German club Hoffenheim have apologised after an employee admitted using a high-frequency sound system to drown out visiting fans' chants during their 1-0 win over Borussia Dortmund on Saturday.
The employee, who now faces disciplinary and legal action, deployed a sound system which emitted a shrill pitch.
The device irritated fans and disrupted their chants targeting Hoffenheim's major investor, billionaire Dietmar Hopp.
"An employee came forward and admitted that he had put in the equipment," the Bundesliga side said in a statement on Tuesday, a day after a Dortmund fan filed a police complaint for bodily harm following the incident, in which play was not affected.
The club said its employee had wanted to create an "antidote" to what he thought were unbearable insults against Hopp.
The statement added: "He also said he was unaware of the seriousness of his action which was mainly supposed to be humorous.
"Neither the club nor the president nor Mr Hopp were aware of the equipment's existence.
"We distance ourselves from such an action and apologise to all football fans if their experience was affected."
Photographs taken during the game showed a man with headphones sitting in a tunnel immediately under the travelling fans' stand with a home-made wooden construction that included two speakers.
Heidelberg police, who have asked for the apparatus to be delivered to them, are still investigating the incident.