What You Need to Know About Headroom

Headroom is a crucial concept for live sound, recording, and also for buying a car that’s comfortable. We can ignore the part about buying a car, but we can’t ignore the importance for audio. Because headroom has different implications for digital and analog audio, we’ll cover both.

Headroom in the Analog Recording World

Technically speaking, headroom (when measured in deciBels) is the ratio of the maximum amount of undistorted signal a system can handle compared to the average level for which the system is designed. For example, suppose you have a home recording setup with a nominal -10dB average level. If you can pump out +8dB signal through your system without distortion, then you have 18dB of headroom.

The key words here are “without distortion.” When you run out of headroom, distortion happens. Granted, this can be a good thing when you run out of headroom in a Marshall stack. But for a PA system or a mixing console, it’s not a good thing.

With an analog mixing console that’s designed for a +4dB average signal level, its VU meters will be calibrated so that the meters indicate 0 VU at +4dB. However, pro gear designed to work at +4dB is capable of delivering +24dB output levels, so at 0 VU, you have a 20dB (24 – 4 = 20) margin of built-in headroom to make sure that unanticipated level increases (screamer singers, anyone?) or strong transients are reproduced properly.

Analog Mixer Headroom

But do we actually need all that headroom? Don’t we want as high a signal level as possible to avoid noise? Well…yes, but it’s important to note that although the VU meter indicates an average (also called RMS) level, music is anything but average because of what’s called the crest factor. This represents the ratio of an audio signal’s peak value to its average value, which can be 10dB–20dB higher than the average value. Although it’s important to monitor average levels because they correlate more with how human hearing perceives sound, you also need to monitor peak levels so you can know if there’s a chance of exceeding the available headroom.

Many people don’t realize that peak transients in music recordings can demand up to 10x the average (RMS) wattage demands. That means that in a system using 40-watt RMS for music program, the peaks can require 400 watts to be accurately reproduced.

Headroom, PA Systems, and Mixers

With a PA system or an analog recording mixer, to minimize noise you’ll want to use the maximum available safe headroom. For the best gain staging, the place to raise levels to the maximum acceptable level is as close to the input as possible. With mics, dial in the right amount of gain at the mic preamp, and set the rest of the mixer around the unity gain zone. With line-level devices like electronic instruments, set the instrument’s output to produce the maximum safe level short of distortion, and then gain stage the mixer appropriately.


I’m Givin’ Her All She’s Got, Captain!

If you know Star Trek, then you know chief engineer Scotty used that phrase a lot. Editor’s note: Star Trek devotees will know that James Doohan never actually uttered that phrase in the original series. And while he was referring to an inability to get to warp speed fast enough to get away from some sort of horrific problem, he was also commenting on headroom — ultimately, the limiting factor in headroom is the power supply. It stands to reason that if the maximum voltage available to a system is, for example, 15 volts, then there’s no way you could reproduce a signal so that its peak exceeds 15 volts.

What About Power Amps and Speakers?

This is where it gets more interesting. First of all, a power amp’s level control doesn’t adjust the output. The amp has the potential to run at full amplification all the time. (The monitor speakers in your studio probably work the same way.) The input control varies the level going to the amp. This is good, because the full headroom is available all the time, so your transients will come through. But it also means you need to be careful not to increase the input level enough to bump up against the available headroom.

Speakers don’t have headroom in the same sense that power amps do, but sending in too much signal can do anything from cause distortion to blow up speakers. Because modern, powered speakers incorporate both the speaker drivers and the amp(s) that drive them, they have protection measures (limiters) in place to limit the audio levels and protect your speakers. Passive speakers and separate power amps are more likely to have issues with this kind of setup — the speakers and power amps don’t “talk” to each other. And if there’s distortion at the power amp that results in clipping, this raises the signal’s average level, creates more power at higher frequencies, and is therefore more prone to do something like blow out a high-frequency driver.


Headroom in the Digital Recording World

This is where it gets really complicated. Please fasten your seat belt, and make sure your tray table is in the upright and locked position.

First off, 0dBFS (FS stands for “Full Scale”) in a digital system means the absolute maximum level the system can handle, so unlike analog systems that build in “invisible” headroom above 0 VU, 0dBFS is as high as you can go. This is why many digital recording pros calibrate their recording systems (DAWs) at -18dB below 0dBFS, because it creates headroom that a digital system doesn’t have inherently. Granted, you’re giving up about 3 bits of resolution by recording at -18 instead of 0 (each bit represents about 6dB). However, with 24-bit recording, you’re only down to 21 bits, and that’s still more than the true resolution of most hardware anyway. (A 24-bit converter doesn’t really have 24 bits of resolution owing to inaccuracies in the converters themselves, hiss, circuit board layout, and the like.) And you gain the headroom to accommodate peaks, resonances, and sudden level increases.

