Section Chair Dan Mortensen opened the meeting with announcements and an overview of the evening's discussions. After introductions around the room, Johnston began the presentation by asking just what that big knob on the front of a home stereo receiver adjusts. Nobody actually fell for it and said either 'volume' or 'loudness', but this was a pretty savvy group! The point was that the knob is ultimately involved with adjusting intensity, not loudness - it adjusts gain and the power delivered to the speakers (Johnston pointed out that "volume" is not even a defined measurement). Some might argue that this is simply a matter of semantics, but Johnston made the point that language exists to communicate, and that in this case the essential difference between the terms "intensity" and "loudness" are highly important to understanding how decisions made in recording or mixing will impact the way the audience hears the result.

Intensity is the external measured level of a sound. There are numerous measures of intensity, but for the purpose of this lecture, Johnston chose to focus on the common psychoacoustic definition of Sound Pressure Level, or SPL, which he described as a measure of the acoustic energy in the atmosphere referenced to a 0 dB SPL of 0.00002 dynes per square centimeter. In terms of electronics, the rules are the same, but the 0dB reference is not universally defined, so specific references such as the dBm and dBv provide a way to measure the intensity of an analog signal. All of these measurements require assumptions to be made about specific conditions in the atmosphere or the electronics such as temperature, humidity, constant impedance, etc., but having ascertained these conditions, the resulting measure of intensity remains an objective one, which can be shown on a meter, and agreed to by multiple observers.

Loudness is the internal subjective experience of how loud a signal is. Johnston reminded the audience that the use of the term dates back at least to the work of Fletcher, if not earlier. In any case, it is clearly not the same as intensity. While loudness and intensity can mostly be related by a complex calculation, Johnston noted several important caveats;

• Every listener has a slightly different perception of loudness.
• Hearing injuries can affect that perception in many ways.
• The calculation of the relationship between intensity and loudness is highly complex.
• In the most severe cases, perceived loudness does not closely track measured intensity.

So, when do we use each measure? Johnston reiterated the essential difference between the objective measure of intensity, and the subjective measure of loudness, and listed specific suggestions as to when to use each. Use intensity when the objective physical measurement is what we need to know, but do not use intensity measures when we want to know how loud a sound is to a listener. Use loudness when:

• We are trying to describe the listening experience
• We are trying to estimate psychoacoustic parameters.
• When someone is shouting "Turn that darn thing DOWN!" and the intensity doesn't seem to be that high.
• When someone is shouting "Turn it UP!" and the intensity already seems excessive.
• When we want to match loudness, not level, across audio selections.

Regarding that complex relationship between loudness and intensity, Johnston reviewed a few essential points:

• When the frequency content of a signal is unchanged, loudness is approximately proportional to the 1/3.5 power of the signal power, or the 1/1.75 power of amplitude.
• The ear has a mechanical filtering system that splits the signal into "critical bands" or "ERBs (Effective Rectangular Bandwidth)."
• Signals in the same critical band convert intensity to loudness with approximately the power law relationship stated above.
• Signals in different bands add linearly in loudness.

So, given a simple sine wave signal, doubling the energy at a single frequency should increase loudness by approximately 2^(1/3.5), or a factor of about 1.21. Adding a second sine wave to the signal, at a frequency removed from the first in terms of critical bands should increase loudness by a power of 2 (it sounds "twice as loud"). So, while these two signals present the same measured energy (intensity), they will have a loudness ratio of 1.21:2. The signal with energy at more than the single critical band will sound "louder" even though it measures as equal in intensity. Further, if this same process is followed to spread the signal out across multiple critical bands, keeping the total energy constant, the perceived loudness will continue to increase as the energy is spread out.

Johnston demonstrated this phenomena by playing a number of sample sounds, beginning with a single sine wave signal, then adding sine waves at additional critical band points, while keeping the total signal intensity constant (to a tolerance of 5 significant digits). As predicted, the audience agreed that the signals spanning multiple critical bands were "louder" than the initial single frequency tone. Johnston commented that the auditory system tends to pick up the peak in ranges as it responds to the spread signal and noted that the "filter bands" in the ear vary in a non-linear fashion as they proceed up the spectrum. The critical bands are roughly 100 Hz in width up to 700 Hz, and 1/3 octave above that point. So, as the frequency goes up, each critical band has a progressively wider bandwidth. He noted that "everything about the ear is violently non-linear".

Interestingly, dog and cat hearing systems appear to have equal bandwidth critical bands, the reason they probably can not distinguish formants in human speech (especially, apparently the word "no") but have very acute sensitivity to small noises of different types.

What then does that "loudness" button or knob on the stereo do? Because of the reduced sensitivity of the ear at low frequencies, some manufacturers add a bass-boost control, sometimes variable, to their products and label it "loudness". But these controls are linear in function, and neither signal nor time variant - that is, they apply a fixed boost based on the position of the knob that neither reacts to signal content, nor to duration of events within the signal. As demonstrated during the meeting, the actual perception of loudness depends on both, so the best that can be said for these "loudness" controls is that they make a portion of the signal higher in intensity.

In summary, there is significant reason to exercise care in using the terms intensity and loudness, and similarly, care must be exercised when applying signal processing which is nonlinear and has the effect of altering the frequency content (spreading the signal across multiple critical bands). We should not forget that when a signal is spread in that way, perceived loudness may well rise faster than expected, and that rise may not be evident on our meters which only indicate intensity.

After a break for refreshments, the group reconvened with the traditional drawing for door prizes. Prizes included:

• a TC Electronics guitar shaped key chain courtesy of Brownestone Marketing and won by Kory Rhodes and Wayne Edwards
• three great CDs provided by Starbucks and Hear Music and won by Darren Cohen, Jon Harris and Bob Smith
• a polo shirt courtesy of Meyer Sound Labs won by Greg Mauser
• a limited edition Opus 4 Guitar mug, donated by Opus 4 Studios and won by Dave Tosti-Lane
• an Audio Geek coffee mug courtesy of Mr. Johnston, won by David Christensen.

Our host for the evening, Dr. Michael Matesky briefly described the elaborate and methodical construction of Opus 4 Studios, and demonstrated the Lexicon Acoustical Reinforcement & Enhancement System (LARES). Two Schoeps microphones suspended in the room pick up sounds and feed them to the LARES processors which control how they are distributed through 40 channels of amplification to the matrix of 72 speakers distributed throughout the room. This system can adjust the acoustical signature of the room in real time to simulate different types of playing environments.

The remainder of the evening was devoted to questions and answers, both with regard to loudness and intensity, and with regard to perceptual encoding, psychoacoustics, masking and critical bands, and the original development of the MPEG family of standards. The next meeting of the Pacific Northwest section of the Audio Engineering Society will be "Born Digital: Will recordings survive the 21st century - Asset management and storage issues for Engineers, Producers, Artists, and studios" with John M. Spencer, President of Bridge Media Solutions (BMS/Chace). The meeting will be presented in conjunction with the Association for Recorded Sound Collections (ARSC) on May 17, 2006, at Glenn Sound Studios in Seattle, WA.