Posted Jan 13, 2010 12:09 PM
In this article I am going to try to explain what the wooden box around your speakers does and how it shapes your sound. The article is aimed at musicians not engineers but should help anyone thinking of buying a speaker or give you a start if you want to start building or modding. I've seen a lot of confusing and inaccurate stuff in the forums and the internet so I'll try to stick to facts and not express opinions. I will keep it simple but as a guitarist you will already know more than you think.
Why Do We Need A Cab
If you get the chance then try this; take a speaker without a cabinet, put it on the floor resting on its magnet and connect it to an amp turned quite low. It will sound really tinny and not very loud, just like headphones when they are not in your ears. Why is this?
The speaker works by moving backwards and forwards in time with the vibrations of your strings. When it moves forwards it squashes the air in front increasing the air pressure, at the back of the cone it is moving forwards and it reduces the pressure. At the edge of the speaker the air will leak around and the low and high pressures will cancel and you get no sound. The pressure wave from the middle of the cone will take longer to reach round the cone and if it completes its cycle before the sound cancels it will send sound into the room. This means fast frequencies with short wavelengths (high notes) can be heard but not long wavelength bass notes hence the tinny sound. The cabinet will stop the sound from the back of the cone interfering with the sound from the front and you hear all the sound.
The other job the cab does is to tune the speaker. As a guitarist you are used to tuned systems. The strings on your guitar vibrate at different frequencies which you can tune. Heavy strings natural resonance is lower but shortening the string by fretting it or increasing the spring by tightening the tuning pegs makes the string vibrate more quickly. The heavy cone of the speaker bounces around on the springy suspension, the air in the cab and the spring of the magnet. Extra air in the cab adds to the mass of the cone as this has to be moved too which is why big cabs go deeper. With the cab providing both a spring and a load there is only one size which will give the tuning the designer wants.
Taking the guitar analogy further, if you play an acoustic then the guitar body has its own resonances. Each wooden panel resonates depending upon its weight, tension and shape and the air inside the body resonates against the hole in the front and is affected by every different dimension inside. All these resonances affect the sound and no two guitars sound alike as a result. Speaker cabs and speakers also have resonances depending on their exact dimensions and different cabs will have different musicality.
Just like a guitar a cab has to be correctly tuned if it is to sound right but even then the sound will vary depending upon the exact design.
Open Backed Cabs
The simplest way of stopping the cancellation effect is to put the speaker in the middle of a large wooden board which we call a baffle The bigger the baffle the deeper it will go. The problem is that to be effective for really low notes the baffle needs to be huge; about 3.5 metres by 3.5metres at bottom E on a bass. This isn't practical. Fender tried an open back bass cab in the 1954 Fender Bassman but it wasn't a success. For a guitar amp though the size needed for an open back is much less and many guitarists loved the sound of the Bassman. A big flat board with a speaker in the middle isn't a very practical shape but adding sides and an open back still force the air to take the long way round and avoids cancellation.
Open backed cabs do have the advantage that the bass only rolls off slowly (6dB per octave) below the cut off frequency and so practical sized cabs can have enough output down to 80Hz which is bottom E on a guitar. Other advantages are that with no panel behind the speaker there are fewer standing waves inside the cab and no sound reflections back through the speaker cone. However open backs mean that half the sound goes out of the back of the speaker and can bounce around the stage by echoing off the back walls of the building so the sound is partly dependent upon their positioning. Speakers designed for an open back will generally have very light cones and stiff suspensions and limited travel which makes them very efficient (loud)
If the problem is the air behind the speaker cone interfering with the sound from the front, then why not close the box and stop it getting out at all? This is how the sealed cab works and because none of the air can get around the sides it is sometimes called an infinite baffle. One problem is that sealing the cab makes the air behind the speaker act as a spring. The springiness of the air and the speaker interact with the weight of the speaker and the weight of the air in the cab to create a tuned unit and you need a different type of speaker to work well in a sealed cab. The volume of the cab has to match the speaker you use. The second problem is that resonances inside the cab can sound boxy and sound reflects off the back of the cab and out through the speaker cone causing distortion of the mids.
