A survey of clubs and pubs in Birmingham's entertainment quarter found that music was routinely played so loud that the hearing of bar staff and glass collectors was at risk of permanent damage.
Thirteen out of 31 premises visited by city council environmental health teams found sound levels above 90 decibels, equivalent to the noise emitted when a power drill is used on concrete.
All but one of the premises were breaking legal limits for noise levels at work.
Employees had not been given hearing protection nor were quiet rooms set aside for staff, contravening the Noise at Work Regulations.
In two cases, the inspectors noticed that glass collectors had devised their own ear plugs made from toilet paper.
Almost 100 members of staff were spoken to from bars and clubs in Broad Street, the Arcadian Centre and other city night spots. Nearly half said they had suffered from ringing ears or a temporary dullness in hearing.
The symptoms are typical of exposure to excessive noise and are likely to result in long-term hearing damage, according to the council.
Public protection committee chairman Neil Eustace is insisting on licensed premsies meeting their obligations under health and safety legislation. Improvement notices requiring owners to comply with noise rules were served on 13 of the 31 premises.
Coun Eustace (Lib Dem Stechford & Yardley North) said: "We have visited a large number of clubs and bars in the city centre. Noise levels in the vast majority of them were way too high.
"This can lead to long-term hearing problems. But there are some very simple steps employers can take.
"Many bar owners place speakers above or behind the bar. One straightforward change is to move them to somewhere nearer the dance floor."
The survey was one of the biggest in Britain.
Coun Eustace said: "We served notices on premises that had problems to be put right. One of them closed for a major refurbishment.
"More than 60 per cent of the premises had an average noise level above 90 decibels over the working day. This is equivalent to standing next to someone drilling concrete for eight hours."
A council spokeswoman said: "Inspectors discovered that the vast majority of these bars and clubs had just ignored the regulations. Owners and managers had simply not thought about the prospect of staff suffering hearing impairment."