Multiple drivers wiring vs total impedance vs amplifier power distribution

[djtetei]

Hello to everybody
I want to put together a stereo system containing 2 cabinets with a total of 8 drivers, each cabinet housing 4 drivers (2 woofers, 1 midrange, 1 tweeter).
Each cabinet will have 3 enclosures (2 separate ported enclosures for the woofers, 1 enclosure for midrange and tweeter).
All drivers are from the same manufacturer and their specifications are:

No	Brand	Model	Type	Size	Impedance	RMS Power	Max Power
1	A	X	Woofer	6.5 Inch	8 Ohm	70 W	100 W
2	A	X	Woofer	6.5 Inch	8 Ohm	70 W	100 W
3	A	Y	Midrange	5 Inch	8 Ohm	60 W	90 W
4	A	Z	Tweeter	1 Inch	8 Ohm	80 W	140 W

The crossover network looks like this:

The cabinet is supposed to have a total nominal impedance of 4 ohm, presented to an amplifier rated at 140 W / channel RMS and the woofers will be wired in parallel.
How should the midrange and tweeter drivers be wired?
How much power the amplifier will distribute to each cabinet driver?
The answers which I hope to get to my questions will help me to model the drivers response in Bass Box Pro with the exact amount of power for every driver in order to control cone excursion above the tuning frequency.
Thank you in advance for your patience and for your kind replies!

[5th element]

This looks like it is a Visaton kit? And the answer to your question is very difficult to answer. The actual amount of power being delivered to the individual drive units is really dependent on the type of music that you listen to.

Power ratings for drivers are not in any way equivalent to the power ratings described on amplifiers. Amplifiers are usually capable of outputting their rated power with a sine wave stimulus, at least with one channel driven, whereas loudspeakers are specified usually for peak power values that are more representative of music.

For example most tweeters will be able to handle say 1-15 watts with a sine wave stimulus, depending on their design. Tiny neo tweeters, for example, will be able to handle a lot less continuous power than a big beefy, full sized affair with a whacking great big ferrite magnet. Still you can feed a transient into a tweeter, typical of high frequencies, at a very high instantaneous power level (say 100 watts into the 1 watt sine wave capable tweeter) and it will be absolutely fine. What's important here is the duration of the applied signal.

To make matters even more complicated the crossover modifies and alters the loudspeakers overall impedance, this does two things. Firstly it band limits the drivers within the loudspeaker, but the other thing that happens is it alters how hard the loudspeaker is to drive. Now the confusing part here is that the drive units themselves are no harder to drive with or without the crossover in place but as the crossover components are resistive and reactive in nature, the amplifier has to drive them as well as the drive units. With a poorly designed xover (with regards to impedance) one loudspeaker might require an amplifier capable of delivering 500 watts into a 3ohm load, whereas the exact same loudspeaker with an impedance optimised xover might only require 150 watts into a 7 ohm load to reach a given SPL.

Power is typically the incorrect way to view things when looking at loudspeakers though, especially if you're wanting to simulate driver excursion vs drive level. Loudspeakers are voltage driven and what you want to look at is how the loudspeaker performs at a given level of voltage drive. The loudspeaker will then draw the amount of current necessary to support said voltage from the amplifier that is driving it. You can then calculate the required power for a sine stimulus from the voltage and current.

The tweeter and the midrange driver should be wired with reverse polarity with respect to the woofers.

Both the tweeter and midrange are high passed via the xover and their excursion levels are protected via the xover. You do not need to simulate anything with respect to them.

At low frequencies only the bass drivers will be being driven and it's also only the low frequencies that could possibly upset the bass drivers with respect to their excursion levels. For bass simulation purposes just assume that the drivers are receiving the full voltage from the output of the amplifier. The bass drivers are wired in parallel, so each driver sees exactly the same voltage.

If you're worried about protecting the drivers from over excursion then the way you'll want to do this is via a signal limiter on the input of the amplifier NOT by using an amplifier of limited output power. Driving a small amplifier too hard, so that it could potentially clip, is one of the worst things you can do for keeping loudspeakers healthy. You are much better off going with an amplifier of ample power and being careful with your volume control, or simply use a limiter or some sort. Limiters are rarely seen with domestic hifi but they are used regularly in the professional world to keep amplifiers from clipping and therefore keep loudspeakers alive.

[djtetei]

The woofer power rating is 70 W RMS, xmax = 6 mm and xlim = 11 mm. Bass Box Pro simulation graph depicted the fact that in an enclosure of 27 liters, with a signal equivalent to 70 W, the cone displacement above the tuning frequency will reach a maximum of 7.88 mm, meaning that at high levels the woofer will present some distorsions, but as long as the cone displacement stays below the xlim value of 11 mm, the woofer integrity should be ok.
Cone displacement below the tuning frequency shoul be atenuated with a subsonic high pass filter.
Please, correct me if I am wrong!

[5th element]

A sub sonic filter is always a good idea for ported loudspeakers. Lots of music and especially movies now contain large amounts of very low frequency information. It isn't particularly easy to damage a ported loudspeaker above, or at, its tuning frequency, but go below this and everything flies out of control.

The most deadly thing that can happen to a driver, from a mechanical point of view, is if the voice coil slams into the back plate during an over excursion scenario. Depending on the driver design this may not be possible. Some have their motor parts sized such that the excursion limit, imposed by the suspension and spider, physically prevent the coil from being able to reach the back plate. If your driver is like this then going to xmech is less likely to result in a dead driver. When the voice coil slams into the back plate it can instantaneously destroy the driver. If the surround and spider prevent this from happening, but instead are stretched beyond their comfortable limits, this will not make the driver explode, but they will fail over time.

[5th element]

I suppose I should clarify here that sub sonic simply refers to frequencies that the loudspeaker cannot really produce rather than frequencies we cannot hear!

[djtetei]

Thank you kindly for your prompt answers.
When I used the term subsonic I was talking about frequencies below the capabilities of the driver and in this case frequencies just below the tuning frequency, because below the tuning frequency the driver will act as if it were in free air.

[fbov]

Minor point...

Sub- and super- refer to speed, as in subsonic and supersonic flight

Infra- and ultra- refer to frequency/wavelength, as in infrared and ultraviolet. An infrasonic filter removes energy at frequencies below the human detection threshold (wherever that is).

Frank

[5th element]

Minor, but very important, point worth taking note of. (And the kind I don't like to slip up on!).

Thanks Frank.