Eminent Technology

The Science of Guitar and Guitar Speaker Sound

DESIGNING A NEW LOUDSPEAKER FOR A GUITAR

In this article, we'll look at a brief history of guitar speakers and what they use for drivers, plus what that means for guitarists, band-mates, and audiences.

Eminent Technology's research shows that their new driver design used in their Model Twenty offers several significant advantages over what has been used for the past 70 years.

A LOOK INTO THE PAST

Back in the Big Band Era of the 1930s and 1940s, the guitar on stage was primarily used as a rhythm instrument, accompanying the music or singer. Used acoustically, it was not able to compete in volume with the brass instruments, drums, piano, et al., the main complement of a dance band. Not until jazz player Charlie Christian began using pickups on his larger-bodied Gibson guitars was there a common way to feed the guitar's signal into an amplifier. At the time, most amplifiers were modified output sections of a radio, with only modest power output. After WW II, Big Bands began to wane, as it was difficult to have enough trained musicians and enough large, paying gigs to keep them all employed.

Les Paul pioneered the use of a solid body guitar, with his famous "log" guitar. A solid body guitar is far more resistant to feedback than the hollow body, pickup-equipped Gibsons of the era, such as the ES-150, ES-250, et al. Gibson denoted "ES" for their Electric Spanish guitars. As amplifiers grew in power, having an amplified hollow body guitar on stage led to more feedback issues--and at the time, distortion and feedback were undesirable to most guitarists and listeners.

So what were these early guitar amplifiers using for a speaker driver? Being a small, niche market, amp manufacturers had to make do with the components available at the time. Given the desire for more volume, these were usually cone drivers from console radios, such as Zenith, RCA, et al. used in their larger home or commercial (usually hotels) units, typically one or more drivers, 12 inches in diameter for the bigger models.

Vintage 12" Guitar Speaker

All of these radios were for reproducing AM radio, which by FCC decree, could only use a limited bandwidth, notably rolling off the high frequencies to better accommodate the inter-station spacing. Now imagine your guitar being played through an AM radio, and note how much of the tone would be missing. That's what the 12 inch cone drivers of the era were designed around, the designated narrow bandwidth of AM radio, not guitars.

Even as amplifiers progressed in design, remaining a small part of the market, manufacturers continued to use a conventional 12 inch cone driver. That continues today, even with "Class D" amplifiers you may have heard about. Many amplifier sections are much lighter than in the past, but in combo amps and separate speaker cabinets alike, nearly every manufacturer still uses the 60+ year old cone driver speaker, often right off-the-shelf.

In fact, most of the famous speaker manufactures, such as JBL and Celestion, had considerable growth back in the day making drivers for, you may have guessed it, console radios. Yes, they may have made bigger magnets, and stiffer suspensions in their guitar products, to take the load that many guitarists dished out, but essentially they're the same product. Note the bigger the magnet, usually the more efficient the speaker will be, but it also means more weight the guitarist, or, if you're lucky, a roadie, will have to lug to band practice, gigs, etc.

So what are the drawbacks that these 12 inch cone speakers have, that the Eminent Technology Twenty solves?

Let's dig into the technical details.

PLUCKING STRINGS WITH FINGERS

ET began by looking at the signal coming from the guitar, which includes the frequency response of the pickup, the frequency response of guitar amplifiers, the directivity (dispersion) of existing guitar speakers, power handling, speaker frequency response, and the distortion of existing speakers.

Electric guitar pickups are surprisingly good devices. Looking at the direct signal from the pickup, they have a usable frequency range from 80 Hz to beyond 6 kHz, with 80-90 dB of dynamic range (what a CD can produce) and very low noise.

Figure 1 above shows a guitar's high E string plucked with a fingertip. The vertical Y axis on the graph represents sound pressure level in dB--how strong in amplitude it is, with the horizontal X axis in Hertz (Hz) using logarithmic spacing commonly employed in audio graphs. You see the fundamental frequency--the base note, and just a few overtones with a finger pluck.

Most musical instruments have overtones in multiples of the fundamental frequency, both even 2x, 4x, etc., and odd multiples, 3x, 5x, etc. Our ear-brain system prefers the even-numbered multiple overtones, whereas the odd-numbered multiple overtones are perceived as sounding harsh and nasty.

The assortment of overtones also are how we recognize various instruments. A Gibson Les Paul Standard guitar's plucked G string sounds different than a Fender Stratocaster's plucked G string, even though they're tuned to the exact same fundamental frequency. Orchestral instruments usually use "tone color" or "timbre" to describe these characteristics.

STRING PLUCKED BY A PICK

In figure two, we see the same strings plucked, but this time by using a pick (formally called a plectrum)--note the stronger fundamental tones, as well as far more complex overtones. As expected, and what our ears tell us, is that a guitar pick elicits many more harmonics. This is a typical example, most pickups have high inductance, we would expect both the gain and upper frequency limit to be affected with different designs.

