The loudspeaker. A reverse microphone 

Reasons to use 1st order crossovers

Vector outputs combine to unity, like 3rd order, but transient perfect

Time and phase coherent.

Widest blend of frequencies between drivers

Greatest control of midrange timbre

Lowers EMF to the amplifier

Solid and well defined soundstage

Partners the Head Related Transfer Function

A crossover is an electrical filter circuit, located inside your speaker cabinet, usually close behind the speaker wire connections.

It receives the music signal from the amplifier and then filters it to pass the relevant frequencies to each drive unit of the speaker. Our 1st order crossover has a capacitor to pass high frequencies to the tweeter and an inductor to pass low frequencies to the woofer.

Crossovers are described as having an 'order', 1st order, 2nd order, 3rd and 4th. The number denotes the strength of the filter, with 1st being the weakest and 4th the strongest.

Inductors and capacitors are 'reactive' components, changing their electrical characteristics as each frequency arrives.

This reactance is the reason they are used. Inductors filter out high frequencies. Capacitors filter out low frequencies.

 

Each of these components share the same value of filter power. For each octave they will lower the signal by 6dB, (-6dB).

A 4th order filter, for a woofer, would feature 4 components, typically 2 inductors and 2 capacitors. 

A 3rd order filter, for a woofer, would feature 3 components, typically 2 inductors and 1 capacitor.

A 2nd order filter, for a woofer, would feature 2 components, typically 1 inductor and 1 capacitor.

A 1st order filter, for a woofer, would feature 1 component, typically 1 inductor.

1 inductor. . . . . . . . . . . . .   (which is a 1st order filter ) that starts to filter at 500Hz will then lower 1000Hz by -6dB and lower 2000Hz by -12dB, 4000Hz by -18dB and 8000Hz by -24dB. 

1 inductor with 1 capacitor (which is a 2nd order filter ) that starts to filter at 500Hz will then lower 1000Hz by -12dB

and lower 2000Hz by -24dB, 4000Hz by -36dB and 8000Hz by -48dB.

3rd = 1000 -18dB, 2000 -36dB, 4000 -54dB, 8000 -72dB

4th = 1000 -24dB, 2000 -48dB, 4000 -72dB, 8000 -96dB

The same would then apply to a capacitor which filters high frequencies as they descend to meet the low frequencies.

A single capacitor (1st order) filtering from 3000Hz would lower 1500Hz by -6dB then 750Hz by -12dB, 375Hz by -18dB etc.

Below is a graph showing each order of crossover for low and high frequencies as they combine. This example is showing a 1000 Hz (1KHz ) crossover point

It all appears quite functional, harmless and unimportant, the crossover is hidden away and we don't see it working. Loudspeaker manufacturers rarely mention them in their specifications, usually just quoting the single crossover frequency and not stating the strength/order of crossover filters used. Reviewers normally take a cursory look using the limited information from the user manual and like the majority of us, do not really want to know the type of filter being used.

For the loudspeaker designer the choice of filter tells them a great deal before the listening begins

When your music arrived at the speaker it was in pretty good shape. Everything arrived on time, in the correct order and each sound started and stopped precisely where it was meant to. It was correct in transients, amplitude, time and phase.

The speaker drive units, tweeter and woofer, now need all the help they can get from the crossover to maintain the original structure of the music because within the transients, amplitude, time and phase are the triggers that help your ear and brain decide how realistic the music appears to be.

I'm sad to say that once the music has passed through the crossover (capacitors and inductors) it emerges with changes to the transients, amplitude time and phase. Every original key part of the musical structure has been altered or moved from its original position. 

Speaker manufacturers do not like talking about their crossover designs very much and if they do talk it is only to focus on the one strength it may possess such as time coherence or phase coherence, power handling or that it is a simple, minimalist design. They would rather talk about the materials used in the drivers, a bespoke component or how the new speaker betters the previous model and generally aspects that are more obvious for the buyer to see and understand.

Crossovers are, however, the single most influential component in determining how the loudspeaker will reproduce the original music signal. They deserve more attention and understanding because they set the parameters for the drivers and ultimately how they will operate as a 'single' unified force in reconstructing the original signal. 

 

We use 1st order crossover filters for the tweeter and woofer drive units in our speakers. We do this because the music reproduced appears very natural, easy to listen to and understand with effortless detail and a realistic timbre coupled to solid imaging of the sound stage.

 

At 3 Square Audio we are very experienced listeners of music playback systems (36 years and still going strong) and we definitely know how we want our speakers to replay music but only slowly are we beginning to understand why 1st order gives us ALL the properties we want. It is a crossover that can help the speaker drive units, more than any other, work as a team.

In brief it outputs a -45 degree vector to the tweeter and a +45 degree vector to the woofer. This is as good as it gets for the speaker drive units, a signal that sums to unity with a combined phase shift of zero.

Below is a visual version of how the music 'reads' when it exits a crossover, that is not 1st order.

Aoccdrnig to rscheearch at Cmabrigde Uinervtisy, it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer be at the rghit pclae. The rset can be a toatl mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.

The above text shows how the timing and order of information has been corrupted but has sufficient structure for the brain to process and make sense of it.

 

The brain will have a similar demand when the music signal undergoes the same alterations. Frequencies are passed at inconsistent times between each drive unit causing errors in leading edge information, transients, image scale, image placement, timbre etc. Many of the subconscious aural cues have been compromised and now diminish the believability of the performance. 

 

Those text/timing errors increase to thier peak at the crossover point between the woofer and tweeter. A 2nd order crossover will turn half of the letters upside down while 4th order would not invert any letters or change their order but instead duplicate the first and last letter of each word i.e. ffirstt aandd llastt lletterr ooff eeachh wwordd.

Almost all loudspeaker manufacturers use 2nd, 3rd, or 4th order crossovers and very often a combination, such as 2nd on the woofer and 3rd on the tweeter. 

1st order

A 1st order crossover will not change the order of letters, invert or duplicate them. They will exit the crossover in the same order they entered it. The letters would look tilted, half leaning forward 45 degrees and half leaning back 45 degrees.

It is this time/order coherence 'locked' to its phase coherence that allows the brain to turn down its corrective work, relax and perceive the music signal as more natural and lifelike.

With 1st order we have retained the fundamental structure of the input signal, preserving and passing to the tweeter and woofer the most accurate time and phase information within the original recording.  

4th order

Our crossover frequency point is 2600Hz, the last E on a piano.

This frequency point that we choose to crossover between the two drive units is often avoided, even by manufacturers using the strong 4th order filters. The ear is very sensitive to the properties of sound between 2KHz and 6KHz.

This brings us to another benefit in using 1st order. As the filters are very gentle in attenuation they allow the drive units to share and blend the largest range of frequencies in the most sensitive range of our hearing. We now have the ability to tune and voice, by ear,* how the blend works. The end result is a sublime neutrality of timbre, excellent placement of instruments in the soundstage and the ability to play at higher levels than would normally not be possible with 1st order. 

* This allows the Equal Loudness Level Contours (ISO 226:2003) and Head Related Transfer Function to play thier vital part in how we voice our speakers. 

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