The absorption coefficient was developed by Dr Wallace Sabine (the father of architectural acoustics), who spent many a late night moving furnishings and materials between his lecture hall and a nearby concert hall in order to measure their effect on the reverberation decay and thus produce a table of values for many common building materials and furnishings.
We use these tables to calculate the reverb time of a given room by measuring the surface area of each different material; however, it’s important to note that the absorption coefficient differs at different frequencies so the reverb time is likely to change depending on the frequency of the sound.
While reverberation can clearly be problematic, it’s not automatically a bad thing. A small to moderate amount is pleasing to the ear and can help to make things sound “musical.”
A good mix engineer can use the reverberation of the room to help glue the mix together – something which can be replicated using artificial reverb when taking a mix outdoors. However, excessive reverberation can quickly make speech unintelligible and ruin the subtle textures and dynamics of music.
If we look at the history of musical composition and performance prior to the harnessing of electricity, it’s interesting to note that music was often written to be performed in spaces with specific acoustics. Church music is an obvious example; everything from Gregorian chants to the organ music of Bach was designed to take advantage of the long reverb times commonly found in churches.
While symphony halls typically have shorter reverb times than churches, they’re still larger than most common spaces and thus lend themselves to grand sweeping arrangements. Chamber music, as the name implies, was designed to be played in the small furnished rooms of patrons and their guests and thus is typically more sprightly with faster moving passages.
There’s even a concert hall in Germany called the Bayreuth Festival Theatre that was designed specifically for the performance of the stage works of Richard Wagner – no other composers’ works has ever been performed there since it opened in 1876. The main hall is mostly wood and has a reverb time of 1.55 seconds, the orchestra pit is large, extends far under the stage and features a unique hooded design which acts as a filter that attenuates the higher frequencies of the orchestra and enables the vocals to come through. The result is a huge, warm and rich sound that compliments the music of Wagner.
By contrast, many of the venues employed for the performance of modern music were never designed for amplified music and thus can present unique challenges. Over the years I’ve worked in venues that were once theatres, cinemas, slaughterhouses, munitions factories and railway turning sheds – all places where the original acoustic demands were quite different (or not even considered).
In any challenging environment the key to the best possible sound is to minimize the unwanted reflections by careful positioning and aiming of the loudspeakers, which can greatly help in increasing the critical distance of any given room. If problems still exist, then reducing the overall level of the sound system can help claw back a modicum of intelligibility.
Plus & Minus
But reverberation is not the only problematic consequence of boundary reflections, whenever we have parallel boundaries we also have the specter of standing waves to deal with. Standing waves exists when the distance between parallel boundaries (or a multiple thereof) equals the wavelength of a specific frequency, which causes the wave to be reflected back on top of itself.
This is shown in Figure 3, where we see a 150 Hz sine wave being fed into a room with walls that are 30 feet apart – this just so happens to be 4 times the wavelength of 150 Hz (i.e., 7.5 feet). The result is a stationary wave that consists of zones of low pressure called nodes alternating with zones of high pressure called antinodes.
If you were in that room and moved into a node, you’d notice the level of the sound decrease dramatically, whereas if you moved into an antinode, it would increase dramatically (the distance between the nodes and antinodes being half the wavelength; i.e., 3.75 feet). Because this is a purely mathematical relationship, we also know that this space would create standing waves at multiples of the base frequency (300 Hz, 600 Hz, 1.2 kHz, 2.4 kHz, etc.) Such standing waves are also known as room resonances or room modes.
If an amplified microphone with broadband content moves into the antinode of a standing wave, it will dramatically amplify that specific frequency and thus increase the risk of feedback. Thankfully this risk can be addressed with the deployment of equalizer filters across the outputs to enable tuning of the frequency response of the system, thus minimizing the impact of the room modes and helping to prevent the formation of standing waves.
The study of the science of acoustics can quickly draw one into a world of complex calculations and mind-blowing mathematics, but a solid understanding of the acoustics of indoor spaces can gradually be built up simply by observing as many different spaces as possible. If you get to enjoy being involved in some great music along the way, then that’s just a bonus.