A sound is a rigid rod used to penetrate body openings. In a BDSM context, it nearly always means a urethral sound, for use in the urethral tube. They are used as another way to control a submissive or slave.
The normal male urethra is as long as the penis, plus the additional inch or two needed to reach the bladder. In addition, there is a sharp turn near the bladder. Sounds for males must be longer than the minimum required for females.
The normal female urethra passes from the vulva an inch or two to the bladder. Sounds for use in females can be smaller than in males. In practice, males sounds are used for females as well.
Materials and shapes
Stainless steel, hard rubber and other similarly rigid sterilisable materials are common. Glass was used formerly, but there is considerable risk of breakage, and consequent internal injury, so glass sounds are very rare in modern times.
Sounds always have rounded tips to avoid cutting or damaging the urethral walls. They can be straight, or curved gently at one or both ends. Expecially for use in men, the curve assists in coping with the sharp bend in the male urethra.
Sizes vary somewhat, but typically sounds are the same diameter throughout their length. Diameter units are typically given in French gauge, in which a larger number means a smaller diameter and vice versa.
As with catheters, problems in using sounds are both mechanical (poor skill in installation, usually) and infection. The mucous membranes of the urethral walls are relatively easy to infect and so sterile technique is required. Reuse requires full sterilization, as in an autoclave. Applicaiton of antiseptic fluids is possible must be very carefully done. Use of sounds can lead to a bladder infection, or urinary tract infection with all of the problems attendant thereto.
Sounds may be inserted and left for brief periods. They may also be moved gently back and forth. Some female submissives, or slaves, report very intense feelings of being used sexually, akin to being fucked. There is at least one Net report of a sumissive being able to reach orgasm from this.