COMMON AMP FREQUENCY RESPONSE (WHO SCOOPED OUT MY MIDRANGE?)

Guitar Amplifier Electrical Frequency Response

Fender Marshall Vox tone control frequency response

Figure three shows the frequency response through three major amp brands: a Fender, a Marshall, and a Vox model AC-30. On the X axis, we have included open and fretted string positions, to show you where these strings' frequencies are on the scale. Note that the Marshall has a rise around the low open E string, and overall, the flattest response. Fender, most famous for having a scooped midrange, has the lowest point just above the open high E string; looking at the plot, you can even see why it got called a "scoop." This particular AC-30 was not measured with Vox's famous Top Boost circuit engaged.

Of course, much of these curves can be changed by adjusting the tone controls. The range of the tone controls may or may not allow the amplifier's response to be truly flat as it would be when the tone controls are centered on a home or pro audio amplifier. When we study the response of the amplifier and speaker together, the "scoop" amounts to heavy equalization, and explains why it is not a good idea to use a typical home audio amplifier as an amp for electric guitar. This "scoop" is just one element of what changes the sound of the amplifiers from one brand to another.

This "scoop" is just the beginning of what changes the sound of the amplifiers from one brand to another.

Cranking it into Over-drive

Another unique characteristic of a guitar amplifier is the amount of low level gain, plus the ability to over-drive a low or high level section of the amplifier. Most often, a low level or preamp section is over-driven or distorted, while not overdriving the output section of the amplifier. This break-up changes the harmonic structure of the amplifier and overall tone and can be adjusted to operate in and out of the region of clipping, depending on how hard you play the guitar. This break-up is a form of distortion, where and how this distortion occurs in the circuitry, is up to the amp designer. Clipping occurs when the amplifier is no longer able to reproduce the full waveform at the desired amplitude (volume), so it "clips off" the top and bottom of the waveform.

Pedal Effects

Pedal circuitry, plugged in ahead of the low level or preamp section, can provide a near-infinite number of sounds, filters, or effects, either clean or over-driven. Figure four is a sample of a signal from a "fuzz" pedal, note that much of the energy is now centered around 150-300 Hz and also 2 kHz, the signal contains a significant amount of harmonic distortion (additional harmonic tones), which is intentional.

EFFICIENCY: 1 WATT IN, HOW MUCH SOUND PRESSURE LEVEL (SPL) OUT?

On (black trace) and 30 degrees off axis (orange trace) 12" guitar speaker frequency response

Typical guitar speaker frequency response

Current guitar speaker efficiency is excellent. They range from 95-107 dB, using the standard one watt of input power, depending on frequency and measured on-axis, typically with the microphone 1 meter directly in front of the speaker cabinet. This allows lower-powered amplifiers to be used in many applications. Figure five's measurement below is with a 12 inch representative guitar speaker in an open back cabinet at 3 meters, with 1 watt input in a large room--the response you might get in the typical gig-type venue.

Two curves are plotted for a typical 12-inch guitar speaker, the top curve is on-axis, that is, directly in front of the speaker, and is what would be published by the manufacturer. The lower curve is the same measurement 30 degrees off-axis, that is, 30 degrees angled off to the side. Only a few front row center listeners in a audience might get the on-axis response, most everyone else, including the guitarist and band, will hear something similar to the off-axis measurement. This reveals a problem in that the tone you hear playing your guitar is altered by the speaker, and is not what your audience hears. Most guitarists are unaware of this.

A FLY ON THE CEILING, OR "LOOKING" AT THE SPEAKER'S SOUND FROM ABOVE

12 inch Guitar Cabinet Polar

Looking from above in a polar graph--imagine being a fly on the ceiling able to see the sound, the North Pole, so to speak. The polar response of a speaker defines how it projects sound into space or how sound spreads around a room. This plot represents a different way to depict the problem above. Guitar players are accustomed to listening to their amplifier and speaker cabinets off-axis, more often than not, aimed at the backs of their knees while they're on stage. This means they are conditioned to a significantly attenuated high frequency response--they are missing hearing the high notes and overtones.

This response is considered normal, and is because the traditional full range 10 or 12 inch cone speaker is not particularly good at projecting higher frequencies off- axis. Here is the polar response of a 12 inch guitar speaker in figure six. Note that the speaker does fairly well in frequency vs. dispersion (the lines are roughly circular) until getting up to the green and blue traces, about 1.2 kHz, and then the projection to the sides falls well off. In practical terms, if you're playing a solo up the frets, only the people sitting front row center are going to hear your work properly. Everyone off to the sides, including your band-mates, aren't going to hear what you're playing nearly as well.

For comparison, here is the Model Twenty polar response in figure seven below. Note that high frequencies are not attenuated off-axis, this means your tone will be the same for the audience, and band members will hear you clearly as you're playing on the higher frets. The Model Twenty has a much wider "sweet spot" for your audience, as well as for you and your band-mates.

