� 2004  H. Davis



As patricide is immoral and illegal, and you really like your dad in spite of it all, you give in. You turn down the stereo whose intolerable din nearly drove the old man crazy. You reduce its average output from 8 watts to 2 watts, and with this 6-decibel volume reduction, peaceful coexistence resumes.


An "intolerable din" at only 8 watts? Most certainly. Just as beauty is in the eye of the beholder, loudness is largely in the ear (actually the mind) of the listener. If you like the music, you turn it up - it just can't be too loud. If you don't, you turn it off. When you play guitar you play loud - at least some people say you do, but you may not hear them. So loudness is often a matter of context. Even though 100 watts can soothe your soul or inspire spontaneous dance, it can also inspire spontaneous eviction.


Regardless of subjective factors, one watt of average power isn't going to satisfy a Rock club or concert-hall crowd - though it could, with better speaker technology than we now have. The familiar electrodynamic paper-cone speaker has an actual efficiency of only several percent. Over 90% of the power amplifier's output is wasted, ultimately being dissipated as heat. The invention of even a 70% efficient speaker of practical size would radically reduce amplifier power requirements - and also put some roadies out of work. You could carry your amp in a lunch box, power it with batteries, and still blow an audience away.


Loudness and volume mean pretty much the same thing - sound intensity. The term loudness is used instead of volume on home audio system volume controls that compensate for certain human hearing characteristics by boosting the bass and treble at low volume settings. With musical instrument equipment, either term can he used. Let's take a close look at sound intensity, power, and gain - and how to get the most sound from the least power.


Sound pressure level, or SPL - the technical term for volume - is measured in units called decibels (db). One db is about the smallest change in level that is perceptible to the human ear. If someone whispers quietly, he is exposing you to an SPL of about 20db. Since the typical background noise level in a home is around 50db, you'll ask him to speak up. Sound level and the power of the sound source do not have a linear relationship; twice as much power expended does not raise a 20db whisper to 40db, hut only to 23db, as each doubling of power only produces a 3db increase. To achieve a more audible 32db, the whisperer must vocalize with 16 times the power he previously employed. (A 12dB increase = 4 x 3dB, which equals 4 power doublings; 2 x 2 x 2 x 2= 16). Even though the 50db background SPL is still a good 18db over the speaker's voice, and contains about 64 times the acoustic power (18db = 6 x 3db; 6 doublings = 64), he can be understood if listened to carefully. Just as we can mentally tune out Muzak if we are not interested in it, we can psychoacoustically focus on what we want to hear.


Normal conversation is in the 60db to 70db range, and unless concentration is important, SPLs in this area are usually not disturbing or objectionable. The background noise in a workplace such as a factory often averages around 75db, which is not harmful or annoying to most people unless there are peaks well above this. A one second peak of 140db averages out to 95db over eight hours, but after one second of exposure to 140db, you won't hear the 95db at all - perhaps ever!


At 90db, prolonged exposure becomes irritating and stressful. Amplified rock in an enclosed space is often in the area of 110db, louder than thunder, and this level is considered a threat to one's hearing by many experts. U.S. Department of Labor noise regulations permit no more than 30 minutes of exposure a day to this sound level in the workplace. The threshold of pain - and rapid permanent ear damage - is 130db. This represents 1,000 times the acoustic power of a 100db sound pressure level.


Now let's consider the reason for using humongous power amps: the inefficient electrodynamic cone speaker. With overall efficiency being defined as the acoustic power output divided by the electrical power input, typical efficiencies for speakers and speaker systems range from under 2% to about 10%. Horn-type speakers are much better, with efficiencies around 40%, but these are limited in frequency response and are often impractical because they must he very large to he able to generate bass frequencies.


The efficiency of an electrodynamic speaker is influenced by such factors as its enclosure, its placement in the listening space, and the acoustics of that space. Considering only those factors not adjustable by the user, speaker efficiencies range from about 85db to l03db SPL with 1 watt of amplifier power delivered to it, measured at a point one meter away in front of the speaker. This 1-watt, 1-meter measurement is commonly used for speaker SPL ratings. To produce 110dB SPL, a speaker with an 85dB efficiency figure requires a transistor-roasting, transformer-frying 316 watts! No single speaker can handle this, so several must share the power. The demands on the amp are enormous, but even if the amp can deliver a clean 500 watts, gross speaker distortion often occurs when SPLs over 112db are attempted. If you need a fairly clean 115db but your 4 x 12" speaker cabinet distorts excessively above 112db, two such cabs will be needed, as the additional 3db requires double the power handling capability.


How much better off are we with the more efficient 103dB-rated speaker? With a 103db SPL at 1 watt, 110db (7db more) requires a little over two doublings of drive power. A mere 5 watts can do it! And if the speaker can take it, 50 watts will generate a 120db SPL. At full power through speakers of this efficiency, the 500-watt amp would produce a 130db SPL - leaving you to forever respond to questions with, "Watt? Watt?"


A while hack I stated that with better speaker technology, one watt of power could satisfy a concert-hall audience. Now I'll explain that. The 103db at one watt speaker has an efficiency of roughly 10%. Were it 80%, there would be eight times the acoustic output for this same one watt input, an increase of 9db. Imagine: 112db SPL with one watt of electrical signal power!


Before concluding that efficiency should be the main criterion for speaker selection, consider that it is only one of many important speaker characteristics. High efficiency is often had at the expense of other desirables, such as a smooth, wide-range frequency response. Advantage can be taken of a poor speaker's resonance or a response peak to claim a misleadingly high efficiency figure, when in fact the efficiency is far lower over most of the speaker's frequency range. To sound musically rich, go for wide range, low distortion at high volume, and desirable tone qualities. To merely be loud, consider efficiency alone.


