat). Also this result is un-gated.
Below is the off-axis coloured polar map for the horn. As you can see the horn is drastically altering the off-axis sonic signature of the driver. There is pattern control (controlled directivity) from about 1kHz where is provides a wide 100 degree coverage up to 20kHz.
Conclusion
The Scanspeak Discovery 10F/4424G02 is an excellent driver. It's amazing to me how much the horn configuration drastically alters nearly every measurable aspect of the driver. After applying the appropriate EQ to flatten the frequency response I briefly listened the the horn with music. The sound was very transparent with excellent treble. I will continue to investigate this configuration since it offers an alternative to the BG Neo8 planar transducers that this horn was originally designed for.