SPEAKER ACOUSTICS
|
HOW LOUDSPEAKERS WORK
Parts of a Loudspeaker. Source: https://edisontechcenter.org/speakers.html
A basic loudspeaker is made up of four parts:
- The frame: Holds the parts in position and attaches the speaker to the cabinet.
- The magnet: A permanent magnet, typically made of Ceramic (strontium carbonate and iron oxide), Neodymium (a very powerful rare-earth magnet), or Alnico (an alloy of Aluminum, Nickel, and Cobalt).
- The voice coil: a coil of wire, usually copper, which, when fed current from the amplifier, induces a magnetic field that pushes against the field of the permanent magnet.
- The speaker cone: a cone, typically made of paper, rubber, or kevlar, but can be made of any stiff but flexible material. This is pushed and pulled by the voice coil, producing sound waves.
Since the speaker cone is moving forwards and backwards, it produces positive pressure on one side and negative pressure on the other, meaning that the waves produced by the front and back of the speaker are identical, but out of phase by 180 degrees. This means that they will cancel each other out if allowed to interact. Because of this, a speaker without any sort of barrier between the front and back, called a baffle, will cancel out much of its own sound, making an inefficient and weird-sounding speaker. In addition, when fed too much power, the unenclosed speaker may be able to "overextend," or stretch beyond it's physical limits, causing the cone to be damaged.
Therefore, some sort of enclosure is required for a speaker to work properly, but what sort? How do we decide the dimensions and design of a speaker cabinet, and how will this change depending on the loudspeaker being used?
ENCLOSURE DESIGN
INFINITE OPEN BAFFLE
For low-frequency reproduction, the ideal speaker enclosure would not be an enclosure at all, but a flat baffle of infinite size, allowing free movement of air on either side of the speaker but preventing any mixing, or interference, between the waves produced by the front and back. Because most of the math about speaker design is based on these ideal engineering scenarios, speakers are tested in a large open baffle, meaning that when you look up the frequency response chart of a given speaker, the information you see will describe the measured performance of the speaker under these near-ideal conditions. It is up to the designer to choose the correct speaker and design the correct enclosure in order to get good real-world performance from a given loudspeaker.
FINITE OPEN BAFFLE
With a finite baffle, sound waves from the back of the speaker travel around the edges of the baffle to interfere with those from the front. This is most pronounced with sound waves that have wavelengths that approach or exceed the dimensions of the baffle, meaning that for any reasonably-sized open baffle there will be substantial interference with the low frequencies. For example, the A string of a guitar produces a fundamental of 110 Hz, which has a wavelength of roughly 10.2 feet. The low E string produces a fundamental of 83 Hz, a wavelength of 13.6 feet. Since it is obvious that a speaker baffle greater than 14 feet wide is wildly impractical, it is clear that some sort of engineered enclosure is required to prevent interference, especially when dealing with subwoofers and bass-guitar speakers.
SEALED/CLOSED ENCLOSURE
A sealed enclosure would seem to solve all the of the issues inherent with the finite open baffle, but it comes with its own challenges, requiring much more careful engineering. Since the back is sealed, there can be no interference, so comb filtering is no longer an issue. However, since there is nowhere for the air to go inside of a sealed cabinet, the movement of the speaker cone causes compression and decompression of the air trapped inside. This causes an equal and opposite reaction of the air pushing back on the cone, with the air acting as a spring, causing increasing force in opposition the further the speaker cone moves. This mechanical speaker damping causes a significant decrease in low-frequency response below the resonant frequency of the box, although this can be tuned by adding fibrous insulation inside the cab, such as fiberglass, to slow the propagation of the sound waves.
PORTED (BASS-REFLEX) ENCLOSURE
This is something of a hybrid of the open baffle and the sealed enclosure. The idea is that a sealed cabinet with small, engineered "ports" or tuned openings, can have the advantages of the sealed cabinet but with improved bass response. Properly tuned ports can even cause the waves from the rear of the speaker to sum additively with the waves from the front at certain target frequency ranges, dramatically boosting the bass response possible from small speakers and compact cabs. This is the technique used for almost all modern subwoofer enclosures, since it can produce impressive low bass from a compact enclosure. However, the boost provided by these ports is imprecise, and only linear in a very narrow frequency range, which can impact the balance and quality of the resulting sound and requires EQ correction on the amplifier.
For low-frequency reproduction, the ideal speaker enclosure would not be an enclosure at all, but a flat baffle of infinite size, allowing free movement of air on either side of the speaker but preventing any mixing, or interference, between the waves produced by the front and back. Because most of the math about speaker design is based on these ideal engineering scenarios, speakers are tested in a large open baffle, meaning that when you look up the frequency response chart of a given speaker, the information you see will describe the measured performance of the speaker under these near-ideal conditions. It is up to the designer to choose the correct speaker and design the correct enclosure in order to get good real-world performance from a given loudspeaker.
FINITE OPEN BAFFLE
With a finite baffle, sound waves from the back of the speaker travel around the edges of the baffle to interfere with those from the front. This is most pronounced with sound waves that have wavelengths that approach or exceed the dimensions of the baffle, meaning that for any reasonably-sized open baffle there will be substantial interference with the low frequencies. For example, the A string of a guitar produces a fundamental of 110 Hz, which has a wavelength of roughly 10.2 feet. The low E string produces a fundamental of 83 Hz, a wavelength of 13.6 feet. Since it is obvious that a speaker baffle greater than 14 feet wide is wildly impractical, it is clear that some sort of engineered enclosure is required to prevent interference, especially when dealing with subwoofers and bass-guitar speakers.
SEALED/CLOSED ENCLOSURE
A sealed enclosure would seem to solve all the of the issues inherent with the finite open baffle, but it comes with its own challenges, requiring much more careful engineering. Since the back is sealed, there can be no interference, so comb filtering is no longer an issue. However, since there is nowhere for the air to go inside of a sealed cabinet, the movement of the speaker cone causes compression and decompression of the air trapped inside. This causes an equal and opposite reaction of the air pushing back on the cone, with the air acting as a spring, causing increasing force in opposition the further the speaker cone moves. This mechanical speaker damping causes a significant decrease in low-frequency response below the resonant frequency of the box, although this can be tuned by adding fibrous insulation inside the cab, such as fiberglass, to slow the propagation of the sound waves.
PORTED (BASS-REFLEX) ENCLOSURE
This is something of a hybrid of the open baffle and the sealed enclosure. The idea is that a sealed cabinet with small, engineered "ports" or tuned openings, can have the advantages of the sealed cabinet but with improved bass response. Properly tuned ports can even cause the waves from the rear of the speaker to sum additively with the waves from the front at certain target frequency ranges, dramatically boosting the bass response possible from small speakers and compact cabs. This is the technique used for almost all modern subwoofer enclosures, since it can produce impressive low bass from a compact enclosure. However, the boost provided by these ports is imprecise, and only linear in a very narrow frequency range, which can impact the balance and quality of the resulting sound and requires EQ correction on the amplifier.