MODEL TWENTY RESPONSE

For comparison, here is the Model Twenty polar response in figure seven. Note that high frequencies are not attenuated off-axis; this means your tone will be the same for the audience, and band members will hear you clearly as you're playing on the higher frets. The Model Twenty has a much wider "sweet spot" for your audience, as well as for you and your band-mates.

DISTORTION UNINTENDED

1 watt harmonic distortion of a typical 12 inch guitar speaker

Speakers themselves add distortion, the cone in a conventional 12 inch driver will flex. It may not be able to keep up with what the voice coil at center is doing, especially at higher volumes. In figure eight, we see the distortion of the 12 inch guitar speaker in the setting above. Even at 1 watt, the distortion is high enough that the lowest guitar octave notes will persist and be accompanied by additional harmonic content at about the same level as the fundamentals. The added distortion would be particularly problematic for anyone trying to play "clean," on an electric, an acoustic, or even a pedal steel guitar. Eminent Technology believes that the speaker should be reproducing what the guitarist is doing, rather than adding extra sound into the presentation.

MINIMAL SPEAKER DISTORTION

Compare figure nine, where the Model Twenty is reproducing the same signal as figure eight. Other than the note you play, and the harmonics from your instrument, no additional signals are added.

So the frequency range, dynamic range, amplifier power, and harmonic structure all define what we need in a speaker to reproduce an electric guitar accurately.

Electrical Impedance of a typical Guitar Speaker

A conventional 12 inch guitar speaker (figure 10 below) looks like an inductor to an audio amplifier, the rise in resistance can be attributed to the design of it's voice coil. The impedance rises as you go up in frequency and has doubled by 5Khz (in this case about 7 ohms to 15 ohms). This explains part of, but not all of the loss in high frequency output. A resonance peak is observed at about 89Hz which is typical.

Guitar Speaker Impedance curve

Electrical Impedance of The Model Twenty Guitar Speaker

The model twenly looks more like a resistor to an amplifier (figure 11 below) with roughly constant impedance across the frequency range and a resonance peak at about 98Hz.
An audio amplifier only sees a load corresponding to the input frequency, both impedance curves are benign to most types of audio amplifiers, hence, very friendly to tube and solid state amplifiers.

Model Twenty Impedance Curve

Acoustic Compression of a typical Guitar Speakers electrical input

The compression test involves doubling power from 1 watt to 32 watts or a 3dB increase in electrical input power per step (an increase of 15dB SPL total) and observing what a speaker does. The ideal speaker should increase it's sound level output proportional to the input power. These measurements were made at 10 meters free field, you would add about 10dB to correct for the 1 meter equivalent sound pressure level.

The results below suggest that a typical 25 watt rated power handling twelve inch guitar speaker starts compressing the signal beyond 16 watts of input power. Below 200hz compression starts around 108dB SPL and above 1.5Khz at about 112dB SPL.

Traditional Guitar Speaker Compression

Acoustic compression of The Model Twenty Guitar Speaker

The Model Twenty is not compressing the signal at all above 150Hz and up to the limit of the test (119dB) at 32 watts. Here is the model twenty result:

Model Twenty Compression

What does this mean? When you want to play louder rather than just adding distortion, the Model Twenty actually gets louder. The model Twenty’s cone area (which equals 4 - 12 inch woofers) and high power handling demonstrates a significant advantage in this test. Low distortion and no compression is a new sound for the electric guitar.

SUMMIN IT UP

Guitar amp speakers have been the essentially same for more than 70 years. They were never designed for the output that a guitar pickup creates, but rather a way to reproduce AM radio sound. As cone speakers were what was available, that's what got used in guitar amps and speaker cabinets. The speaker, amplifier, guitar body, pickup, and strings coalesce to create a musical instrument, but there is considerable room for improvement on the speaker side. So what can be improved?

What can be improved?

Bandwidth, distortion and dispersion are areas that can make a significant difference in both the user and audience experience.

Typical open back 12 inch guitar speakers are usually flat in response to about 200 Hz, and then the output starts decaying between 6 and 12 dB per octave below that frequency. This means that the last octave of a guitars' musical range is significantly reduced in output. Since the low "E" string on a guitar is tuned to about 80Hz, the Model Twenty has a switch which adjusts low frequency response to 100Hz or 200Hz. The frequency response or bandwidth of a guitar speaker can be improved at both the low and high end of the usable range. Figure 10 shows the frequency response of the Model Twenty.

The design parameters for the Model Twenty are to have uniform frequency response in all directions, so both the performer, band, and the listener get the same tone. To have low harmonic distortion means clean tones from an acoustic or electric guitar are amazing. Distortion is additive to any tone you create, low distortion means pedal and amplifier over-drive effects are not constantly altered by you speaker cabinet. The Model Twenty's thin and lightweight cabinet means user-friendly gig or home use. Even so, it has the same diaphragm cone area as a 4 x 12 cabinet so getting loud is not a problem. The impedance friendly design allows you to use whatever amplifier you prefer.