Acoustic Ramp™ vs. RPG’s QRD 734

Posted on January 2, 2012 by Hendrik

We now have a way of comparing apples to apples! Using the methods described in AES-4id-2001 (r2007) “Characterisation and measurement of surface scattering uniformity,” we now have a direct comparison of the Acoustic Ramp and RPG’s QRD 734.

The Acoustic Ramp vs. RPG's QRD 734

The black line is the QRD 734 and the green lines are the Acoustic Ramp™. Please note that BOTH of the green lines are happening at the same time, so you’re comparing both the green lines to the black line. As you can see the Ramp™ extends the frequency range way above and below the 734.

So we’re actually comparing a super-yummy Ginger Crisp apple to a mealy Red Delicious. Which is which?

How Was The Acoustic Ramp Tested?

Posted on October 26, 2011 by Hendrik

A sketch of testing a Ramp in the vertical position

Acoustic testing on a budget is a daunting task. Many measurements need to be made, and sometimes several batteries of tests are required to get valid data. In the case of testing the Acoustic Ramp, I actually did an entire battery of tests that proved later to be invalid. The good news, is that I learned from my mistakes and the second battery provided good repeatable and valid data.

My test process is essentially this: Shoot a swept sine wave from a high quality studio monitor at the diffuser and record what bounces back from the diffuser with a high-quality microphone placed on the circumference of a circle with the diffuser at its center. Then I move the microphone 5 degrees along the circumference and repeat the test. Software assembles all of the data into impulse responses that when combined can create a sonogram which shows amplitude, frequency and the position of the microphone in relation to the diffuser.

Here are some photos showing the process of the testing:

Laying out the test locations. Each piece of white tape is marked with the exact location of the microphone for each test position.

The test computer, audio interface and microphone preamplifier. I am using a PC laptop with Windows XP, ARTA Acoustic Testing Software, a MOTU Traveler, and a Daking Mic Pre One microphone preamp for the testing.

The first test is with a flat panel to be used as a control to compare between the effect of the diffuser and that of a flat reflective surface

Testing in the vertical position with a single acoustic ramp

A second view of the vertical position

Testing in the horizontal position using a Mackine HR824 loudspeaker

Another view of testing in the horizontal position

Acoustic Ramp™ Test Results

Posted on September 17, 2011 by Hendrik

Sonogram Directivity Results

Flat Panel Test 200Hz-18000Hz

Horizontal Ramp Test 200Hz-18000Hz

Vertical Ramp Test 200Hz-18000Hz

How Does It Work?

Acoustic Diffusion is not an easy thing to test or describe. In fact in the academic acoustics world there is a big controversy about the best way to describe and measure diffusion: the Scattering Coefficient or the Diffusion Coefficient. “You will know it works when you hear it,” is true but probably not a good explanation. I am going to avoid this whole dispute by publishing the actual data of all frequencies and amplitudes in my tests.

I did several rounds of acoustic testing on the Ramps and here is what I found:

The Acoustic Ramp scatters frequencies horizontally from about 300 Hz to about 4000 Hz (4kHz), but doesn’t do a lot below or above this bandwidth
The Acoustic Ramp scatters and reflect frequencies vertically from about 300 Hz all the way up to 20000 Hz (20kHz)

The Ramp does both of these things at the same time. So if you install the diffusers on their sides, it only changes which orientation scatters and which one reflects and scatters.

Why Do We Want To Scatter the Sound Anyway?

When sound strikes a hard flat surface most of the sound bounces off the wall as an echo. That echo can interfere with the sound that is headed towards the wall by cancelling out certain frequencies and emphasizing other frequencies. This effect is called comb-filtering. You just cannot hear sound accurately in a room that has comb-filtering problems. Scattering the sound breaks up echoes into many little tiny echoes diminishing their effect. The Ramp also reflects sound away from the sound source, further minimizing the effect of comb-filtering.

Reflections can also be controlled with absorption of course, but absorption-only treatments tend to yield rooms that sound dead and lifeless.

How Do you Read Those Diagrams?

Those diagrams are called “sonograms” and they were generated from data gathered using the software package ARTA. There will be a later post about the testing procedure, but for right now let’s focus on the diagrams.

Flat Panel Test 200Hz-18000Hz

The Sonogram shows Amplitude (color), Frequency (y-axis), and Directivity (x-axis). The above diagram shows what happens when you place a loudspeaker directly in front of a flat panel. The bright yellows and reds show loud levels and the blues and dark blues show quiet levels. The stripe of brightness that goes horizontally across the center of the diagram shows that sound is bouncing right back towards the speaker. At about 500 Hz the sound is bouncing at about 30 degrees and 50 degrees off-center. And from about 300 Hz and below the reflection pattern is pretty wide.

If there is pretty good diffusion at a specific frequency the diagram will show a vertical band of similar colors. Horizontal bands of similar color means that there is an intense specular reflection at a specific angle. For instance, in the above flat reflector model, there is a very strong reflection at 0 degrees, meaning that sound hitting the reflector is bouncing straight back to the loudspeaker. If the reflector had been angled at 45°, we would expect there to be an intense band at around 45° as well.

In the horizontal test of the Acoustic Ramp™ you can see that nearly all of the energy is directed away from bounding back towards the sound source because the center of the diagram shows darker blue colors.

Horizontal Ramp Test 200Hz-18000Hz

In the vertical test of the Ramp you can see the scattering of frequencies from about 300 Hz to around 4 kHz.