Recording 101 teaches us that the audio spectrum is 20-20,000 Hz and it is our job as recording engineers to manage those frequencies. For introductory level classes, that is a usable definition, but it often leads to misunderstandings. >Do we hear 20 Hz as much as 20,000 Hz? Do we hear those frequencies as well as 2,000 Hz? The answer to both is no. In fact, given contemporary technological limitations, it isn’t even possible to accomplish most of that.

For those of you who read Jay’s Primer on Audio Frequency Bands and made it all the way the bottom, you would have read some interesting things about broadcast standards and encoding algorithms.  Broadcast standards here in the US actually cut off frequencies above 15 kHz.  That is, radio and television broadcasts don’t even bother with the top 5000 Hz of the audible spectrum!  If there were such a thing as radio anymore, you’d know to laugh off any audio engineer who promises you “radio quality mixes.”  Also, cutoffs are employed in almost all digital encoding algorithms in order to prevent aliasing of upper frequencies.

On the other end of the spectrum, most playback systems are not designed to go below 30 Hz.  Currently, the lowest reproducible frequency by any JBL system is a live sound reinforcement loud speaker with woofer that goes down to 25 Hz.  They also have consumer and studio woofers with roughly the same specs.  You’ll notice that these are all woofer systems and not standard speakers for desktop and meter-bridge monitoring.  The standard studio monitors without a woofer falloff sharply at ~45 Hz.  With this in mind, you should know not to expect to hear anything below 40 Hz on a standard system without a woofer.  Furthermore, you should know that about 90% of your audience will not be able to physically reproduce anything below 50 Hz given the standard consumer set up.

This is not to downplay the psychological impact of low or high frequencies.  These play a very important role in psychoacoustics.  Low-lows, though inaudible, help us perceive lowness partially through feel rather than sound.  High-highs also help us perceive presence and therefore clarity by giving more emphasis to the minutiae of a sound that you’d only hear by being close to it in the real world.

Next week, I’ll clearly define the component regions of the audio spectrum and talk about the various ways to treat undesirable maladies afflicting them individually.

More from Phil’s Audible Spectrum series:

• The Audible Spectrum, Part 1: Introduction
• The Audible Spectrum, Part 3: Components
• The Audible Spectrum, Part 4: Sub-Bass Issues
• The Audible Spectrum, Part 5: Harmonics, Overtones, and Formants Primer

Over the course of hundreds of interactions with clients through Fix Your Mix, both in a mixing and mastering capacity, I have noticed that there is a great disagreement out there on the practical frequencies in audio.  This is strange to me because we have such a vague lexicon for our enterprise (boomy, boxy, tinny, etc.) that you’d think we’d all latch on to terms with such defined parameters as Low, Low-Mid, High, et al.

But nevertheless, every couple months I get a client who says “I love the mix, but I’d really like to hear more bass, can you boost 10 Hz by like 5 dB?”  So for all of you loyal readers out there and as a reference for future clients, I have composed a series of articles describing the portions of the frequency spectrum.

Here is an excellent primer for discussing frequency ranges. Jay works in post-production (television, film, etc.), so his end goals are different from those of us in the music business. He also neglects to emphasize the importance of upper frequencies for imbuing a recording with presence, clarity, and professional quality.  But other than that it is an excellent breakdown of the frequency bands.  For this week though, we’ll be talking about the audible frequency spectrum at large.

The audible frequency range is generally accepted to run from 20 to 20,000 Hz.  Some people hear more, most people hear less.  However, it is important to understand that this broad frequency range is supposed to include the frequencies that the average person is physically able to hear.  For the purposes of experimentation, frequencies outside of the range can be heard, but they have to be amplified to such an extreme that they are not worth measuring.

To the left is the Fletcher-Munson Equal Loudness Curve, established in 1937.  It is probably the most cited graph in psychoacoustics (although the Robinson-Dadson Equal Loudness Curve of 1956 has been shown to be more accurate, since Fletcher-Munson is the most widely used, the following commentary will focus on that).  This graph plots sound pressure level (SPL) in phons against frequency.  The lines indicate equal apparent loudness.  That is, if you were to follow each line, from 20 to 20k, you’d see the variation in amplitude necessary to make each frequency sound equal in loudness.  For example, on the top curve, take 1000 Hz sounding at 120 phons as the baseline.  In order to hear 20 Hz at the same apparent level, you’d have to amplify it to 130 phons.  The same goes for 20k.

Another interesting phenomenon about this curve is how exaggerated the differences become at lower amplitudes.  For instance, when you look at 1000 Hz at 20 phons (the third line from the bottom), you can see that it takes almost 80 phons to sound at the same apparent level.

Now bear in mind, this is not to say that you want to go and quadruple your bass content to get a booming mix.  On the contrary, this is to say that you really shouldn’t expect to hear anything beyond a certain points in the mix.  In almost all instances of music recording, there will be frequency content below easy audibility.  The point of mixing is not necessarily to make them audible.  Sometimes these frequencies are meant to be felt rather than heard.  Other times, these frequencies don’t really add much to the mix at all—eating up large portions of the usable power spectrum and overloading your mix with unnecessary content that either will hurt fidelity due to digital encoding or broadcast algorithms, or will be cast off anyway due to physical limitations of sound reproduction systems.

Here is a graph of all the frequency ranges for common instruments and their notes as shown on a piano.  What you’ll notice is that the range for a concert bass is from ~90 Hz to ~350 Hz.  The absolute lowest note on the piano is around ~28 Hz, and that is a note that you will likely never hit.  Practically all the action in musical instruments occurs between 60 and 5000 Hz.  Allowing for formants, harmonics, and other sonic phenomena outside of the fundamental frequency of the note, it is safe to say that practically all usable and desirable sounds fall within 20-20K and that range could even reasonably be made smaller.

In next week’s article I will examine these specific limitations and discuss why the low frequencies are the most problematic.

More from Phil’s Audible Spectrum series:

• The Audible Spectrum, Part 2: Capture and Reproduction Limitations
• The Audible Spectrum, Part 3: Components
• The Audible Spectrum, Part 4: Sub-Bass Issues
• The Audible Spectrum, Part 5: Harmonics, Overtones, and Formants Primer