Courses:PHYS341/2021/Project8

Speakers

A speaker is an electroacoustic (transfer electrical signal to acoustic signal) transducer (a device that converts energy from one form to another) (Newell, Philip Richard, 2019). That is, it transforms an electrical signal into an acoustic sound. Speakers are found in home entertainment systems, mobile phones, computers and many more. Speakers can be categorized by different characteristics, such as the speaker's pitch (subwoofer, midrange, treble), speaker's purpose (home entertainment, car audio). It is important to note that the term speaker can be used for either individual transducers (driver) or the entire speaker system that consists of one or more drivers and the enclosure (box).

History

This prototype is finished in May 1921. The speaker cone is damaged. The cabinet also contains a power amplification system. Source: https://edisontechcenter.org/speakers.html#timeline

Johann Philipp Reis - a German teacher - developed the first simple type of electronic loudspeaker in 1861, it was just an experiment, and his invention can only crudely produce noise. In 1876, Alexander Graham Bell patented his electric speaker to be used in his telephone based on Reis’ work. There are many different versions of speakers developed from then, but no one was successful. Even though they understood very well the basic principles of a speaker, which is still applied in speakers up until today, they didn’t have the aid of modern technology and materials.

A speaker driver consists of a permanent magnet, voice coil, dust cap, inner and outer suspension, cone.. Source: https://soundcertified.com/how-do-speakers-work/

It is not until 1924 that Rice from General Electric and Kellogg from AT&T patented a speaker that can produce nice and crisp audio sound. Within a short period, they perfected the commercial product and sold it under the company name of Radio Corporation of America (RCA) for US$250 in 1926 (about US$3000 today). There was a great demand for their speaker, especially in the movie theatre industry.

The modern-day speakers are still using the same principle to produce sound as Rice and Kellogg’s speaker used a century ago. However, modern speakers are equipped with advanced technologies such as can connect to a digital source wirelessly or has a built-in smart assistant, as in the case of Google Nest and Amazon Alexa.

Speaker components

There are different types of speaker design, namely moving-coil cone speakers, dome speakers, compression drivers. However, all of them use the same fundamental parts and design.

The main components of a speaker are:

• Permanent magnet: This magnet creates a fixed magnetic field around the voice coil to accommodates movement.
• Bobbin: The bobbin is a cylindrical tube attached to the bottom of the cone.
• Voice coil: A very long and wound coil of wire. It is often made from copper. When electricity from through, the voice coil creates a magnetic field.
• Inner suspension: It is also called the spider. It is a wavy thin woven material that holds the voice coil assembly and helps restore the cone back to the initial position as it moves.
• Speaker cone: This is a cone-shaped component that's moved by the magnet and voice coil assembly. The cone displaces air around it and creates a variation of pressure in the air, called sound waves.
• Dust cap: A thin material covering the cone's opening to keep dust & dirt from entering the driver.
• Outer suspension: Also called surround, is a flexible circular component that attaches connect the top edge of the cone to the basket. It is often made from rubber.
• Basket: This is the speaker frame, often made from cast metal or stamped metal. It is used to attach every part of the component to and keeps everything aligned.
• Speaker enclosure: A box containing one or more speaker driver assembly sometimes accommodates other electrical devices such as amplifier or crossover.

The working principles of a speaker

When the amplifier's electrical signal enters the speaker, it passes through the voice coil and creates a magnetic field. This magnetic field combines with the magnetic force from the permanent magnet inside the speaker to move the cone back and forth in the direction of the electrical signal. The electrical signal changes direction many times per second, pushing the cone back and forth continuously. Because the cone has a large area that contacts with air, it will displace air surrounding it and create sound waves that the human ears can capture.

The cone will move according to the electrical signal. When the electrical signal voltages increase towards the top of the sine wave, the cone will reach the maximum displacement in the corresponding direction. The stronger the electric current, the further displaced the speaker, and the louder the sound created. The rate at which the electric current changes direction is directly responsible for the cone's vibration rate. Therefore, a faster rate of current change direction will increase the sound's pitch and vice versa.

