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sound transduceres STW7 blue line/ STW7 red line: Differences in sound and material of diaphragms and historical background

We admit that proving differences in sound regarding the material of diaphragm of STW7 blue line and STW7 red line is practically impossible for us with the measurement opportunities we can use. STW7 red line has got diaphragms made of Polyester PET also known as Mylar and STW7 blue line stands for our sound transducer skinned with diaphragms made of Polyvinylchlorid (PVC). We limit ourselves to the free field measurement of frequency response in our small semi-anechoic room. As a matter of principle we make an effort to achieve a nearly linear horizontal frequency response for both versions in the range 40-15000Hz primarily in pattern cardioid. For measurement we install our sound transducer STW7 in a little reflective housing. We use the Neumann U67 housing. Using STW7 together with the lollipop M7 housing causes that the frequency response of STW7 is less linear but equates to the characteristic frequency response of Neumann M7.

Differences in frequency response are primarily caused by differences in electrode (tolerances). For example our model STW7H that consists of a modified electrode has got a linear horizontal frequency response that ends a little bit curved (higher frequencies are accented). This is independent of the diaphragm's material. We favor the frequency response of our standard STW7 because it comes as close as possible to the original-vintage sound transducer.

Due to our repair and reskin service we made the experience that the tension of diaphragms (e. g. red line) reduces a little bit over a long time whereas the tension of old PVC diaphragms could have increased. One can assume that the aging of PVC is the reason for small differences in sound like less bass, strengthened high frequencies, loss of sensitivity and reverse attenuation caused by a tauter tension of diaphragm. So differences in sound are caused by alterations in tension of diaphragm due to aging process and not directly by chemical composition of diaphragm's material. This small alteration is possibly accepted because these old sound transducers with PVC diaphragms were sometimes attested having the best sound. We use the designated tension of diaphragm for STW7 blue line, STW7 red line, backplates produced by FLEA (only red line material) and for reskinning original Neumann sound transducers. We don't necessarily give the tension that we found before repair because we always give the designated tension. This can be the reason for small differences in sound between PET and PVC used in combination with old (PVC: higher tension of diaphragm, PET: less tension) and newly skinned sound transducers.

More than 20 years ago we started the reskinning of M7 electrodes with Mylar respectively MCF diaphragms. Many years before Neumann Berlin modified the M7 sound transducer for U47 (K47) to make it possible to screw a Mylar diaphragm to the backplate. In contrary to Microtech Gefell they ceased the production of M7 sound transducer. Apart from another diameter (34mm instead of 32mm) the K47 sound transducer is identical with M7. That's why the question of differences in sound between M7/PVC and K47/PET and also regarding a larger diameter can be interesting particularly because the U47/48 was produced with M7 as well as with K47. With the help of our ring STR32/34 the outer diameter of M7 style sound transducer can be adapted to K47 assembly. Acoustically the increasing of outer diameter hasn't a positive influence on sound. That's why holders should be as thin as possible or segment holders should be used (e.g. our offer STS7.1). As a result the sound transducer isn't completely enclosed and not thickened and a short way for sound around the sound transducer is maintained. Our sound transducer STW7.1 in combination with the holder STS7.1 complies with this requirements because segment holder STS7.1 thickens only a small part of the sound transducers and allows that the main part can free. Consequently the frequency response is hardly influenced by the holder.

To define differences in sound regarding the diaphragm's material differences in acoustic color would have to be identified. We didn't examine frequency analysis (harmonics) and unit pulse response. In our opinion the acoustic color should be judged subjectively by comparisons of what can be heard. We aren't sound engineers but we can give the experiences that our customers made. Experiences with STW7 blue line can reach back only to 2008 whereas we used the red line material due to our reskin service much longer.

It's not possible to identify the better sounding STW7. Some of our customers find small differences in sound but some of them prefer STW7 red line other prefer STW7 blue line. It's not possible for us to describe these differences. At this point we want to remind of differences in sound between amplifiers with transistors or tubes. The version with tube is more popular although measurable properties often aren't as good as them of the transistor version. Neumann Gefell respectively Microtech Gefell whose authorized repair shop we were preferred PVC for a long time. In contrast we started the usage of Mylar because we firstly weren't able to manufacture PVC diaphragms. The M7 sound transducers reskinned with PET turned out to be working very well. That's why there wasn't any rush to manufacture PVC diaphragms too. In the meanwhile producers of cheap microphones use diaphragms made of Mylar for condenser microphones too. The demand for diaphragms made of PVC increases and therefore we started with the manufacturing of STW7 in red and blue in 2008.

The differences in material and manufacturing of both synthetic diaphragms are significantly. Therefore we assume that differences in acoustic color can be based upon this. Diaphragms made of metal produce another sound than diaphragms made of synthetic material. Of course additional factors have to be considered. We also manufacture nickel foils and as a result diaphragms made of nickel in our company. PVC diaphragms are harder than PET-diaphragms. The structure of Mylar can be described as inhomogeneous because it's extruded and stretched. The PVC foil that we cast by our own can be signified as homogeneous. Its thickness is approx. 6Ám. Optically the vaporized surface of our blue version shows a very good capability of mirroring. Differing properties of expansion and elasticity regarding length and width caused by the industrial production process are reduced by a special finish treatment we give to the material that we use as "red line". Our manufacturing of PVC diaphragms contains the casting of every single diaphragm in a circular area and don't cause differences in length and width.

Mylar is a little bit more porous than PVC in respect of the absorption of water and also the absorption of dust. Especially on non-vaporized spots of diaphragm this can cause problems over time. Due to the humidity of close vocal use noise occurs earlier and makes it necessary to reskin the sound transducer. We like to use MCF instead of Mylar. This material is has got better properties regarding the absorption of water. In case of our red line version we don't distinguish between MCF and Mylar The absorption of water due to air humidity and causes that tension of diaphragm decreases temporarily. We assume and hope based upon our own technology of manufacturing that our STW7 red line and blue line will have a long duration that changing of diaphragms is insignificant and that the sound is preserved with no alteration.