How much bass do I need for stereo TV? How low should I go? Why do I need to listen with a subwoofer when everyone streams video on their phones? In this article, Damian Kearns looks at the case for bass management as the bedrock for excellent stereo mixes.

Good Old 2.0

I decided to concentrate this article on stereo mixes, as all too often, stereo monitoring is done through small to mid sized monitors in post production studios. 5.1 systems require a subwoofer at least for the LFE content so there’s no debate there. We don’t have LFE content in stereo TV mixes, whether they are derived from a multichannel mix or whether the stereo mix itself is the deliverable. Yet, I argue subwoofers are an integral part of stereo mixing for TV. Sure, I’m a huge fan of pounding bass but there are practical reasons why I not only have a subwoofer with an optional bass management switch in my 5.1 system, I have another subwoofer exclusively on my stereo monitors.

The Bass-ics

We humans collectively have this hearing range, defined generally from 20 Hz to 20,000 Hz. The Hz stands for Hertz, named after German scientist Heinrich Rudolph Hertz, who in the 1880’s, worked with electricity to produce and detect radio waves. Our hearing occupies a piece of the frequency spectrum much slower than radio waves but nonetheless, ‘Hertz’ is synonymous with cycles per second and this is how we quantify soundwaves. ‘Hertz’, just like ‘Foley’, is a term derived from someone’s last name and because of this, the first letter of both these terms should be capitalized when written or abbreviated, out of respect for the individuals’ achievements (Pedantic rant over).

We don’t all hear the frequencies in the human hearing range and no two of us hear them the same way. We’re all actually not so great at the limits of the human hearing range, specifically with regards to the highest frequencies and lowest frequencies. Our ability to hear or feel low bass does improve the louder the overall sound level is but at the point we hear all frequencies as equal as we can, it’s too loud to sustain listening for very long. Americans Harvey Fletcher and Wilden Munson presented their findings about our hearing in 1933 which graphically illustrated (above in blue) just how non-linearly humans perceive sound at various sound pressure levels. Today, we are guided by ISO 226 which is a much-refined model inspired by the original 1933 findings. In a nutshell, we only feel or hear low bass well when it’s really, really loud. 

Regarding sound reproduction, most bookshelf or nearfield monitors (aka speakers) are lucky to get down below 80 Hz and it’s usually not a very linear frequency response once the loudspeakers are tasked with reproducing lower frequencies. There are more than a few reasons for this: Power consumption aka efficiency, driver size (how big your woofer physically is), speaker shape, speaker size, materials used, inherent distortion in the drivers, and build quality, etc. And then, there’s the room.

Audio Monitors are boxes placed inside another box, a room or space. Boxes resonate and then the box they’re in resonates sympathetically, as well as all the other objects inside (and often outside) the space. This changes the nature of how speakers perform and many would successfully argue that the space is the one thing over which we, as audio engineers, have the most control. That is, unless you live next to a busy highway or near a tube station. The louder the outside world, the more it encroaches. And bass moves through walls very well.

Getting In Tune

The basis of our stereo systems is a pair of loudspeakers. There are hundreds of different makes, models, sizes, designs, vintages of monitors out there and unless the speakers you own have built-in subwoofers, chances are, at least part of the bottom two octaves of that benchmark human frequency range is missing or not being reproduced evenly. I’m talking about 20 Hz to 40 Hz and 40 Hz to 80 Hz. That’s 2 out of 10 octaves we hear that may be missing or underrepresented in our critical decision making processes. Considering that musical instruments can pound, resonate, quiver and waver most often between 20 Hz and 2500 Hz before we even start to talk about overtones, we’re already starting to lose fundamental low frequencies, important for determining pitch and how in tune our music beds are.

Why do we care about music in post audio? Well, it’s a fundamental element in the majority of our mixes, for starters. It’s a driving force, it’s a scene setter; music is the emotional floor, walls and ceiling for our work. So if we leave in location rumble or start cutting ambient or specific sound effects that eat up some of that space or work dissonantly against the music track, we’re already falling apart, at ground level. As a trained musician, prior to my post career, I can’t help but think of my mixes as constructs akin to musical arrangements and as such, performance, pitch, time and loudness are my elements.

To this point, I have dumped or swapped or asked for sounds to be recut on many occasions when I thought the pitch of a sound effect clashed with the music I was working with. I also tend to tune any voice processing effects to sit in the mix along with all the other elements. For instance, If I’m pitching a voice down, it has to work with everything else down there below 80 Hz as well as it does above 80. My subwoofer tells me when and if I need to apply EQ or compression to ‘pop’ an element into a ‘spectral pocket’.

Bass In Its Place

I have bass-managed a sub into my 2.0 path and yeah, it makes a huge difference to my mixes. I wasn’t sure how much bass management would help until I started looking at what it was bringing to my stereo speakers. It’s important to understand that since this subwoofer is not being used for an LFE channel, the +10 dB gain we typically apply to LFE channels is not relevant here. This is strictly a monitoring path.

For a variety of reasons, mentioned above, the left and right monitors in my system display significant nonlinearities below 200 Hz, including a couple of prominent notches below 100 Hz. This is probably all due to the room I mix in, which is not acoustically treated.

