how does one decide on crossover point(s)? say for example you have a woofer that can do 40-6000 and a tweeter that does 2000-20000. looks like you could do it anyplace between 2000-6000. if both woofer and tweeter are nice 2000-6000 does it matter? how do you decide where to crossover? also besides frequency how do you pick order?

thanks, zonkers

I always try and use tweeters beginning as high a crossover frequency as possible for protection.

I'd say that without more info on the drivers themselves that 2.5khz would be my best "guesstimation".

Please provide info on the drivers if you can.

Regards, Ron

i'm really looking for a general set of rules if possible. from what you say it sounds like a balance. tweeters like higher crossover better for protection but woofers like lower crossover best for good response and not beaming. is that the general idea?

thanks, zonkers

For example, suppose your tweeter is a 90 by 40 degree horn, and your woofer is 8" in diameter. So the woofer's radiation pattern is roughly 90 degrees wide at somewhere around 2000 to 2500 Hz (depending on the characteristics of that particular woofer). In this case, I'd put the crossover somewhere in that ballpark.

If you're using a direct-radiator or bullet tweeter, you won't be able to match up the radiation patterns. In that case, the crossover frequency is less critical than when you're trying to match up radiation patterns but keep in mind that the ear's sensitivity peaks around 3.5 kHz to 4 kHz, so you don't want any peaks in that region or else you'll have a fatiguing loudspeaker. I'd probably want a good 3 or 4 dB or more dip on the tweeter's side of the crossover, this because I place a higher priority on the power response than on the on-axis response.

Duke

there is one thing i don't follow. can you explain this?

"I'd probably want a good 3 or 4 dB or more dip on the tweeter's side of the crossover, this because I place a higher priority on the power response than on the on-axis response."

what's the difference between power response and on-axis response?

thanks, zonkers

Thanks for asking for clarification.

The "on-axis response "measures the frequency response along one axis only, typically assuming anechoic conditions (in other words, reflections are excluded from this measurement).

The "power response" is the summed omnidirectional response of the loudspeaker; it's total output taking into account all angles. It is impractical to measure, but because the ear hears the reflected sound in a room it matters. The power response is strongly influenced by the speaker's radiation pattern.

Assuming a two-way with a dome tweeter, the woofer will be beaming somewhat at the crossover frequency but the tweeter's radiation pattern will be quite wide, possibly more than 180 degrees if the speaker's front baffle is less than 1/2 wavelength wide at the crossover frequency. So if the speaker measures "flat" on-axis, the tweeter will be putting out quite a bit of extra energy off-axis just above the crossover frequency. Typically, this is the lower treble region, maybe 3-4 kHz or so, right smack where the ear is most sensitive. Because the reverberant energy contributes to perceived tonal balance, such a speaker may measure "flat" but sound bright and in extreme cases even harsh due to all the extra lower treble energy in the reverberant sound.

Such a speaker cannot have a smooth on-axis response and a smooth power response at the same time, and in my opinion the power response corresponds more closely with perceived tonal balance in a normal listening room. So, my suggestion (and it's hardly original) is to design in an on-axis dip on the tweeter's side of the crossover, in that lower treble region, as this will smooth out the power response. Because of the way frequency response is typically measured this approach will look less smooth on paper, but it will sound smoother under most listening conditions.

Let me know if you have further questions.

Duke

if i understand you, you are saying the speaker sprays sound differently at high frequency than low so the total sound in the room is what matters. that makes sense.

why are speakers made this way? what causes it?

tia, zonkers

The radiation pattern of a direct-radiator driver depends on how physically wide the driver's diaphragm is compared to the wavelengths being reproduced. If the diaphragm is more than 1/2 wavelength wide, it will start to "beam" - that is, the radiation pattern will start to narrow.

Let me try to explain why beaming occurs. Let's assume we have a 5" diameter cone that is reproducing a sine wave at 1350 Hz, so at this frequency the cone diameter equals 1/2 wavelength (sound travels 13500 inches per second, so one wavelength at 1350 Hz is 10 inches long). The sound from the right-hand edge of the cone actually radiates in all directions, including straight across the cone towards the left-hand side. But by the time it gets to the left-hand side, which is 1/2 wavelength away, the left-hand side of the cone is now moving exactly out-of-phase with the sound that originated on the right-hand side. So, it gets cancelled. This is happening all across the surface of the cone. As a result, less energy is radiated to the sides of the cone than out in front of the cone. The higher up in frequency we go the narrower that main frontal lobe becomes (though we do get some side-lobes at higher frequencies).

So let's take a hypothetical speaker with a 6.5 inch woofer and a 1" dome tweeter, crossed over at 2700 Hz. The actual cone diameter of the woofer is about 5 inches. At this crossover frequency, a sound wave is 5 inches long . Our woofer's diameter is thus one wavelength at the crossover frequency, so the woofer will be beaming - in this case it's radiation pattern will be roughly 90 degrees wide (that's not a "brick wall" at 90 degrees; the anechoic sound pressure level will be down by 6 dB by the time we get to 45 degrees to either side of the centerline.)

Now our 1" dome tweeter's diameter is much less than 1/2 wavelength, so its pattern will be very wide. In fact, it will probably want to be close to 360 degrees (omnidirectional), but the front baffle of the enclosure acts as a 180 degree "horn" and confines its radiation to a 180 degree angle (this assumes the tweeter is not mounted in virtually free-air atop the enclosure, like on some B&W speakers).

Very few driver manufacturers publish polar response plots of their drivers, but prosound manufacturer Selenium of Brazil does. At the link below you'll find the spec sheet for one of their 12" woofers. Note that the radiation pattern narrows as we go up in frequency until we get to 3.125 kHz - where suddenly, the pattern widens! The reason is cone break-up; now the cone is flexing severely, and acting as if its diameter is much smaller than it really is. Note also that in the 2 kHz plot we see side-lobes starting to form. Finally, the Selenium woofer's pattern is generally wider than rigid piston theory would predict even below well 3.125 kHz, and this is because the cone is not perfectly rigid so some flexture is occuring.

Let's look as some implications of this beaming phenomenon. At low frequencies the woofer diameter is much smaller than a wavelength so the bass will be omnidirectional. At 13,500 Hz the tweeter's diameter if equal to one wavelength, so its radiation pattern will be about 90 degrees at that frequency, and will continue to narrow as we go up higher.

What about an MTM? Well, the vertical woofer arrangement will result in beaming setting in in the vertical plane at a much lower frequency than in the horizontal plane. And look at the traditional sideways MTM used for a center channel - now the dual-woofer beaming is in the horizontal plane! This is exactly what you don't want - you want the center channel to have correct tonal balance for everyone in the room, but instead it's now the speaker whose tonal balance changes the most with different listening positions.

Let's look at a ribbon tweeter, with its relatively tall, narrow diaphragm. A ribbon will have a very wide radiation pattern in the horizontal plane, but will beam badly at high frequencies in the vertical plane.

Well I've probably rambled enough. Hope this helps. And if I've made any mistakes here, I welcome correction or clarification.

Duke

• Selenium 12 inch woofer - polars on page two