Ask Phil Jones: Why Does The Sound Of My Amp Change In Different Venues?

Master amp creator Phil Jones is here to answer all of your sound questions.

Ask Phil Jones: Why Does The Sound Of My Amp Change In Different Venues?

Master amp creator Phil Jones is here to answer all of your sound questions.

Nobody has to fight room acoustics in venues like bass players. The primary reason is that the acoustic waves produced from a bass guitar start at one inch in length for snappy high-end overtones all the way up to 37 feet for an open B string. If you go even lower, the sound waves are massive — 65 feet in length for the C# fundamental below open B. Personally, I feel that maybe A0 (the A one octave below the open A string) is the lowest note that works in amplified music, as any frequency lower than that almost becomes low-end noise, as in thunder, waterfalls, and earthquakes. Almost no commercially available bass rigs can produce these subsonic fundamentals, anyway.

The thing that makes your sound, the loudspeaker, is physically tiny in comparison to the size of the sound waves, and this is one reason why it is so hard to get rich, deep bass economically. The better-quality bass sound you want, the more it will cost you. My goal here is to help you get the most from what you have (or what you intend to pay for) by understanding, in simple terms, how sound waves work and how your amp interfaces with the acoustics of a venue.

Why Good Sound Goes Bad

In the mid ’70s I was a full-time bassist, and one thing that constantly frustrated me was that my sound would change from venue to venue, and even in the same venue depending on the number of people in it. (It seemed to improve if I added some alcohol to the mix!) On some nights, my bass would sound thin, and no matter how much I boosted the low end, I could never get that weight I wanted. On other occasions, the bottom end was muddy and bloated, or the vocalist would tell me to turn down, even though I could hardly hear myself. Bass playing is what really got me into learning about acoustics; I wanted to understand and control my own tone, regardless of where I played. If this has happened to you, it’s not your fault or maybe not even your gear’s fault. As soon as those sound waves leave your cabinet, they are airborne and at the mercy of the environment they happen to be in. You spend your hard-earned dollars on a beautiful bass and get the best amp for your needs, but after that, everything goes out of your hands. I will explain why and tell you what is going on and some possible solutions to fix or at least improve your bad sound. So, let’s talk about what makes good sound go bad.

First, every room you play in has its own sonic signature. Complicating matters, things change depending on where the sound source is and where the listeners are; the sound is different from the front to the back and also side to side. One example from my own experience: I was touring the U.K. with the legendary Chuck Rainey. We were doing a clinic in Cardiff, Wales (both Pino Palladino’s and my home town). Chuck had set up his bass and amp, and standing next to him, I could hear he was getting that rich, smooth “Chuck Rainey tone.” I went to the back of the hall to greet Marc Palladino (a drummer friend of mine and Pino’s brother), and some other bassist friends standing around asked me, “Phil, what happened to Chuck’s tone?” I noticed that it was profoundly muddy at the back of the hall. This had nothing to do with Chuck — it was the result of all of the reflected sound in the room merging at the spot we were standing in. Why was this happening? Yes, the venue had lousy acoustics, but what does that mean? What was happening was that the room was highly reverberant, and the sound from Chuck’s amp was combining with the reflections from the room’s walls and ceiling.

Reverberation refers to the persistence of sound in a space after the sound is produced. In acoustical terms, it has a value expressed as Rt60. This stands for reverberation time and how long it takes for a sound to diminish 60dB (decibels) below its initial level. Typically, the highest frequencies decay the fastest, which means that the low frequencies linger on a lot longer. That’s bad for us bass players! A good auditorium may have a reverberation time of one to two seconds in the midrange. Acousticians try to get this time even across the frequency range, but the low end is always problematic. Materials with high sound-absorption coefficients, such as rock wool or foam tiles, are often used to acoustically treat halls, and the thicker the material is, the better it is for attenuating lower frequencies. Nearly all sound-absorption techniques work above the 300Hz range, but below that, it gets exponentially more expensive to treat a room (even a small recording studio), and invariably, the cost exceeds the budget. The result is always longer Rt60 times for bass — no matter what space it is.

Unfortunately, there really is no remedy to fix over-reverberant halls, except to avoid playing in them. I have found that just reducing the low midrange (from 200Hz–500Hz) can bring back some clarity, but it can also take out the body of your tone. My advice, if you’re playing in a highly reverberant room, is to get someone to play your bass during soundcheck while you walk around. If reverberation is muddying it up, try cutting frequencies anywhere from 160Hz–300Hz. That should clean up some of the mud that the audience will be hearing.

Reverb, though, is not all bad; it can actually enhance your overall sound, and it makes everything louder. I have taken really large loudspeakers into my anechoic test chamber (which absorbs everything with a flat response down to 30Hz!), and even 1,000 watts doesn’t sound very loud in there — and the overall tone is actually awful. Our ears need a reverberation field for anything to sound good. Having absolutely no reverb sucks the life from music. Still, for decent bass sound, you want the Rt60 for the low end to be under three seconds. (By the way, there are many phone apps you can install to test the reverberation of a venue, such as RT, Reverberation Time, and SabSound Reverb calculator; most are free.)

How Sound Waves Propagate

Bass guitar cabinets become increasingly omnidirectional below around 200Hz (near the fundamental of the G string on the 12th fret). In other words, they push low-frequency sound waves in all directions — not only side to side but up as well as backwards. Walls, floors, and ceilings guide the wave and in some ways act as acoustical mirrors. Here’s why a speaker cabinet is omnidirectional at the bass end of the audio spectrum.

