Submission 3 (Week 11)
Group project – Headphones Material Selection
Group number: 18
Members:
Xiao Yi Huang -3414839
Phillip Moser -3420965
Mark Lestrange -3414730
Hari Ganesan -3419257
Justin Hui -3372210
2.1 Speaker Housing……………………………………………………….…….3 2.2 Speaker Magnetic Element…………………………………………………...3 2.3 Ear protection on Speakers………………………………………………..…4
2.0 Preliminary Material Selection..…………………………………………………......5
3.4 Speaker Housing……………………………………………………………..5 3.5 Speaker Magnetic Element…………………………………………………...6 3.6 Ear protection on Speakers………………………………………………..…7
3.0 Final Material Selection………………………………………………...……………8 4.7 Speaker Housing…………………………………………………..…………8 4.8 Speaker Magnetic Element………………………………………….……….9 4.9 Ear protection on Speakers…………………………………………………11
Choice of Suppliers……………………………………………………………..………12
Bibliography…………………………………………………………………..………...13
Group Contribution Declaration…………………………………………………...........15
1.0 - Design Criteria
Head phones are small loudspeakers designed to play music or sounds clearly within close proximity of the human ear so that the user can listen to music/sounds while remaining relatively inaudible to others. Some of the components of a headphone include a speaker, speaker housing, ear protection cover, cord and connection point. The connection plugs into many devices, receiving electrical signals that translate into sounds through the interaction of coils containing current and electromagnets within the headphones. The sounds can be heard by wearing the headphones by having the speaker be placed within the ear as a bud style (earbuds), a muff style (circum-aural) where the headphones cover the whole ear, or placed against the ears (supra-aural).
1.1 - Speaker Housing
The function of the speaker housing is to contain and restrain any electrical components residing within, hold the output cable securely and provide a base for the speakers itself. It must restrict all movement of its components – including the cable – as adequately as possible, as any movement (walking, turning of the head, etc) may produce unwarranted noise (also known as 'microphonics') and interfere with the sounds being transmitted. The material must be non-toxic or safe for constant touch as the speaker housing will be handled often. They must also be able to withstand devaluation due to use over time. Typical speaker housings are designed to last at least two years of constant wear.
The speaker housing must be fairly durable, lightweight or have low density but high relative strength, while also being resistant to impact. While it must be able to remain durable over time, they are generally not designed to be placed under a great amount of stress and thus can have low to medium yield strength, elastic modulus and fracture toughness. It must have some level of corrosion resistance otherwise the two year lifetime will be shortened considerably. As earphones and headphones are widely used, it is also very important for this component's material to be cost efficient and easy to manufacture to be economically viable for high volume manufacturing. Additionally, it must not only be ergonomic in its design but aesthetically pleasing as well.
1.2 - Speaker Magnetic Element
The function of the magnetic element in a speaker is to cause the cone or diaphragm to pulse back and forth rapidly, producing a pressure wave that propagates through the air that the ear processes as sound. This is done typically through the interaction of a permanent magnet and an electromagnetic magnet. This element must be able to rapidly produce alternating forces to act on the diaphragm in the two directions, back and forth, to produce soundwaves.
Furthermore, the forces produced cannot be excessive enough to cause the diaphragm to tear or split, requiring a reasonably high enough fracture toughness and yield strength as a result. The diaphragm would also need to be able to withstand high cycle fatigue, due to varying vibrations, amplitudes and frequencies. However, the magnetic field of the magnet must be powerful enough to ensure that both the amplitude of the soundwaves (volume of the sounds) and the frequency of the soundwaves (pitch of the sounds) are controllable in a well within reasonable range. The magnetic element must also be as corrosion resistant as possible to withstand years of use.
The electromagnet must use a coil of wire that is able to carry a current in order to magnify the magnetic field of the magnet and interact with it to produce alternating forces. This must be done without a high rise in power consumption for energy efficiency. The coil must be ductile enough to be wound around the magnets without fracture, while the magnets themselves must have strong magnetic fields while remaining small and having a low density for portability. They must also be able to resist high cycles of fatigue to prevent fracture. They are not required to have high yield strengths and fracture toughness’ as they are not designed to withstand large loads, but they still must be high enough to withstand forces produced by the interaction with the current.
1.3 - Ear protection on Speakers
The function of the ear protection cover is to act as an interface between the speaker and the user’s ears. This must be a comfortable fit as it is to be worn frequently, and cannot cause any irritation or allergic reactions to the user, or be toxic in nature. It must be flexible and soft to fit many different ear shapes and sizes, as they are typically a one size fits all design. The cover design and material must also be able to be easily cleaned in order to maintain hygiene.
