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Acoustic Transmission

Jun 26, 2014 11:14 AM, By Bob McCarthy

Moving sound from point A to B

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Figure 3: Humidity and temperature effects: The standard transmission loss rate is affected by changes in humidity and temperature. See larger image

Temperature effects

Temperature change affects sound speed. Hot air is fast air. We can’t see the “normal” sound speed, which makes it difficult to sense such a change in the direct sound arrival from a single source. Sound is always behind light, so even natural audio is out of sync to the visual. The question is simply how much. When multiple paths of different lengths are involved, a change in temperature moves into audibility because the summation is modified by the speed change. Reflections and delay speakers are examples of this.

How much does the sound speed change over temperature? Not much. If we did a concert that went from a sauna to ice cold, the sound speed would have fallen by 10 percent. It takes a rise of 10°F to increase the sound speed by a mere one percent (a 100 millisecond path would now become a 99 millisecond path).

Temperature change adjusts sound speed by a given percentage. Therefore longer paths change more than short ones. As temperature rises the transit time between a main speaker and a delay shrinks and the sound gets to the walls and back again faster than before. The wavelength of sound is rescaling with temperature. This is hard to visualize, since sound is invisible. Instead we can do a reality polarity reversal and visualize temperature as rescaling our drawings of the room, with wavelengths staying the same. Either way, the reality is that a hot room fits fewer wavelengths of a given frequency. The longest paths have the most accumulated change over temperature. The longest audible paths indoors are LF reflections, which is the range we are most likely to find detectible changes related to temperature.

How much will temperature change modify the acoustic response? Not enough to fix a bad sound design or break a good one. One immediate concern is the settings on delay speakers. A quick math exercise will put this in perspective.

An example main/delay combination has respective paths of 100 milliseconds and 20 milliseconds. Let’s sync them with 80 milliseconds of delay (100 milliseconds to 20 milliseconds). It’s perfect for one seat and then time-offset errors begin from there. Sad, but true. Later the temperature rises by 20° F, a big temperature change that accelerates the speed by 2 percent. The new paths are 98 milliseconds and 19.6 milliseconds, respectively, which yields an offset of 78.4 milliseconds, an error of 1.6 milliseconds. How bad is that? The location of precise synchronicity might have moved one seat.

Temperature effects on reflections are similar, but multiplied tremendously in quantity. All direct and reflected sound paths accelerate as a room warms up. Let’s do a similar exercise. We’ll use those same numbers for the direct sound (20 milliseconds) and reflection path (100 milliseconds) so the time offset is 80 milliseconds. This creates a comb filter frequency of 12.5Hz (peaks at 25Hz, 37.5Hz, 50Hz, and so on). The same 2 percent temperature shift changes the time offset again to 78.4 milliseconds. Now the comb filter frequency is 12.75Hz (peaks at 25.5 Hz, 38.26 Hz, 51 Hz). Not exactly a game changer, eh?

The paths must be extremely long to be worth monitoring for delay speakers and many long reflection paths for us to hear the sound change solely because of temperature.

Wind effects

Sound is moving through air. Wind is moving air. Sound moving through moving air is a mess. There is no simplistic way to describe any particular moment of the wind’s effects, but we can explore the principal mechanisms. Standard sound speed is 767mph. Gale force winds begin in the 32 to 38mph range, which could add or subtract 10 percent to the speed of sound if the wind is traveling in the same direction as the sound. Wind going in the direction of the sound is simple to visualize, while a crosswind is less so. All of this is blown out the window when we remember that sound transmits spherically through the air, which is impossible for wind. If wind moved spherically away from a location it would leave a vacuum. Wind moves in a direction and air is pulled in behind it to fill the lower pressure. Since sound propagates spherically we will have every relationship possible to the wind direction, all the time. Our sound waves are moving with, against, and across the wind.

The concert with the gale force winds was cancelled, so let’s consider a moderate breeze (13-18mph). Now we have a ±2-3-percent change in sound speed, in the range we find with changing temperature. The effect is not uniform over the space, even if the breeze were rock-steady, because of the propagation issue. Let’s run through some possibilities of one path: the one to our ears.

A baseball pitcher throws straight (fastballs) and curve balls. For the fastball he aims at the target and sends it. For the curve ball he aims somewhere else, knowing that the path will have a bend in it. A third pitch is the changeup, an unexpectedly slow pitch. A speaker in still air throws fastballs, a straight line from speaker to listener. If there is wind our speaker starts throwing the tricky stuff. A headwind slows it like a changeup. A crosswind causes our sound path to bend like a curveball. The sound that we hear was actually aimed somewhere else and bent in our direction. Our sound went to our neighbor. That wouldn’t matter if the wind was constant, but it never is, so we get a variety pack of sound aimed at others arriving at our location. It sounds as if somebody is wiggling our speaker. There is a small amount of pitch shifting due to the Doppler Effect. If the source has a uniform directional response then there will be a minimal change in frequency response.

Speaker arrays have a fine grain of amplitude and phase interactions between the elements. These are measurable in the best of times, and audibly degrading in the worst. Our brains do a remarkable job of adapting to stable combing effects. Shifting comb filter interaction (flanging) is profoundly audible. Bending the paths from an array puts the fine-grained flanging in motion at our location, and differently at everyone else’s.

We can encounter two types of wind in indoor spaces. HVAC blowers are indistinguishable from a steady breeze of outdoor wind (modulating phase response, moving impulse response and decreased coherence in the HF). The second is moving speakers, such as a newly hung cluster that is still swinging. It’s not really wind, of course, but it walks likes a wind and talks like a wind, and we can hear it. Outdoors, the wind may actually be moving the speakers, in which case the solution is to secure the speakers (for safety, not sound quality).

We discussed wind-induced bending and temperature effects on sound speed. These can come together in long-distance applications to create a larger scale form of bending, similar to the way light bends through the air. It is a form of refraction that results from layers of air at different temperatures (termed thermal gradients). Consider the distant train horn that some nights is faint and other nights feels close by. The refraction on one night bends it down to us and on others sends it skyward. Large-scale outdoor concerts can sometimes encounter mysterious disappearances of their sound and extremely disturbing landings into unhappy neighborhoods.

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