Bass Roll-off: What Does it Mean?

Part of the challenge of becoming a good music producer is understanding the seemingly-endless technical terms and data associated with digital audio production and recording. It can be extremely overwhelming, and the amount of work and consideration that goes into a finished track isn’t immediately apparent from the outside looking in. On top of this dilemma, it isn’t enough to just know what these terms are. We must also know what they do, how they work, and how we can utilize them to make better music. In this informative article, we’ll uncover a common term that anyone interested in audio production will encounter: the bass roll-off. Let’s jump right in.

Understanding Frequency Response

In order to understand bass roll-offs and how they work, we need a deep understanding of frequency response, sound, and waveform physics.

Basic Sound Physics

Every sound you hear, from music in your car, to an exploding firecracker, to the roar of a jet engine, is made by the vibration of molecules and atoms in the air passing over small hairs in your ear. These complex mechanical vibrations are further broken down into individual, simple sine waves that, when combined together, create the complex sounds of a cat meowing or a dog barking.

A sine wave is the simplest of wave forms. It is defined by three main parameters or variables:

1. Frequency – the number of waveform cycles (one cycle is shown below) the sine wave goes through in 1 second measured in Hertz (Hz)
2. Amplitude – the height of the waveform above 0 or the equilibrium point (A_o)
3. Phase – the position of the waveform in its cycle measured in degrees

Below you can also see λ which is the wavelength. Wavelength is the physical distance one full waveform cycle covers, and it’s related to frequency f by the speed of sound V_s in the following equation:

V_s = f × λ

The important takeaway here is that all sounds are composed of many simple sine waves added together, all having frequencies that fall within the range of 20 Hz to 20 kHz.

Sound to Electricity

These mechanical sound waves in the air can be transformed into electrical wave forms using a transducer such as a microphone. The microphones job is to encode the frequencies and amplitudes of sound waves in the air into an electrical waveform with as much accuracy as possible – to capture the individual frequencies present in the sound with no distortion.

A 100% perfect conversion from sound to electricity is physically impossible. But we can get pretty close.

This is where frequency response comes into play. Frequency response is a way for us to tell how much distortion is present when we send information through a device such as a loudspeaker or microphone, and at what frequencies the distortions are present at.

• When we send a signal through a filter, we usually specify the frequency response curve of the filter in order to remove parts of the frequency spectrum that we no longer want present in the signal.
• When we send a signal through an EQ, we specify the frequency response curve of the EQ in order to boost or cut certain frequencies in the signal.
• When we send a signal through a loudspeaker, the frequency response curve is usually* permanent and changes the way the signal will physically sound to a listener
• When we record audio with a mic, the frequency response is usually* permanent and changes the way the sound is transformed into electricity.

*Note: Loudspeakers and microphones can have switches that will change their frequency response slightly. These can be filters, pads, or basic EQs.

Reading the Frequency Response Curve

So, in a nutshell, the frequency response curve tells us how the amount of any given frequency present in a signal will change as it passes through a device. Frequency response curves can be permanent charts that are provided by the manufacturer of physical devices like microphones or studio monitors, or a curve that the engineer modifies in things like virtual filters and EQs. But how do we read them? Let’s take a look.

Here, we can see the frequency response curve of an EQ plugin in the popular DAW, FL Studio. In this case, the EQ is virtual and it’s just changing the signal. There is no conversion from sound to electricity going on here.

• On the bottom, we can see the frequency range of 20 Hz to 20 kHz. This is the range of human hearing, and anything above or below these frequencies is inaudible.
• On the top, we can see the regions of the spectrum commonly referred to when talking about music: Sub bass, bass, low mids, mids, high mids, presence, and treble
• On the right hand side, we can see the gain in decibels (dB)

The white curve is telling us what frequencies (horizontal X-axis) are experiencing what increase or decrease in gain (vertical Y-axis) as they pass through the EQ.

We can see a band stop filter at ~175 Hz – this area where the curve disappears means frequencies in that area are removed from the input signal completely.

