A Short Tutorial on Sound Level and Loudness for Voice
Ingo R. Titze · Journal of Singing · 2013
SOUND PRESSURE LEVEL (SPL) is often used as a physical measure of vocal loudness. It measures an acoustic (oscillatory) pressure in reference to an internationally agreed upon standard, 20 micro Pascal (20 µPa). The exact relation isSPL = 20 log^sub 10^ (P/P^sub 0^) dBwhere P is the measured pressure and P^sub 0^ is the reference pressure. The logarithm and the multiplication by 20 are chosen so that the dB numbers fall conveniently into a 0-100 range for many sounds we are exposed to on a regular basis, but numbers up to 150 dB are possible for very loud sounds.In most cases in free space, SPL also measures the sound intensity, the sound power distributed over a surface area. Sound intensity level is defined asSIL = 10 log^sub 10^ (I/I^sub 0^) dBwhere I is the sound intensity in watts per square meter ( W/m^sup 2^) and I^sub 0^ is another internationally agreed upon standard, 10^sup -12^ W/m^sup 2^. The two standards are chosen such that SPL and SIL give the same number in dB. In fact, one can generally drop the reference to pressure or intensity and simply talk about sound level (SL). A sound level meter (SLM) does just that. Any dB reading can be converted to either pressure or intensity.Sound level can help explain several important phenomena in singing. Questions of interest are: (1) How does sound level change with fundamental frequency and lung pressure? (2) How does sound level change when there are multiple sound sources? (3) How does sound level change with distance from a source? (4) How does sound level relate to perception of loudness? To answer these questions in detail would require a complete textbook, or at least a chapter (e.g., Principles of Voice Production, Chapter 9).1 Some quick rules of thumb can be given here, however.With regard to fundamental frequency, SL increases about 6 dB/octave, all else being equal. This is the primary reason why females often outsing males on the opera stage if they sing an octave higher. Males may produce more glottal airflow, but it is the rate-of-change of airflow that determines the acoustic power. Higher frequency produces a higher rate of change of airflow.With regard to lung pressure, SL increases about 6-9 dB with every doubling of lung pressure.2 The major phenomenon here is increase in peak glottal airflow. Higher peak airflow results in a greater rate-of-change of airflow (from peak flow to zero prior to glottal closure). The rate of change is also known as maximum flow declination rate (MFDR), which is measureable.3With regard to the addition of two or more sound sources, the instantaneous pressures add together, but this addition is complicated because the waves may differ in amplitude (the amount of increasing and decreasing pressure each wave has), in frequency (how rapidly the pressure increases and decreases), and in phase (where in the cycle of oscillation the wave is at a given instant of time). A good microphone is responsive to all of these variables. For example, if the amplitudes and frequencies are the same, the pressure doubles if the waves are in phase and the pressure is zero if the waves are out of phase. For the in-phase situation, the SL increases by 6 dB over the SL of either of the two sounds individually (20 log^sub 10^ 2 = 6). For the out-of-phase situation, the SL meter would read nothing. …