speaker design.docx

7/29/2019 speaker design.docx

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GENERAL AND SPECIFIC PROJECT OBJECTIVES:

To be able to design and construct a 2-way speaker system with ported boxenclosure.

To be able to test if the constructed speaker systems meet the expected outputbased on the design.

PROJECT DESCRIPTION:

There are four kinds of speaker enclosures in general use: closed boxes, ported boxes,

labyrinths and their variations, and horns. The first two types, closed boxes and ported boxes are

used for the great majority of stereo speaker systems.

Ported Box Enclosures Speaker System

The ported box or bass reflex is a tuned circuit in which a mass resonates against

compliance. The box with the hole in it acts as a resonator, properly called Helmholtz resonator,

after the nineteenth-century German physicist who first described the behavior of tuned

acoustical resonators. The frequency of resonance for any Helmholtz resonator is determined by

the compliance of the air in the container and the mass of the air in the port. At the Helmholtz

frequency of resonance, the air in the port vibrates easily, compressing and decompressing the air

in the box.

When a speaker is installed in a ported box and is coupled closely to the tuned circuit ofthe box, the original speaker resonance is replaced by two new resonances, one at a higher

frequency than the original, and the other at a lower frequency figure 1 below. At the upper

resonance, the air in the port moves in phase with the cone, but because this frequency is well

above that of the box resonance, the damping action is reduced. This is the frequency at which

some reflex systems produce too much output, particularly small enclosures whose upper

resonance peaks occur within the frequency the frequency of the male voice.


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Figure 1

If the box is properly tuned, the vibrating air piston works in phase with the cone over a

selected band of frequencies to reinforce bass output. This action is most effective at theHelmholtz frequency.

To tune a ported box, change the area or the length of the port/s. compact ported boxes

usually are tuned by a ducted port because the use a duct increases the mass of the vibrating air,

tuning the box to a lower frequency. One of the peculiarities of a reflex system is that port

radiation does not vary with the size of the port. A small port will radiate just as much sound as a

large port, but at higher velocity. If the port is too small, it can produce unmusical noises by its

high air velocity.

Importance

The most important advantage of the ported box is its damping control on the speaker. It

also offers more efficiency by permitting a more efficient woofer. A woofer designed for a

ported box does not have to have as heavy a cone or as long a voice coil as a sealed box woofer,

so it can have more extended mid-range response. This makes a 2-way woofer-Tweeter system a

practical possibility.

The disadvantage of the reflex is the complexity of speaker-to-box relationships,

requiring careful design for good performance.

An example of ported speaker enclosure


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Figure 2

Speaker Characteristics and Type

Subwoofer Speaker

A typical woofer will have an excursion of +/-8mm, push it further and it will still work

but introduce non-linear distortion due to the voice coil not being fully inserted in the magnetic

field of the magnet. Non-linear distortion obviously is unwanted, as it introduces harmonics inthe reproduction which were not in the original recording. But of course you cannot simply keep

increasing the cone diameter either, as the cone needs to remain stiff and move as a whole, rather

than the center piece being driven and the edges trailing behind it. Obviously larger cones are

also heavier requiring more powerful magnets and voice coils to drive them. Realistically 15" is

about the limit where conventional materials and engineering can be used to construct a properfunctioning woofer. In our case 10" and 12" woofers offer a good balance between accuracy,

effective cone area and maximum sound pressure level (spl).

Figure 3 image of a typical Subwoofer Speaker
http://www.hardwareanalysis.com/content/image/11490/http://www.hardwareanalysis.com/content/image/11489/http://www.hardwareanalysis.com/content/image/11490/http://www.hardwareanalysis.com/content/image/11489/http://www.hardwareanalysis.com/content/image/11490/http://www.hardwareanalysis.com/content/image/11489/


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Actual image of the Subwoofer speaker used for the project shows below

Selecting the right 10" or 12" woofer is not easy though. So before we make a list of

criteria for the woofer it is important to set a few design requirements for the subwoofer as that

will largely determine what specifications the woofer will need to have. These design

requirements are listed below and selected from personal and professional experience from the

author.

1. Compact design, with about 30-litre effective volume.

2. Closed, vented or passive radiator box.3. Active system with amplifier equalization and active filters.

4. Usable frequency response down to 20 Hz at max spl above 100 dB.

5. Total price

The 10" or 12" woofer we are looking for has a large excursion, allowing it to move lots

of air, but is also equipped with a powerful enough voice coil and magnet system so sensitivity is

not affected. It also needs to have a low resonance frequency (fs), a low equivalent volume

(VAS) and a low Q factor (Qts) so it can be mounted in a small box and still be able to offer

accurate and deep bass reproduction.

A closed box is not best suited for low frequency reproduction as the roll-off can be as

low as 12dB/oct. below the -3dB point (F3, fsb), which means that the sound pressure level

drops rapidly below the -3dB point. However actual roll-off is largely determined by theenclosure volume in conjunction with the Q factor of the loudspeaker, the lower the Q factor the

lower the roll-off. A closed box has a few advantages though which are quite suitable for our

design. For example a woofer with a low Q factor offers a very fast and accurate response in


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small closed box. Furthermore the air inside the closed box will limit the excursion of the woofer

at low frequencies.

Types of Tweeter Speaker

A tweeter loudspeaker is used for high-frequency sounds, typically those sounds used torepel animals or to produce high-frequency noises in songs, movies and other audio. There aremany tweeter speakers, such as the cone speaker, which has a small and fast cone and looks like

a woofer. The dome tweeter is formed like a dome, has larger voice coils than most other

tweeter speakers and is normally used for high-quality systems. A ribbon tweeter uses a rapidlymoving ribbon that is lightweight, but it typically this needs more power than the other tweeters

to make high-frequency sounds. Air motion transfer (AMT) systems use air to create high-

frequency sounds, and they tend to have fewer parts but can be difficult for designers to create.

