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Condoms aren’t foolproof, and vasectomies may be too much so. Now researchers say they’re working on another contraceptive option for men that offers them more flexibility and control over their fertility. It’s based on ultrasound.

Using sound waves in medicine isn’t new — obstetricians rely on them to take noninvasive pictures of a developing fetus, and cancer doctors use them to image tumors hidden deep in the body. Physical therapists employ ultrasound to heal damaged muscles. Now new data on rats shows that exposing testes to ultrasound can shut down sperm production — which could lead to an effective contraceptive.

James Tsuruta, an assistant professor of pediatrics at University of North Carolina School of Medicine, and his colleagues report in the journal Reproductive Biology and Endocrinology that commonly used doses of ultrasound can lower rats’ sperm concentrations to 3 million per milliliter of semen; experts define low sperm counts in men as anything less than 20 million sperm per milliliter of semen.

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“When we treated the rats in the study, it only took two weeks to shut down a process that is essential to the survival of any species,” says Tsuruta. “Males produce millions of sperm every day. So it’s a very, very robust system. To be able to turn that off — we are really excited to learn how this actually works.”

Ultrasound generates heat by physically vibrating tissues with sound waves — similar to the way that microwave energy shakes up water molecules to heat up food. But the sound waves may also be working at a deeper level to change the tissues they affect; Tsuruta says when he compared rat testes exposed to ultrasound to testes heated to the same temperature without ultrasound, the ultrasound-treated testes showed a 10 times greater drop in sperm concentration.

Research by other scientists suggests that ultrasound may disrupt the proteins in cells and even their gene expression, leading to alterations in the way these cells work. “Ultrasound can definitely change [the cells’] state,” he says. “So to learn whether any of these things are happening if we use ultrasound as contraceptive is going to need future studies.”

In the study, the rats’ testes were exposed to high frequency ultrasound at 3 MHz for 15 minutes each, two days apart. The sessions were enough to kill the existing sperm in the testes and stop the development of additional sperm. The first study to look at the effect of ultrasound on sperm production, in the 1970s, showed that the depletion was temporary, and Tsuruta hopes his studies will show the same result.

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Why the need for zapping sperm? In the U.S., 70% of couples use some form of contraception, with the vast majority relying on female methods, including the birth control pill. Only a quarter of men use condoms or have a vasectomy, which means 75% don’t use any method to control their fertility. There are probably some women out there who’d like to see the balance shift.

So far, the technique has too many unknowns to be tested safely in people. The only men to undergo ultrasound of their testes — not to test its effectiveness, but only to test its feasibility — were prostate cancer patients who were scheduled for castration to remove tumors in the 1970s. These men reported that the ultrasound treatment was tolerable and didn’t cause them any pain.

If Tsuruta’s results hold, and the technique proves safe, Tsuruta says ideally men would go to a clinic for treatments that last about six months or so. But there is a lot more that scientists have to learn about ultrasound and its effect on fertility before that happens. “We need to do more studies to be sure that a man can use this over and over again, and that when he does decide to father children, there is no problem at all,” he says.

In the meantime, Tsuruta stresses that the procedure isn’t something you should try at home, despite the fact that commercial ultrasound machines are available online and men are apparently purchasing them for this purpose. “I get emails asking me what conditions men should use,” Tsuruta says. “This is really not something you should do at home because we don’t know nearly enough about its safety and reversibility and what other effects there might be long term.”

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.