The Care & Feeding of Tweeters
In another advice article, we looked at some causes of automotive loudspeaker failure. Now we'll look specifically at tweeters: why they fail, and how keep them healthy longer.
Tweeters reproduce the very highest frequencies (treble). High frequencies require that the speaker move back and forth very fast, up to 20 thousand times per second! To move that fast, the moving parts (voice coil wire, former and cone) must be lightweight. Music is delivered to the tweeter in the form of electrical current. As current flows through the tiny voice coil wire in the tweeter, the coil gets hot and may burn out if fed too much current. A crossover network filters out the lower frequencies so the tweeter is only asked to reproduce frequencies and amounts of current that it can safely handle. Most tweeters should be crossed over at 3,000Hz or higher. The lower the crossover point you choose, the greater the chance the tweeter will fail.
In coaxial speakers the "crossover" is usually a single capacitor. A capacitor reduces the level of low frequencies at a rate of 6dB per octave. That means that if the capacitor's filter value is 3000Hz, the current will be reduced by 6 dB one octave below 3,000Hz or 1,500Hz and by 12dB two octaves below at 750Hz (Fig. 1). More sophisticated crossovers use a capacitor with an inductor (coil) to make a "second order crossover" that rolls off the lower frequencies at a rate of 12dB per octave. To use our previous example, with a 12dB per octave cross-over, the signal is down 12dB at 1,500Hz and 24dB at 750Hz. There is far less low frequency energy getting to the tweeter with a second order crossover than a first order. A tweeter with a second order crossover will play louder and last longer than one with a first order filter. At the lower, more dangerous frequencies, much less energy gets to the tweeter, and thus much less heat.
Rule Number One
NEVER EVER RUN A TWEETER WITHOUT A CROSSOVER NETWORK!
In more sophisticated installations the passive crossover network supplied with the tweeter or component system is replaced with an external, electronic crossover. In this case the low frequencies are filtered out before a dedicated tweeter amplifier. While this method has multiple performance advantages, there are potential dangers.
The crossover point is usually adjustable on an electronic crossover and the possibility for setting it incorrectly goes up exponentially. If you set the crossover "by ear" at low levels, it'll sound great at crossover frequencies as low as 1,000Hz, but as soon as you turn up the volume, Poof! Bye, bye tweeter.
The calibration settings on an electronic crossover are rough at best, just because the knob says it's at 5,000Hz doesn't mean it is exactly. To be absolutely certain that you have chosen a safe and appropriate hi-pass filter setting, use a signal generator and volt meter to measure the filter characteristics of your crossover. Many professional car stereo installers have equipment capable of confirming cross-over filter characteristics.
Rule Number Two
NEVER SET A HI-PASS TWEETER CROSSOVER LOWER THAN 3,000Hz.
Equalizers (or EQs) are electronic devices that can adjust the frequency range to achieve "flat" response. When misused, they are notorious tweeter killers. Many people adjust the EQ to yield flat response as measured by a RTA meter. But meters don't "hear" like people, and "flat" response is often too bright. Over adjusting an EQ for measured flat response is a sure-fire way to fry your tweeters. If your system isn't "bright" enough for you, don't compensate by turning the treble control or EQ all the way up. Either move the tweeters so they are more in line with your ears, or add another set of tweeters.
Rule Number Three
BE CAREFUL WHEN USING EQUALIZERS OR TONE CONTROLS. NEVER TURN THEM UP TO, OR EVEN NEAR, THE MAXIMUM SETTING.
Loud Vs. Distortion
We understand if you want to play your car system REAL LOUD. We also understand that if you want to play it that loud you'd better have plenty of speakers and amplifier power. Asking your one set of itty-bitty tweeters to keep up with the eight 12" woofers in your trunk is unrealistic. As soon as you hear any distortion, it's a sign that some part(s) of your system is operating at or beyond its limits.
Rule Number 4
WHEN THE SYSTEM DISTORTS-TURN IT DOWN! THEN BUY MORE STUFF!
This article was last modified Dec 7, 2013