do tweeters need to be protected when making measurements on them?

[JLC7]

Should tweeters be protected with a cap or something when making measurements or is it typically not necessary at lower volumes?

[Curmudgeon]

A cap is always a good idea. Particular care needs to be taken around the tweeter's resonance, as even a very low input can result in a substantial cone displacement. If the tweeter's resonance is, say 1,000 Hz, a 1 uF cap will be ~160 Ohms, plenty of attenuation at 6 ohms. But at resonance, some tweeters' impedance will reach 50 ohms, so that there is a lot less attenuation. So even a cap has protection limitations.

At higher frequencies, the tweeter's operating range, a one volt signal will be fine, about 100-200 mW, with no danger at all of the voice coil frying.

At resonance, start at a very low level signal, and increase it slowly until you have the minimum level needed for your measurements. If the nature of the sine wave out changes audibly, (or on the scope if you are using one) back down to perhaps half that level.

[JLC7]

Thanks for the reply curmudgeon. I was going to use speaker workshop for FR measurements of my vifa XT25TG30-04 tweeter. I think it uses an MLS signal so I'm not sure if you can limit the frequencies you test at. I can get away without a cap if I'm careful with the volume levels and only go up as high as needed to get a usable measurement?

[Curmudgeon]

yes, with a carefully limited signal. What is the source impedance of your signal source? A very low source impedance as from a power amplifier will keep the voltage constant, regardless of tweeter impedance. A higher impedance, such as from a signal generator, will show a marked increase in V across the tweeter at resonance, which is not really what's wanted...

[JLC7]

For the frequency response measurement I'll be hooking the speakers up to my power amplifier and measuring with a mic. For impedance measurements, I'll be driving them with the sound card.

So I should take care to limit the volume while testing?

[Dave Bullet]

For the frequency response measurement I'll be hooking the speakers up to my power amplifier and measuring with a mic. For impedance measurements, I'll be driving them with the sound card.

That's exactly what I do with SW. Impedance measurements using only the sound card's grunt - no external amplifier.

for tweeter amplitude (frequency) response, I set the amp at 1V output for all my tweeters. Haven't fried one yet. The Vifa D19 3/4" dome I tested was fine with a 1V output level.

Just put a DMM set to AC voltage measurement across your amp speaker terminals and play an MLS tone. Increase the volume on the pre-amp until you read 1V and you're ready to go.

Don't change the volume level when switching to woofer measurements, otherwise you won't know how much to pad the tweeter (unless you scale after the fact)

David.

[JLC7]

Good stuff. I'll definitely keep that in mind. Just to doublecheck by 1V you mean 1V RMS?

[Curmudgeon]

Yes, RMS.

[Undefinition]

I haven't used any protection on any tweeters I've tested yet, and haven't had any ill effects. However, I have heard it's a must on ribbons and planars. That may be, but it makes me wonder this, then:

Won't putting a cap in-line with the driver cause a 90 twist in the phase of the measurement? How would one account for that?

[Salas]

Originally posted by Undefinition
I haven't used any protection on any tweeters I've tested yet, and haven't had any ill effects. However, I have heard it's a must on ribbons and planars. That may be, but it makes me wonder this, then:

Won't putting a cap in-line with the driver cause a 90 twist in the phase of the measurement? How would one account for that?

Yes its a must on Ribbons and planars especially with Pink noise.

Yes it will turn the phase. You can compute minimum phase using only the SPL data though. Soundeasy has Hilbert, LEAP has Minimum Phase Transform and tail correction in its Processing menu.