The materials used must be very elastic, flexible and ductile, whilst having a very low stiffness, as well as having a smooth and pleasant enough feeling along the surface of the material. It must have low density to achieve this (although high enough for noise cancelling properties), while also being highly resistant to wear, as it will undoubtedly depreciate due to abrasion during its lifetime. Cost efficiency must also be taken into account, as for mass production, the materials must be relatively cheap to produce.
2.0 – Preliminary Material Selection
2.1- Speaker Housing
The material of the speaker housing is largely dependent upon the design priorities and specifications of the speaker itself and are not restricted to a single materials class, however the most common class of materials to be used worldwide are polymers. From a manufacturer’s perspective, we must consider which to be important, form or function? The next step to take into consideration is the cost and ease of manufacturing in large quantities – what are the manufacturers most interested in producing? A material that is low in cost or one that is easy to mould? Depending on several design factors, these decisions can shift the material selection into different directions.
In the case of the manufacturer favouring form over function, there are very few materials that can be ruled out, as despite natural presumptions, it is even possible to obtain glass headphones (or headphones with specific parts made from glass). While not recommended for the higher quality speakers, ceramics are not entirely unconsidered. Another odd but fairly popular material is wood, often ebony wood for the darker colour and aesthetic appeal, so composites are not ruled out either.
Perhaps the manufacturer has placed a greater significance on function. If ease of manufacturing is an issue, metals are often the best choice, typically aluminium. Aluminium fits the design criteria for the speaker housing perfectly, having medium yield strength, elastic modulus and fracture toughness while remaining low in density, especially when compared to other metals. Higher grade speakers often include chrome as a main constitute in the speaker housing. Chrome is a stronger metal overall than aluminium and has a fairly sought-after high polish. Both aluminium and chrome have a high natural corrosion resistance (aluminium because of its oxide layer) which prove beneficial in prospects of long term use. Metals can also be favoured for their metallic lustre depending on the design as it gives the appearance of a 'sleek' or 'modern' headphone. However, they must be ensured to be insulated by polymers to ensure the safety of the user from electrical mishaps.
However, the most widely used materials are polymers. Polymers and plastics are heavily considered due to their great cost efficiency and high ductility during moulding. Although they may not be as durable and have as high tensile strengths, fracture toughness’ and be as stiff as metals, they can be mass produced for a much lower cost, causing them to be the most widespread in use. One of the more popular polymers is polycarbonate, again favoured for its durability and lightness. They are also corrosion resistant, far more than metals, often having no problems at all. Polycarbonate is slightly more costly however, although the higher end (and higher priced) speakers are often exclusively made of polycarbonate.
What has become increasingly common though is a polycarbonate/aluminium fusion. That is to say, the backside of the speaker is made of polycarbonate (the flexibility of the polycarbonate is preferred for holding the cable) while the front side is comprised of aluminium (as it is sturdier and stiffer). Considering these fusions and different use of a variety of materials for different parts within the same set of headphones, even more design options become available to manufacturers.
Overall, the material selection is largely dependent on the design and manufacturing needs rather than for specific purposes, but in general polymers are the easiest to manufacture on a large scale due to them being cost effective and readily made.
2.2- Speaker Magnetic Element
The speaker’s magnetic element headphones can be separated into two parts – an electromagnet with an alternating magnetic field and a permanent magnet with a static magnetic field.
Electromagnet
For the electromagnet, the material of the coil would have to be a metal due to metals being the only class of material out of polymers, ceramics and metals with the common ability to conduct at normal voltage and temperatures, hence producing an electric field and the corresponding magnetic field that is required for the speakers to function within the headphones. The magnet must be a metal, as only metals are magnetic in nature and produce magnetic fields.
The coil must be ductile as explained in order to be wrapped around the electromagnet, whilst also being conductive in order to carry an alternating current that will interact with the magnetic field of the magnet to produce forces that translate into sound. Preferred metals include copper, aluminium and silver. There are issues with the latter surrounding the weight and the cost of the material, making the more practical and viable solutions copper and aluminium, although silver is used for higher end speakers.
Polymers and ceramics do not meet the criteria to perform as the speaker magnetic element as they cannot conduct at reasonable voltages, with the exception of some superconductors. These superconductors however have their own issues of cooling, ductility and practicality, and thus are not suitable to be widely used, resulting in a metal being the only reasonable choice.
