From the Desk of Art Noxon

Art Noxon is a fully accredited Acoustical Engineer with Master of Science degrees in Mechanical Engineering/Acoustics and Physics. A Professional Engineer since 1982, he is licensed in Oregon to practice engineering in the public domain with the specialty area of acoustics. A prolific inventor, he developed and patented the iconic TubeTrap, the original corner-loaded bass trap/treble diffuser, 150 other acoustic devices, and counting. Lecturer, writer, and teacher of acoustics, he has presented 7 AES papers, numerous magazine articles, white papers and blogs. He is president of Acoustic Sciences Corporation, the company he founded in 1984.

What is Sonic Shimmer?

When it comes to measurement or assessment tools in acoustics, we have a problem when the tools we have do not agree with subjective interpretations. In this case we need a shimmer meter to better study and understand concert hall acoustics and guitar pedal sound, because in both venues, the concept of shimmering sound is a valued commodity.

Vocal shimmer is measured in speech analysis testing (Praat speech analyzer from Phonetic Sciences). During a long or held tone, the voice signature is actually a sequence of very short tone bursts. The variation in the loudness of these tone bursts during a held sound is called shimmer. Using reverb circuits, shimmer is created by introducing lots of early reflections in contrast to late reflections and long reverb (Sound on Sound, Advance Reverberation Series, Oct 2001). Shimmer in musical composition is created when tones of various instruments playing the same note are slightly out of tune, which created a beating kind of sound, a tone whose amplitude rapidly varies.

There is reported to be a substantial component of what appears to be a random energy experience a fluctuation in the low frequency range associated in true, physical reverb spaces. It is referred to as an acoustic shimmer. There is interest in an electronic reverb shimmer effect but I'm not sure how it is or isn't the same as acoustical hall shimmer. See ValhallaShimmer which is pretty close to what I'm talking about. Except I'm a little concerned about modulation rate adjustments. However, such an experience is not measured or explained. I have developed a conceptual perspective on this perception. It goes like this:

If you make a bass tone for a short duration, say 1 second, you have a huge expanding bass wavefront which reflects off of various large surfaces in a variety of different locations, then what returns to a seat in the audience is a series of intersection bass wavetrains, each arriving at different time delays and coming from different directions. This creates a phase add and cancel type of experience at the listening position, but because of the duration time, the number of different delays, strengths and directions, the phase add/cancel effect at the listening position becomes very complicated.

We have the direct tone duration of one second. The latter portion of the direct duration is modified at the listening position by the phase add/cancel effect created by the onset of a series of time delayed reflections, arriving still before the end of the tone duration. Once the direct has passed by the reflections continue to arrive for a short period of time as before but the interference with the direct is over, and they only interfere with each other. But soon this simple shimmer condition due to intersecting wavetrains at various delays and their interaction fades as the wavetrains become diffused, weakened and multiply reflected.

Finally the condition of true reverberation takes place, where we will have any number of weaker wavetrains intersecting, still creating the shimmer effect due to rapidly changing phase add and cancel effects, but more random due to the more random intersections and more frequent but weaker wavetrains.

Now, if we want less bass mud and dry up the bass, then will we lose bass shimmer?. And if so, how do we retain bass shimmer without overloading the hall in bass mud? Certainly not by absorbing the bass at the rear wall. That removes a major component of reflected bass energy in the room which removes shimmer creating reflections.

It’s useful to remember the classic "functional sound absorber" by Harry Olson yet, and to notice how he proposed to distribute his TubeTrap like product throughout the overhead volume of the hall. Distributed acoustics in this case means bass range energy is uniformly weakened through out the volume of space. This kind of acoustics does not change the reflective nature of the hall, it just quiets more quickly all the various aspects of the hall acoustics in the bass range.

If we want to enhance the shimmer quality of a hall I suggest we alternate wall sections of bass absorption and huge bass reflecting cylinders, or semicylinders. The sequence of wavetrain reflections can be cultivated while the buildup of reverb mud are minimized.

And so, when we talk about bass traps, we have to be very careful what we are talking about. A bass trap is a tool, it is not a sound. We are trying to cultivate a sound effect, and we use the tools at our disposal to do so. But to cultivate the effect, we have to first know what the effect is and how it comes into existence and what the noise floor associated with the effect also is. Generally the goal is to improve the signal to noise ratio, even when the "signal" is a shimmer in the afterglow of sound.

I did some work on and published about musical articulation test tones (MATT) which is an MTF type of test for small room acoustics. I wrote 2 AES papers on the subject. MATT is a measure of "Musical Articulation" which is similar to speech articulation. It measures rapid dynamic changes in the sound level of a playback environment. It turns out that it is the MTF of a room that mostly matters to the audiophile and recording engineer and not so much how flat the frequency response happens to be. Note that a reverb chamber has a zero MTF or articulation rating and a perfectly flat frequency response.

Along the lines of testing for and measuring acoustic shimmer, I've spent a lot of time measuring what happens when sound levels are varied in level in the range of 8 times a second. This is easily within the realm of the shimmer experience, measuring rapid fluctuation in sound level. The test is basically a modulation transfer function (MTF) test. It is a frequency response sweep that measures how well rapid sound level variations are transmitted through a signal process line. It is most frequently recognized as a form of a speech intelligibility or articulation test. Shimmer might well be measured by a some sort of MTF test. More on this later...........