Do loudspeakers automatically block frequencies they can’t handle?

roystonchris

Standard Member
Sorry if the question is simple, I’m new to all this stuff.

I know that the tweeter has a capacitor in line to block lower frequencies, but I was wondering, does a speaker divert everything else to the woofer, or does that also have some form of capacitor/filter to block anything below its optimum range? Reason I’m asking is that a HiFi amp will (I assume) output all frequencies and leave the speaker to sort them out, but with an AV amp you specify the speaker range. Or is that done just so the amp knows when to add in the sub, and the signal to the speakers actually contains all frequencies?

Was wondering because, in a car for example, you might have different speakers in the front and back, and you don’t specify the range on the headunit.
 
S

Snagglington

Guest
Well, I'll have a go here - I'm a complete novice at buying home cinema but have messed with hifi all my life so I feel reasonably confident to answer you - please point out if I make any errors please experts!!

So - the passive crossover network can be either very simple or complex little piece depending on what you buy, but basically:

If your tweeter has just one capacitor then this is a very simple crossover which will 'roll off' into the midrange at 6db down per octave that the frequency drops.

Now, as you say, that filters out the midrange and below and allows the tweeter to perform happily, but what about the mids etc?

Well - as frequency climbs (in respect of the mids) the impedence of a speaker generally increases as the speaker gets up a long way from its natural operating range. This naturally reduces the output of a speaker without doing anything at all, so from this perspective nothing is needed to make the output swap over happily to the tweeter..... BUT

However, in practice there are problems with this because at the frequencies where the output from the mid has 'rolled off' the amp can electrically still 'see' the load from the speaker - so this would 'load down' the amp at higher frequencies.

The solution is for the speaker designer to insert a coil (kind of the opposite of a capacitor!) in the line to the midrange. This way the mid now rolls off load wise and the tweeter sweetly takes over.

To take this to the next level so that you understand it's not that simple we here have only looked at 6db per octave (single component) crossovers. The aspiring crossover designer has the ability to add multiple components to the crossover to create different roll off slopes - 12db per octave, 18db per octave and even a very steep 24db per octave are all possible with varying degrees of difficulty and cost!

In addition to that, it is possible for the designer to add 'l pads' to the crossover which actually drops the output of a complete speaker in relation to another by increasing the resistance of that complete speaker load - this allows the designer to 'match' the output of a speaker set.

Finally, there are other complex possibilities such as 'series notch filters' and 'zobel networks' - these are designed to compensate for the natural impedence (and forced impedence changes) created when a speaker is made -(and other things - have a google if you're interested!)

The final thing to remember is that the actual 'elbow' of the crossover as well as the 'slope' is configurable depending on how far the designer goes and the way the crossover is built.

So - in summary -

1) a capacitor and coil is about as simple as it gets.

2) It is not a given that an 'active' system which is little more than an electronically adjustable slope in many cases will outperform a hand crafted and carefully engineered passive crossover.

3) The design of a crossover is a science that occupies the best speaker designers for a great portion of the development and contributes to many of the systems you read about on here that perform in an outstanding manner.

I hope this helps you to understand a little more.

(Now if you know anything about choosing a decent set of 5.1 speakers for upto a grand feel free to help me because having come to this market fresh from many years of only buying stereo I'm completely bewildered by what's available!!)

Cheers,

Peter
 

BlueWizard

Distinguished Member
That was actually a very good explanation for someone who claims to be a novice.

Looking at the entire speaker system as a single unit, the system relies on the natural ability of the speakers to roll-off at frequencies outside their working range.

So, on the low end, nothing limits the bass other than the speakers ability to respond. As frequencies go lower and lower, the speakers is putting out less acoustical energy.

On the high end, we have something similar, but more so is the fact that as frequencies go up, there is simply less and less music available to the tweeters. Fundamental musical notes don't go much above 5khz, so the first octave harmonic is 10khz, and the second octave harmonic is at 20khz. These harmonics, within a certain context, are faint echoes of the lower fundamental note. The higher the harmonic, the fainter the echo.

