Fostex 025H27 Horn Tweeter
The Fostex 025H27 horn tweeter is very popular in PA speakers. Due to the shape of the horn, it is also known as a bullet tweeter. In HiFi, bullet tweeters are normally seen in super high frequency tweeters but with this Fostex, it’s range is more of a tweeter. There’s very little information available except that it’s rated for 150W. With no further information, the power rating is quite meaningless. This prompted me to do some measurements which I hope will help professionals optimize the use of this tweeter.
Fig 1 – Fostex 025H27 Impedance
First off is the Impedance of the Fostex 025H27. Fig 1 shows it doesn’t go below 6.5Ω. It is therefore safe to assume this tweeter is rated at 8Ω nominal. The resonant frequency (Fs) is at a highish 1.9kHz. In it’s working range, the lowest impedance is about 8Ω at 6kHz.
Fig 2 – Fostex 025H27 RAW Frequency Response.
The Frequency Response sweep in Fig 2 shows the Fostex is capable of up to about 14kHz before dying off. In Pro Sound, 15kHz is the max. So this Fostex is a tweeter as well as a super tweeter. Impressive.
There is a slight hump from 3kHz~6kHz which is easy to flatten with EQ. However, this can be left alone if the user prefers some midrange presence. The horn itself unloads at 1.5kHz. From the plot, a safe frequency to cross would be at 3kHz~3.5kHz. If I want to cross lower, 2.5kHz is about it and that is with a 24dB/oct slope.
Fig 3. REW Waterfall of Fostex 025H27. Rise Time = 0.10 msec. Window = 1.3 msec ( 769Hz ). Time Range = 1.0 msec
The Waterfall plot in Fig 3 shows a longer decay from 4kHz~6kHz. From 7kHz upwards, it is exceptionally clean.
Fig 4 – Toneburst Energy Storage of Fostex 025H27
The Toneburst Energy Storage plot In Fig 4 shows stored energy (light blue slices) from 4kHz~7kHz. This verifies the extended decay seen in the Waterfall plot in Fig 3. Above 7kHz, there’s virtually no stored energy present.
Fig 5 – Spectrogram of Fostex 025H27
The Spectrogram in Fig 5 is a 2-dimensional view of the Fostex’s response. It shows minimal smearing, contributing to a very clean treble. At 3kHz, the decay is already gone by 2.5ms. Above 3kHz, no more than 1ms.
I have yet to listen to this Fostex 025H27 but the measurements are outstanding. It is quite remarkable that it stretches from 3kHz~14kHz. I didn’t expect that. I can see it being used in a pro 2-way speaker, especially in 6″ or 8″ satellites. Another use is as a super tweeter to support compression drivers.
Even though the 025H27 is meant for the PA market, I see no reason why it cannot be used for HiFi. More so when high sensitivity speakers are desired. They will fit right in with Home Theater speakers or low powered chip amps.
Note: All measurements were made with the mic at 1m, tweeter axis. Impulse Window=5ms. No smoothing applied.