Understanding Speaker Driver Specifications 101

If you are into high-end home theater, check out our Display and Audio Calibration Guides to maximize your experience.

Intro

Speaker drivers, which transform electrical signals into the sounds we hear, are the most critical component of any speaker system. For speaker designers, selecting the perfect driver from the multitude available is a crucial challenge. This decision is one of the first and foremost steps in the design process, coming right after defining the overall goals for the speaker system.

In this article, we will explore some of the essential basic specifications of speaker drivers and share insights from our experience selecting drivers for our DIY speakers.

Driver Type

Let’s discuss what the most common speaker drivers out there are, to help you understand some of the specifications that we talk about in this post. Each type of driver is optimized for a specific range of frequencies and has unique characteristics and specifications.

• Cone drivers are the most common type of speaker driver and are typically used for mid-range and bass frequencies. The cone shape allows for efficient movement of air, producing sound waves over a broad frequency range. Cone drivers are versatile and can be found in a variety of speaker systems, from small bookshelf speakers to large subwoofers.
  • Applications: Woofers (low frequencies), mid-range drivers (mid frequencies), full-range drivers (broad frequency range).
  • Key Specifications: Size, material, frequency response, sensitivity, impedance, power handling.
  • Material: Influences the sound quality, durability, and efficiency. Common materials include paper, polypropylene, Kevlar, and aluminum.
  • Size: In general, the larger the cone, the more efficient the driver.

• Dome drivers are typically used for high-frequency reproduction. The dome shape allows for a wider dispersion of sound compared to cone speakers, providing a broader and more even soundstage. Dome drivers are good at producing the high-frequency details, but are ineffective at low frequencies.
  • Applications: Tweeters (high frequencies).
  • Key Specifications: Material, size, frequency response, sensitivity, impedance, power handling.
  • Material: Affects the high-frequency performance and durability. Common materials include silk, textile, metal (such as aluminum or titanium), and composite materials. Silk and textile domes often produce a smoother, more natural sound, while metal domes can offer greater detail and accuracy but may sound brighter. Each material has its own unique sound signature.

• Horn drivers consist of a driver (usually a compression driver) and a horn structure to amplify and direct the sound from the driver. This design increases the efficiency and output of the driver, making horn drivers ideal for applications requiring high sound pressure levels (SPL), such as public address systems, theaters, and concert venues. Horn drivers can cover a wide frequency range depending on their design, but are commonly used for mid to high frequencies. They are also harder to design because both the driver and the horn affect the sound signature, while the horns tend to take up a lot of space.
  • Applications: Midrange (mid frequencies) and tweeters (high frequencies) in professional audio systems and some high-end home audio systems.
  • Key Specifications: Horn shape and size, driver material, frequency response, sensitivity, impedance, power handling.
  • Driver Material: Similar to other types of drivers, the material affects performance and durability. Horn drivers often use compression drivers with diaphragms made from materials like titanium, aluminum, or phenolic.
  • Horn Shape and Size: Influence the directionality and dispersion of the sound. Larger horns can control lower frequencies and provide a more focused sound, while smaller horns are typically used for high frequencies and offer wider dispersion.

These are the most common type of drivers out there, but there are additional options available such as planar magnetic drivers, electrostatic drivers, and ribbon drivers. Each of these have their own unique set of characteristics and applications.

Frequency Response

One of the major characteristics of a speaker driver is how well it handles producing different frequencies. The frequency response refers to the range of frequencies a speaker driver can reproduce, from the lowest bass notes to the highest treble notes. This range is typically measured in Hertz (Hz) and there are two common ways that the frequency response is displayed:

• Text: Most manufacturers will display the frequency response range by text, which basically states which frequencies the speaker can reproduce without a significant change in level or distortion.

• Graph: This displays how the speaker produces each frequency and can be harder to find for some drivers. This is usually shown with the dB scale on the y-axis and the frequency on the x-axis. When designing speakers, it is highly recommended that you have access to the frequency response graph before you pick a driver, as it can provide a good amount of information on how the driver will sound and act throughout the entire range.

Example of woofer’s frequency response

A flat frequency response over the range that you want the driver to operate is generally desirable because it indicates that the speaker can accurately reproduce sounds across the entire audio spectrum without favoring or neglecting any particular frequency range. However, different types of drivers specialize in different parts of the spectrum. Woofers are designed to handle lower frequencies, and their frequency response typically degrades at higher frequencies. Conversely, tweeters are optimized for high frequencies so their response tends to be smoother at those frequencies, but weaker in the lower ranges

Power Handling

Power handling is a crucial specification that indicates how much power a speaker driver can handle without being damaged. Power handling is usually specified in two ways: RMS power and maximum power. This specification is usually expressed in Watts (W), which is the unit of measure for electrical power.