This is not a technically perfect explanation, but it illustrates the principle. While we can hear up to 20khz, in a perfect world, there are no fundamental musical notes that are even remotely that high.

So, my point here, is that the music itself is naturally rolling off as the frequency goes up.

As to the individual speakers that make up a speakers system, the crossovers can be as simple as a single capacitor filtering bass from the tweeter. In this case, we rely on the natural roll-off of the woofer to reduce high frequencies in the woofer itself.

But, this can't be done with all woofers. Many woofers, and especially hard cone woofer have a nasty breakup area above their working range.

Notice the frequency response graph (top right) of this aluminum cone Dayton Reference woofer -

http://www.parts-express.com/pdf/295-362s.pdf

Nice and smooth across the bass region, it starts to roll off, then the response goes nuts and jumps all over the place. This speaker can not be expected to roll off naturally, it needs heavy filtering.

In this case, you would use a filter that rolls off faster, and starts lower, so that when you get to that break up region, the volume is extremely low.

Also, keep in mind that these crossover frequencies are not brick walls. They are not absolute barriers. When a given speaker gets to its crossover, it gradually fade in or fades out.

Using one crossover component on each speaker gives you a 6db per octave slope. One octave up or down, and the output from the speaker has dropped by 6db. That is a very small amount. Two octaves down, it is down by 12db. Three octaves down, it is down by 18db.

While that can work in a properly designed speaker, it is typically too slow, it doesn't rise or fade fast enough.

Far more common, is a crossover that uses two components per speakers, given a 12db rise or roll of. Now the volume comes in and goes out faster.

Occasionally, if you are close to the spec limits of a driver, an 18db per octave crossover is used, and only in the rares instances, as 24db per octave crossover is used.

Occasionally, in the middle of the frequency response curve, there will be a slight peak, for whatever reason, the peaks can be flattened out with a notch filter.

The Zobel networks are components added to flatten the impedance of various speakers.

For example, in the Dayton speaker specs linked to above, has another graph, just below the first (lower right). The line with the high peak in it is the impedance graph. Notice on the high end, well out of the working range of the woofer, the impedance starts to rise.

Why would this matter? Well, the crossovers are design to interact with a specific impedance, if the impedance shifts, so does the crossover point. To keep the crossovers stable, a Zobel network is added to flatten the impedance, even though the impedance that is is flattening is outside the working range of the woofer.

Zobel Networks are actually pretty simple. They are just a capacitor and resistor in parallel with the woofer (as an example). When the frequency goes up, the impedance of the capacitor goes down, placing the resistor in parallel with the woofer, thereby stabilizing the impedances seen by the crossover.

So, in general, there is nothing on the top end or bottom end of a speaker system to control frequencies outside its rated range. The system simply relies on the natural roll-off of the speaker itself, or the available music.

In the middle, especially the midrange driver, there are filters limiting what frequencies are fed to the various drivers.

But again, the frequency limits are not brick walls, they don't just start and stop, they gradually ramp up or roll-off. At a give crossover frequency, both speakers are playing equally, though with a slight attenuation to make up for the fact that there are two speakers.

I'm not sure if we've answered your question, but you should have a better idea of how crossovers and filters work.

Steve/bluewizard
 
S

Snagglington

Guest
Thanks for adding that Steve - I'm certainly a novice at BUYING home cinema and home cinema in general but as I say I've played with hifi (stereo) all my life so have a reasonable idea about it technically ! (Though when you come to actually purchase being a new entry to 5.1 it's amazing how little technical knowledge helps!)

I hope that between us we have answered the OP's question without making it too complex.

Unfortunately it is a very complicated subject :-(
 

roystonchris

Standard Member
Thanks both for taking the trouble to answer that so thoroughly.

Interesting that a speaker will ‘self block’ all the lower frequencies it can’t manage – my setup is using the Jamo style-speakers, which obviously lack the mid/lower ranges. You can certainly hear the lack of bass when using pure direct on the amp, yet the treble sounds clear, so I guess the speakers aren’t straining to play frequencies they can’t manage.

