What defines a mid range

[corElement]

A question which struck me while I was reading up about half way crossovers that most contemporary speakers that say theyre 3 way are actually half.. with

Low-bass
Mid-bass
High-high

For example say a cross over of
220/2200
where everything between 220 and 2200 is mid, where as in a full 3 way speaker it would be like
900/5000
where everything between 900 and 5000 is mid range.

So what's the magical/technical/main call it whatever you want factor that seperates the mid range, mid bass, and low bass driver?

Is it the material? the coil? the size?

 

[Hari Iyer]

There could be number of factors that defines the cross-over point beween the low-mids-highs viz. the frequency response of the drivers, the impedance curve of the drivers, the resonating frequency of the drivers, the order of the cross-over filters (-6dB, -12dB, -18dB, -24dB / octaves), they type of enclosures used (sealed, Vented, Band-pass, Transmission Lines), Octave -to-Octave balance that is required during the transition between lows-mids-highs.

Typically for a first order filter (-6dB/octave) the mid and highs are usually crossed about 2.5 to 3 octaves above the resonating frequency of the drivers so that the gentle roll-off the cross-overs are able to handle the frequnces below the cross-over point. Also as there is a overlap between the low-mids-highs, these designs are usually time-aligned and are coherent sound sources. The -6dB/octave filters are the only filters that do not have any phase shift (-24 dB/octave filters have a phase-shift of 360 deg). It is important to have the -6dB/octave filters to be time-aligned otherwise there will be phase shift due to the non-coherent sound source from the woofer, midrange and tweeters. All the drivers need to have a very wide bandwidth to do justice to the roll-off.

I usually prefer a 6dB/octave filters because of the simplicity of the design and other advantages of having a gentle roll-off and zero phase shift. I usually go for a transmission line 2 way design so that the woofer can be used till the mid-range and use the cross-over only for the tweeter. This kind of design allows me for to avoid all the vices of the cross-overs ( i use a plenty of compensating circuits though) and still get a zero phase-shift design aming at a coherent sound source.

Best Regards,

Hari Iyer

 

[corElement]

Man, thats some heavy duty knowledge right there, I could understand a part of it but not entirely, I couldnt understand the octaves roll off thing (maybe because I dont know what octaves are lol!). I understood the time issue but curious what determines the shift ( or maybe thats what you explained but it went over my head ). I need to read some audio glossary maybe we should make one on this forum.

 

[Hari Iyer]

6dB/octaves mean at the cross-over point say 1000Hz for a mid-range the lower octave will be 500Hz and the gain will be down by -6dB/octave. For a -12dB/octave filter the gain will be down by -12dB and so on.

The 6dB/Octave filter usually uses only one capacitor for the tweeter to roll-off from the cross over point and in a capacitor there is a phase-shift of -90 deg. Hence the output from the woofer and tweeter will be out-of-phase and will cause destructive interference cancelling some of the sounds in the music. Hence the tweeters are usually reverse connected for cancelling the affect of the phase-shift (i assume here that there is no inductor connected to the woofer). If the woofer is connected with the inductor then there is a phase-shift of +90 deg (-6dB/octave) and hence both will eventually cancell out there by giving zero phase shift.

As the attenuation is only -6dB/ocatve on either side of the low and high there will be some kind of overlap of frequencies in both the mids and highs in this type of filter. Hence the driver needs to be wide-band. Typically the tweeter used should have a low resonating frequency so that they can be crossed 3 octaves above that. Assuming that the resonating freq of the tweeter is 600Hz, it will typically be crossed at 1800Hz (600x3) so that the tweeter is able to handle higher power (the level will be low only by -18dB at 600Hz here). Since the woofer is also crossed at 1800Hz, it should have a resonable response till 3.6KHz (ie. 3 octaves above the cross-over point) to do justice to this kind of filter. Since there is an overlap of the frequncies you need to time-align the baffle. Since the radiating source of the tweeter and woofer is not at the same point in the baffle some kind of angle is given to the baffle to align the woofer and tweeter in time so that they operate as a chorent sound source arriving at the listner at the same time. If the drivers are not aligned it will cause a phase shift.

It does not make any sense to time align the baffle for higher order filters ie 12dB/octave and above as there is already a phase shift introduced by the filters. Hence it will not help if you time-align the baffle here.

-6dB/octave filters (1st order) when time-aligned will sound very open and coherent and you can actually listen to the music between the spaces and locate the instruments and due to the gentle roll-off are more natural to listen. Also 1st order filters can handle trainsent response very good and there are few ringing and fast settling time as energies are not stored in the passive componets (inductors and capacitors).

I belive that cross-overs are kind of a necessary evil which we have to live with and try and minimize the usage to the bare minimum and keep the mechanical design of the enclosure (like a TL enclosure) so that vices of the cross-overs can be avoided. You need more compensation in the cross-over only if you have a medicore drivers for which you need to add more passive components for the response shaping.

Best Regards,

Hari.

 

[thevortex]

Very well put, Hari! And welcome to the forums.

 

[Rajiv]

6dB/octaves mean at the cross-over point say 1000Hz for a mid-range the lower octave will be 500Hz and the gain will be down by -6dB/octave. For a -12dB/octave filter the gain will be down by -12dB and so on.......................................................................................

....................................You need more compensation in the cross-over only if you have a medicore drivers for which you need to add more passive components for the response shaping.

Hi,

Hari,very well explained.


Regards
Rajiv