The Short Answer and Why It Matters So Much
Let’s get straight to the point and answer the core question that likely brought you here: The Shure SM58 is definitively not an omnidirectional microphone. It is, and has always been, a cardioid microphone. While this might seem like a simple technical detail, it’s actually the single most important factor behind its legendary status and why it has dominated live stages for decades. Understanding this distinction isn’t just trivia; it’s the key to using the SM58 effectively and achieving professional-sounding results.
So, if you’ve ever wondered why the SM58 sounds great for a lead singer but maybe not for capturing the sound of an entire room, you’re about to find out. This article will take a deep dive into the SM58’s polar pattern, explaining not just *what* it is, but *why* it’s designed that way and what it means for you and your sound.
Understanding Microphone Polar Patterns: What Does “Omnidirectional” Even Mean?
Before we can fully appreciate the SM58’s design, we need to quickly cover the basics. A microphone’s polar pattern (or pickup pattern) is essentially a map that shows how sensitive it is to sounds coming from different directions. Imagine a 360-degree sphere around the microphone; the polar pattern illustrates where the mic is “listening” most effectively.
There are several types of polar patterns, but for this discussion, we’ll focus on the two most relevant ones.
- Omnidirectional: The word “omni” means “all,” and that’s exactly what this pattern does. An omnidirectional microphone captures sound equally from all directions—front, back, and sides. Think of it like a bare light bulb, casting light evenly in every direction. It’s fantastic for capturing the total ambiance of a space or a group of people talking around a table.
- Cardioid: This is the pattern of the Shure SM58. It gets its name from its heart-like shape on a polar pattern chart. A cardioid microphone is most sensitive to sound coming from directly in front of it and least sensitive to sound coming from the rear. You can think of this one like a flashlight; it brightly illuminates what it’s pointed at, while the area behind it remains dark.
While other patterns like supercardioid, hypercardioid, and figure-8 exist, the fundamental difference between the SM58’s focused cardioid pattern and a wide-open omnidirectional pattern is the most critical concept to grasp.
The Heart of the Matter: The SM58’s Cardioid Polar Pattern Explained
So, we’ve established the Shure SM58 has a cardioid polar pattern. What does this actually mean in a real-world scenario, like on a noisy stage? It means everything. This specific design choice provides two monumental benefits that are crucial for live sound reinforcement.
- Incredible Source Isolation: When a singer is using an SM58, the microphone is primarily focused on their voice. It’s actively “rejecting” or ignoring, to a large degree, the other sounds on stage. The thunderous drums behind the singer, the wailing guitar amp to the side, and the cheering crowd in front are all picked up at a significantly lower volume than the voice aimed directly into the front of the mic. This allows a sound engineer to make the vocals clear and present in the main mix without also turning up the volume of the entire band.
- Excellent Feedback Rejection: This is arguably the most celebrated feature of the SM58’s cardioid pattern. Feedback is that awful, high-pitched squeal that happens when a microphone picks up the sound from a speaker that is amplifying that same microphone. Since stage monitors (the speakers pointing back at the performers) are typically placed on the floor in front of a singer, they are firing sound directly into the *rear* of the microphone. Because a cardioid pattern is least sensitive at its rear (its “null point”), the SM58 naturally ignores much of the sound from the monitor, allowing for much higher volume before feedback occurs. This is a lifesaver in live situations.
The Engineering Genius: Why is the SM58 a Cardioid Microphone?
The SM58’s cardioid pattern isn’t a happy accident; it’s a masterpiece of acoustic engineering centered around its famous Unidyne III capsule. To understand why it’s directional, we have to look at how it works on a physical level.
Omnidirectional microphones are typically pressure-operated transducers. Their diaphragm (the thin membrane that vibrates with sound) is sealed at the back. It only reacts to changes in sound pressure at its front, regardless of where that pressure change originated. This is why it picks up sound from all directions equally.
The cardioid SM58, on the other hand, is a pressure-gradient transducer. This is the clever part. The diaphragm is not sealed at the back. Instead, the capsule is designed with an intricate system of openings, or rear entry ports, that lead to an acoustic labyrinth inside the microphone’s handle. Here’s how it creates the cardioid pattern:
- Sound from the Front (0°): Sound waves hit the front of the diaphragm directly, causing it to move.
- Sound from the Rear (180°): Sound waves from the back of the microphone have to travel around the mic to hit the front of the diaphragm. At the same time, they also enter the rear ports and travel through the internal acoustic labyrinth to hit the *back* of the diaphragm. The labyrinth is precisely engineered so that this journey takes the exact amount of time for the sound wave to arrive at the back of the diaphragm perfectly out of phase with the sound wave hitting the front.
When two sound waves that are perfectly out of phase meet, they cancel each other out. This acoustic cancellation is what creates the “dead zone” or null point at the rear of the microphone, giving it its cardioid directionality. It’s a brilliant, purely mechanical solution that makes the SM58 an expert at rejecting sound from behind.
