How to Choose a Crossover Frequency Using Wavelength for a High End Loudspeaker

There are several confusing factors when deciding on your crossover frequency using wavelength… Its best to choose a crossover point using wavelength and common acoustics principles that will apply to your design. Let me explain before you hit the back button…

All sound frequencies have a wavelength that can be measured. For example; 2400 Hz has a wavelength of 5.6266 inches. 100 Hz has a wavelength of 135.039 inches or 11.25 feet.

So you can see by this example that higher frequencies have wavelengths that are shorter than lower frequencies.

Here are some examples of wavelength and frequency:

20Hz = 56.2664 feet

200Hz = 5.6266 feet

1000Hz = 1.1253 feet

5000Hz = 2.77 inches

10,000Hz = 1.35 inches

20,000Hz = .6752 inches

To use this calculator go here… set any crossover frequency or distance and find out the opposing calculation.

What this indicates is that the distance between drive units will determine where your best crossover frequency using wavelength. (or you can reverse engineer the speakers by placing drive units at the spacing needed for the chosen crossover point)

If you use a 7” driver and tweeter has a 4” flange your closest center to center point will be 5.5 inches. Or 11 inches divided by 2 = 5.5. You may need a 1/8” thick rib between the drivers for strength also which means that you would have 5.675” from center to center.

In order to get perfect integration at the crossover point you would need a maximum 2379Hz crossover between midrange and tweeter. If the crossover point chosen is higher than this, there would be lobing issues at the crossover region depending on the distance between them and also depending on the listening distance from the speakers.

choosing a crossover frequency using wavelength

There is some good news even if you choose a higher crossover point and cannot place the drivers closer. As distance from the speaker increases the wave of both drivers begins to melt into one wave.

This is very similar to when you throw two rocks into a pond side by side. At first there are two waves going out from each rock’s point of entry. As the waves flow outward the waves converge into one wave just as if only one rock was thrown.

Sound waves from two drivers work very much in the same way. At the crossover point each driver is playing nearly at the same level, but the further you are positioned from the speaker the more you will hear only one sound from the speaker as opposed to two different sounds.

There are other factors at play here

The baffle width is another contributing factor to how the wavelength is influenced by frequency and dimensions. A baffle also has its own wavelength. If the tweeter is positioned in the center of the baffle, there will be two supporting waves supported by the distance to the edge of said baffle.

This is why on some speakers the tweeter may often be mounted slightly to one side. This is actually beneficial to the design as there would be no duplication of support for the same wavelength.

Another way to combat this problem is to have not just a curved front baffle but gently curving edges as well. This does not look like a half inch round on the baffle edge. It is much more drastic.

This would be a curved front face that gently leads into a more aggressive radius of say 1.5” to 2.5” inches.

Obviously, this is difficult to work into the average speaker cabinet as router bits don’t go much over 1” at the most. So this type of rounding of the front edges must be done with CNC or by actually molding the shape instead.

An example of this would be the RockPort Altair loudspeakers. They are made completely out of a molded shape to achieve a good flow from the front wave to the side wave without causing diffraction problems on the edges.

This actually will have an effect on both dispersion of the system and also diffraction. These two acoustic properties are often confused at least in regard to how the baffles play a role in both.

The wavelength is what creates the issues for both problems but they are corrected in differing ways

Dispersion is a component of the width of the baffle, angle and curvature of the baffle, distance between drive units, size of the drive units creating the sound, and the distance between drive units and the edges of the baffle.

Diffraction is a buildup of spurious energy on edges of the baffle created by the wavelengths reproduced by the speaker drivers. In essence; the sound waved get confused not knowing where to go so they can cause ‘cavitation’ at the edges. The crossover frequency using wavelength approach will affect both dispersion and diffraction in differing ways.

I’ll talk more about dispersion in a later article.

So why don’t most speaker system address wavelength both in the baffle design and driver spacing?

Good question. But I suspect the truth is, most companies know about these issues but it is too expensive to deal with the problems at a given price point. This isn’t always the case.

Some speakers that are very high end seem to break all the rules as if the designer were unaware of the affects they have on the sound. Others, seem like they break the rules for means of appearance of the product.

I believe that choosing a crossover frequency using wavelength within a given design should be taken into consideration in any speaker.
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