Acoustic Feedback Induced Directly Through the Tonearm

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lavallee
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Acoustic Feedback Induced Directly Through the Tonearm

Post by lavallee » 16 Apr 2018 16:49

Hi All,

I have some questions about acoustic feedback. I am a newb to restoring turntables and am getting back into vinyl after getting away from it in the early 90s. My current project is restoring a Thorens TD 165; however, my questions are not really specific to that project, or even makes and models of turntables (although they could be, relative to specific design characteristics). Keep in mind, when I say 'directly' or 'direct,' I mean vibrations directly absorbed by the tonearm itself, as opposed to vibrations it might receive indirectly from a chassis, sub-chassis, or arm board. Also, I am familiar with the protection provided by isolating the tonearm (i.e.., the whole assembly, the arm board, etc.) and platter, from the motor and the rest of the turntable.

So, with all that, here goes:
  1. How much acoustic feedback can be induced directly through a tonearm?
  2. Are low mass tonearms more susceptible to a wider band of frequencies than higher mass tonearms, or even just more susceptible in general?
  3. Does a dust cover assist or even completely protect the tonearm from direct vibrations?
  4. If a dust cover doesn't afford the tonearm full protection from direct vibrations, would a raised wall of say, the sides of a plinth, Help any?
  5. Is exposure to direct vibrations even considered much of an issue?
Thanks in advance for any thoughts and knowledge on this subject.

Spinner45
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Re: Acoustic Feedback Induced Directly Through the Tonearm

Post by Spinner45 » 16 Apr 2018 17:54

in response:

1 - low but related to speaker placement.
You wouldn't put a turntable ON or NEAR a speaker, that's dumb.
2 - all depends on the resonant frequency along with material of the tonearm.
3 - no, dust covers offer no shielding to air-born vibrations.
4 - no - again, air-born vibrations go everywhere, even through walls.
5 - yes, common sense regarding turntable placement is important.

Use common sense and you'll have nothing to worry about.

Woodbrains
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Re: Acoustic Feedback Induced Directly Through the Tonearm

Post by Woodbrains » 16 Apr 2018 18:13

Spinner45 wrote:
3 - no, dust covers offer no shielding to air-born vibrations.

Of course they do shield some vibrations, to what extent is difficult to say.

4 - no - again, air-born vibrations go everywhere, even through walls.

Vibrations do go through walls, but if walls provided no reduction in sound transmission, the next door neighbours would have a lot to complain about. Dust covers are walls of sorts, and must reduce external sounds from affecting the tonearm.

Every time there is a passage of sound from one medium to another, there is a reduction in energy as it is dissapated through the material.
Mike.

Sunwire
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Re: Acoustic Feedback Induced Directly Through the Tonearm

Post by Sunwire » 16 Apr 2018 19:09

You ear is a sensitive detector of airborne vibrations.
Put a dustcover or wall between your ear and your loudspeaker.
See if either one reduces the level of airborne vibrations reaching your ear

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Re: Acoustic Feedback Induced Directly Through the Tonearm

Post by analogaudio » 16 Apr 2018 19:14

Because the phono preamplifier (RIAA equaliser) includes a lot of bass boost, about 20dB at 20Hz (this is amplification of 10x) it is the low frequencies where acoustic sensitivity of the arm is most important. This is the low bass region, starting below the frequency of hearing and including the musically important frequency regions between 40Hz and 250Hz.

Several things are going on at the same time. The combination of the arm mass with the springy suspension of the cart cantilever usually produces a resonance below audible limits in the region 5Hz to 15Hz. This resonance is an inherent property of the tonearm and cart, and is common to all pivoted arm designs. The resonance magnifies (very strongly) any very low frequency vibration present at the stylus, including vibration to to unevenness of the disc surface and vibration due to acoustic sound in the room. Note that the resonance frequency and the frequency region of maximum amplification in the preamp overlap. The consequence is that there are now two systems that magnify sub-sonic energy and they are connected one after the other. Sub-sonic energy is magnified by the resonance and the result is magnified again by the low frequency amplification boost in the preamp. The acoustic output of the speakers that reaches the arm and stylus adds to the already resonant system and sometimes the result is an acoustic feedback loop that oscillates, sometimes at audible frequencies, sometimes sub-sonic and described as "woofer flapping". This system design could be described as a disaster because the likelihood of such a set-up being able to reproduce accurately the bass content of the music can become low. This is why some choosy and deep-pocketed listeners relocate the turntable in an adjacent room, to sever the sub-sonic acoustic feedback loop.

Compared to the sub-sonic and bass regions where resonance dominates the situation sound energy in the middle and upper regions of the musical spectrum suffer less from this effect and the arm is usually treated as a rigid ideal support in these frequency bands.


I can report from measurements made using my own setup that the presence of a closed cover does have an effect on the sub-sonic resonance (it could hardly be otherwise) in this case the sensitivity to acoustic feedback centered around 30Hz was significantly increased by closing the cover.

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Re: Acoustic Feedback Induced Directly Through the Tonearm

Post by Sunwire » 16 Apr 2018 19:34

I think a larger (surface area)tonearm should be more sensitive to airborne vibration than a small tonearm.
A low mass tonearm should be more sensitive to airborne vibration than a high mass arm due to the lower inertia of the low mass arm (it takes less energy to move it).

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Re: Acoustic Feedback Induced Directly Through the Tonearm

Post by Pryso » 16 Apr 2018 20:13

lavallee, "Is exposure to direct vibrations even considered much of an issue?"

Yes, they can be so placement within the room, as well as what surface the table is mounted on, can be important. My turntable is placed on a shelf 60" wide which fits within an alcove along one wall. I discovered placing the table to the far right end of the shelf resulted in the cartridge/arm being too close to a corner of the alcove. That corner generated more bass waves than I suspected. Moving the table near the left end of the shelf resulted in the cartridge/arm being about at the mid-point along the shelf. With the cartridge/arm away from any corner it removed a small bit of muddiness, thus increased clarity. I would never place a turntable where the business end was close to bass node buildups -- wall meets wall, wall meets floor, wall meets wall meets floor (the worst condition).

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Re: Acoustic Feedback Induced Directly Through the Tonearm

Post by MikeyB456 » 16 Apr 2018 20:58

analogaudio wrote: I can report from measurements made using my own setup that the presence of a closed cover does have an effect on the sub-sonic resonance (it could hardly be otherwise) in this case the sensitivity to acoustic feedback centered around 30Hz was significantly increased by closing the cover.
Interesting. I've noted that my own system sounds better with the dustcover open rather than closed, and have speculated that this is because closing it produces a resonant 'chamber' around the arm and turntable assembly. It also sounds better with the bottom cover of the plinth removed for similar reasons, I think. How you go about measuring which bits (arm, platter, subchassis, etc) respond to what frequencies and in what proportion, is beyond me.

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