Cone tweeter speakers normally look circular on the outside, but there is a small cone

inside that is built from stiff or soft material, or both. Stiff material, such as ceramic, is more

durable; softer materials such as fabric have a better dampening effect. When a high-frequencysound is in the audio, the cone rapidly moves to produce the noise. While these are cheap and

easy to produce, they are uncommon because they cannot disperse sound as well as other

tweeters.

Dome tweeter speakers are usually larger than other tweeters, and they also have a longer

voice coil. The voice coil, which is usually 1.5 inches (3.8 centimeters), is hidden behind a soft

or somewhat soft material, such as thin metal mesh or fabric. This dome protects the voice coil,and the softness of the dome makes it easier for high-frequency sounds to be produced. They are

normally good at dispersing high-frequency sounds, so dome tweeters are commonly used.

Ribbon tweeter speakers require a thin and lightweight ribbon wire to produce high-frequency sounds. These tweeters tend to use much more power than other tweeters to make the

wire move quickly enough; otherwise, the ribbon will not be able to function properly. High-

powered ribbon tweeters have higher dispersion rates and can be stacked together for powerfulresults.

AMT tweeter speakers function by using air and pushing it through a mesh. Air isresponsible for making the high-frequency sounds, so these tweeters have fewer moving parts

and tend to be more durable. They are typically only made as high-end equipment, so these

tweeters are generally expensive and can be difficult for designers to create without interrupting

the high-frequency sound production.


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Figure 4 dome tweeter use for the project

Figure shows Front View and Back View Of the 2 way ported enclosure


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Figure 5 shows the Left and Right Side View

Figure 6 shows the Top and Bottom View


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Figure 7 shows the different side and disassembled plywood and assembled one w/ the used of

Computer Aided Design


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Fs = Resonance frequency of the driver. In free-air, the driver's impedance will peak at this

frequency.

Pe = Thermal capacity of the driver, in Watts. If continuously driven above its rated Pe, the

driver may prematurely burn out and fail.

Qes = Electrical Q of the driver at Fs. Qes is a measure of the driver's tendency to resonate at

Fs, based on its electrical characteristics, e.g. magnet strength, magnetic circuit

characteristics, etc.). The driver's overall resonance characteristics are usually

dominated by Qes.

Qes=Qms/Ro-1

Qms = Mechanical Q of the driver at Fs. Qms is a measure of the driver's tendency to resonate

at Fs, based on its mechanical characteristics, e.g. surround compliance, the compliance

of the spider, weight of the cone, etc.

Qms

Qts = Total Q of the driver at Fs. Qts is a measure of the driver's tendency to resonate at Fs,

based on its overall characteristics. Qts can be calculated using the equation:

Qts= Qms*Qes/(Qms+Qes))

Re = DC resistance of the driver's voice coil. Re is less than the driver's rated impedance

(normally 4 or 8 ohms).

Sd = Effective surface area of the driver. Roughly equal to the area of the cone plus 1/3rd of

the surround.Vas = Equivalent air compliance. The volume of air that has the same compliance

("springiness") as the driver's suspension. Because less air is more "springy" than moreair, a large Vas represents a "loose" suspension

Vb = The net box volume

Zmax = Impedance value at frequency of resonance Fs.

Ro = Computed resistance in terms of Zmax and Re

Ro

Konzert Subwoofer Specifications:


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Fs= 30 Hz

F1= 25 Hz

F2= 35 Hz

Re= 7.8

Zmax= 60

Max. Power Rating= 300watts

SPL= 93 dB

Rs= 8

Dia=10 inches

Plywood Board= inches

To Compute for Ro,Qms, Qes, Qts

Ro= Zmax/Re =60/7.8

Ro=7.692307692

Qms

=

Qms=8.320502943

Qes=Qms/Ro-1

=8.320502943/ (7.692307692 -1)

Qes=1.243293543

Qts= Qms*Qes/(Qms+Qes))

= (8.320502943*1.243293543)/ (8.320502943+1.243293543)

Qts=1.081665383

Vas external dimensions


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Height=62.5 cm.

Width=36.5 cm.

Length=31.0 cm.

Vas= LxWxH

Vas= 31cm. x 36.5 cm. x 62.5 cm.

Vas= 4315.522903 cubic inches

Vas= 2.497409087 cubic feet

Vas internal volume

H= 24.60629921 inches- 1.5 inches

H= 23.10629921

W=14.37007874-1.5

W= 12.87007874

L= 12.20472441-1.5

L=10.70472441

Vas internal= L*W*H

Vas= 3183.36977 cubic inches

Vas= 1.842227876 cubic feet

Port Displacement

Diameter= 2.75 inches

Subwoofer 10 diameter

Approximate cone area= 50 sq. inch.

Minimum port area= 3.5 sq. inch.

Minimum pipe diameter= 3 inch.

Computation on Crossover Network

Cut Off Frequency= 2000 Hz


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Xc = 8 ohms @ fH

XL = 8 ohms @ fL

For High Frequency: Capacitor Value

Xc = 8 ohms @ fH

C = 1/2x2000x8

C= 9.94718943 uF

Use 10 uF---Commercial Value of Capacitor

For Low Frequency: Inductor Value

L=8/2(2000)

L= 636.6197724 uH

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