Got interested in this because I was reading a thread on a car forum about a stereo setup in an older car, where the author used the example of having 2-way speakers in the front, 6x9 in the back, and a sub. He suggested manually soldering in capacitors to block lower frequencies from the fronts, and also the 6x9s so that the speakers don’t exert themselves trying to do things thay can’t manage…but going on what you’ve said, that should be strictly necessary since the speakers ‘self block’.

One final Q, when you set a speaker to ‘small’ or ‘large’ in an AV setup, I presume this makes no difference to the frequencies sent to the speakers, it’s just so the amp knows to start adding the bass to fill in any gaps according to the crossover frequency you specified?
 
S

Snagglington

Guest
- mmm not quite - the speakers at the bottom end won't actually 'self block', it's just that as the frequency drops down lower the response of the speaker becomes weaker as it moves out of its happy operating range.

Unfortunately as this happens the cone 'excursion' for a given input wattage increases as the power handling of the box/speaker combo drops off... this means that you will need to have a crossover (this time a high pass again like when you stopped the tweeters playing midrange earlier with your capacitor - except that this time your capacitor will be much bigger).

The practicality of doing this 'passively' is not so good because you can't easily adjust it to taste. With an active crossover (ie one built into your amp) in this case you, the user, can fine tune the slope and frequency (have you seen the thread where it is being discussed what frequency to cross over to the sub?) to switch into the sub as seamlessly as possible.

Generally this low end crossover would be more practical therefore to be done actively, whearas the 'in speaker' crossover we talked of earlier was better left to the speaker designer.

Hope that makes sense :)
 
S

Snagglington

Guest
One final Q, when you set a speaker to ‘small’ or ‘large’ in an AV setup, I presume this makes no difference to the frequencies sent to the speakers, it’s just so the amp knows to start adding the bass to fill in any gaps according to the crossover frequency you specified?
I don't know what processing the amp does for this - (as I said before I'm new to AV and the stereo amps I have used through my life didn't have this level of super-dooper trickery! - come to think of it whenever electronics are concerned in my experience the more 'gadgets' in there the worse the sound!!!) But as I say that's hifi not AV.

Maybe Steve or someone else could fill in this one?
 

roystonchris

Standard Member
After a lot more reading, I’ve discovered that when you set the speakers to ‘small’ on an AV amp, the lower frequencies are blocked, according to your crossover setting. The sub then takes over.

On my setup, the sub goes up to 150Hz, and the speakers, which are the small ‘style’ type, only go down to 150Hz also, which would be fine, except on the amp, there’s a gap, the crossover options available are 120 or 160. So, either I lose between 120 to 150, or 150-160. Hypothetically, if I set the front speakers to large and the crossover to 160, presumably I’d then get the missing 150-160 from the fronts, even though the speakers themselves would then have to do a lot more passive filtering. Might try this and see how it sounds. Fortunately, according to the manual, it’s still possible to get the sub to add the low frequencies from the fronts in tandem, so these shouldn’t be lost.

Cheers both for all your help with this,
Chris
 
S

Snagglington

Guest
Gotcha - it's just basically a variable crossover presety kinda thing.

I'm with you - makes sense:smashin:
 

BlueWizard

Distinguished Member
After a lot more reading, I’ve discovered that when you set the speakers to ‘small’ on an AV amp, the lower frequencies are blocked, according to your crossover setting. The sub then takes over.

... the crossover options available are 120 or 160. So, either I lose between 120 to 150, or 150-160. ...

Cheers both for all your help with this,
Chris
Remember that the crossover settings are slopes not walls. At the crossover frequency of 160hz BOTH speakers are playing equally. So, you don't have a gap in the 150hz to 160hz range, just a very slight dip in the response. A dip so small that you are not likely to even know that it is there.

Here is a link showing graphs of what the slopes look like. The most shallow is a 6db per octave, next 12db per octave, then 18db, and 24db. Most common are the one and two element 6db/octave and 12db/octave -

Passive Crossover Slopes

Color coded in the graphs, blue or cyan is a 6db, red is 12db, green is 18db, and pink/magenta is 24db per octave slope.

So, the transition between the subwoofer and the front speakers is very gradual.

Steve/bluewizard
 

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