Caltrain's horns have finally returned to their original volume level, but residents are still groaning that the blasts can be heard farther from the tracks — a headache that will not be soothed for some time.

Caltrain moved its horns from underneath its locomotives to atop its trains in mid-July after an internal inspection revealed they were not making the distinct "tweet and toot" blasts required by the Federal Railroad Administration. As a result, the horns became two to four times louder than usual.

In the ensuing weeks, residents and businesses unleashed a firestorm of complaints, saying the racket was waking them up at night and forcing them to insert earplugs during the day.

The agency on July 31 began installing regulator valves to quiet the horns and, as of Wednesday, the volume level of the blasts had returned to the previous standard of 96 to 100 decibels on all cars, spokeswoman Christine Dunn said Monday.

But the noise produced by the horns, since they are now 14 feet higher than before, is still traveling farther than those along the tracks had become accustomed, Dunn said. Also, she said residents will continue to hear four distinct sounds: two long blasts, a short one and a long one. Previously, the horn sounded more like one continuous blast, which is a violation of federal safety regulations.

Though some residents have expressed satisfaction with the softer horns, the agency is still receiving grievances about the sound, she said.

"I think that's because of the range (increase)," she said.

Caltrain officials are working with an independent engineering consultant to determine whether it is feasible to move the horns back underneath its trains, Dunn said. The key, she said, will be figuring out how to move the horns while ensuring they still make the four distinct sounds required by federal regulators.

A project to move the horns back to the undercarriage would also take longer than the 3½ weeks it took to quiet the horns, she said.

For now, residents will have to get used to the sound, as Dunn said officials have no timeline for completing the study.

Dunn, as Caltrain CEO Mike Scanlon and Deputy CEO Chuck Harvey did in early August, expressed further apology for the disruption the horns have caused. Scanlon and Harvey have defended the decision to move the horns, saying safety is the agency's first priority.

Staff writer Mike Rosenberg covers San Mateo, Burlingame, Belmont and transportation issues. Reach him at 650-348-4324.