We can also see a peak at ~2500 Hz – if we draw a horizontal line across from the top of the peak, we can see that this frequency is being boosted by +12 dB.

Basically, we take any point located on the curve and draw lines vertically down to the frequency, and horizontally over to gain, to determine what frequencies experience what increase or decrease in gain as they pass through the device. Let’s look at the frequency response of a real physical device – the AT2035 condenser microphone.

Here is the frequency response curve of the Audio-Technica AT2035, one of many popular entry-level condenser microphones. In this case, we are changing sound to electricity, and the curve tells us how different the actual sound will be in comparison to the electrical waveform the mic produces.

This frequency response curve is read the same way as before – frequency on the bottom, and gain in dB on the right. Keep in mind that the frequency range on the bottom uses a logarithmic scale. We can see some peaks in the treble region that add to the presence of those frequencies in the recorded signal. We can also see what we all came here to find out about – the bass roll-off which is activated by the filter switch on the AT2035.

Bass Roll-off Explained

So, with everything we’ve learned up to this point, it should be clear to you now what the bass-roll off is, exactly:

Bass-roll off is the area of the frequency response curve of a device that exhibits a gradual tapering in the low-frequency region of the frequency spectrum.

Bass roll-offs are useful for lowering the low-frequency content of an audio signal. Let’s say you’re using the AT2035 to mic a bass guitar, and it seems like the bass is just too muddy. You can activate the bass roll-off filter (also called a high-pass filter or HPF in this case) on the mic to remove the excessive low-end in the signal.

dB per Octave

Taking this another step further, you may encounter a term called “dB per octave” when talking about bass roll-offs. The spec sheet of the AT2035 from the above example lists a roll-off of -12 dB per octave starting at 80 Hz.

An octave in music theory is a set of 12 keys on a keyboard. But more technically, an octave is a range of frequencies. 880 Hz is one octave above 440 Hz, and 220 Hz is one octave below 440 Hz. Doubling or halving any frequency will produce a tone or note that is one octave above or below the original tone or note.

The 12 dB per octave bass roll-off specification means that for every halving of the starting frequency of 80 Hz, the gain decreases by 12 dB. So at 40 Hz, we’re at -12 dB from the gain at 80 Hz,, and at 20 Hz, we’re at -24 dB from the gain at 80 Hz.

When to Use the Bass Roll-Off Switch

We touched on this earlier, but it’s worth mentioning specific situations where we’d use the bass roll-off switch on a microphone.

Obviously with excessively bass-heavy instruments or deeper voices, the bass roll-off may come in handy to achieve a more balanced signal. But something to consider that isn’t immediately clear is the proximity effect.

The proximity effect is the tendency of the sound source to have an increasingly exaggerated bass response as the mic is moved closer and closer to the source. For close miking, the bass roll-off switch can be activated to counteract the proximity effect. The proximity effect isn’t all bad, though. Activating the bass roll-off is a decision you should make based on what instrument you’re recording and what kind of sound you want to achieve.

Bass Roll-off Explained: Main Takeaways

We covered a lot of technical information here in order to explain exactly what a bass roll-off is. Lets recap a few critical points that should solidify your understanding of this term:

1. Sound can be broken down into many individual sinusoidal wave forms at different frequencies between 20 Hz and 20 kHz
2. A the goal of a microphone is to change sound pressure waves into electrical waves with as much accuracy as possible – the right frequencies and the right amounts (amplitudes) of each frequency
3. The frequency response curve is a chart that shows how “inaccurate” a device is when converting sound to electricity, or vice versa. It shows what frequencies are amplified and what frequencies are attenuated by the device.
4. Bass roll-off is a characteristic of the frequency response curve that exhibits a gradual decline in the bass region of the frequency spectrum
5. Some microphones have bass roll-off switches (or high-pass filters) that help to cut back on excessive bass – whether it be due to the instrument itself or the proximity effect