The answer is perception based, and may not be simple. It’s influenced by many factors. We can measure sound pressure, and it’s useful, but correlating that to human-perceived loudness can be confusing. Here’s some clarity on the topic. A bit of tech talk is necessary first, then some practical conclusions.
Loudness is the subjective perception of sound pressure, and it’s influenced by the frequency content (and duration) of sounds. Our ears are definitely not flat! They’re quite nonlinear. We’re most sensitive to midrange frequencies and less sensitive to higher and lower frequencies. This is especially true at lower listening levels, and become less true as volume increases. So, the tonal response of our hearing is rather complex. Figure 1 is a well-known chart, going back nearly a century, that demonstrates this behavior by showing the required SPL (in decibels) for different frequencies to sound equally loud:
Fig. 1 (courtesy chegg.com)
Sound level meters use weighting curves to compensate for this nonlinearity, allowing them relate better to human hearing, usually with an “A” or “C” mode switch. “A” weighting relates to rather low listening levels (largely ignores bass energy) and “C” weighting takes more low frequency information into account, better relating to louder listening levels. Measuring SPL with music that includes lots of sub-bass energy will measure much higher on the C scale than on A.
As illustrated, if listening to a midrange sound of 1kHz at 70 decibels, we would need to turn an 80 Hz bass tone up another 10dB to sound equally loud, and an 8kHz high frequency sound up about 6dB to sound equally loud. While these may sound about the same “loudness”, they measure 70dB SPL, 80dB SPL, and 76dB SPL. And again, this curve changes at different levels. Notice that at very loud levels (particularly 100, 110, and up) our ears are closer to flat (the “loudness” button on some home stereos compensates by boosting the lows and highs and is intended to be used at low listening levels). There are also less commonly used weightings of B, C, D, and Z (which is essentially flat, or no weighting). No weighting reference implies an un-weighted measurement. Clearly, the non-flat frequency response of our ears is a huge factor in measuring loudness.
Human hearing averages levels in very short durations. And live music is constantly changing in level, sometimes several times per second. A very loud but brief repetitive sound (such as a rock snare drum) may not sound as loud as a longer or sustained sound (keyboard pad or rhythm guitar) even if it measures, briefly, at a much higher sound pressure level. For this reason, meters should have a time selection – at least a “fast” and “slow” response option. Fast, Slow, and Peak, are common modes. Do we want to know the instantaneous level of the snare strike (a few milliseconds) or the average music level sustained over a few seconds? Both are useful. But slow response metering often correlates best to human perception.
Decibels in sound pressure level measurements
dBs can be relative or absolute and are used in acoustics, electronics, optics, and several other fields. Turning a vocal up 3dB is an example of a relative dB change. We don’t need to know the actual level of the vocal before, or after, only that it changed by 3dB. Absolute dBs refer to a known reference (0dB SPL), and must be denoted with the suffix “SPL”. 0dB SPL is the theoretical lowest human hearing threshold and sound pressure levels are measured in decibels above that threshold. A face-to-face conversation may be 60-70 dB SPL and a diesel truck might be 90dB SPL. Here’s a chart of some common SPLs:
Fig 2 (Google images)
Any meaningful SPL measurement must denote: 1) the weighting curve, 2) the time constant, and 3) the reference level. The statement “I mixed at 92dB” is simply invalid (and not uncommon). It’s missing the weighting (or implies no weighting), the time factor, and the reference level (indicating a relative measurement, not absolute). Properly expressed, this might look like “92dBC SPL Slow”. Stating only “92dB” and comparing that to “92dB” in another venue or application is not absolute, but is relative, and invalid. Someone once made this analogy: “saying 100dB is like saying I got paid 1500 money last week. What is the value of the money?” That’s really meaningless.
There are different types and qualities of sound level meters (hardware, software, integrated handheld, mobile app, etc.). Any good meter should at least offer a choice of weighting and time modes. There are also Type I and Type II meters, which are built to different standards (Type I meters cost more). Any meter is only as good as the measurement microphone in use. Also, measuring absolute loudness requires calibration by a “microphone calibrator” tool.
The variables explained above must be taken into account in any valid measurement. Other considerations:
- microphone type – there are big variations
- microphone position – the front row, especially near a live drum kit, will probably measure louder than the back row. Sometimes, engineers measure multiple listener positions across the venue and average them.
- Measurement duration – measuring during several loud songs will result in higher measurements than during a quiet song or a sermon (in most cases). Measuring over time is very useful.
Say you are mixing around 94dBA Slow SPL on an arrangement with full bass energy. Switching the meter to C weighting may result in readings of 10dB higher, or more, because C weighting factors in more low frequency information than A. Neither is necessarily wrong, as long as the user understands the difference. Keep in mind that +10dB sounds about twice as loud, and -10dB sounds about half as loud.
Some SPL measurement tools offer data logging. These store periodic SPL values (every few seconds, minutes, or whatever is preferred) as a data file. Want to know if today’s worship set was really that much louder than last week’s? Review that data.
Another way of expressing loudness in sound reinforcement, which I believe relates best to human perception in live worship, is Leq (Equivalent Sound Level). This averages the level over a stated period of time. A 20-minute music set bouncing between 90-100dB SPL followed by 40-minute sermon that ranging 78-86dB SPL might produce an Leq in the range of 84dB Leq for the whole hour, or something in that range. Listener exposure of 84dBA over a one hour duration is not unsafe.
Measuring loudness is often misunderstood and expressed incorrectly, making measurements between venues or applications improper. Proper measurements, expressed correctly, can be compared. Comparing measurements of the same type is okay, but comparing measurements with different weighting or time factors is often misleading.
If someone feels an experience is too loud, it may be (for them) and they won’t care what any meter indicates.
If someone feels an experience is too loud, it may be (for them) and they won’t care what any meter indicates. But often “it’s too loud” really means something like “I don’t like the musical style” or “I can’t understand the words” and the listener may not even realize this is what they really mean to convey. These may or may not be actual loudness issues. Some other factors, which are out of the mixer’s control, are:
1) Direct-to-reverberant ratio changes across the audience area. Indoors, a listener further from the loudspeakers hears relatively more room reverberation (a lower D/R ratio) and may perceive the lower intelligibility as tougher to understand and, sometimes, mistake this for an inappropriate loudness. Changing level won’t fix that.
2) Some people are hypersensitive to sound. It’s a real thing. Hearing protection or a quieter seating area (if available) are possible solutions.
Relative measurements aren’t useless. Say you measure one day with an un-calibrated meter, or you don’t notice which weighting or time mode is selected. If you measure again at the same location and same meter settings you can compare that difference meaningfully. While the actual numbers aren’t absolute, the difference is useful. Stating that “this song is about 4dB louder than the last one,” for instance, without knowing the absolute SPL is ok. But comparing that measurement to one from another venue or a different application is invalid.
Mixing live sound (including understanding loudness) is all about listener perception. When mixing live worship, we are mixing “feel”. The responsibility of the mixer is to make it feel “right” while conveying the stage performance across the house. This is highly subjective, and judging loudness is similar to judging mix balance…get away from the booth, walk the house, listen, and know your room. Sound level meters are a useful reference tool when understood, but should never replace good listening skills.