The Complete Guide to Speaker Selection

Speakers are critical in audio systems, functioning as transducers that convert electrical signals into the audible sound waves a user experiences. To fully understand their role in sound reproduction, it's essential to explore the engineering principles behind their operation and the key components involved. This article offers a detailed examination of speaker technology, including fundamental operation principles, important specifications, design considerations, types of cones and magnets, and more.

Speaker basics

Sound production in a speaker is a carefully engineered process, leveraging the principles of electromagnetism and mechanical motion. It all starts with an electrical signal, which represents the audio content needing to be reproduced. This signal could come from various sources, such as a smartphone, laptop, or any other audio device designed to generate electrical impulses. Whether it's the melody of a song, the clarity of a podcast, or the immersive effects of a movie, this signal serves as the electrical blueprint of the auditory experience the speaker is about to create.

At the heart of the speaker are key components: the voice coil and the permanent magnet. The voice coil, typically a tightly wound wire, is connected to a flexible cone or diaphragm. This coil surrounds a permanent magnet, establishing the core of sound production. As the electrical signal passes through the speaker's wires, it energizes the voice coil, setting off a sequence of actions driven by electromagnetic forces. The electrical current generates a magnetic field around the coil, with its polarity shifting in accordance with the changing electrical signal.

The speaker's operation hinges on the interaction between these magnetic fields. As the fields fluctuate, they alternately repel and attract, causing the voice coil and its attached cone to move back and forth rapidly. This oscillating motion of the cone interacts with the air molecules around it, causing them to vibrate in harmony with the cone's movement.

These vibrations generate pressure waves in the air, which travel until they reach human ears, where the brain interprets them as sound. This completes the process of sound perception, transforming electrical signals into rich auditory experiences.

Figure 1: Working principle of a speaker. (Image source: Same Sky)

Exploring the anatomy of a speaker reveals several critical components that work together to produce sound: the cone, voice coil, plate, magnet, frame and yoke, and printed circuit board (PCB). The following illustrations highlight the internal structure of the speaker in a disassembled (Figure 2) and assembled (Figure 3) state.

Figure 2: Exploded view of speaker components. (Image source: Same Sky)

Figure 3: Assembled view of speaker components. (Image source: Same Sky)

Important speaker specs and considerations

With the foundational understanding of speaker operation and structure established, it's crucial to consider several key specifications and performance criteria when selecting a speaker. These parameters are vital in ensuring the speaker meets the desired audio performance and reliability standards.

• Sound Pressure Level (dB): Sound Pressure Level (SPL) quantifies the intensity of sound waves in the air, essentially indicating loudness. Distance from the sound source and environmental conditions both impact SPL. Decibels (dB) are the standard unit for measuring SPL. Understanding SPL is critical, not only for designing speakers but also for assessing noise levels in various environments, which is vital for noise control and regulation.
• Max Input (W): This specification indicates the maximum power the speaker can handle for a short period without sustaining permanent damage.
• Nominal Input (W): The nominal input wattage refers to the amount of power the speaker can safely handle during continuous, long-term operation. This is a key factor in ensuring the longevity and reliability of the speaker under normal operating conditions.
• Impedance (Ohm): This measures the resistance to the flow of current from the amplifier to the speaker. Lower impedance means the speaker will draw more power, so it’s important to match the speaker’s impedance with that of the amplifier. Proper impedance matching is crucial for optimal performance, efficient power transfer, and maintaining high-fidelity audio reproduction while preventing damage to the equipment.
• Resonant Frequency (Hz): This is the frequency at which the speaker vibrates most efficiently, providing insight into the low-frequency response of the speaker. Understanding resonant frequency helps in comparing different speakers' performance, particularly in the low-frequency range.
• Size (mm): The physical size of the speaker directly influences its frequency range. Smaller speakers are optimized for higher frequencies, while larger speakers perform better at lower frequencies. This relationship is key when designing systems for specific audio outputs, such as deep bass or clear mid-range vocals.
• Total Q: The Thiele-Small parameter, known as Total Q, serves as a broad reference for selecting the ideal enclosure type for a speaker. A Total Q value of 0.4 or lower suggests that the speaker is best suited for a vented enclosure. A value between 0.4 and 0.7 means a sealed enclosure is recommended, while 0.7 or higher indicates that the speaker is appropriate for free-air, semi-open back, or infinite baffle setups. However, these guidelines are not absolute, and it’s important to consider all relevant parameters in a design to achieve the desired audio performance.
• Ingress Protection: Speakers can come with a variety of ingress protection (IP) ratings for designs dealing with moisture and contaminants found in harsh environments.
• Mounting Style: Depending on the application need, speakers are available in a variety of connection styles, including through hole, wire leads, spring terminals, and more.

