Pt. 2: Audibility of Crossovers

[cuibono]

This is the second installment of my investigation into the audibility of crossovers. The first part is here, where I asked people what crossovers they considered least audible. The premise is that I may rebuild my current dipole speakers as a 4-way, which would then introduce a crossover at 700Hz.

The primary concern is that introducing a crossover at 700Hz would put it right in the middle of the human voice range, where we are very sensitive to sound. But there are several reasons for doing so: it would allow me much greater flexibility in choosing midrange drivers; it would allow me to move my other two crossover points out of the midrange (from 275Hz and ~2kHz out to about 140Hz and 2.8kHz); it would allow greater regularity of the off axis response; and finally it would allow much greater output SPL and lower lower nonlinear distortion. Those are some pretty good reasons.

The results of the previous poll were a little surprising - almost nobody used higher order crossovers (above 4th order) and many people don't go for any particular alignment.

So what I have done now it to try and develop a test to help determine just how audible a crossover is, particularly at 700Hz.



What I've done is take two 6" drivers, specifically Eminence Alpha6a. I EQ them to be approximately flat between 250Hz and 10kHz, and as similar to each other as possible. Here are the two on axis response curves:



As you can see, they are very close - note the 3dB per vertical division. Here is one driver's polar response, just so people know what we are dealing with. The overall sound is okay - good for a single driver system, but poor compared to full range speaker systems.



My intention was to be able to listen to a single driver, and then using A/B type switching in my active crossover software (Reaper), introduce a crossover by using both drivers. The crossovers of interest were LR2, LR4 and LR8, all at 700Hz.

Here is an example of the LR4 crossover - one driver gets the lower half, the other gets the upper half.



Here is a comparison of both drivers individual frequency responses, overlaid with the LR2 response.



Here are the three FR's of the LR2, LR4 and LR8 overlaid. As you can see, they are exceedingly close:

[cuibono]

The results? There were some audible changes compared to a single full range driver.

There are two issues with crossovers as I see it - phase and power response. The phase (group delay) nonlinearity of LR type crossovers is fairly well known. Linkwitz discusses it here. His conclusion is that the phase errors introduced by LR2 and LR4 type crossovers are inaudible.

Perhaps less know is John Kreskovsky's analysis of the power response dip due to crossovers. In summary, his calculations suggest LR type crossovers introduce a dip of at least -3dB around the crossover region in the speakers total acoustic output (power response), with the width of that dip narrowing for higher order crossovers. He postulates that higher order crossovers are more audible than lower order crossovers due to the more abrupt change in power response.

So what I did was take a listen to the various alignments, with various types of music. I would select two alignments, listen for some time, and then do a quick switch between them - one alignment was always the full range driver, and the other was one of the three crossover variations. I did this over several days, both sighted and blind. Interestingly, classical music, which is usually very revealing, was quite unrevealed to me. But human voice was very revealing - both male and female. Also interesting, sighted and blind tests gave the same conclusions, but sighted tests took less time to make a determination.

So what were the results???

The basic trend was that the higher orders were more audible. LR2 was indistinguishable from the single full range driver, while LR8 was easily identifiable. LR4 fell in the middle - it was slightly audible, only on very close examination.

How did they sound different? Higher orders made human voices sound more forward and direct, over a very small part of their spectrum. The full range driver (or LR2) sounded smoother, with better ambience. I should add here that this may be a feature of the 700Hz crossover frequency, and I would expect different results much lower or higher (like 150Hz or 3000Hz).

I want to emphasize that the differences ranged from very to extremely subtle. A poorly done crossover is many time more audible than the differences between these crossover alignments. The sense of forwardness was hard to pinpoint - the basic frequency response of the music didn't change - but it was like a small little section of the voice stood out more compared to the full range driver. It seemed apparent that this was due to a reduction in the ambient sounds associated with the regions of change - basically a reduction in the voices' reverberant field. These differences would probably be impossible to judge without quick A/B switching.

But I will add something that I felt was important - LR8 always had a sense of unpleasantness to it. The degree that part of the voice jutted forward was always got on my nerves. Because of this, I've decided to avoid higher order crossovers (above LR4), at least in this application. Like I said earlier, LR8 was also fairly easy to discern while switching.

I feel this test is evidence supporting John K's conclusions - that the crossover's main issue is the reduction in the total sound output at the crossover region, with higher order crossovers being more intrusive. Initially, I had thought the opposite would be true, but I'm glad to know just how little audible difference the LR4 crossover at 700Hz made. I'm guessing Linkwitz's test didn't show these results because it was a test of electrical phase changes, not acoustic summing of multiple drivers.

[David Gatti]

Brilliant work, much appreciated.

[Pano]

Very nice test - thanks! I certainly have never seen it done that way before.

Yesterday I was experimenting with crossover slopes, too. My crossover is at about 600Hz. Changing types had a similar effect - moving things closer and farther away. It seemed to me that the better the phase, the farther back things moved.

Similar to MP3 vs uncompressed. MP3 often sounds more forward to me.

Thanks for posting that.

[bear]

You are considering amplitude response only in your tests.

If you consider other ways of looking at the output it may illuminate other factors?

_-_-bear

[cuibono]

I'm interested in other points of view. What other ways and factors are you thinking of?

[catapult]

A couple of Dutch guys did a similar test with 4" drivers. They ended up preferring a BW3 crossover to the LRx. You might give that a try just for fun. Try it with same and inverted polarity.

[Keriwena]

You show a chart for the on-axis response with the xover, did you also chart the off axis response?

[john k...]

Originally Posted by cuibono

Perhaps less know is John Kreskovsky's analysis of the power response dip due to crossovers. In summary, his calculations suggest LR type crossovers introduce a dip of at least -3dB around the crossover region in the speakers total acoustic output (power response), with the width of that dip narrowing for higher order crossovers. He postulates that higher order crossovers are more audible than lower order crossovers due to the more abrupt change in power response.

This is true when the crossover is at a frequency where off axis nulls appear. If the separation of the drivers is significantly less than a wave length at the crossover frequency then the dip for the LR crossover will be reduced and ultimately disappear as the wave length gets longer. At 700 Hz the wave length is 1.6 feet so for drivers as you have them positioned I would not expect to see much of a dip in the power resposne. For a dipole set up you will have to compare the vertical and horrizontal response and see if the vertical deviates for the horrizontal. If it does, it is an indication of a null resulting from the crossover and a dip in power response.

[cuibono]

Originally Posted by catapult

A couple of Dutch guys did a similar test with 4" drivers. They ended up preferring a BW3 crossover to the LRx. You might give that a try just for fun. Try it with same and inverted polarity.

I've considered it, but haven't felt it high enough priority - as far as I can see from just data, the only differences between LR and BW type crossovers are BW have a small on axis bump up, less power dip, and a different orientation of the vertical off axis nulls. While LR2 was inaudible (to me), and LR4 only slightly, I would guess BW3 would be in between - but the differences are so small, it probably doesn't matter (again, at least to me).

I'd love to see someone else try this experiment, to see if they get similar or different results than I did. If you have a computer and some measurement capability, it was actually pretty easy to put together...