Why can't I plug a base guitar into a guitar amp?

[Roly]

1. speakers have their seal on the front of the mounting flange, and it is desirable to minimise the volume in the speaker back chamber; but front-mounting with added weatherseal strip, silicon, felt, or whatever is fine.

2. As the frequency goes down the radiation shifts from the front of the cone to the horn, so the time difference doesn't really come into play.  Even at the transition frequencies you have two sources radiating with a time difference and the effect isn't cancellation but a shift in the radiation pattern.

Whenever you have two sources radiating the same signal you get interference, cancellation at some points and reinforcement at others - this is simply the unavoidable mechanics of waves, but all a delay in one does is move this pattern of cancellation and reinforcement.


Two point sources radiating the same signal

Some figures to keep in mind.  The velocity of sound in air is roughly 1000ft/sec (depending on the temperature, barometric pressure, humidity, and the state of your aunt's trick knee).  That's 1 foot per millisecond, or one full cycle of 1kHz per foot.  For Middle-C the wavelength is about 4 feet; at 100Hz the wavelength about 10 feet.  So you can see that for high frequencies delays can become critical, while at low frequencies they aren't really significant.

Where you have multiple forward facing direct radiators working at higher frequencies, the upper musical register, time-alignment starts to become important, and with tweeters at 10kHz (wavelength about 1.2 inches) it gets pretty critical.

This is an example of the d'Appolito configuration where the radiators are arranged from the highest frequencies in the middle going out as the frequency comes down (note the vents at extreme top and bottom).  Practically speaking the vents only radiate below their resonant frequencies which is normally set to match the resonant frequency of the associated driver and will almost always be somewhere below 100Hz.  Above that the vents are effectively blocked by the air in them going "solid".



This is very much a Hi-Fi configuration, but if you happen to see a Flying-J PA stack undressed you may notice that they do something very similar, but sideways, the trebles up the middle and the lower frequency drivers on each side, upper-mids then lower-mids.


One unit module of a vertical Flying-J array

The PA world has rediscovered the Line Source (which I won't go into the detail of) which makes use of the fact that sound tends to beam and be more directional as the frequency goes up.


Model of the plan radiation pattern for two twelves side-by-side, 400Hz-4kHz
Wide at low frequencies, narrowing to a beam as the frequency rises

Still on the floor will be the lower frequencies coming from J's and/or W-bins.

This is an example of a typical stage bin, bass at the bottom, then a mid horn, and the treble tweeters at the top.


Three way stage bin

The reason for this arrangement is because bass is pretty well omni-directional and the radiator tends to beam more as the frequency goes up.  In fact it doesn't matter much which way a bass bin is pointing because it tends to radiate in all directions.

What does matter however is what it is radiating into.  If it is suspended high in the air it will be radiating into a whole sphere; sitting on a floor or stage, into half a sphere; and if placed in a corner into a quarter eighth of a sphere.  This makes a large difference to the "radiation resistance" it "sees", and a bass bin jammed into the corner of two walls and the floor is a hell of a lot more effective than standing in the middle of a stage.  Up against the back wall is a quarter of a sphere, half way in between. {ed to correct 130402}

At the same time the surroundings, people, walls, furnishings tend to absorb more sound (if they are soft) and reflect more sound (if they are hard) as the frequency goes up and the wavelength comes down.  For mid-band frequencies each person is roughly equal to half a square meter of open window, so when you have more than twice as many people as there are square metres of wall and roof in the venue you are effectively playing in the open air and the venue should be quite flat and non-reverberant.  Ever noticed that the sound seems to get better as the venue fills up?  This is the reason.

Now if we turn this bin upside-down a couple of things happen; the bass gets worse because it's now up off the floor and radiating into a larger area, and so does the treble.  If it's on a hard wooden stage the trebles will reflect off the floor, so it might not be too bad, but if the band is playing on a carpet (fairly common here these days) there will be little reflection and the radiation will be mainly straight ahead - into the punters knees, where it will quickly be absorbed.  In fact tilting a bin like this back a bit (up the right way) can get some useful bounce off the ceiling and carry the trebles to the back of the venue.

Sometimes you may see a guitarist put their combo up on a chair and tilt it back a bit, and some amps like Fenders actually had legs to make this easier.  The bass loss won't worry a tenor guitarist, and the roof bounce can give better projection. {halls with very high ceilings are a special case and not exciting the reverberations above the audience is much more important in this case; get the amp up, above audience head height if possible, and direct it down towards the rear of the audience}


Low/mid/high dispersion (Fane)

Hopefully the above will provide the answers to Q3.  If the mouth of the horn is taller than it's width then laying it on its side will give a slight gain in the bass, but the lower position of the mid/high front radiation from the driver will be somewhat disadvantaged.  So your surmise is right, yes, upright give better overall dispersion.  "Stacking" bins like these side-by-side can be very beneficial.

It is very helpful to understand how sound behaves at different frequencies because it allows you to "game" difficult environments and venues for best results.

I've heard more than a few soundies say things like "I need ten kilowatts to ride over the reverberation", but this is seriously wrong. The more energy you pump into a reverberation the more it will come back to bite you.  The only way to treat a badly reverberant space, such as a cavernous hall with a very high ceiling, is not to excite the reverberant modes in the first place, and you do that as in the diagramme above, direct the sound into the audience, not the "head space".

I also discuss some of these issues here.

HTH

[mexicanyella]

First off, apologies for already asking the question about horn length/phase relationships before...I just stumbled across a thread where I asked you the same damn question, and you answered it in just as detailed a manner, and my response sounded like I read and understood what you said.

Then I apparently filed it in a mental black hole. So, thank you for taking the time twice!

Both the post above and the link to your comments elsewhere were interesting and informative; thanks for those too. I'll try not to ask all the same things over again in six months this time. Maybe I'll have undertaken a J-bin project by then...

[Roly]

Since I don't remember you will get some idea of how often I answer questions like this in detail.  {one of these daze I'm going to boil all my forum posts down into the mother of all FAQ's.}

I try to answer questions with one eye on the fact that Google can drop anybody into the middle of a thread.

Anyway, I'm glad you found my responses consistent.

Stumbled across the attached on the local eBay this evening, Altec-Lansing theater speaker, asking $AU500.

Edit to add:

Just a footnote about time alignment.  In a big venue the time it takes for sound to travel from the stage to the back of the auditorium is quite significant, and in situations where multiple sources are used along the walls to serve the middle and rear there can be a significant advantage to voice intelligebility by having delay units in the rack so that the radiators get a time-staggered feed and radiate just behind the primary wave passing.

Ref; the Haas Effect for location of the source by the first wave arrival.

[Jack1962]

Great Answers Roly,

Mexicanyella , here's what you need if yo want to play guitar and bass (or anything else) thru the same amp and cabs , buy a power amp (PA) buy monitor cabs not instrument amp cabs power amps just amplify they do care what the source is , monitor cabs have better frequency response .