prk@charm.UUCP (Paul Kolodner) (06/12/85)
Ahem. I have a question. Every now and then (as recently on this net), I see comments about turntable mats and record clamps. These seem like a fine idea, but I don't understand why people talk about resonances in the same connection. The air space underneath a record has a thickness of a millimeter or less, putting its resonance frequency way above the audio range. Vibrations of the record might be resonant, but the speed of sound in vinyl is so much higher than in air that the resonance frequency ought to be real high there, too. So, I'm a bit confused. Any comments?
herbie@watdcsu.UUCP (Herb Chong [DCS]) (06/12/85)
In article <682@charm.UUCP> prk@charm.UUCP (Paul Kolodner) writes: >Ahem. I have a question. Every now and then (as recently >on this net), I see comments about turntable mats and >record clamps. These seem like a fine idea, but I don't >understand why people talk about resonances in the same >connection. The air space underneath a record has a thickness >of a millimeter or less, putting its resonance frequency >way above the audio range. Vibrations of the record might be >resonant, but the speed of sound in vinyl is so much higher >than in air that the resonance frequency ought to be real high >there, too. So, I'm a bit confused. Any comments? perhaps resonance is the wrong term. anyway, vibrations in the air column itself are neglible for all intents and purposes. the reason for clamps and exotic mats is to damp vibrations induced in the vinyl of the record by the stylus and by acoustic feedback through the air to the record. having an airspace underneath the record allows a greater amplitude of motion due to rapid air pressure fluctuations (sound). removing that airspace increases the force neccesary to induce record surface motion. a mat alone will not increase coupling. a clamp is required unless the turntable is designed to accomodate the added wear of a weight. vibrations in the vinyl are induced by the stylus assembly. if you put on a record and turn the volume all the way down, you will still be able to hear the record playing. the vibration of the stylus is enough to excite air and vinyl movement so that is is audible. the sonic energy introduced into the vinyl of the record propogates until it meets the edge of the record where most of it reflects back because of the mechanical impedance mismatch of air and vinyl. having an airspace underneath the record also has the same impedance mismatch. a clamp and a properly designed mat provide closer mechanical impedance match to the vinyl and more vibrational energy is transmitted to the mat so that less is reflected. the mat has a lot of internal friction so that the vibrational energy is dissipated as heat. the wave front that is normally propogated to the record edge is damped before it is able to reflect. a properly designed clamp and mat must make the record contact as much of the mat as possible, including the label and edge areas. the mechanical impedance of the mat must be matched to that of the vinyl so that the transfer of vibrational energy to the mat is maximized. the mat must have high internal losses so that the vibrational energy is dissipated as heat instead of being allowed to reflect off of the platter and return to the record. a very soft rubber surface provides a good compromise between all the conflicting requirements. a conductive rubber surface helps by reducing static buildup if your turntable platter or spindle is grounded. although vinyl is not as good a conductor of vibrations as steel or aluminum, it certainly is much better than rubber. you must remember here that the record stylus is very sensitive to vibration of any kind. vibrations on the order of the wavelength of UV light can be detected by the stylus and easily heard in reproduction. the best mats i've seen are the Platter Matter and the Angstrom Ultimat. the Angstrom comes with a clamp that looks suspiciously like the Nagaoka clamp. hard mats provide inadequate contact surface and sometimes have lower internal losses than the vinyl itself (there are ceramic platter mats that do this) so that the vibrations are reflected even better. fibrous mats provide only limited damping (like the Linn and Rega mats) and i personally recommend replacing them because they pick up dust. many people that i know who own Linns have replaced their mats for the dust reason alone. no platter mat which purports to reduce record vibration should be used without a clamp or (if your table can take it) a weight. the benefits are better resolution of low level information, especially in the midrange. this usually is perceived as better ambience or more detail. Herb Chong... I'm user-friendly -- I don't byte, I nybble.... UUCP: {decvax|utzoo|ihnp4|allegra|clyde}!watmath!water!watdcsu!herbie CSNET: herbie%watdcsu@waterloo.csnet ARPA: herbie%watdcsu%waterloo.csnet@csnet-relay.arpa NETNORTH, BITNET, EARN: herbie@watdcs, herbie@watdcsu
mohler@druxu.UUCP (MohlerDS) (06/13/85)
I have found that the air trapped below the disc and the disc damping are not the problem, but rather the cause of two other problems. A moving coil cartridge can have problems if high frequencies (20K & up) get in the generator circuit. This can have the effect of causing intermodulation distortion and high frequency mistracking which are definitely audible! I suspect that the audiophiles that use special mats and clamps are reducing intermod. and low level high frequency tracking problems rather than damping "vinyl ringing" and removing air space vibrations. Moving magnet cartridges tend to be less sensitive to this but it can still be a significant problem. I have experimented with mats and clamps and I believe my results support my opinions listed above. Another comment to those that have been reading the postings on clamps. Beware of the EON POD! Depending on the shape of the tonearm and the length of the lead-out grove the arm can hit the POD!!! When I worked at an audio store during college I saw 3 very expensive cartridges "meet their maker" due to the POD. David S. Mohler AT&T - ISL @ Denver drune!mohler <---- Notice new path. "At the approach of daybreak...the tweeks disappear. They probably returned to a certain castle in Transylvania." - AT&T - BELL LABS RECORD 1984 !!