Permanent Magnet
The magnet would evidently have to be a magnetic material in order to produce a magnetic field. This limits the materials class to metals – polymers and ceramics simply do not have the capability to be utilised as efficient magnets. Typically these magnets are ferrite or cobalt alloys, though some ceramics may be used in conjunction with these metals. These magnetic materials provide the required property of magnetism and produce a magnetic field, but also need to be lightweight and the magnetic fields produced must be fairly strong. Possible magnets may be ferrites (iron based) mixed with ceramics or rare earth magnets (such as Neodymium). The former is cheaper, whilst the latter is both lighter and produces stronger magnetic fields in excess of 1.4 Teslas, far stronger than the typical 0.5 to 1 Tesla of ferrite based magnets.
Considering that the permanent magnet is attached onto the diaphragm and adds to its weight, rare earth magnets are the better choice as they best meet the criteria of being lightweight and producing strong magnetic fields – Neodymium is very effective in situation which require a lightweight yet powerful magnetic field – although they do suffer from corrosion unless protected. These strong magnetic fields are needed to effectively allow volume and pitch control within the headphones.
Polymers and most ceramics do not exhibit magnetic properties and hence simply do not meet the criteria to be a permanent magnet.
2.3- Ear protection on Speakers
Different types of headphones will often use different materials for ear protection on speakers.
The speaker ear cover for headphones would need to be made of a polymer rather than a metal or ceramic. This is because polymers provide the required properties of the speaker cover which is high elasticity, flexibility and soft to touch, also allowing a “one size fits all” approach needed in order to sell to a mass market of different consumers, while allowing consumers to wear the headphones for long periods of times without discomfort. Metals and ceramics in general do not have such a high elasticity, and are too stiff to be suitable for ear protection.
Popular polymers for a soft cloth type cover for circum-aural headphones are textiles, including velour, velvet and plush. Supra-aural headphones are typically composed of materials such as foam, leather, rubber and vinyl. In ear headphones are designed to be inserted into the ear, and therefore the material selection must be more controlled to help reduce discomfort or reactions. They are typically made from polyester and polycarbonate. A natural or synthetic rubber can be used as it is flexible, elastic and ridged enough to stay in position. Higher quality ear phones may use medical grade silicon instead as it has the same properties needed but also reduces the chances of allergic reactions or infections within the ear canal. These polymers are also insulators, making them comfortable at a varying degree of different temperatures.
Metals and ceramic do not meet the criteria as their stiffness and rigidity is too high, making them unfitting as they are not flexible, elastic or soft enough for the application as ear protection.
3.0 – Final Material Selection
3.1 - Speaker Housing
For the speaker housing, it will be assumed that the manufacturer wishes to produce the common headphone that will be highly marketable and easy to produce.
Polycarbonate is often the best choice of material mainly due to manufacturability. As the design criteria specify only low-medium yield strength, elastic modulus and fracture toughness, aluminium does not possess a great advantage over polycarbonates as extra strength is not necessarily a required function. Polycarbonate is still sturdy and durable within its own right to last the typical short lifetime of the headphone. Additionally, its 'softness' can be favourable feature as it can be comfortable to touch.
The ultimate advantage polycarbonate has over aluminium is its greater potential for high volume manufacturing. Not only is polycarbonate cheaper than aluminium but polycarbonate can be moulded into a wider variety of shapes more easily and for a lower cost. Polycarbonate typically costs around $1.5-$4 per kilogram while aluminium costs about $3-$5 per kilogram. This means that not only are there more design options, but due to the lower cost to produce a headphone, larger profits can be made. It is for this very reason that the most common type of material for any kind of speaker housing is polycarbonate.
Aluminium still has great ductility during moulding and as such can still be a good manufacturing choice. For buyers more interested in longer lasting headphones, aluminium (with chrome for higher priced speakers) can be the solution. Below is a table illustrating the yield strengths and elasticity aluminium, chrome and polycarbonate. Note the yield strength of polycarbonate is actually the ultimate tensile strength as plastics deform as a function of time and temperature, and so the ultimate tensile strength is used in place of yield strength (the values correspond to a test at room temperature).
As is evident, the metals have high yield strengths, especially when compared to the polycarbonate. It is especially impact resistant, with aluminium having a fracture toughness of approximately 21 MPa.m1/2.
Ceramics and composites (e.g. glass and wood) are not usually suitable as due to the higher production costs resultant from greater costs in material and machining processes, as well as the low market/customer demand, it is far less profitable for the manufacturer to produce headphones from these materials as opposed to the other choices. Furthermore, glass and wood suffer from their own shortcomings in yield strength, elastic modulus and fracture toughness, causing them to have lower durability and shorter lifespans in comparison to polymers.
