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Home Theater Handbook

Loudspeakers

Part 1


See also: all questions, part 2, part 3

Why are loudspeakers crucial components of home theater?

Loudspeakers play a crucial role in a home theater setup. Loudspeakers must recreate the three dimensional audio environment the movie director intended, within the limited space of our home theater's room. Loudspeakers are called upon to render the soft sound of a breeze in one scene, and the roaring sound of a helicopter, the next. Loudspeaker performance should never draw attention to the loudspeaker. The sound should seamlessly emanate within the room instead.

Why price is not necessarily related to speaker performance?

Loudspeaker price and performance are not related. There are excellent affordable loudspeakers, superior in perform­ance to much more expensive models. The reasons are com­plicated but it suffices to say that loudspeaker design and manufacture is still more of an art form instead of craft.

Why is it important to understand sound and its physical characteristics?

Before we discuss loudspeakers and their characteristics in detail, it is important to understand sound and its physical characteristics. After all loudspeakers generate sounds, which follow the rules of physics to propagate within our home theater room. These rules define to a great degree the ideal loudspeaker characteristics and their placement posi­tions within our room.

What is sound?

Sound is a form of energy, the acoustic energy that is generated by an audio source.

How is the acoustic energy transferred to the air?

The audio source (i.e. speaker) transfers the initial acoustic energy to the air particles and sets them into motion, caus­ing their oscillation. The movement of the air particles caus­es instantaneous pressure variations that are transported to ever-distant points via the air.

How does sound propagate?

Sound propagates through the air in the form of an acoustic wave. This acoustic wave carries the audio information of the initial disturbance of the air particles caused by the loud­speaker, according to the rules of wave propagation.

How do we describe a wave?

According to the rules of Physics, wave is described by its wavelength, its period, its frequency and its speed.

What is the wavelength of an audio wave?

Wavelength is the distance the audio wave travels until it completes a full cycle and it starts repeating itself. In Physics the wavelength is symbolized by the Greek letter Ñ‘ and is computed in meters.

What is the period of an audio wave?

Period is the time required by the audio wave to complete a full cycle, or to cover a distance equal to its wavelength. It is symbolized by the letter T and is computed in seconds (sec) or milliseconds (msec).

What is the human hearing frequency range? What is the frequency of sound?

The frequency of an audio wave indicates the number of cycles generated in one second, which is how many times the audio wave, repeats itself in one second. It is symbolized by the letter f and is computed in Hertz (Hz) or kilohertz (kHz). One Hz means one cycle per second. One kHz is one thousand cycles per second. Humans can hear sounds with a frequency between 20 Hz and 20.000 Hz (20 kHz). Age and long term exposure to loud environments reduce this range.

What is the speed of sound through air?

The speed of a wave indicates how fast it travels through the medium it propagates. The speed of sound is measured in meters per second (m/sec) and is symbolized by the letter �. The speed of sound through air is influenced by the temper­ature of the air, which affects the air density. At 21°C the speed of sound is equal to 344 m/sec.

What is the relationship of the basic sound wave parameters of the period T, the wavelength Ñ‘, the frequency f and the speed c?

The period T, the wavelength ё, the frequency f and the speed � are related according to the following formulas: F = 1/T = �/ё or ё = c/f or � = ёf, where � = 344m/sec.

How is the frequency related to the wavelength of the audio wave?

We deduce that the frequency is inversely related to the wavelength, i.e. the lower the frequency (bass), the greater the wavelength of the sound. This means that the lower the frequency the greater the distance needed by the wavelength to complete a full cycle. In order for a low frequency to be generated the cone of the loudspeaker must travel a long distance and must be of suf­ficient size to generate such a wave. For this reason low fre­quency speakers are bigger in size than high frequency speakers.

Which frequencies are most difficult to control and why?

The wavelength of a sound wave is equal to the speed of sound divided by the frequency of the wave. For a 20 Hz wave the wavelength is 17.2 meters, while at 20 kHz the wave­length is 1.72 cm.

A simple comparison is enough to understand why low frequencies, due to their long wavelengths, are difficult to control in a typical size room and cause most problems related to audio reproduction and acoustics.

What is the role of a loudspeaker?

Audio reproduction is a very complicated process, which aims to fool the human hearing into interpreting the repro­duced sound as a live sound.

Speaker manufacturers must produce speakers that gener­ate realistic audio reproductions in an environment where sound waves interact with the room and furniture before reaching the human ears.

What complicates the task of speaker manufacturers?

Sound behaves differently depending on its frequency and level, which complicates the task of accurate audio repro­duction for the manufacturers.

In addition sound interpretation by humans is very subjec­tive. For example, humans perceive two sounds with an equal level but different frequency as having different lev­els. If this was not complicating enough, this perception is also influenced by how tired we are or if we like the sound we hear.

What do speaker manufacturers aim to do?

Speaker manufacturers aim to formulate the acoustic behav­ior of the speaker in such a way as to enhance the audio per­formance of the speaker by utilizing the combined knowl­edge of acoustic physics and psychoacoustics. Speaker man­ufacturers must take into consideration the laws of physics and the way humans interpret what they hear to manufac­ture speakers with the best possible sound reproduction behavior.

What energy conversion do loudspeakers perform?

Speakers convert the electric energy they receive from the amplifier to acoustic energy; they convert the electric input signals to acoustic waves (variations of air pressure). Most speakers base their operation on some kind of vibrat­ing diaphragm that pushes air particles, causing them to oscillate. This oscillation generates sound waves that carry all the characteristics (frequency, amplitude etc.) of the elec­tric input signal.

What are some of the challenges speaker manufacturers face?

Speaker manufacturers face many challenges. High frequen­cy reproduction requires fast and accurate diaphragm movements. These diaphragms must have low weight to minimize heat generation and increase speed and control. On the other hand, low frequencies require large size diaphragms, capable of moving the required volume of air to generate low frequency sound waves. Large diaphragms however weight more and have greater inertia and decreased sensitivity, while they also require more power to be set in motion.

What is the function of the crossover circuit?

To overcome the conflicting drive requirements for low and high frequencies, speaker manufacturers resort to two or more way designs, where distinct diaphragms reproduce dif­ferent frequencies. This technique requires the use of a crossover circuit, which separates the frequencies and routes them to the appropriate drives. Crossover circuits are responsible for many of the distortions caused by speakers, especially in low cost units.




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