Graph courtesy of jensentone.com

Many years ago, while playing on an album project at Eldorado Studios in Hollywood, I had a discussion with the great producer Dave Jerden about mixing. He told me that a mix is like a painting, where the canvas is a fixed size and can only hold so much color. Once you’ve filled up that space, the only way to add more of a particular color is to remove some of another to make space for it. As it turns out, this is a great analogy for the way speakers behave in guitar amplifiers.

A speaker has a finite set of operating parameters—frequency response, power-handling capacity, and efficiency—and we’ve spent a lot of time discussing the first two here in Speaker Geeks. But that last one is a clincher and needs a little attention.

Understanding efficiency helps us understand the limits of tonal behavior. We don’t normally see “efficiency” listed on most spec sheets, at least in a form we would recognize. What we do see is something called “sensitivity,” and it’s this quality that we often associate—or confuse—with efficiency. It’s easy to conflate these two because, in fact, they’re closely related by what’s known as the law of conservation of energy. Gosh, that sounds serious!

Actually, the law of conservation of energy is a fundamental concept of physics that simply states that the total amount of energy remains constant in an isolated system. It implies that energy can neither be created nor destroyed, but can be changed from one form to another. What that means in lay terms is that a 100-watt light bulb takes 100 watts of energy in the form of electrical current and changes some of it to light. Whatever electrical current didn’t get turned into light got turned into something else. If you’ve ever changed a light bulb right after it conked out and burned your fingertips, you learned the energy that didn’t go to produce the light was instead turned into heat, courtesy of the law of conservation of energy.

All right, how’d we get way over here?

Think of a speaker as an energy-conversion device. It takes electric power modulated by a conglomeration of frequencies (audio signal) and converts it into physical motion modulated by that same conglomeration of frequencies. In very simplistic terms, if 50 percent of that electrical power actually becomes sound, and 50 percent is lost somewhere along the way, the efficiency of the speaker is 50 percent.

In reality, speaker efficiency is much lower than this, but for our purposes, this is irrelevant. In fact, when it comes to guitars speakers, efficiency in general is pretty much irrelevant since there is very little actual difference in mechanical behavior and energy conversion across guitar speakers, generally. What that means in the real world is that you already have a sense of how loud a given amp is at a given volume and the small differences in efficiency between one guitar speaker and another isn’t going to make a dramatic difference in the volume of your amp. What does distinguish speaker personality in ways gear freaks can relate to is sensitivity.

Sensitivity is how much sound pressure level (SPL) can be measured at a specific distance away from the speaker, at a specified amount of power. Sensitivity is typically measured as X dB @ 1 kHz @ 1 watt @ 1 meter. So standing roughly three feet away from a speaker being driven by a single frequency (as opposed to a full range of frequencies) at 1 watt, you might look at your dB meter and see a number like 98. That’s the sensitivity rating of the speaker.

All efficiency tells us is that a speaker will convert our fixed amount of amplifier power to a fixed amount of sound. Sensitivity tells us how the speaker will behave at various frequencies at a given volume.

Now, there is a mathematical equation that will convert sensitivity to efficiency. So why don’t they just do that and save us all a lot of confusion? Because sensitivity is the real indicator of a speaker’s tonal performance. All efficiency tells us is that a speaker will convert our fixed amount of amplifier power to a fixed amount of sound. Sensitivity tells us how the speaker will behave at various frequencies at a given volume.

In my previous installment, I took a little heat from a commenter for committing the treasonous act of referring to speaker sensitivity as efficiency. What’s really at issue, in my opinion, is not what the terms actually mean, but that we’re just trying to get a sensible answer to a simple question along the lines proposed by one of Premier Guitar’s senior editors, Andy Ellis: “If I want to make my Fender Deluxe louder, can I simply replace the stock speaker with one that has greater sensitivity? Or should I move to a 35-watt amp instead?”

I hate to say it, but to answer that question I need to ask you how loud you normally play, how much tonal difference you’re willing to accept, and how much more weight you feel like flinging around, because sometimes trying to explain this in terms of the physics just doesn’t fire up the mental light bulb.

