1 inch compression drivers test, BMS vs 18Sound

The 1 Compression Drivers

We will test several 1" exit compression driver in this article:

• 18Sound ND1095N-16: 1.75 inch Nitrided titanium diaphragm
• 18Sound ND1TP-16: 1.75 inch titanium diaphragm
• BMS 5530-16: 1.75 Kapton annular diaphragm
• BMS 4552-16: 1.75 Kapton annular diaphragm
• RCF 350ND
• Faital HF10AK
• Faital HF108R

For interpreting results about break-up, it is advised to read the breakup article.

In essence: Stiffer materials, like metals, will exhibit a higher but more violent break-up point, while materials with inherent damping, like treated paper or composites (Kapton), will break up at a lower frequency but affect a wider range of frequencies.

It’s all a mater of compromise, the larger the diaphragm is lower he can goes (theoretically) and the sooner the breakup will occur.

The horn used for testing is a X-Shape X33 with his return for doesn’t be impacted with midrange narrowing or beaming.

For the RCF 350ND, as she cannot goes beyond 1250hz a X25 is used.

SPL

Here is the SPL alligned on 4kHz witout EQ of crossover.

Here there is no “more” or “less” of something, the drivers gives an initial energy then the horn distribute as we have explained it here : Energy in horns.

If a driver was flat it will simply mean that the horn is not constant, exactly what we do not want.

So the “bell” response is normal, the two 18sound come from the familly even if the generation is not the same, the shape of the BMS is very different.

We can see that the two Faital HF10AK have problems around 2.1kHz, especially one of the two units, that have even worst distortion than the other one.

The main juge here will be the distortion measurement after have EQ it flat.

Temporal measurements

Here we will do Burst Decay at 32cm of the mouth with all driver EQ flat, we will see here the breakup impact on temporal measurement.

Nothing audible on 18Sound and BMS, but on the RCF it’s not very good even if it’s also not really in audibility range, the BMS 5530 is a champion on this test.

The BMS 4552 has a little problem around 10kHz and we will see later that that has an impact on the polar map, as always.

Both Faital are not very good, especially the 108R and we will see that this will not help for polar measurement.

Distortion

Here is an overlay of all the compression drivers, the measurement is taken at 31.5cm to the horn mouth, every driver is EQ flat, the volume is 95 dB SPL (so the dB at 1m).

A point about audiblity

Our article about audibility and masking effect has explain that a H2 distortion will create a sound at Fr*2, and a H3 at Fr*3.

So some high frequencies rise up in H2 are in fact not audible, we have to put our attention at the low end to the half part of the measurements.

The two 18Sound ND1TP have been let on the graphs for show possible divergence beetween unit, the same divergences occurs with BMS too.

H2

The 18Sound ND1TP are partculary good here, as an improvement of the 1095N, the BMS 5530 a little less.

H3

The Faital HF10AK is catastrophic, the distortion is very audible and make the driver not usable bellow 2100kHz, I take the less worst of the two I have.
The HF108R is good but not very low, so in a X-Shape X25 but not with a bigger horn.

For the RCF 350ND I have lost the measurement and I don’t have the compression anymore, it remain a lower measurement one that just show that H3 goes up very fast after 1250/1300hz :

Here we see the an advantage of the ND1TP but in fact all performed very well. It’s now time to talk about divergences on the same unit and diaphragm adjustment.

When I receive brand new ND1TP pair with one producing 2% of THD at 80 dBSPL, she was sounded broken, both unit have been re-tuned about diaphragm height, as 18sound diaphragm are auto-centred it’s easy to tune diaphragm height.

More a compression driver is tiny more it will be sensitive to it, even between my several BMS 5530 there is differences in THD and polar.

18Sound use 3 wedges: 0.2, 0.1 and 0.05 mm, to have these performances in the test I remove 0.05mm on one and 0.1mm on another, a difference of 0.05 completely change THD but also polar map as we will se it just after.

Polar Plot

Why show polar on different driver?

A horn starts from the principle that the compression driver is a perfect plane wave radiation on his exit.

But the breakup put the driver out of plane wave radiation as we have seen in breakup and his distortion.

By analysing the moment when breakup occurs on a perfect wave propagation horn as the X-Shape X33 is we can detect when and how breakup affect wave-front propagation in the horn, and by deduction the quality of this propagation and the respect of plane wave radiation.

Polar on X-Shape X33

Polar conclusion and diaphragm impact

We can see a huge difference is very high frequencies if the diaphragm is not at the right height, after tuned diaphragm height on the ND1, his polars are better than 1095N, the 5530 and 4552 unit show a very low variation between unit so I let just one the both polar for each.

For the Faital, the two HF10AK are very different due to diaphragm tuning also, the HF108R is not very good as his temporal measurement are relatively bad so it’s mean that the compression driver goes out of plane wave radiation too soon and , as a result , has erratic behavior in high frequencies.

The RCF 350ND is not the better in this test, it’s a driver that goes out of plane wave radiation sooner than others, but on an X25 she goes out of plane wave radiation very high.

The BMS 4552 has a very little temporal problem around 10kHz as we see above, we can see that the new BMS 1" driver generation (the 5530) is a little bit better and that is linked to the temporal behavior so the plane wave behavior.

Conclusion

On an X-Shape X25 (so cut at 1250Hz) the 18Sound 1095N and BMS 5530 are more interesting, this last one is very good but just a little bit of H2 on the low end, otherwise it would be perfect.

On a X-Shape X33 the 1095N is a clear winner, it’s the only one that can be really cut low at very high SPL, the BMS 5530 will perform well to but if a very high SPL is needed and cut very low the 1095N will be a little better.

For the ND1-TP case, they are very sensitive to height diaphragm placement so if a problem is detected it’s better to return it, the H2 is a little bit high in % and frequencies.

For both Faital the HF108R may be interesting but the HF10AK is a clear no go.

Polar plot is the finest way to detect at 0.05mm precision if it’s well tuned or not but a simple sweep can tell you if everything is fine: If you have all distortion harmonic orders very hight centered at the same frequency in one violent spade form, only a problem of diaphragm can be at the origin of it. I may have bad luck as other customers don’t suffer from the same problem out of the box, moreover this kind of thing can happen to every brand.

The 4552 is good and is an older generation of the 5530, we can see the improvement on the plane wave radiation behavior on the new generation, the 4552 temporal polar measurements are not as good as the new 5530.

The best 8/9.5" woofers for be used with these compression drivers are here: 8/9.5 inch woofer test.