Q:

Can an eight ohm speaker produce ultrasonic sound?

I am currently making an ultrasonic dog repellent circuit using a 555 in astable configuration generating a frequency of 30khz. Most circuits that I saw use a piezo buffer. I was thinking that since a speaker connected to computer can generate ultrasound, why not  connect the same to a 555 output. Would my speaker burn out?

A:

Most standard speakers such as those used in your stereo or the little speaker inside your desktop computer will NOT work well at ultrasonic frequencies.
The speaker impedance has nothing at all to do with the frequency response.  What matters is the size and mass of the speaker cone and the inductance of the speaker coil.
Piezo transducers are an entirely different story.  One of my favourite ultrasonic transducers is the piezo tweeters that used to be commonly available in the '70s and '80s for use in stage monitor speakers and such.
There are several reasons why piezo transducers work well at ultrasonic frequencies.
1) low diaphragm mass
2) impedance that goes DOWN as the frequency increases

A:

This one can.
Most 8 ohm speakers described as "supertweeters" can produce some ultrasonic output - this one seems reasonably competent up to 40kHz according to its frequency response (below).

So can this one which claims "up to 38kHz" .  These links do not necessarily constitute a recommendation for either product.
Any normal audio speaker other than a supertweeter ... forget it.