3.2 - Speaker Magnetic Element
As was the case for 2.0 – Preliminary Material Selection, the Speaker’s magnetic element will be split into two sections, that of the electromagnet and that of the permanent magnet.
Electromagnet
For the electromagnet, copper would be the material of choice. There are many reasons for this, including reactivity, ductility, conductivity and cost.
Copper is a very unreactive metal, as is evidenced by its comparatively high ionisation energy (745.5 kJ.m-1) and the lack of reaction upon contact with water. Furthermore, whilst it does react with atmospheric oxygen, the copper oxide produced forms a barrier against further corrosion. As such, copper is unlikely to corrode and hence fail when being used as an electromagnet in the speakers, making it suitable for this purpose.
Copper is also quite ductile, as can be seen from the Stress vs. Strain graph above. This means that drawing the metal into long, thin wires is relatively easy to do, and also enables the copper to be coiled around the magnet to allow forces to be created from the electromagnetic field controlled by the current.
For conductivity, copper has an electrical resistivity of 16.78 nΩ.m at 20˚C, well below that of aluminium (28.2 nΩ.m at 20˚C). This means that less energy is required to produce the electromagnetic field, as less energy is lost whilst travelling through the wire.
Finally, the cost of copper wire is quite cheap, being AUD $9.50 for a kilogram.
Silver and aluminium are two other materials that can be considered for use in a speaker system. However, these are determined to be inferior to copper for the following reasons: * Silver is prohibitively expensive, costing well over one hundred times that of copper. This cost also fluctuates, making it difficult to base a long term manufacturing business on said material. * Aluminium is a better option than silver, as it is more ductile than copper, less dense and similarly costed. However, it has a higher resistivity and is not easily soldered, making securing the coil an issue. Whilst there are methods of doing so, such as crimping, these are more susceptible to failure than the soldered copper, making it unsuitable for the function within the speaker system.
Permanent Magnet
For the permanent magnet, rare earth magnets such as neodymium would be used due to its strength and very low weight.
Neodymium magnets are the strongest permanent magnets known, with field strength of 1 to 1.4 Teslas. This strong field means that less of the magnet is required to produce the same force acting upon the diaphragm. This in turn lowers both weight and cost.
Weight is also comparatively low, with Neodymium having a density of 7.01 g/cm3. Samarium cobalt, another rare earth magnet, has a higher density of 8.4 g/cm3.
The only problem with Neodymium is its higher reactivity in comparison to other rare earth magnets.
Samarium cobalt and ferrite based magnets could also be considered for use in the speaker system. However both are of poorer quality compared to Neodymium due to their inferior power to weight ratios. The samarium cobalt, whilst comparable in magnetic field strength, is denser. The Ferrite based magnets, whilst less dense is generally four times weaker than the Neodymium magnet. Considering the purpose of the permanent magnet (to move the diaphragm through the forces produced by a magnetic field), weight and power are prioritised, leaving Neodymium as the top choice of material as permanent magnet within the speaker magnetic element.
3.3 – Ear Protection on Speakers
As discussed in the preliminary materials selection, the materials that could be used for in ear head phones are synthetic/natural rubber or hospital grade silicon. The costs are as follows: * Medical grade silicon 5.7 – 9.98 $/Kg. * Natural rubber 3.55 $/Kg * Synthetic rubber 2.2 – 4.76 $/Kg
The best choice of material for the in-ear speaker protection is the Medical grade silicon. All the above materials would satisfy the property requirements to a sufficient level, though the quality of the silicon rubber (they have been tested for biocompatibility) makes it the most appealing material. On average, the synthetic rubber is cheaper than the medical grade silicon by a fair amount; however its quality and biocompatibility are in turn also lower. The medical grade silicon is also a much more reliable material in terms of availability and price than that of natural rubber. The price of natural rubber can vary on a day to day basis as it is in high demand and supplies can be limited due to its appeal as a natural product. The price of natural rubber can also spike very high in times of drought or other natural disasters where large plantations are lost.
The properties of medical grade injection silicon rubber that makes it suitable for the application as a speaker cover on in ear headphones are: * Viscosity (Pa.S): 400-1000. This viscosity will allow it to easily injection moulded. The manufacturing process of injection moulding will allow this material to be made into the speaker covers at high production rates. * Tear strength (KN/m): 10-40. This will provide enough strength for the cover to resists tearing when being transported in bags, pockets, in the car etc. * Tensile strength (MPa): 5-8. This tensile strength is what gives the material its tear strength. * Appearance: Highly transparent. This transparency will allow the material to be coloured to any specification that is desired, making it a desirable material due to the ability for customisation options for the consumer. * Hardness (shore A): 30-60 * Density (g/cm3): 1.05-1.3
The supplier of choice is located in Guangdong China as the material can be bulk produced for a low cost. There are possible suppliers in Australia that may be an option but the manufacturing cost is higher. This is because there is only a small market for the material supply and the industry set up is small.