Omnidirectional vs. The SM58’s Cardioid Pattern: A Practical Comparison
To make the differences crystal clear, let’s compare the features of the SM58’s cardioid pattern directly against a true omnidirectional microphone in a handy table.
| Feature | Shure SM58 (Cardioid) | True Omnidirectional Microphone |
|---|---|---|
| Directionality | Picks up sound primarily from the front, rejects sound from the rear. | Picks up sound equally from all directions (360°). |
| Feedback Rejection | Excellent. The rear null point is ideal for use with stage monitors. | Very poor. Prone to feedback in live sound reinforcement settings. |
| Stage Bleed / Isolation | Good. Isolates the intended sound source from other nearby sounds. | Poor. Captures all nearby sounds, including bleed from other instruments. |
| Proximity Effect | Pronounced. Bass frequencies are boosted as the mic gets closer to the source. | Virtually none. The tonal character does not change with distance. |
| Handling Noise | Lower, thanks to its internal pneumatic shock mount designed for handheld use. | Generally more sensitive to handling and wind noise if not designed for it. |
| Off-Axis Coloration | Some. Sounds from the side may sound slightly different (less bright) than sounds from the front. | None. The tone is consistent regardless of the sound’s angle of arrival. |
| Typical Application | Live vocals, miking guitar amps, snare drums—any situation requiring isolation. | Capturing room ambiance, recording acoustic ensembles, broadcast interviews, lavalier mics. |
A Special Note on Proximity Effect
The proximity effect mentioned in the table is a crucial characteristic of the SM58 and nearly all directional mics. It’s the phenomenon where bass frequencies increase as the microphone moves closer to the sound source. Singers often use this to their advantage, moving closer to the mic for a warmer, more intimate sound and pulling back for a brighter, less boomy tone. An omnidirectional microphone does not exhibit a proximity effect, so its bass response remains consistent regardless of distance. This is another fundamental difference stemming directly from the polar pattern design.
When Would You Actually Want an Omnidirectional Microphone?
Given the SM58’s success, you might wonder why anyone would ever choose an omnidirectional microphone. Well, in the right context, an omni mic is a far superior tool. You might want one if you are:
- Recording a Room’s Ambiance: Trying to capture the beautiful, natural reverb of a concert hall or church. An omni mic will capture this space in a much more realistic and immersive way than a directional mic.
- Miking a Round-Table Discussion: For a podcast or meeting, a single omni microphone placed in the center of a table can capture everyone’s voice clearly.
- Recording a Choir or Acoustic Ensemble: When you want to capture the sound of the entire group as a cohesive whole, rather than isolating individual members, an omni mic is perfect.
- Using a Lavalier (Lapel) Mic: Many lavalier mics are omnidirectional so that they still pick up the speaker’s voice clearly even if they turn their head away from the microphone.
Common Misconceptions and FAQs about the SM58 Polar Pattern
There’s a lot of chatter about the SM58, and some common questions and misconceptions pop up frequently regarding its pickup pattern.
“But it feels like my SM58 picks up everything on stage!”
This is a common feeling, but it’s a matter of degree. While the SM58 is excellent at rejecting off-axis sound, it’s not a magical sound eraser. A very loud sound source, like a crashing cymbal or a loud guitar amp, will still be picked up even if it’s at the side or rear of the mic—it will just be significantly quieter than the on-axis source. The cardioid pattern provides attenuation (volume reduction), not complete elimination.
“Can I make my SM58 omnidirectional?”
The simple and firm answer is no. The cardioid pattern is an integral part of the microphone’s physical construction. Some people mistakenly believe that covering the rear entry ports on the capsule with tape will make it omnidirectional. Do not do this! Blocking these ports will destroy its directional characteristics, but not in a useful way. It will disrupt the delicate pressure-gradient system, severely compromising its sound quality, frequency response, and transient response. You won’t get an omni mic; you’ll just get a broken-sounding cardioid mic.
“Is there an omnidirectional version of the Shure SM58?”
No, Shure does not produce an omnidirectional version of the SM58. The microphone’s entire identity and purpose are built around its cardioid pattern for live performance. If you need a rugged, handheld dynamic microphone that *is* omnidirectional, you would look at different models specifically designed for that purpose, such as the Electro-Voice RE50/B, which is a standard in the broadcast journalism world for its omnidirectional pickup and low handling noise.
Conclusion: The SM58 is Cardioid for a Very Good Reason
So, to bring it all home: Is the SM58 omnidirectional? Absolutely not. It is a purpose-built cardioid microphone, and that’s its greatest strength.
Its heart-shaped polar pattern is the direct result of brilliant engineering that allows it to focus on a single sound source while simultaneously rejecting unwanted background noise and feedback-inducing monitor wash. This directional nature is precisely what has made it the most trusted and widely used microphone for live vocals across the globe. Far from being a limitation, its cardioid pattern is the very soul of the SM58. By understanding and respecting its directionality, you can harness its true power and join the millions of performers and engineers who rely on it every single day.