How might chamber music presenters adjust their HVAC systems to optimize the acoustics of the performance hall?
This question occurred to me when I attended an ‘early music’ concert in Minneapolis. The humidity in the church was very low, probably because of the wintry air temperature outside—and the fact that the church heating plant and thermostats were set so as to conserve energy. Too, there probably was no humidifier in the airflow ducting of the furnace, although I don’t know for certain whether this was the case or not. Within 10 minutes of taking my seat, my eyes felt dry. The performance was delayed because of snowy road conditions (much like the situation on 13-DEC-2007 in Boston). Within 40 minutes, still waiting for the performance to begin, the inside of my nose was crispy and sore. The room was like the Sahara, maybe 5% to 10% relative humidity!
But eventually the concert started. And, oh, the acoustics! The acoustics were unusually crisp. Scintillating! The extraordinary timbre of the room was all the more noticeable because I was seated about 30 meters from the performers—far enough away for the reverberation and acoustic absorption properties of the arid air between the performers and my ears to have a big effect on the sound waves.
By contrast, Boston and Kansas City and New York are temperate cities and always fairly humid (40% to 100% relative humidity outdoors, and 30% to 70% humidity indoors). The altitudes of all of the cities where I’ve ever attended performances or played are low, so I think we can safely exclude barometric pressure differences as the source of the differences I’m referring to.
Chamber music performance venues in these humid cities all sound warm and ‘liquid-ey’. There’s a high-frequency ‘roll-off’ on everything—starting, I think, at about 1 KHz (C6) and dropping maybe 3 dB per octave above that.
The warming, fluid effect at increased humidities that I’m referring to is not a pan-spectrum ‘deadening’ of the sound. (The effect of new-fallen snow, like we have in K.C. today, is deadening—a severe attenuation across all frequencies; no distinct low-pass cut-off frequency or ‘shoulder’—and that’s not what I’m talking about at all. ) The high-humidity timbre is just sweet and warm.
Reverberation is actually enhanced in humid environments. The reflected sounds are louder. And this enhanced sound pressure level of the reflected sounds seems to be what is mostly responsible for the warmer sound in a humid environment.
I retrieved the papers by Harris and Bohn, extracted the data contained in them, and performed log-normal statistical regressions on the data. (The data are for ‘direct’ reception of ‘flat’ wavefront sound propagating horizontally from a single localized sound source to a receiver at the same elevation above the floor as the source. In other words, the data—and my regressions—do not consider any reflections/reverberation, do not take into account the height difference between the performers and the listener, and do not make any assumptions about hall dimensions, geometry, wall-coverings, etc.) For your interest, I’m making the results of the regressions available in the following spreadsheet.
If you download the spreadsheet and play with it (adjust the relative humidity and your distance from the performers with the scroll-bar control sliders), you’ll find that the low-pass shoulder frequency shifts to the right (to higher frequencies) as the humidity increases. You’ll find that the slope of the sound pressure level (SPL) roll-off in dB/octave is maximum around RH=25%. You’ll find that the slope is about 2 dB/octave at very low and very high humidities, but can be 7 dB/octave or more (for listeners seated far from the stage in a typical-size chamber music concert hall) when RH is between 20% and 30%.
What this suggests to me is that, to optimize the listening experience for as many concert-goers as possible, presenters should try to avoid humidities in the 20% to 30% range—tough to do in wintertime in climates with outdoor ambient temperatures below –10 ºC.
Hypothetically, it would be wonderful to perform in very low-humidity air—say, chamber music festivals in the mountains at 4,000 meters or higher; or venues (Arizona? New Mexico? inland California? Alberta?) having peculiarly dry air like what I encountered in Minneapolis. Or, conversely, the timbre could be similarly sweet at low altitudes, in air that has been conditioned to between 55% and 70% relative humidity.
(For humans, relative humidity less than 25% feels too dry. The normal recommended comfort range is 25% to 65%. Above 65% many people feel clammy or uncomfortably moist. Many concert halls and other public buildings are conditioned so as to stay close to 45% relative humidity.)
An unexpected surprise from my statistical modeling and analysis is that tropical locations with relative humidity in the 70% to 90% range should have even sweeter, more lively timbres. Yes, such high humidity levels would have some adverse effects on tuning for stringed instruments and pianos, but the liveliness of the sound might be pretty interesting—noticeably warmer than usual, especially for listeners 20 meters or more away from the stage. And, of course, choral and wind instrument performance—no problem at all at such high humidities.
I’d be grateful to hear from you regarding your own acoustical experiences in unusually dry or unusually humid indoor performance venues. If you live or perform in equatorial locations, your impressions would be especially interesting. Any chamber music presenters or organizers of festivals in warm, humid locations out there with comments to share? Do any architects out there reading this blog post have advice based on your concert hall design projects?
With global warming, I suppose the future for most of us will tend to be more humid on average, compared to conditions to which we’ve been accustomed…
- Bohn D. Environmental effects on the speed of sound. J Audio Eng Soc 1988; 36:223–31.
- Harris C. Absorption of sound in air versus humidity and temperature. J Acoust Soc Am 1966; 40: 148-59.
- NOAA National Climatic Data Center, Relative Humidity by U.S. City by Month table [(M)-morning; (A)-afternoon]
- NOAA National Climate Data Center mean annual humidity [1MB pdf, source of Fig. shown below]
- ASHRAE website
- Steinway & Sons. Caring for Your Steinway. [references a recommended range of relative humidity between 45% and 65%]
- Hardy-Holzman-Pfeiffer LLC; Fisher & Marantz, Inc.; Jaffe Acoustics, Inc. Design Guide: Music and Drama Centers, Ch. 3. U.S. Army, Office of Chief of Engineers, 1981. [1MB pdf]
- McQuay Air Conditioning Acoustics Manual, 2006. [1MB pdf]