Ask me some techy questions! Email barefacedbass@gmail.com

Sound and the production of it is a pretty complicated subject so over the coming weeks/months I intend to explain the following subjects in the hope of creating a world of better informed and thus better sounding and better playing bassists! High hopes maybe but no point aiming for mediocrity...

An explanation of stage/floor coupling:

Part 1 - mechanical coupling

The key point to understand regarding coupling is that there are two forms of coupling - mechanical coupling and acoustic coupling. With mechanical coupling the stage/floor is excited by the vibrations from the cabinet or by the acoustic output from the speakers/port and the floor itself becomes a resonating surface like a giant drum skin or speaker cone. The acoustic output from the resonating floor is very unpredictable, centred around the modes of resonance of the floor, which almost always results in a boomy onstage sound and unpredictable offstage sound (depending on how it reinforces and cancels the output from the speaker cabinet). You should generally try to avoid mechanical coupling and this can be done by using a well-braced speaker cabinet which vibrates less, isolating the speaker cabinet by reducing the amount of physical contact with the floor (isolating foam / tilting cab / raising onto beer crate) and placing the cab at a more solid part of the stage (i.e. a corner). If you are on a small hollow stage see if it's possible to place the cab off the stage - also make sure that the PA subs are not on the stage, though any sound engineer with a clue will not have done this.

Part 2 - acoustic coupling

The second form of coupling is acoustic coupling and this works by the direct output and the reflected output from the speaker cabinet combining to give increased output. As you can read in the 'Understanding sound dispersion' section the low frequency output from any loudspeaker is omnidirectional which means that your bass cab is sending low frequencies not just to the front but also out to both sides, upwards, backwards and downwards! The best way to picture this is as a sphere of energy surrounding the cabinet with the cabinet in the middle of the sphere. If you place your cab on the floor then the bottom half of the sphere is blocked off and that energy is reflected by the floor, combining with the upper half of the sphere and increasing your low frequency output by ~6dB - that's equivalent to going from a 100W to a 400W amplifier. This kind of coupling is thus a really good thing! So what happens if you put your cab on a beer crate so you can hear it better and/or to stop some boomy sounding mechanical coupling with the stage - does that mean you lose all of that 6dB of acoustic LF coupling? Fortunately the answer is no - what happens is you still get the 6dB gain but the cut-off frequency goes down, so rather than you getting the gain below about 250Hz you might only get it below 100Hz.

Understanding room acoustics

Continuing on from stage/floor acoustic coupling, also known as boundary reinforcement, there are a number of other boundaries in venues that you should consider, most importantly the back wall behind the stage and the side walls either side of the stage. If these are solidly built walls (not flimsy partitions) they too will be 'seen' as boundaries by the low frequency output from your speaker cab. So if we return to that image of the sphere of sound surrounding the cabinet, picture what happens to the half-sphere created by placing the cab on the floor when we move the cab backwards so it is close to the back wall. The half-sphere is now a quarter-sphere and the sound that was previously going backwards is now going forwards and this reflected sound is reinforcing the direct sound from the cabinet. This gives us another low frequency gain of ~6dB. Now push the cab sideways so it is the corner of the room and the quarter-sphere becomes an eighth-sphere with another ~6dB gain in output in the lows. Just as when you lift a cab off the floor the frequency below which you gain reinforcement drops, when you move a cab away from a wall the same effect occurs. This gives you an excellent way of acoustically controlling the low frequency sound from your cabinet without using EQ.

So that covers how acoustics can reinforce your sound but there is another issue to consider - those horrible moments when acoustics actually cancel out part of your sound! Again this is to do with direct and reflected soundwaves colliding. To understand this we need to firstly consider what a soundwave actually is - the best way to do this is to picture is as being like waves on the open sea, rolling up and down with peaks and troughs. When a reflected peak meets a direct peak then they combine and you get a peak that's twice as high (3dB). When a reflected peak meets a direct trough or when a reflected trough meets a direct peak you get a deep narrow cancellation, usually about 18dB deep. So how do we know when we're going to get reinforcement and when we're going to get cancellation?

Understanding power handling

Understanding sound dispersion

The difference between on-axis response and power response

The difference between ported and sealed cabinets

An explanation of the relevance of speaker size

An introduction to power, gain, volume, decibels and SPL

Clipping - what it is and what it causes

The effects of high SPL demands on loudspeakers

And now some questions and answers which originally made up the FAQ but are probably a little too deep for most visitors to bother reading them!