• RMS Power: RMS (Root Mean Square) power, often referred to as continuous power, is the amount of power a speaker driver can handle on a continuous basis without overheating or suffering damage. This specification provides a realistic measure of the speaker’s capability, as it reflects the power the driver can handle during prolonged use. Usually, this is the power you design for as any power over this could risk damage to the driver.
• Max power: Maximum power, sometimes called peak power, refers to the highest amount of power a speaker driver can handle in short bursts (less than one second) without being damaged. This specification could come into play for handling dynamic peaks in audio signals such as loud transients in music or sound effects in movies. However, it is important to note that continuous operation at or near the maximum power rating is not advisable, as it can lead to distortion or permanent damage to the driver.

Impedance

Speaker impedance refers to the opposition a speaker presents to electrical current flow from an amplifier, measured in Ohms (Ω). Unlike regular resistance, speaker impedance changes with frequency due to the nature of audio signals (AC current). Manufacturers provide a nominal impedance, or an average value for reference. Common ratings are 4Ω, 6Ω, 8Ω, and 16Ω, which are crucial for ensuring compatibility between speakers and amplifiers.

Lower impedance (e.g., 4Ω) allows more current to flow, which can produce louder sound. However, this increased current flow can be demanding for the amplifier, raising the risk of overload and excessive heat generation. This requires careful management through speaker design to ensure the amplifier can handle the load without compromising performance or longevity.

Higher impedance (e.g., 8Ω) restricts the current flow, which can make the amplifier’s job easier by reducing the risk of overload and excessive heat generation. This typically results in a more controlled and stable performance but might produce less output volume compared to lower impedance systems.

Another factor to consider is speakers with a crossover, as all passive components, like resistors and inductors, will have to be able to work with the driver’s impedance.

There is a common misconception that speakers maintain their rated impedance across all frequencies. This is not the case! While the specification sheet provides an average value (nominal impedance), the actual impedance fluctuates with different frequencies. The real burden on the amplifier depends on the minimum impedance, not the average. This is why it is crucial for the speaker’s minimum impedance to fall within the amplifier’s capabilities to ensure optimal performance and to prevent overload.

Author’s Note: An international standard (IEC 60268-5) ensures some level of consistency: a speaker’s minimum impedance must be at least 80% of its nominal rating. For instance, an 8Ω speaker should not drop below 6.4Ω.

Impedance curve of the 4ohm driver

A speaker’s impedance curve, which shows how impedance changes with frequency, can indicate how accurately the speaker produces sound across its range. For example, a large spike in the curve represents the resonant frequency, where the driver is most efficient at producing sound. This resonance is crucial for designing the speaker enclosure. Multiple spikes and dips in the curve suggest additional resonant frequencies, making it harder to achieve a predictable response. A smooth impedance curve is generally more desirable, as it indicates consistent and predictable driver performance.

Sensitivity (Efficiency)

Sensitivity, also known as efficiency, is a measure of how effectively a speaker driver converts electrical power into sound. It is usually expressed in decibels (dB) and indicates how loud the speaker will be for a given input power, typically measured at 1 watt of power at a distance of 1 meter.

A higher sensitivity rating means the speaker will produce more sound for less power, which is advantageous in many situations, especially when trying to achieve high volume levels without requiring a large and powerful amplifier. Most speakers will tend to be around the 87-92dB range, but there are speakers that go above and below that number.

For example, a speaker with a sensitivity rating of 88dB at a distance of 4 meters will require about 500W to reach reference peak levels (105dB). In contrast, a speaker with a sensitivity of 95dB will only need about 130W to achieve the same volume. This is why general higher sensitivity speaker drivers are desired as it puts less strain on the amplifiers and often helps reduce the cost of the amplifiers as well.

For our speaker designs, we prioritized creating highly efficient speakers with a sensitivity rating of over 90dB. This allows them to be powered by an AVR and still reach high volumes. Therefore, sensitivity was one of our major factors when designing our speakers.

Directivity

Directivity refers to how a speaker driver’s frequency response changes at off-axis angles. A speaker with wide directivity maintains consistent amplitude (sound pressure level, SPL) between on-axis and off-axis sound. Conversely, a speaker with narrow directivity shows significant differences in amplitude between on-axis and off-axis sound.

