Compression Driver

What is a compression driver and comparison with dome Tweeter

A compression driver is a plane wave generator high volume capable, a regular tweeter is not, so the solution cannot be the same and the usage is not the same either.

We can constrain the radiation of a compression driver and choose a radiation angle, but with a regular tweeter it needs a phase plug or we will create some problems.

Phase Plug

It’s why we talk about waveguide for a tweeter and horn for compression driver, with a tweeter we just “guide” the tweeter but with the use of modern phase plug we can reach 100°,90° or even less without problems: see our tweeter waveguide

In addition to the fact that the SPL is limited with tweeters, it’s why we dedicated tweeters for close distance or monitoring usage and compression drivers for medium/long distance usages.

At mid-distance, so with a 90° coverage, a horn with compression driver will be more appropriate.

Throat Size

The throat size will in most cases dictate the top end control of the horn, hence the constant directivity behavior of the horn. The larger the throat size is, the lower top end control we will have, but there are some tricks to circumvent that, like on our bi-radial horn or X-Shape/Grand Cinema Horn.

1" compression driver:

They are relatively cheap, they cannot go very low, the best of them have a limit of 950/1000hz (BMS and 18Sound), the throat is tiny so the radiation control is high in frequency.

2" compression driver:

It’s an historic size, the control in high frequency is limited, today we continue to see some of this throat size for very specific usage : Have a high SPL, low end device (instead of a tinier compression + midrange couple) without top end control into account.

These need to find an use case such as Line Array, so for a very long throw, it’s typically the usage of the Celestion Axi2050, which was created for this very specific purpose.

Using 2" compression drivers is not recommended for our usage.

1.5" or 1.4" compression driver:

It’s an ideal, they go lower in frequency and the throat size is not too big. In fact some 2" inch driver are just 1.5" driver with built-in adapter, like the JBL 2450 or TAD TD4002. From the perspective of the horn it’s way more efficient to use an 1.5" version of a driver rather than the 2".

1.4" drivers have a 3" or 3.4" diaphragm, so the break-up frequency is upper. 1.5" drivers have a 4" so better max SPL capacities and lower THD at the bottom range.

On our 1.4/1.5 Bi-radial horn we use a smooth pinch in order to push the control like it’s an 1", this way we have all the advantages.

We found a similar principle in X-Shape and Grand Cinema.

Exit Angle & Flare

A compression driver has an exit angle, sometime we talk about rapid flare vs slow flare. It’s the exit angle of the device in degrees, we talk in semi-angle, for example a TD 4002/4003 has a semi-angle of 4/4.5°, so the “full” coverage is 8/9°.

The more the flare is important, the more it’s difficult to smoothly bring the flare to reach the coverage angle of the horn, semi-angle 45° for example, so coverage 90°. Some compression drivers are designed with an important semi-angle, called rapid flare, that is not an advantage for our usage.

We prefer slow flare angle, aka an angle as close as possible to 0°, the JBL 2450SL is 0°, it’s a no-flare device, it’s why I advise it for 1.5" usage generally, it’s also find-able used at an acceptable price. The 1.4" 18Sound ND3 family or FaitalPRO HF1440/1460 are slow flare, it’s a good point for us also.

Note: about diaphragm materials, Truextends Be is embossed Be, not vaporized Be like TAD does (more expensive), so even if it’s Be it’s far from what TAD does.

It’s hard to set precisely a new diaphragm in a compression driver, even if modern driver facilitated it. Diaphragm characteristics are taken into account in the full device, sometimes changing by another doesn’t bring a lot of improvement (and sometimes it’s worse), so be careful with this.