Choice of Suppliers
Choice(s) of suppliers for materials selected | Contact details | Source of supplier
(e.g., web search, yellow pages etc) | AMF Magnets | Tel: 9700 0055 | Web search | Badger Wire | Tel: 9644 2390 | Web search | Capral Aluminium | Tel: 9725 3351 | Yellow Pages | Matra Glass | Tel: 9622 4488 | Web Search | Injection mouldable medical grade silicon rubber | Telephone: 86-20-15986331376 Mobile Phone: 86-18665053594 Fax: 86-20-86833916 Address: First floor, A1 building, Tangkou 236 industrial park, Tuanjie village, Xinhua town, Huadu district, Guangzhou city Country/Region: China (Mainland) Province/State: Guangdong City: Guangzhou | Web search |
Bibliography:
Amf magnetic, “Magnets & Magnetic Equipment - AMF Magnetics”, Online store, Last revised August 2012, Accessed 22 August 2012 http://www.magnet.com.au/ eHow, “What Materials Are Headphones Made Of?”, Online Article, Last revised 2011, Accessed 22 August 2012 http://www.ehow.com/facts_5009182_what-materials-headphones-made.html Head-Fi, “Glasses and headphones, a bad combination?”, Online Forum, Last revised 10 March 2010, Accessed 23 August 2012 http://www.head-fi.org/t/515734/glasses-and-headphones-a-bad-combiniation Lowther, “Silver or Aluminium?”, Technical Information Page, Last revised 2 September 2011, Accessed 23 August 2012 http://www.lowtherloudspeakers.com/specfeat.html iLounge, “The Complete Guide to Earphones, Part 3”, Online Article, Last revised 13 March 2007, Accessed 24 August 2012 http://www.ilounge.com/index.php/articles/comments/the-complete-guide-to-earphones-part-3/ Dick Smith, “TDK In-ear Headphones MT-300 – Black”, Online Store, Last revised 2011, Accessed 24 August 2012 http://dicksmith.com.au/product/A2151/tdk-in-ear-headphones-mt-300-black HowStuffWorks, “How Speakers Work”, Online Article, Last revised 24 August 2012, Accessed 24 August 2012 http://electronics.howstuffworks.com/speaker6.htm TheMinerals, Metals & Materials society (TMS), “Metallurgical Principles and Practise of Engineering”, Online article, Last revised 4 October 2012, Accessed 4 October 2012 http://www.tms.org/Education/PE/question24.html London Metal Exchange, “Copper”, Online Article, Last revised 4 October 2012, Accessed 4 October 2012 http://www.metalprices.com/metal/copper/copper-wire-rod-india Alibaba, "Aluminium Raw Materials", Online Retail Store, n.p., n.d, Accessed 5 October 2012 http://www.alibaba.com/product-gs/642509339/aluminium_raw_materials.html ESPI Metals,"Chromium", Online Article, n.p, n.d., Accessed 5 October 2012 http://www.espimetals.com/index.php/technical-data/63-chromium The Engineering ToolBox, "Elastic Properties and Young Modulus for Some Materials", n.p., n.d., Accessed 5 October 2012 http://www.engineeringtoolbox.com/young-modulus-d_417.html Alibaba, "Polycarbonate Plastic Raw Material; polycarbonate Hollow/solid Sheet Manufacturer", n.p., n.d., Accessed 5 October 2012 http://www.alibaba.com/products/542810195/polycarbonate_plastic_raw_material_polycarbonate_hollow.html?s=p Alibaba, Aaron Xu, “Medical Grade Silicon”, Last revised 22 August 2012, Accessed 5 October 2012 http://www.alibaba.com/product-gs/580891007/injection_silicone_rubber.html index mundi, “Rubber Daily Price”, Last revised 5 October 2012, Accessed 5/10/2012 http://www.indexmundi.com/commodities/?commodity=rubber Alibaba, Lincy Cheung, “Synthetic rubber price”, Last revised 22 August 2012, Accessed 5 October 2012 http://www.alibaba.com/product-gs/629009957/Synthetic_rubber_price.html?s=p Declaration
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Name: Xiao Yi Huang, Phillip Moser, Mark Lestrange, Hari Ganesan, Justin Hui