At sea level, sound waves move at approximately 1,117 feet per second, and I say approximately because the atmospheric air pressure isn’t constant (just look at any weather report). Sound waves have a specific length that depends upon on the frequency. The open B string on a 5-string bass is almost 31Hz, so to find out the wavelength, you just divide the speed of sound by 31Hz, which equals a wavelength of 36 feet. But, consider the largest dimension of your cabinet. If it’s a 4×10 cab, it may be 30 inches across — this is 14 or 15 times smaller than the wave it is trying to produce. As the speaker cone moves forward, the pressure wave literally rolls across the baffle and wraps around the cabinet, resulting in an almost identical pressure wave in all directions. So, the energy of the sound wave at this frequency is propagated as much backward from the cab as forward. This effect of the wave bending around the enclosure is called diffraction, and it physically distorts the wave front. All bass cabs suffer from diffraction, and this means that the shape and size of the venue, and the location where you position your cabs, will drastically affect your bottom end. Conversely, midrange and upper frequencies beam forward like car headlights, so you can pretty much control where they are going — but stand beside your cabinet and you’ll notice it sounds much duller than standing in front of it.

The worst-case scenario of losing the bottom end is at an outdoor event, where there are no walls or ceiling near your cabinets. Since the lower frequencies are going in all directions (other than down into ground), with no reflections to bring them back toward the audience, the wave’s energy loses its punch over a shorter distance. Acoustically, this is the simplest kind of situation you’re likely to encounter, so let’s analyze what’s happening to the waves. The waves are basically hemispherical, spreading out from your amp at the center over the area of that hemisphere. For any given distance, we can use the formula for the area of a hemisphere (2πr2, where r is the distance from the cabinet) to find out how much the wave’s power has diminished. For example, at ten feet, the area of the hemisphere is about 628 square feet; at 50 feet, the area is 25 times greater, or about 15,700 square feet. This means that if your amp was pushing 200 watts into the speaker to produce 100dB at a distance of ten feet, then at 50 feet your amp will sound like it was pushing only eight watts! Unless you have a wall of subwoofers with a massive baffle area, your wave front won’t have the power to propagate effectively. (Keep in mind that’s the worst-case scenario, and at an outdoor event, let’s hope your bass is being reinforced through a PA system.)

Acoustic Loading In Venues

Let’s talk about how loudspeakers react in rooms. The sound waves in a room are constrained by walls, the ceiling, and the floor, so they will be guided along these boundaries. Low ceilings will turn the sound field to a vertical cylinder (rather than a hemisphere), so the sound wave will still radiate backward and sideways but be at the mercy of the reflections off the walls.

Suppose you position your cabinet around six feet from the back wall in a venue. There will be a reflection from the back wall that combines with what your speaker is reproducing, and depending on the frequency, those waves will combine both in phase and out of phase and everything in between. When sound waves combine in phase, they add to each other; the opposite is true when they are out of phase, resulting in some notes being louder and some quieter. At a distance, the fundamentals of some notes may be nonexistent, because the wave reflected off the back wall is out of phase almost exactly and therefore is fighting with what your speaker is doing. Placing the cabinet closer to the wall will give you a smoother low-end response, but as a tradeoff, it will affect upper bass notes. Putting the cabinet right up against a wall will allow all the low-end energy to move forward only, resulting in a smoother bottom end. In this case, the sound is radiating outward as a quasi-quarter-sphere, and that means you need less power to get a good bass response. It’s even better if you can place your cabinet in a corner, where the walls almost start to act like a horn loading on the cabinet. If you are ever in a position where you can do this, it will mean your amp needs to work even less hard.

When I was growing up in Cardiff in the ’70s, my older brother Steve and I played in rock bands. Occasionally we had gigs at the New Moon Club, located in a seedy part of town. The building had a flatiron shape, with the stage at the pointy end. One of the local bass players had left a massive custom-built 4×15 on the stage as the house bass cab. One night, when I was playing for Paul Chapman, the guitarist in the rock band UFO, I got to use this cab with my Acoustic 370 head. The bass sound was just crushing, and I was easily able to keep up with Chapman and his very loud Marshall stack. A 4×15 cab is going to move air, but I had never expected just how much! The primary reason why the bottom end was so powerful had more to do with the room than the cabinet: The side walls were at 30 degrees to each other, so the only way the bass wave could travel was forward. Also, the ceiling was only about ten feet high, so there was very little energy lost at the back of the venue, and that was why the bass sounded so good.

Using Amp EQ

If you want to get a fuller and more even bass response from your rig, it could come down to where you position your cabinets. But on many gigs, it may not be possible to have a choice of where you put your bass amp. In that case, you may have to resort to using equalization (EQ), which is nothing more than frequency-selective gain controls. In other words, when you boost your bass control or EQ slider, the amp will deliver more power to your speakers at those frequencies. Likewise, when you cut back a control, your amp will produce less power in that range. Boosting your bass control by, say, 6dB will make your amp work four times harder at that frequency range. Excessively boosting your EQ will not only rob your amp of power, it can also cause damage to your loudspeakers from to much cone excursion or overheating the loudspeakers’ voice coils.

Getting great bass is indeed a challenge. I hope that my knowledge and experience can guide you on the path to great tone and therefore better gigs.

Phil Jones is the founder and president of Phil Jones Bass, a leading manufacturer of bass amplifiers and cabinets. Check out PJB’s product line at

Phil Jones   By: Phil Jones

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