When the cone moves back and forth, it will create a sound wave from both the front and back. As it moves outward, it creates a positive pressure in the front of the speaker. But at the same time, it creates a negative pressure in the back of the speaker. These two sound waves at the front and back of the cone have the same intensity, frequency, but they are out of phase. As a result, they will cancel each other out if they interact with each other, according to the superposition of wave. The speaker enclosure prevents the waves from the back of the cone from interacting with the waves in front of the cone, avoiding the cancellation.

Bass versus Treble

The graph above shows how our ears perceive loudness for different frequencies. The curves present the sound level loudness over the whole audible spectrum of sound waves.

The founder of Wharfedale, Gilbert Briggs, wrote in his 1955 book: “It is fairly easy to make a moving-coil loudspeaker to cover 80 to 8000 Hz without serious loss, but to extend to the range of 30 Hz in the bass and 15,000 Hz in the extreme top present quite a few problems”. As we usually observe, the bass speakers which produce lower pitch sound typically are much bigger than the treble unit, which produces the sound at the top range, and there is a reason why.

Bass speaker

It is possible to run an electric signal with a frequency of 20Hz through a small speaker to vibrate the cone 20 times per second. However, at this low frequency, the cone moving relatively slowly, so the air in front of it simply slips aside instead of being pressurized, and when the cone returns in the opposite direction, the air slips back. A larger cone is needed to make it harder for the air to move out of the way. The second reason a larger cone is necessary is that the human ears do not hear bass frequencies as well as the higher frequencies.

To hear a lower frequency with the same loudness as a higher frequency, the intensity of the low-frequency sound need to be higher than the intensity of the high-frequency sound. To achieve this, the cone needs to have a larger displacement, and therefore, a larger cone diameter is required.

Interesting fact: Many portable Bluetooth speakers, such as the Bose SoundLink Mini, can produce decent bass sound with a cone diameter of just over 3 cm. It does not actually make a low-frequency sound wave, but instead, it produces a series of higher harmonics that trick our brains into hearing a lower fundamental frequency. This phenomenon is called the “missing fundamental.”

Treble

5.1 surround sound system: the left speaker is a subwoofer, which has one large driver. The right speaker case contains a treble speaker and a mid-range speaker.

When the cone vibrates at 20 kHz, it must change direction 40,000 times per second. A large speaker that can produce bass sound cannot act like this, as the momentum would be too great to

accelerate and decelerate that fast without an enormous amount of electrical input power. A smaller cone is needed because lower mass objects have smaller momentum when moving at the same speed. It is easier for the magnetic force to accelerate and decelerate the cone. A smaller cone cannot produce a high-intensity sound due to the smaller displacement. Fortunately, the above Fletcher-Munson Curves shows that higher pitch sounds don’t need a very high intensity for the human ear to hear them.

A good sound system

Due to speakers' limitations, one speaker cannot produce the sound on the entire audible spectrum. Therefore, a good sound system usually consists of more than one speaker, often one bass, which produces lower pitch sounds; one mid-rage speaker; and one treble, which produces higher pitch sounds. Depending on the type of speaker system, the bass driver and the higher range drivers can be in the same or separate enclosure. A music listening stereo system often has all the drivers in the same enclosure. A surround sound system used to play movie sound needs to have a separate enclosure for the bass speaker to achieve the best surround effect.

Conclusion

The structure and the operating principle of the speaker are relatively simple. Making a device that vibrates under an electric signal to create sounds that can be used in a laptop or pocket music player is easy. However, if you want to make a speaker system that can produce a full-frequency range, high-fidelity sound, low distortion at all volume, then the physic around that increases in complexity at an exponential rate. However, physics is not the only factor in creating a good speaker system. It is also the subjective perception of each person toward the music they hear. The science in creating an “audiophile” speaker is still one of the most debatable topics in technology.