Frequency Response of Vifa PL27TG-35

The PL27TG35-06 represents Vifa's top tweeter in it's time. It features a silk diaphragm which is reputed for better damping, ferrofluid to minimize mechanical damping in the motor, a specially designed bulged faceplate for good on-axis and off-axis response and a vented magnet system with rear chamber for a low resonance frequency.

PL27TG35

One of the most appealing features I find with this tweeter is the frequency response bandwidth.

According to Vifa's literature, it extends from 1,500Hz to 30,000Hz (Figure 19).

Fs (Free Air Resonance) is listed at 750Hz. This indicates the PL27 can be crossed as low as 1,500Hz

Furthermore, its' relatively flat response is much desired.


(Figure 19) Frequency Response of Vifa PL27TG35

Gated Sweep PL27TG35

With the PL27 mounted onto the box, a 300 Points Precision Gated Sweep was taken at 1 Meter, on axis with the tweeter.

Figure 20 is the result of the sweep.

Simply unbelievable. Ran the sweep a few times, just to be sure. No change. Replaced the PL27 with another unit. Same result. Replaced with a 3rd unit - no difference. Finally, connected my 4th and last tweeter, again no difference. Four units of PL27, all displaying the same curve.

The reason for this huge discrepancy lies in the measuring method used in the industry.

To standardize measurements, tweeters are mounted onto an infinite baffle. Because of this requirement, manufacturers naturally design tweeters based on infinite baffle response.

However, in real use, tweeters are mounted onto a box. As can be seen from our measurements, the differences are tremendous, enough to render the manufacturer's frequency response useless.

To put it succinctly, a ruler flat response of a tweeter on an infinite baffle is not an accurate indication of the tweeter's response when mounted onto a box.

Now that we know what we are up against, we shall proceed with the crossover


(Figure 20) Gated Sweep of PL27TG35 mounted in box (2.83V, 1M)

 


(Figure 21) Impedance Sweep of PL27TG35



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