What I was able to do with my subwoofer was essentially use it as a form of crossover, so that my main monitors can focus on the aspects of sound they reproduce best and leave all the heavy lifting to the sub. Bass management allowed me to fill in notches and flatten the whole curve of my room. I did this with physical placement, subwoofer phase, software EQ adjustments (cut only, no boost), and by listening to four hours of different mixes to find that point where everything was well represented and felt balanced.

What I noticed is that until the bass was properly tamed, it was too loud over a couple of ranges and I felt this was obscuring top end and midrange clarity. The moment I popped 40 Hz and 75 Hz to the right levels, all of a sudden, the image sounded tight and clear.  Even though the software auto-alignment did 95% of the work for me, that last 5% had to be all me. After all, I own the ears that are the target for all this sound. My perception has to count in the final balance. 

The balance I achieved through all my attention to detail is impactful, tight, and clear. The key for me was the understanding that a subwoofer isn’t supposed to be heard as much as it’s supposed to be felt. Even though I love a lot of music with heavy bass, I’m not mixing that content all the time. So my technical target has to be linearity, not a bass bump. Too much bass and I’d be making my mix too bright to compensate. 

Real World Implications

Mobile phones don’t reproduce much if anything below 150 Hz through their speakers. Our earbuds don’t reproduce much below 100Hz and that’s being kind. Same for most TV’s. So, if bass is out of sight it’s out of mind?

Not really. It’s probably still working as far as our mix compressors and filters are concerned. In fact, when it comes to bass, nothing drives a compressor like it. This is why when I mix, my high pass and low pass filters come prior to my compression and EQ, so that I can dump unneeded rumble to keep the compressor from kicking in too much. 

Has dialogue ever been obscured or dropped in level on your tv because dynamic range compression kicked in? This can happen in your set top cable box, in the TV or both! Just because you can’t hear it doesn’t mean it’s not there. 

To this end, I’m supplying three audio files here for testing purposes. All three have the same piece of voice on them. One is just me at -24LKFS talking through the cheap talkback mic I have on my desk, another is the same audio with a static 20 Hz tone applied through at -10 dB full scale, and the other is the same audio but with a uniform frequency sweep from 20 Hz to 80 Hz at -10 dBFs. I chose this level because even with a properly aligned subwoofer, 20 Hz would be very tough to sense at the same level as my voice. The sweep file really illustrates just how non-linear our bass perception is. By the time I hit 80 Hz on good monitors, I was finding it really loud!

Using these files I first started in my mix session and watched the toned files nail my buss compressors, even though I couldn’t hear much of it at all since I decided to monitor through an Avantone Mix Cube. Without a subwoofer, that really loud 20 Hz content isn’t audible so this is a good indication of how what we don’t hear has an effect on our equipment.

On my tv’s, the files with the tones tended to have some effect on the dynamic range of the voice but I was only able to hear part of the tone sweep starting just below 80 Hz. So just like our mix compressors, what we don’t hear certainly can have an effect on how our audiences experience our work. If you use these files, please be very careful with the ones with tones attached, as the tone level is quite loud.

If low bass isn’t properly mixed it can still make it through the quality control process (QC) and straight on through to broadcast or streaming without being tamed. DRC can potentially kick in and possibly affect everything we can actually hear. It’ll also eat up dialogue normalization (dialnorm) headroom in the lower end of the human voice and the result could be a lower volume, muddier, reproduction of your hard work. Consider that the 20 Hz tone I put through the test files added slightly more than 1 dB to my dialnorm. The tone sweep from 20 Hz to 80 Hz added 6 dB to my dialnorm, ate up all my headroom and increased my loudness range by 2 dB! Remember, on my tv, I could only hear the tone sweep as it approached 80 Hz. And the 20 Hz tone and the sweep were set to exactly the same levels.

The bass we’re not hearing can be significantly louder than the midrange of the explosion that is now being buried in our mixes thanks to limiters and compressors and at the consumer end, we’ve got zero control over this. 

Rounding Things Out

In my opinion, a properly bass-managed, linearly-aligned audio reproduction system to mix with is essential to accurately gauge the weight and punch we’re including in our creative projects. There’s a peace of mind that comes with knowing that every frequency and every sonic element has been slotted perfectly into place. We are, after all, painting a picture with sound. Best to hear all the colours while we mix, rather than see only red when we realize we’ve missed something that is ruining the streaming or broadcasting experience.  

This is not to say that we shouldn’t listen without bass management engaged from time to time or through various monitors with limited frequency ranges while we work to hear how things will translate to various devices but our low frequency content is vital whether we hear it, feel it, or not.

Thanks

I reached out to various manufacturers of loudspeakers and subwoofers to gain a bit of insight before writing this article. Thanks to Dynaudio, Harman and Neumann for all the insight. It really helped me understand a lot.

I also read many great articles on subwoofers, Heinrich Hertz, and human hearing. One of the articles I read was written by my colleague, Julian Rodgers, showcasing an excellent subwoofer offering from Adam, the Sub12, titled “Does Your Small Mix Room Need A Big Sound.” Another great article was by Eli Krantzberg, called “Do You Need A Subwoofer? If You Do, EVE Audio Has Four Different Models For You To Choose From” and really provides some great tips.