(Updated 2/4/12) Despite a formal education and real world training at some serious recording studios, I’ve always been amazed at how far along I had to come as an engineer just to learn some very basic information and points of confusion (a big reason for starting this blog by the way). When you do something for the better part of your life, you sometimes forget how much you really know. That’s part of why the best audio engineers are not always the best teachers.

When I started out, the decibel confused me.

I had read that a jet engine was 140 dB, a library was 30 dB, a rock concert was 115 dB, etc. Why then, I wondered, do digital and analog meters go from negative infinity (silence) to 0 dB (absolute loudest)? And why is it that track faders can go up to +6 or +12 dB?

I’m not going to get into all the gruesome details here, I’m just going to set forth one basic, but extremely important concept: there is more than one kind of decibel.

Decibels in the digital realm are called dBFS or “Decibels Full Scale.” Decibels in the real world (the kind used to measure a jet engine) are called dB SPL or “Sound Pressure Level.”

Now let’s apply this knowledge to a WAV file you might receive back from a mastering engineer. In the end, how loudly people hear your master is going to be determined by the playback system (iPod, car stereo, studio monitors, etc.) and whoever controls the volume knob.* This means that in the real world, even a recording that peaks at 0 dBFS and is -5 dBFS on average (very loud) can be as quiet as a library or as loud as a jet engine upon playback.

So the dBFS of your master really only indicates how loud your song will be relative to other songs when played back under identical circumstances. So if someone plays a Notorious BIG track that’s -8 dBFS on average, and your track is -6 dBFS, you’ll be louder than Biggie every time (unless some asshole decides to turn it down when your song comes on).

dB Adjustments Within Pro Tools and on Actual Recording Consoles

It’s also important to note that when you set your snare drum’s volume fader to +6 dB in Pro Tools or on a console, you’re not making the snare 6 dB SPL, your snare has not become one twenty-third of a jet engine or one fifth of a library. You’re also not making it 6 dBFS (which doesn’t exist) either. You’re just adding 6 dBFS to whatever the sound file’s initial volume was. So if you record that snare into Pro Tools hitting -5 dB on the meters, and set the fader to +4 dB, your snare is now -1 dBFS. And like I said before, the loudest anything can ever be in the digital realm is 0 dBFS. This also called “digital zero.” (Going over digital zero is what causes clipping.)

There are other types of decibels, but these two are the most important to grasp for now.

Any questions?

*Further still, final perceived loudness must also take into account the distance between the listener and the speakers, as well as dB SPL.

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