Question: "I've heard that underpowering is dangerous to speakers, how can I avoid this?"

Answer: Underpowering is one of those myths that will not die. You cannot damage a speaker through putting too little power into it.

Question: "What about clipping, does't that damage speakers through putting DC into them?"

Answer: To clear up the DC issue, a correctly functioning amp, however badly it is clipping, should never generate DC. However clipping does have two risks - firstly that a fully clipped amp can produce almost double its rated power. This is only a problem if you keep the amp running fully clipped for a length of time and your speaker's thermal power handling is less than twice the amp's rated power output. Fortunately in the real world this will never happen - we play bass, which by its nature has loud moments and quite moments, thus power output is never constant. If you use an amp within the recommended power range for our speakers then you will not have a problem unless you are seriously abusive.

The second risk with clipping is that most of the extra power is actually in the higher frequencies. This means that if the cab has a tweeter it is likely to receive more power than it can handle. However if you have a tweeter then you should be able to hear the clipping and react by turning the amp down. The plus side for our designs is that the rising impedance of a woofer with frequency actually protects the woofers from having to handle extra power (amplifiers are voltage amplifiers and the power that comes out is simply the voltage squared divided by the impedance at that frequency). So to recap, although your amp can produce more power when it is clipping, the majority of that potential extra power won't actually flow through your speaker unless you have a tweeter, in which case it will make a really ugly noise before the tweeter dies (which is why a well designed speaker should incorporate tweeter protection circuitry).

Question: "What about overpowering?"

Question: "Don't 15" speakers sound slow?"

Answer: Bigger speakers sounding slow is another one of those myths. The speed at which a speaker can move is what limits its high frequency response. There are no speakers that I know of that have such limited high frequency response that they sound slow across their working frequency range. If a speaker can reproduce a given frequency then it will not sound slow at that frequency. If it can't reproduce a frequency then you could claim that it sounds slow at that frequency but because it can't reproduce that frequency then it doesn't sound like anything at that frequency, least of all slow!

Question: "Aren't 10" or 12" speakers a lot punchier than 15"s?"

Answer: The concept of "sounding punchy" means many different things to many bassists but the gist of it appears to be about midrange output and clarity. If a speaker has good midrange response then it will have plenty of punch. The Compact has plenty of midrange output and clarity - comparable to any high quality 10" or 12" cabs. The Big One's dedicated midrange speaker has incomparable midrange output and clarity - enough punch to roar through the densest mix. Many bassists' first giggable rig was a cheap 15" combo or big old dated 15" cab - this has left many believing that 15" speakers sound slow, woolly, muddy, etc, especially when they then buy a better 10" or 12" rig. The problem with these lower quality 15"s is a lack of motor strength resulting in a lack of midrange output - and thus punch! Also these 15"s probably had no more excursion than the 10"s or 12"s they were replaced with, so no advantage in the lows.

Question: "Why don't you use 10" or 12" speakers?"

Answer: Quite simply there are no 10" or 12" speakers than can move as much air for their size and weight as our 15" speakers - and reproducing the fat sound of a bass guitar at high volume requires moving lots of air. The speaker in The Compact also manages to achieve excellent midrange output comparable to 10" or 12" speakers though there is a trade-off in the treble response not going quite as high. I should add that as a bass player I use a wide variety of sounds, slap, play chords, etc and still find the single 15" in The Compact to have enough clarity. The Big One has a dedicated midrange speaker which the woofer solely has the job of reproducing the lows and low-mids - if there was an 18" speaker with better performance than our 15" then we would use that but as there isn't then we use an extremely high output 15".

To quantify the difference when it comes to creating high volume bottom, lets look at the 15" woofer in The Big One and compare it to a 10" woofer which is very similar to the speaker to used in the classic sealed 8x10" cab. The amount of air a speaker can move is equal to its cone area (Sd) multiplied by its maximum excursion (Xmax). This is called volume displacement (Vd). The 10" speaker has Sd=355.4 square centimetres and Xmax=3.2mm therefore Vd=114 cubic centimetres. Our 15" speaker has Sd=881 square centimetres and Xmax=9.6mm therefore Vd=846 cubic centimetres. So the single 15" woofer we use can move as much air as seven and a half 10" woofers. No bad at all! (We should also point out that because The Big One uses a tuned port to supplement the low frequency output that this can almost double the amount of air a cab can move compared to a sealed cab, so The Big One can produce comfortably more low-end than the big classic sealed 8x10" that you see on every stadium and festival stage).

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