Sealed cabs generally are slightly less efficient (loud) than open backs but are usually punchier (engineers describe this as being well damped) and give the possibility of much deeper bass and tweaking of the low frequency response. Below the cut off frequency the bass reduces more quickly at 12dB per octave. Speakers designed for sealed cabs generally have heavier cones and softer suspensions and are designed to be longer throw (they move backwards and forwards further) which explains both the deeper bass and why they are less efficient. They often have bigger (and more expensive) magnets to compensate for this.
Blow across the top of a bottle and you will hear a note. The weight of the air in the neck is bouncing on the spring of the air in the bottle and is resonating. Putting a port in a speaker cab makes it resonate like the bottle. Bigger volumes and heavier air plugs will give a lower note so you can tune a cab to any note you want. If you tune a cabinet to resonate at the exact frequency that the speaker cuts out then the sound from the port will reinforce the speaker at that point and it will go lower and louder at the bottom frequencies. You get free bass. There is a price to pay though, tuning is critical, tune too high and the cab will boom and the bass will cut out early, tune too low and the bass will be weak.
The other problem with reflex cabs is that you have removed the air spring from the back of the speaker. This means that below the resonant frequency the cone can end up flapping around too much and will distort and even become damaged. Speakers for reflex cabs have to be specially designed to avoid this.
The third problem that you can't design out so easily is that a bass reflex speaker will be less well damped than a sealed box so well and will sound less crisp.
Reflex or ported speakers promise more low bass and higher efficiencies at the expense of a little crispness and more design problems. Properly designed they give great bass output for a compact cab. Below the cut off point the bass drops very quickly at 18-24dB/octave.
Speakers for reflex cabs often have lighter cones and their excursion can be less at low frequencies because of the output from the port. They should be protected from excessive excursion at subsonic frequencies. They tend to have big magnets to increase the electrical damping which compensates for the poorer transient response compared with the sealed cab.
Think of a 15 speaker working really hard on stage in a football stadium. To be heard at the back of the stadium it needs to stir up all the air in the stadium. Even this big speaker is a tiny point in the context of the stadium and however hard it is driven by the amp it is going to struggle to transfer all that energy to the air. Roll up a piece of A4 or A3 paper into a cone and speak through it. The home made bull horn directs your voice and makes it louder by making it couple more efficiently with the air. A horn on the front of a speaker works exactly like this acting as a kind of acoustic transformer which dramatically increases the efficiency of any speaker. Horns aren't really relevant for instrument speakers though because they have to be huge to reproduce bass. 2.8mx2.8m and nearly 4m long for bottom E! High frequency horns are much smaller and you will find them as high frequency speakers (tweeters) and you may find them in PA's.
For guitar you want a cab that is good from 88Hz to 6000Hz and which is punchy and well damped. You are going to want an open cab for its efficiency or a closed cab for its control and accuracy. If you are using a closed cab with a low powered valve amp you might want to look for something really efficient like a decent 4x12. Frankly ported cabs are a bit of a gimmick for guitar as the port will only add to the output below the 88Hz of bottom E in most cases. If you want decent bass look to a sealed cab but ultimately if it sounds good to you then go for it.
For bass the cab needs to go from 44-5000Hz (33-5000 for a 5 string). The choice is between the control of a sealed cab and the potential extra output of a reflex design. If you are buying then the design problems will have been solved for you but if you are building then you have a higher probability of success with a sealed cab and a lot of homework if you want to build a decent ported speaker.
If you are swapping or replacing speakers then make sure they are the right designs for the cab you are using. They will have to match the enclosure type and the volume of your box. Most of the manufacturers will tell you this either on their websites or via email.
So there you are, for any of you technical experts out there I know how superficial this has been and how much I've missed out but I hope it has been clear enough to be useful for the rest of you. If there is a lot of interest I will write up more articles, how to design sealed boxes, reflex speakers and how to choose drive units. I could even publish some specific designs if anyone is keen.