To reiterate, sensitivity is highly conditional: remember that 1 kHz @ 1 watt @ 1 meter business? Wouldn’t it be better to use a more realistic measurement, like, say, 100 watts or 50 watts or even 10 watts? If you’re wrapping your head around this, you might already be thinking that to compare a large cross section of speakers, we need a good common denominator. And when you’re looking for a large common denominator, you are, for better or worse, usually looking for a smaller number.

As we learned in previous installments of Speaker Geeks, we use 1 watt because we need a low power level to obtain a test result that isn’t polluted by the speaker’s mechanical limitations or the test environment. And we use 1 kHz because each unique speaker responds differently at different frequencies. A certain speaker, for example, may have what appears to be a low sensitivity number overall, but exhibits an increased low-mid response on the graph. That speaker may sound punchier in the low mids, even though the average power-to-sound ratio didn’t change.

For example, take a look at this interactive spec sheet for the Jensen P12N. This 12" speaker has a sensitivity rating of 100 dB. Hover your cursor over the speaker graph at 1 kHz and bingo—there’s the 100-dB sensitivity (see chart). Now move the cursor around and notice how drastically the sensitivity changes with frequency. The sensitivity goes from a relatively low 86 dB @ 1.6 kHz to a whopping 109 dB @ 3.6 kHz. That’s a pretty radical change and clearly demonstrates how misleading the average figure can be when you’re looking to fine-tune your amp’s behavior.

Also, there’s the nexus between sensitivity and efficiency. For a speaker to deliver on its technical parameters, it must be operated within those parameters. But guitar players could care less about that. We just want to know how it’s going to sound at the threshold of, “Oh look, there’s a trail of smoke coming out of my grille cloth!”

You’re going to push a tube amp harder if the new speaker’s sensitivity is lower because your sound is the sum of the behavior of the speaker coupled with the behavior of the amp, which is determined by the behavior of the tubes, which are impacted by the electrical feedback from the speaker back to the amp, and the hip bone’s connected to the … you get the idea. When the volume goes up and the amp starts to max out, the tone flattens out. The “sweet spot” on your amp is the point where the volume is at a comfortable level, the power amp stage is starting to saturate, and the speaker’s frequency peaks and dips tend to compress slightly.

A higher efficiency speaker will allow you a little more volume before you max out, but not that much. If the sensitivity is higher, your amp will sound louder at low volume (especially around the speaker’s frequency peaks), which may be a godsend until you get to actual gig volume. That’s when you find out that your amp previously hit its sweet spot with the volume at 5, but now does so at 3. If the new speaker is a lot more efficient, you’re probably expecting a visit from the sound engineer any minute.

If the speaker is less efficient, the entire balance of the amp’s sweet spot and volume level has changed, and you possibly ended up in more or less the same spot as you were before the swap. If you’re lucky, maybe the midrange emphasis just shifted up or down a bit overall and that’s exactly what you were hoping for. That’s not a hard rule, but is more often than not the case because speakers that are designed for optimum efficiency are generally not guitar speakers.

A speaker that effortlessly converts electrical energy to sonic energy does so because a lot of purposeful design considerations were brought to bear for that to happen. Unfortunately, that great product graveyard in the sky is full of examples of very efficient, durable speakers that we guitarists didn’t take too kindly to. On the other hand, we all have a place in our hearts for a speaker that has a pleasant personality, makes us sound like rock stars, looks cool, and, to some, smells good when pushed to its limits (my hand raised).

So, Andy … the answer is that changing the speaker will alter the amp’s personality, but unless you’re willing to accept a significant departure from the sound you prefer, it likely won’t solve your power deficiency. So, get a bigger amp? A higher-powered amp could solve the power problem, but it will definitely alter the sweet spot and may not deliver the tone and feel you’re after, so you may soon find yourself shopping for speakers that will do the opposite of what you were trying to accomplish with the smaller amp.

The truth is, there’s never a perfect answer, and that’s what keeps us in hot pursuit of perfection. There are clever solutions that help get us a lot closer to our ideal tone and power balance. And the next time it’s my turn to author this column, I’ll tell you all about them!