“For someone who is supposed to make things quiet, you sure do make a lot of noise!”
We rarely hear this, but imagine it runs through every client’s head when we’re on site conducting tests. Between our 750 watt loudspeaker, signal generators, tapping machine, and various heavy objects to drop, we tend to draw attention to ourselves and not just because we look cool. The simple fact is that to measure many acoustical quantities, a high quality, consistent sound source is required. Read on to learn more about the types of sounds we make and why we make them.
So why are we so loud? Well, to make sure that we’re measuring what we intend to and to reduce the influence of other sounds around us. When conducting acoustical tests, we need to get our receiver room level at least 10 dB above the background at all frequencies of interest. So when we’re tasked with measuring the noise reduction of a well-performing wall, instead of turning the background sound level down (not easy sometimes), we turn the source room level up (easy and awesome)!
The simplest test signal is a sine wave: a pure tone at a known frequency. If we tested every pure tone one by one, we’d never accomplish anything, so we need to get clever. One way to test many frequencies in a short amount of time is to use an impulsive sound source. An ideal impulse consists of all frequencies in an infinitely short time; in practice this is impossible to achieve but can be approximated with a balloon pop or a swept sine wave. The idea of a swept sine wave is that by taking a pure tone – the sine wave – and changing the frequency over time – the sweep – one can cover all the frequencies of interest without having to run a test at every frequency.
An impulse sound source allows an acoustician to measure an impulse response: the room acoustics equivalent of a fingerprint, capturing reverberation time, clarity, discrete reflections, and a host of other acoustical parameters. Acoustical testing purposes aside, one very cool use of impulse responses is to use digital signal processing to simulate the room acoustics of famous performance venues and worship spaces. This was done using a balloon pop in the Hagia Sophia (below photos), in Istanbul, and the acoustics of the space can be imprinted on any sound. Impulse response libraries such as this are widely available; who hasn’t wondered what it would sound like to belt out their karaoke go-to on the stage of Carnegie Hall?
Impulse sources have their uses but aren’t particularly beneficial for diagnostic or sound isolation testing. For these cases, acousticians turn to pink noise. This common test signal consists of random sound energy with equal energy per octave. Pink noise has more “rumble” compared to the “hiss” of white noise. There are a bevy of “colored” noise sources, but because pink noise is steady state, covers the audible range, and has enough low frequency “oomph” to punch through most assemblies, it’s a favorite in the acoustics world and is useful in identifying sound leaks or tonal deficiencies of a tested assembly.
So keep calm and let us carry on and make some noise. Like a two year old temper tantrum, it will be over soon.