Figure 4: Popular speaker connection configurations. (Image source: Same Sky)

Measurements and tests for speakers

After selecting a speaker based on the key specifications mentioned earlier, it's important to conduct additional measurements and tests to ensure the speaker is properly integrated into the design and performs according to expectations. Same Sky’s audio design services offer support for these critical measurements and tests:

• Frequency Response: Visualizes how well the speaker reproduces the full range of audible frequencies. Same Sky’s two blog posts on audio frequency range and resonant frequency discuss frequency response charts in greater detail.
• Stepped Frequency Sweep: Targets alias frequencies for a more detailed frequency response analysis.
• Level & Gain: Determines the speaker’s energy output capacity. It is measured by dividing a device’s output level by its input level.
• Total Harmonic Distortion plus Noise (THD+N): Harmonic distortion occurs when unwanted tones are added to the audio signal, degrading sound quality. THD+N is a single number mark of performance, widely understood and accepted.
• Phase: Describes the time offset between a waveform and a reference signal. The two primary phase measurements in audio design are device input/output phase and interchannel phase, which ensures proper synchronization between multiple speakers in a system.
• Rub & Buzz: Detects higher frequency harmonic products generated in response to a low-frequency stimulus, which can help to identify defects causing unwanted noise.
• Thiele-Small Parameters: This set of measurements captures the complex impedance of a loudspeaker under test and provides electromechanical parameters that define the speaker's low-frequency performance. Thiele-Small parameters, including the total Q, are essential for designing speaker enclosures and for production testing, as they describe how the speaker will interact with its enclosure.
• Impedance: Measuring speaker impedance is crucial for understanding the resistance the speaker offers to the flow of an AC signal, such as audio from an amplifier. Impedance is measured in ohms and affects how much power the speaker will draw from the amplifier, as well as how efficiently it will perform.

Speaker cone and magnet types

The material of a speaker cone greatly affects sound quality. While data is useful, listening and testing are key. Durability varies by material: plastic is the most durable, followed by paper & cloth, and then foam. Lifespan also depends on humidity, environment, and application. Here's a summary of common cone materials:

• Plastic: Durable and resistant to dust and water, with precise manufacturing tolerances that reduce distortion and improve sound quality. Costs vary with thickness and production techniques.
• Paper & Cloth: Offer great sound and self-damping but can be affected by humidity. Made from wood fibers with additives, they are often used in larger speakers for their light weight.
• Foam: Rarely used alone, foam is combined with materials like metal or paper to enhance internal loss, resulting in a more natural sound.

Magnet type is also a crucial factor in a speaker's construction and performance. Here’s a summary of the most common types used in speakers:

• Ferrite: Low-cost and durable, with good corrosion resistance, but heavy and not ideal for portable use. They perform well near maximum capacity.
• AlNiCo: Offer a classic tone and are less prone to cracking but are more expensive. Less common today, they’re used in high-end applications.
• Neodymium (NdFeB): Lightweight with high field strength, ideal for small, high-SPL speakers. They offer good frequency response but are prone to shattering.
• Samarium Cobalt: Expensive but excel in corrosion resistance and temperature stability. They are less strong and prone to shattering.

Summary

Understanding speaker mechanics helps engineers create immersive audio experiences. This article highlighted the various components and specifications involved in selecting the right speaker. However, even with a solid grasp of these parameters, thorough testing and measurement in the final design are crucial. Same Sky offers a range of miniature and standard speakers, along with audio design services, to assist in this process.