Speaker Design Q&A

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richidoo

Re: Speaker Design
« Reply #20 on: 23 Feb 2021, 04:59 pm »
I was planning on not having a high pass filter for my midrange driver in a 3-way design while having a low pass filter on the woofer. Based on what you just said, this would be a bad idea because the woofer would be out of phase from the midrange at specific frequencies. Correct?

If the drivers had flat FR a cople octaves above and bleow the crossover point then yes, the midrange driver would stay flat while the low pass on the woofer would rolloff the woofer, so you would have unequal change in attenuation, which equals unequal rotation of phase.  But you could have phase coherence if the midrange driver had a natural rolloff (without electrcial filter) to match the rolloff of the woofer with electrical low pass filter. Acoustic response is what really matters. Whatever causes attenuation also causes corresponding phase rotation. In this case, the most likely method by which the midrange would roll off with decreasing frequency is loss of acoustic impedance, meaning the cone is too small to grip the air at the low velocity of low frequency. This is exactly why larger cones are used to play bass freqs and smaller for treble. A small midrange cone, like 2" will roll off significantly at a frequency that could be used for a crossover. But it s rarely perfect and some kind of electrical filter is usually added as high pass or as EQ to tweak the response a little to better match the other drivers response.  But there are some such designs like that. See Troels Gravesen DTQWT, no high pass filter on the 8" midrange.

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Also when I’ve been saying “at the crossover point” I should have been saying “at the roll off point”.


The rolloff point, also called the "knee" of a high pass or low pass filter is -3dB. This is the nominal frequency of a filter outside the loudspeaker world, and early crossover designers used this standard to cross drivers at -3dB, with such XO designs being called Bessel, Butterworth, etc. but there are some problems with crossing at -3dB, which were solved by Siegfried Linkwitz and Russ Riley.  Most modern loudspeaker crossovers take advantage of their invention and cross at the -6dB point with Linkwitz-Riley crossover, aka "LR4" abbreviation to mean Linkwitz Riley 4th order XO.

In your case, before you decide midrange gets no high pass, measure all the drivers electrically and acoustically to determine their natural behavior without electrical filter. Then import the data into a loudspeaker design simulator (like X-Sim) to assist in choosing the electrical filters and physical arrangement of drivers that will give you the acoustic response you want. Then, build it, listen, cry, measure to find out what went wrong, repeat.

richidoo

Re: Speaker Design
« Reply #21 on: 23 Feb 2021, 05:23 pm »
FRM is correct on all counts. I started by building full range speakers and learned a lot from that before I felt ready to try multiway. There is a lot to learn about speaker basics with full range speakers. In some ways they are harder to make sound good than multiway. And they have certain sonic attributes that are worth emulating in a multiway design.

New question! When building a 3-way speaker where each driver has an impedance of 8 ohms, how can I get the whole speaker to be 8 ohm without sacrificing sensitivity? I know you can use resistors to up the impedance, but won’t this have an effect on efficiency? Also, more speakers in parallel=more efficiency, and more speakers in series = less efficiency, right?

Impedance is never perfectly flat through the entire speaker bandwidth. A speaker is a reactive load so the impedance changes with freq. As you work with the simulator and read about XO design you'll figure out how to make the changes needed to have impedance curve that your amplifier can handle. Voltage source amps don't care about impedance as long as they can source the current, keeping impedance above 4 ohms is industry standard, but some of the best designers deliberately dip below that. Fine if the amp has adequate current. If you use current source amplifier, or an amp kinda in the middle, like low power SET tube amps which can't source much current then you have to design for flattish impedance response. Few designers do this due to the prevalence of SS voltage source amplifiers, but there are some that still think a speaker should aim for flat impedance.

Technically your simple examples are correct, but in reality, nothing is simple about loudspeaker design. A driver's impedance is not flat at 8 ohms throughout the entire frequency range, nor is the frequency response. Combining drivers together in parallel, the warts on one affect the others. Adding electrical filters also affects everything. Once you start simulating your design you'll gradually learn how everything interacts.