Example of a woofer’s directivity response curve

Using the image above as an example, you can see that after about 2 kHz, this speaker’s off-axis response (red) starts to deviate significantly from the on-axis response (blue). This indicates that if you listen to this speaker at frequencies above 2 kHz and move around the driver, the sound will change.

When designing a speaker, it is preferable for the speaker to sound the same both on-axis and off-axis. However, the driver is not the only factor that affects the speaker’s directivity. Other factors include the design of the speaker enclosure, the crossover network, and the positioning of the drivers. All these elements contribute to how evenly sound is dispersed in different directions, impacting the overall listening experience.

To learn more about directivity, Erin from Erin’s Audio Corner has a great video detailing how to read these graphs in detail.

Enclosure Compatibility

The type of enclosure a driver is designed for greatly impacts the overall sound profile and efficiency of the speaker system. You will typically see this on woofers or speaker drivers that are capable of producing low frequencies. The enclosure refers to the type of box you put the speaker driver into. Different enclosures influence the speaker’s bass response, efficiency, and overall tonal balance.

• Sealed Enclosures: Also known as acoustic suspension enclosures. These speaker boxes have no holes, meaning they are sealed from the space around them. Sealed boxes tend to have less bass extension but a more gradual bass roll-off, and are typically smaller in size compared to a ported enclosure for the same driver.
• Ported Enclosures: Also known as bass reflex enclosures. These designs have holes called ports that are designed to increase the efficiency of the speaker at lower frequencies. Ported enclosures tend to have lower bass extension but a much steeper roll-off below the tuning frequency compared to sealed designs. Ported enclosures are also more difficult to design as the port adds complexity to the speaker.
• Infinite baffle: Infinite baffle designs aim to eliminate the rear wave interference from the driver by using a large baffle or wall, effectively creating an “infinite” enclosure. This way, the waves created by the back of the speaker never meet the waves created by the front of the speaker, so no resonances and no diffraction. With a good speaker, this sounds like the recipe for a great audio system. However, this is usually impractical in most applications, therefore, we do not see many designs for this type of enclosure.
• Other Enclosures: These include bandpass and transmission line designs, each with unique acoustic properties and design complexities. Bandpass enclosures are designed to produce very specific frequency ranges, while transmission line enclosures use long, folded paths to improve bass response and reduce distortion. These are usually way more complicated to design and build.

Sometimes manufacturers will include suggestions for the enclosure type, indicating what type of enclosure they believe is best for their product. This is not a mandatory rule; if a manufacturer suggests a sealed enclosure, it does not mean the speaker cannot work in a ported enclosure. It is a recommendation based on the speaker parameters and what they think will work best.

EBP Calculation: The Efficiency Bandwidth Product (EBP) is used to determine the best type of enclosure for a driver. It is calculated by dividing the driver’s resonant frequency (Fs) by its electrical Q (Qes). An EBP around 50 suggests a sealed enclosure, while an EBP above 100 indicates a ported design. Understanding EBP helps in selecting the most suitable enclosure to match the driver’s characteristics and desired sound profile. Again, this is not a hard and fast rule, but merely a way to estimate the characteristics of a driver in a certain enclosure type.

Thiele-Small Parameters:

Thiele-Small parameters are a set of electromechanical specifications that define the performance characteristics of a loudspeaker driver. These parameters, named after engineers A. Neville Thiele and Richard H. Small, include values such as the resonant frequency (Fs), total Q factor (Qts), and equivalent compliance volume (Vas), among others. They provide critical information about how a speaker driver will interact with an enclosure and the acoustic environment.

While Thiele-Small parameters might not be of immediate concern for casual listeners, they are crucial for speaker builders. These parameters accurately predict the driver’s behavior in various enclosure types, allowing for precise tuning of the speaker system to achieve the desired sound quality and performance. We will explain each parameter in an upcoming article, which will be linked here when available. For now, understand that these specifications enable designers to perform simulations with software, modeling the speaker’s response before it is ever built.

To Wrap it Up

This overview does not cover every speaker specification that exists; in fact, there are many more factors to consider when designing a speaker. Covering them all here would make this post overly long and technical. However, the specifications discussed here are the most basic and essential ones that should be considered in the beginning stages of speaker design. In future articles, we may delve deeper into the Thiele-Small parameters and other advanced specifications.

If you are looking to get into the DIY space yourself or just want to learn more about speakers, we hope that this post helped you get a basic understanding of what each of the driver’s specifications mean.

Thank you for reading. If you are into home theater, do not forget to check out our Display and Audio Calibration Guides to maximize your experience.