View Single Post
Old 03-04-2009, 05:45 AM   #2
Big Daddy Big Daddy is offline
Blu-ray Champion
Big Daddy's Avatar
Jan 2008
Southern California

Chris Seymour of Audioholics performed a series of tests on common M-T-M speakers. He placed the speakers horizontally and vertically and measurements of their on-axis and off-axis frequency performance. He reached the conclusion that almost all of them perform better in vertical orientation. Even a relatively inexpensive 2-way bookshelf speaker had better off-axis results than most center channel speakers. Here is a short summary.

I played pink noise through each speaker and mapped its frequency response every five degrees from zero to 40 degrees off axis with 1/24 octave resolution. The results below 80 Hz, a common and typically good crossover frequency to your subwoofer, and above 20 kHz were discarded. I’ll show the frequency maps of the speakers across their entire 80 to 20k bandwidth, but will focus on the frequency range of redundant drivers and measure their frequency variation as we vary the angle.

$250 MTM Horizontally Oriented Measurements

The first speaker we’ll look at is a very common MTM design, available from a big box retailer where you can reportedly get some best buys.

The horn-loaded tweeter crosses over to the dual 5.25 inch midrange drivers at 2400 Hz. This means that while we will see the performance of the tweeter’s off-axis response, we will focus our analysis in the bandwidth of 80 to 2400 Hz.

What we can see from the chart is that the two midrange drivers exhibit significant wave cancellation from around 800 Hz to where the crossover kicks in at 2400 Hz. Some of the variation near the crossover point is from all three drivers playing the same frequencies. A higher-order crossover would reduce this problem, but just placing the tweeter above the midrange would fix it completely as we’ll see later on.

If we take a slice of one frequency from the 1/24 octave measurements we can plot out the frequency response of the loudspeaker and get a visual representation of lobing. The lobing effect is from the appearance on a polar map of the peaks and valleys of the frequency response. In a real polar graph, it looks like lobes, or flower petals.

$250 MTM Vertically Oriented Measurements

What would happen if we take that MTM speaker and turned it on its side?

The horn tweeter isn’t as happy, exhibiting worse off-axis response from 4 k to 10 kHz, but you can see the frequency response from the midrange drivers shows much less wave interference.

If we look at the same slice at 1244 Hz to compare its lobing to the horizontal orientation, you can see from the below chart that for the specific frequency it was successfully eliminated.

This company’s horn tweeter should be used in the orientation as it was designed, but if you were keen on using their speakers, how would you avoid the wave interference that the horizontally aligned MTM center channel had? One solution is to completely avoid their center channel and use a bookshelf speaker

$600 MTM Horizontally Oriented Measurements

This is a higher-end M-T-M speaker.

You can see that the interference between the two midrange drivers is audible. In the upper midrange we can see over -6 dB of cancellation and almost 4 dB of wave reinforcement. You can also see the tweeter has audible off-axis attenuation, but it’s important to emphasize that these tests only reflect variation from the on-axis response and how well we can avoid wave interference.

$600 MTM Vertically Oriented Measurements

In order to easily test if the drivers or cabinet are primarily to blame for the off-axis response, or if the MTM configuration itself is the limiting aspect, we then rotated the speaker vertically and ran it through another set of measurements

you can see that the wave interference from the two midrange drivers was virtually eliminated. The off-axis response is smooth and consistent up to 40 degrees off axis. The lower treble from the tweeter has even improved, likely due to it now having a narrow horizontal baffle and reduced cabinet diffraction. In this orientation, this speaker would do a terrific job at maintaining intelligibility and clarity across your room. This

$115 Bookshelf Speaker Measurements

A bookshelf speaker from the same horn speaker company was used. The price was $230 a pair or $115 per speaker.

Below is the chart showing the frequency response deviation from on-axis to 40 degrees off-axis. The chart shows the same off-axis roughness of the tweeter, but there may be other reasons why you would otherwise like the speaker’s sound. Importantly to this endeavor, the midrange doesn’t show any significant change in frequency response at different angles.

The 1/6 octave chart below shows that the bookshelf speaker performs very well off-axis by only having one midrange driver.

Last edited by Big Daddy; 03-05-2009 at 04:27 AM.
  Reply With Quote