Guitar amplifiers are not equipped with wattmeters, nor sold with SPL meters as accessories. Aside from your own subjective judgment of loudness - with which the neighbors never agree - the only indication of the sound level you are creating is the position of your gain/volume controls. Equipment designers generally aim to make these a fairly good indication, in that the increase in apparent loudness when cranking up from 3 to 4 should be about the same as the increase from 7 to 8, even though the volume level at 7 is much higher than at 3. Gain refers to the voltage amplification of the instrument's signal. If a 1 volt signal comes from your guitar, and the amp delivers this to the speaker as 10 volts, the overall gain of the amp is 10. How much power is delivered though depends not just on signal voltage but also on the speaker impedance. Ten volts delivers 25 watts into a 4-ohm speaker; but into an 8-ohm speaker, only 12.5 watts, or 3 db less. To calculate wattage from voltage, you square the voltage (multiply it by itself; 10 x 10), and then divide by the impedance (100/4 = 25).


The numbers on the control dials do not indicate actual gain; instead, they only tell you relative levels, and only become meaningful when you learn by experience how your amp sounds with a particular instrument and various settings. When you get down to the real playing situation, ears, yours and others, are your only valid guides. When ears tell you all is right, you look at your control settings and note where to start from the next time. Subjective as they are, it is ears and minds you seek to please - not numbers, theories, or amp manufacturers. Bach never owned an amp, and although his Inventions have been switched on, they were never plugged in. Yes, this musical genius was an electronic virgin.


Knowing about SPL and how gain and power relate to it is all well and good, but chances are you already have chosen your favorite amp and speakers, and want to make the most of them. Well, knowledge indeed is power!


Consider a "point source" of sound, one that radiates sound equally in all directions. Put this source in au open field, or in an anechoic chamber (a room treated with deadening materials that eliminate all sound reflections), and the sound level drops off rapidly as you move away from the source. Double your distance from it, and the acoustic power drops to one fourth of its previous value, a loss of 6db in SPL. This is the situation of the acoustic guitar player outdoors, and is why an audience tends to form a close circle around him.


With amplification, all this changes. Aside from the added power, the sound is now directional - most of it is projected by the speakers over a limited angle, rather than allowed to dissipate in all directions. Double your distance from the speakers, but stay in front of them, and far less than 6db is lost - especially at the more directional higher frequencies. Due to power and directionality, the audience of an amplified guitar player, when unconstrained by seat placement, tends to stay in front and at greater distances.


Indoors, especially in an acoustically live space, things are much better for the acoustic guitarist. For one thing much of the sound that was radiated away outdoors is now reflected back, increasing the acoustic power reaching the listeners' ears. Seated in a corner, the player utilizes a natural acoustic amplifier, the walls, floor, and ceiling forming a horn like the large horns found on the most efficient of speakers. Outdoors, he or she can enhance loudness by choosing a similar location, or at least by playing where there is a wall in back to reflect the sound like a band shell.


As loudspeakers are already somewhat directional, some of the beneficial acoustic effects of a room are not as pronounced with them as they are with the unamplified instrument. The amplifier more than compensates for this however, and the proper placement of speakers can squeeze out a few more decibels before distortion becomes intolerable - and remember, an increase of only 3db effectively doubles your power!


The audiophile school of thought for "good sound" at home tells us that a room should be fairly dead (devoid of echoes), and that frequency response - particularly in the bass range - is smoothest when the speakers are placed away from the corners of the room. If the speakers are small and the bass is weak, putting them in the corners boosts the sagging low end at a tolerable loss of response flatness. The resonances of a very live room destroy fidelity by introducing mountainous peaks and valleys in the frequency response, and the reverberation has an unpleasant quality far from the natural open sound of a large concert hall.


These rules do not hold up under the conditions in which live music is usually performed, primarily because when indoors the space is usually much larger than the typical living room. Music is being produced, not reproduced; reverberation is not only tolerable here but desirable. Should the placement of speakers to maximize efficiency have any deleterious effects in other respects, chances are that tone controls or equalizers can adequately compensate for it.


Due to the lack of reverberation in open spaces, outdoor concerts and those in very large or acoustically dead halls demand the use of large, efficient speakers and amps with plenty of muscle. The surroundings are no help at all here, and so the sound people must know what they're doing. If you are the typical band doing an indoor club gig though, you probably don't have such expert help. Sound efficiency requires ingenuity, especially in a dead space like a club or bar packed with customers. To reduce demand on your own equipment, the house sound system should be utilized as much as possible. Use the structural features of the space to your advantage, creating a band shell or horn effect. Should there be no stage, and if the space is longer than it is wide, setting up at one end with the speakers facing down the length of the room is acoustically advantageous. If necessary to achieve adequate volume draw back any drapes or sound-absorbent panels that have a deadening effect, and consider speaker placements more efficient than usual, such as room corner locations rather than in a closely spaced stage back line.


Loudness is no substitute for musicianship, and partial deafness is no badge of honor. I was once able to hear this advice, and I wish I had heeded it. An understanding of sound levels, power, gain, and efficiency enables you to get the most out of your equipment, but exercise discretion to protect the delicate, irreplaceable inner ears of your audience and yourself. Use those ear condoms to protect yourself from irreversible hearing loss. The sweet, soft song of the nightingale enraptures, pounding thunder excites, but 130db only destroys.