Christopher Blair – full time acoustician, part-time conductor, last time blogger. We come to Friday and the end of Drew’s absence from these pages. I have really enjoyed my time here (Thanks, Drew for providing this forum!) and look forward to possible future exchanges.
This being my last day, I thought I’d try something different by offering a series of short thoughts, one or more of which might spark some interaction from the readers. These will range from discussing acoustical misconceptions to the difference between acoustic measurements and what we hear. And, please, if you have a question about a topic that has not appeared here this week, go ahead and ask using the comments section. So now, welcome to “Open Line Friday!”
“Wood is good”
Despite the fact that most the great concert halls of the world are constructed of plaster on masonry, there persists a notion that wood improves the acoustics of a music room. This is not necessarily true. Wood paneling with an air-space behind in a large concert hall can soak up large amounts of low frequency energy, muting the most strenuous efforts of the lower strings, and shifting the balance of reverberant energy to the harsher higher frequencies.
What wood can do very well, however, is offer a rich appearance, and the appearance of any environment can shape a listener’s perception of it acoustics. Not the actual sound, just the perception of that sound.
There are many studies that have documented this effect. Most of us gather about 85% of information about the world around us through the sense of sight. For example, take the case of a soloist appearing with and orchestra: most listeners will judge a soloist to be playing louder if a spotlight highlights them than if they appear in the same light level as the rest of the ensemble.
The late theater designer George Izenour designed orchestra concert shells out of steel. But, realizing there might be both musician and audience biases against such an “industrial”-looking material, usually covered the steel with a thin wood veneer. He referred to it as “acoustic wallpaper”.
“Concrete is bad”
It certainly is, if the stage flooring is laid directly on top of it. That particular construction detail will win you no friends in the cello and bass section. Just ask orchestras that have played in the concert hall of the new Grand Theater in Beijing (popularly known as the “Egg”). Yes…one was laid there. Ouch!
But other than that sort of faux-pas, what characteristic of concrete should make it worse for acoustics than the plaster on masonry found in the best concert halls? Other than slight differences in thermal conductivity (which theoretically might cause a tiny increase in absorption at very low frequencies), I can’t think of what would be the problem with the material itself.
Well, there is that appearance aspect of course. Big slabs of concrete offer little visual enchantment. But I think what may have put concrete in the acoustical doghouse (in the mind of the public, anyway) is the poor shaping of rooms that were designed in the mid-20th Century which happened to coincide with increased use of exposed concrete as an architectural design statement. In addition, the aesthetic of that period for finishes was clean lines and smooth, flat, un-modulated surfaces. Consequently there was often little in the way of diffusion to distribute sound evenly and reduce harshness, other than perhaps the occasional element applied to a wall as a sort of acoustical band-aid.
“Thicker is better?”
Back in the mid-1990’s I began to notice a trend among acousticians regarding wall thickness recommendations for preserving bass energy in large auditoria. In the belief that if massive thick walls were good for providing low bass absorption, even thicker walls would be better, several were calling for concrete walls in some cases well over 20” thick.
Not an unreasonable assumption to make on the face of it, unless one digs into some thorny theoretical details of absorption mechanisms at heavy non-porous surfaces at low frequencies. One finds there is a limit, caused by surface heat transfer effects, as to just how low, in terms of absorption coefficients, that you can go.
I was employed at Artec Consultants at this time, a sometime participant in these “wall wars”, and wondered just how much thickness of concrete would be required to make the absorption loss component that can be assigned to sound escaping through the material an order of magnitude less than the minimal, and unavoidable, losses generated by heat transfer. The frequency of most interest for evaluation was 16Hz, since the human ear cannot perceive sound below this limit.
So what was the answer?…drumroll, please…..six inches of concrete! More than this makes no audible difference for room acoustics.
Part of my role as a consultant is to save clients’ money when I can. So it a pet peeve of mine when someone takes a rather ordinary product, slaps the word “acoustical” on it in some way and jacks the price 400%. For example, there is a 2” thick open-cell foam product, you probably have seen it, with its surface sculpted to look like the absorptive wedges in an anechoic chamber. These wedges are about 1” deep which theoretically can only increase acoustic absorption values above about 1000Hz. However, a 1” thick piece of foam absorbs almost 100% of acoustic energy above 1000Hz anyway, so what is one actually getting here for the price markup?
Answer: A product that in its appearance self-consciously proclaims: acoustics!. So here’s a bit of free advice for any basement recording engineers in the audience not so concerned about appearance: know that almost any 2” thick piece of open-cell foam or glass fiber panel (flame and smoke resistant please!) provides as much absorption at high frequencies and significantly more absorption of the lows, at a fraction of the price per unit area.
And then (not too often, but it happens) I hear about acclaimed merits of some special “acoustical paint”? Oh, come on! Do we really need to talk about this one?
“Design by Numbers”
Acousticians use descriptors such as reverberation time, early decay time, clarity, initial time delay gap, etc. as a shorthand way of discussing individual aspects of concert hall sound. These standard metrics were developed within the limitations of the measurement devices on hand at their creation. (For early studies of reverberation time, the measurement tool was a stopwatch.) New ones have been added as knowledge has increased and computational power has found its way into measurement devices.
In the reliance on these single-number descriptors we lose a lot of critical detail. Psycho-acoustical topics that we have discussed in this series of blogs such as forward masking, two streams of auditory information and others yet to be discussed ,such as binaural unmasking, time dependent loudness integration, critical bandwidth effects, and many others that explain how and what we actually hear are totally absent in these data.
The fact remains that we can hear things that we cannot measure, and we can measure things that we cannot hear. However, the obsession with single number metrics as critical to the design process follows a large body of measured data at halls great and small taken over the past half-century. We are a species that likes to keep score, comparing this to that as a way of determining worth.
It is one thing when these metrics are used by people cognizant of their limitations. It is another matter as a designer to be faced with a specification for desired acoustical performance of a new hall that lays the numbers out in excruciating detail. (This seems to happen more in Europe than the USA for some reason). I can take you to several existing halls with almost identical reverberation times that sound astonishingly different, so why all the fuss?
Rather than try to fit new spaces with a numerical straight jacket of conformity, why not celebrate their differences? Good acoustics, like ice creams, come in more than one flavor. Disney Hall may be strawberry sorbet. Tartly flavorful, with a clean aftertaste. Nashville’s new Schermerhorn Symphony Center? Perhaps a rich chocolate?
Lead me to the freezer!