1 inch compression drivers test, BMS vs 18Sound

The 1 Comrpession Drivers

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

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.


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

On X-Shape X33: SPL
RCF 350ND on X25: SPL RCF

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.

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.

Burst Decay
18Sound ND1TP:
18sound ND1TP burst decay
18sound ND1TP burst decay
18Sound 1095N:
18sound 1095 burst decay
18sound 1095
BMS 5530:
BMS 5530 burst decay
BMS 5530 burst decay
RCF 350ND:

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.


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.



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



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. wedges 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

18Sound ND1TP not re-tuned (diaphragm height): X33-polar-ND3SN
18Sound ND1TP unit 1 tuned: X33-polar-18sound ND1
18Sound ND1TP unit 2 tuned: X33-polar-ND3SN
18Sound 1095N: X33-polar-ND3SN
BMS 5530 unit 1: X33-polar-ND3SN
BMS 5530 unit 2: X33-polar-ND3SN
RCF ND350 on X25: X33-polar-ND3SN

Polar conclusion and diaphragm impact

We can see a huge difference is very high frequencies if the diaphragram is not at the right height, after tuned diaphragm height on the ND1, his polars are better than 1095N, the 5530 unit show a very low variation beetween unit and is very good.

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


On an X-Shape X25 (so cut at 1250/1300hz) an aftermarket RCF ND350 is interesting otherwise the 18Sound ND1TP and BMS 5530 are more interesting in all cases, 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.

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.

The 5530 is very good, just a little bit of H2 in low end, otherwise it would be perfect.

note: I may had bad luck as other customer doesn’t suffers with the same problem out of the box, morehover this kind of things can appens for every brands.

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