Schalldämpfer. This German word is the only differentiation given to the 'silent' cousin of the world's most popular submachine gun. Literally translated as "Sound Dampened," the MP5SD has a large number of fans all over the world, as well as detractors. "Silencers" as they are commonly called, both in legal definitions for their regulation by the National Firearms Act of 1934, and in popular vernacular do not actually do what their name suggests. Sound suppressed or even dampened is a much better way to describe what actually occurs when a firearm is discharged with one on the muzzle. But they are, with the exception of .22 caliber suppressed guns, far from silent.
How would one describe what an SD sounds like being fired? Many gun rags that are available over the counter describe them as being so silent that only the bolt opening and closing is audible. That is pure pablum. Sound suppressed firearms are relatively loud, but suppressed .22 pistols and rifles are very quiet. Most all can be fired without hearing protection, but the key to what the suppressor does, in layman's terms, is to make a gunshot sound like something other than a gunshot. This is an accurate description: They do not sound like gunshots. More like a pneumatic staple gun. As sound suppression systems go, the SD is one of the more quiet available. The SD suppressor is to my knowledge, the only sound suppressor actually manufactured by HK. HK markets many others, primarily by Knight's Armament Co. and Brügger &Thomet of Switzerland (for the new UMP.)
Key to the uniqueness of the SD suppressor is that it is coupled with a barrel that has 30 2.5 mm ports in it to drop supersonic bullets to subsonic velocity for even greater noise reduction. It is often confused, but shooting subsonic bullets in an MP5SD is a definite no-no for what is described in ballistic performance as the world's most expensive .380. The average reduction in velocity is 200 feet per second. You can thus see why subsonic is not a good idea, apart from the unreliability that you are introducing the gun as well. Supersonic always for the SD, but subsonic is in order for all the other muzzle mounted suppressors marketed by or manufactured for, HK submachine guns.