Installation problems

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Diaphragm installation problems.
By Carlos M. Hernandez

Although most diaphragms of a compression driver type are very well made under strict manufacturing quality supervision, the tight tolerances and precise aligmnments needed in their magnetic gap makes it very difficult to fit any given diaphragm to a driver, and just by dropping a new diaphragm in does not guaranty that a perfect fit is attained, even some old original diaphragm are not always ideally aligned, causing very high distortion levels, either the driver or the diaphragm itself may be off a bit, rendering a less than ideal reproduction.

There are 3 potential problems in a diaphragm compression driver interaction.

I call them:

Concentric misalignment
Tilt misalignment
Excursion restriction

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Concentric misalignment. Is the off center alignment of the diaphragm in relation to the center of circumference of the magnetic gap, may be corrected by adjusting the dowel locating pins in the drivers with such a provision, changing the diaphragm or more difficultly done by opening the driver and loosening the center piece and phase plug, then it can move and aligned to the diaphragm with the help of a "spider" shim that sits between the voice coil former and the phase plug,
When interrupting or opening the magnetic circuit the magnet will loose almost all it’s strength, a re-magnetization of the driver’s magnet is a must.

Concentric misalignment causes rubbing or in some cases pinching of the voice coil against the walls of the magnetic gap, rendering partial functioning and restricted travel of the diaphragm resulting in very high distortion levels and eventual erosion and failure of the voice coil.

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Tilt misalignment is mostly caused by a less than perfect true “square” of the voice coil former and the diaphragm plane failing to be truly perpendicular to the driver’s magnetic gap walls. Therefore when the diaphragm moves or travels certain distance rubbing of the voice coil against the walls of the magnetic gap takes place, causing very high distortion levels at certain frequencies, in some severe cases the diaphragm must be replaced with a good one. In most instances is the diaphragm causing the problem by poor manufacturing techniques or mistakes in the process, rarely is a driver causing the trouble, but in that rare instance the driver must be disassembled, inspected and machined if needed to correct the problem, then assembled and re-magnetized in case of a permanent magnet driver.

Tilt misalignment causes rubbing of the voice coil against the magnetic gap walls resulting in very high distortion levels at certain frequencies and power output, with eventual erosion and failure of the voice coil.

	Clearly shown in the photos, tilted diaphragms / voice coil.
A deformed voice coil and former will also result in concentric and / or tilt misalignment and the diaphragm must be replaced.

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Excursion restriction, is the restricted travel of the diaphragm caused by either the wrong diaphragm location with respect to the base of the mounting clamp ring or the driver’s phase plug protruding from it’s base excessively or a combination of both.
When the problem is caused by the diaphragm the surround may be deformed or elongated when subjected to very high output levels of low frequencies, excessive travel and / or excessive power dissipation causing the surround to deform under excessive heat and by mistakes or poor manufacturing techniques in both, drivers and diaphragms.

Excursion restriction limits the travel of the diaphragm towards the phase plug at the positive phase of the incoming signals, by the diaphragm been so closed to the phase plug and colliding with the same, in severe cases when in permanent direct contact with the phase plug the diaphragm only moves with the negative going side of the incoming signal! away from the phase plug.

The results is unacceptable distortion at certain power levels at lower or all frequencies, depending on how severe the diaphragm movement restriction is, with eventual deformation, abrasion and rupture of the diaphragm.

Shown in the photos, erosion and deformation of the dome due to excursion restriction.
The dark area collided with the phase plug and eroded by friction with years of operation,
the dome was slightly deformed.
The sound generated by this diaphragm was seriously distorted but was kept inadvertently
in service for many years!

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Some other problems such as dirt or metal particles located in the magnetic gap or in the phase plug are also possible if not cared for when installing the diaphragms or in old drivers with broken or missing bug screens, exposing the diaphragm to insects and weather.
Corrosion is always a possibility when the drivers were use outdoors.

The phase plug and magnetic gap must be free of any strange matter at all times for any driver to perform as it was intended.

Most compression drivers will perform with no equal, delivering music with the least amount of distortion, if you are not happy with yours maybe is because the driver/s is having one or more problems as discused here. It is time to undust that old driver and check for missalignments !

Severe corrosion failure of a diaphragm.

The statements discussed hereto are not taken from any reference, book or media, but the direct observations, experience and logic of this author, nor I know of any work that states it in any form. If the reader, disagrees or likes to add comments or existing references to this article, please contact me at mail@chaudiodesign.com

I reserve the right to make changes without deviating from the original ideas as set forth.

I do not intend to mislead or scare any reader with false statements but the sole purpose to educate and create awareness and solutions to real problems.

No part of this article can be changed or modified in any way without written consent from this author.

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