what are the advantages of berylium usedin speakers, oposed to say titanium?

Answer 1

Short answer:
Beryllium is the element on the periodic table that allows sound to pass through it the fastest, clearest and loudest, while also being superstrong, lightweight, and non-magnetic.

Long answer:
Notable characteristics
Beryllium has one of the highest melting points of the light metals. The modulus of elasticity of beryllium is approximately 1/3 greater than that of steel. It has excellent thermal conductivity, is nonmagnetic and resists attack by concentrated nitric acid. It is highly permeable to X-rays, and neutrons are liberated when it is hit by alpha particles, as from radium or polonium (about 30 neutrons/million alpha particles). At standard temperature and pressures beryllium resists oxidation when exposed to air (although its ability to scratch glass is probably due to the formation of a thin layer of the oxide). The speed of sound in beryllium (12,500m/s) is greater than in any other element.

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Applications
Beryllium is used as an alloying agent in the production of beryllium-copper because of its ability to absorb large amounts of heat. Beryllium-copper alloys are used in a wide variety of applications because of their electrical and thermal conductivity, high strength and hardness, nonmagnetic properties, along with good corrosion and fatigue resistance. These applications include the making of spot-welding electrodes, springs, non-sparking tools and electrical contacts.
Due to their stiffness, light weight, and dimensional stability over a wide temperature range, beryllium-copper alloys are also used in the defense and aerospace industries as light-weight structural materials in high-speed aircraft, missiles, space vehicles and communication satellites.
Thin sheets of beryllium foil are used with X-ray detection diagnostics to filter out visible light and allow only X-rays to be detected.
In the field of X-ray lithography beryllium is used for the reproduction of microscopic integrated circuits.
In the telecommunications industry, Beryllium is made into tools that are used to tune the highly magentic klystrons used for high power microwave transmissions for safety.
Because it has a low thermal neutron absorption cross section, the nuclear power industry uses this metal in nuclear reactors as a neutron reflector and moderator.
Beryllium is used in nuclear weapons for similar reasons. For example, the critical mass of a plutonium sphere is significantly reduced if the plutonium is surrounded by a beryllium shell.
Beryllium is sometimes used in neutron sources, in which the beryllium is mixed with an alpha emitter such as 210Po, 226Ra, 239Pu or 241Am.
Beryllium is also used in the making of gyroscopes, various computer equipment, watch springs and instruments where light-weight, rigidity and dimensional stability are needed.
Beryllium oxide is useful for many applications that require an excellent heat conductor, with high strength and hardness, with a very high melting point, and that acts as an electrical insulator.
Beryllium compounds were once used in fluorescent lighting tubes, but this use was discontinued because of berylliosis in the workers manufacturing the tubes (see below).
The James Webb Space Telescope (Beryllium related details from NASA here) will have 18 hexagonal beryllium sections for its mirrors. Because JWST will face a temperature of −240 degrees Celsius (30 kelvins), the mirror is made of beryllium, a material capable of handling extreme cold better than glass. Beryllium contracts and deforms less than glass — and thus remains more uniform — in such temperatures.
Beryllium is also used in the Joint European Torus fusion research facility, to condition the plasma facing components.

Answer 2

extract from voice coil 2001, septembre 2001:
Substitute the silk dome on your tweeter, or the aluminum
or titanium diaphragm on your compression driver, with beryllium,
and the frequency response will become more extended
and generally smoother. Look at the response of any tweeter or
compression driver and you will see a transition at the upper end
where the smooth response becomes ragged. This is the dome
upper resonance, where the dome stops working as a piston.


However, this magic comes at a price. Aluminum (in
ingot form) costs about $4 per lb, and titanium (grades 1
and 2 used for diaphragms) is over $100 per lb for 1 mil foil.
But bulk beryllium block costs over $500 per lb!