But this isn’t the only place where headroom comes into play with digital systems. Headroom relates to dynamic range, and once you get signals inside the computer, today’s DAWs have audio engines with practically unlimited dynamic range. It’s almost impossible to exceed the headroom within the audio engine, which is why individual mixing channels can go “into the red” without appearing to create distortion. However, every DAW has a virtual day of reckoning when it comes back to the real analog world, via D/A (digital-to-analog) converters and hardware. These have anything but an unlimited dynamic range. It’s considered a best practice to keep the master fader at 0 and use the channel faders to create the best balance, rather than run the channel faders “hot” and bring down the master fader to compensate. If you keep the master fader at 0, thanks to the extreme resolution of your DAW’s audio engine, there’s no penalty in leaving individual channel levels hovering around -18 on the faders.

For a deeper dive, check out our Understanding Signal Levels in Audio Gear article.

Uh-oh…Intersample Distortion
This is a particularly sneaky way to exceed headroom, because it can create problems and you may not even be sure where they’re coming from.

This issue occurs because most digital metering visualizes the actual numerical value, so the numerical value that represents 0dBFS shows 0 on the meter. However, converting digital audio back to analog may result in higher actual values than the samples themselves, which creates the potential for intersample distortion.

This type of distortion can occur if the level of some samples uses up the maximum available headroom, and these high-level samples then pass through the D/A converter’s output smoothing filter to reconstruct the original waveform. This reconstructed waveform might have a higher amplitude than the peak level of the samples, which means the waveform now exceeds a playback system’s maximum available headroom. Unless your channel’s meters can alert you to intersample distortion, you should leave a few dB of headroom to avoid this. Many mastering engineers recommend absolute peak levels not exceeding -1.0dBFS.

Intersample distortion


If you create a digital file (or a CD, for that matter) for playback with the potential for intersample distortion “baked in,” then the listener’s D/A converter will create this distortion. But another problem is that when you’re mixing, if you’re not aware of the possibility of intersample distortion, your monitoring system may be producing distortion, and your mix will be based on a distorted sound.

Whether this is a serious problem or not depends on the musical material itself. I’ve seen projects with no intersample distortion, and others where peaks often went over +3.0dB. Fortunately, today’s better metering systems, which often include True Peak metering based on the European Broadcast Union’s R128 standard (for more information, please see the article What Is LUFS, and Why Should I Care?), can show if intersample distortion is occurring.

So…when mixing, this is another good reason to leave a few dB of headroom at the master output and not run levels right up to 0 dB. Leave that for the mastering engineer.

DC Offset: The Headroom Thief

DC offset isn’t a particularly sexy topic, nor is it a particularly common problem. But it can be the culprit behind issues related to reduced headroom (mastering oddities, pops and clicks during editing, effects that don’t process properly, and other gremlins).

Back in the days of analog circuits, if a circuit had zero input signal, in theory the output would also be at 0 volts. When op-amps (operational amplifiers), with their massive amounts of gain, became popular, imperfections within the chips, or DC present at the output of the stage feeding the op-amp, could sometimes produce a static DC voltage of several millivolts at an op-amp’s output.

Normally this wouldn’t matter, but if the stage with DC offset was followed by a stage providing lots of gain, even a tiny voltage at that stage’s input could end up as a significant output voltage. For example, if there’s +0.002 volts of offset at the input, and a circuit like a mic preamp has 60dB of gain (an amplification factor of 1,000), now there’s +2 volts of offset at the output. This reduces the available headroom by 2 volts (remember what we said earlier about power supplies limiting your headroom?).

The simple solution was capacitive coupling — we don’t need to get into the weeds with this; it simply meant using a capacitor that blocked DC voltage, like offset, but passed AC voltages, like audio. However, some devices had a frequency response down to DC, so these voltages could indeed be present.

With digital audio, there are two main ways DC offset can infiltrate a signal:

Recording an analog signal that has DC offset into an interface (e.g., MOTU, PreSonus) whose low-frequency response goes down to DC, rather than rolling off below a low frequency like 20Hz.
Issues beyond your control that add DC offset to a file brought into the computer.
In either case, offset appears as a signal baseline that doesn’t match up with the “true” 0-volt baseline.

DC Offset

There are two main ways to solve DC offset problems with software-based digital audio programs.

Most pro-level digital audio editing and multitrack recording software includes a DC offset correction function, generally found under a processing menu along with functions like change gain, reverse, flip phase, etc., or made available as a plug-in. This function analyzes the signal and adds or subtracts the required amount of correction to make sure that 0 really is 0.
Apply a steep highpass filter that cuts off everything below 20Hz or so. (Even with a comparatively gentle 12dB/octave filter, a signal at 0.5Hz will be down more than 60dB). In practice, it’s not a bad idea to cut the subsonic part of the spectrum anyway. Most speakers can’t reproduce signals this low, so they just use up amplifier power and bandwidth.
DC offset usually isn’t a major problem. But every now and then, DC offset will rear its ugly head and reduce the available headroom.

Okay, Headroom Isn’t Very Exciting, But…

…it’s something that’s a part of our audio world, whether that world is analog or digital. Sometimes, headroom issues won’t have audible results, but sometimes they will. So make sure you’ve gain staged and analyzed your system to make the best use of the available headroom — and don’t exceed it.

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