Cabinets and IRs

Loudspeaker cabinets are a major part of the sound of an amplified instrument.

In guitar and bass systems, the cabinet, loudspeaker, microphone, and microphone position can alter the sound at least as dramatically as many changes made to the amplifier itself.

Capture technology can handle this part of the signal chain in several ways. A capture may include the cabinet and microphone, or the amplifier may be captured directly and combined later with a separate impulse response (IR) or cabinet model.

Understanding which approach was used is essential when configuring a capture for playback.

What is an impulse response?

An impulse response, commonly called an IR, is a measurement of the response of a system.

In guitar and bass applications, IRs are widely used to reproduce the frequency and time-domain characteristics of a loudspeaker cabinet, speaker, microphone, and microphone position.

A cabinet IR commonly represents a chain such as:

Loudspeaker → cabinet → microphone → microphone position → recording path

The resulting IR can be applied to another signal using a process called convolution.

In practical use, this allows a direct amplifier model or capture to be combined with the measured response of a cabinet and microphone.

Captures and IRs are not the same thing

An IR and a nonlinear equipment capture solve different problems.

An amplifier, overdrive pedal, or similar device changes its behavior according to signal level. Distortion, compression, clipping, saturation, and other nonlinear behavior may change dynamically with the input.

A conventional cabinet IR is fundamentally a linear representation.

It can reproduce characteristics such as:

Frequency response
Phase response
Resonance
Timing information
Cabinet and speaker coloration
Microphone coloration
Microphone position

It does not, by itself, reproduce the complete nonlinear behavior of an amplifier or distortion device.

This is why a common signal chain is:

Nonlinear amplifier capture → cabinet IR

The two technologies perform complementary jobs.

Direct amplifier captures

A direct amplifier capture normally excludes the physical loudspeaker cabinet and microphone.

A simplified capture path might be:

Amplifier → suitable load/direct capture device → capture system

During playback, a cabinet IR or cabinet model can then be added:

Amplifier capture → cabinet IR → output

This provides substantial flexibility because the same amplifier capture can be used with many different cabinet and microphone responses.

For example, one amplifier capture might be auditioned through IRs representing:

Different speaker models
Different cabinet sizes
Open-back and closed-back cabinets
Different microphones
Different microphone positions

The amplifier does not need to be captured again simply to change the cabinet response.

Captures that already contain a cabinet

A capture made using a physical cabinet and microphone may already contain the complete cabinet/microphone response.

A typical path is:

Amplifier → loudspeaker cabinet → microphone → preamp/interface → capture system

The resulting capture may therefore contain characteristics of:

Amplifier
Speaker
Cabinet
Microphone
Microphone position
Microphone preamp
Other equipment in the return path

Such a capture normally does not require an additional conventional cabinet IR.

Adding another cabinet IR can result in two cabinet responses being applied in series.

This is commonly described as double cabbing or a double cabinet configuration.

The result is not necessarily unusable as a creative effect, but it is normally not an accurate reproduction of the original captured signal chain.

How cabinet IRs are created

An IR is created by measuring the response of the system to a known signal.

Several measurement techniques are possible. A common method uses a swept sine signal.

A simplified measurement chain is:

Test signal → power amplifier → loudspeaker cabinet → microphone → recording system

The recorded response is mathematically processed to derive the impulse response.

The resulting IR is usually stored as an audio file, commonly in WAV format.

Once created, the IR can be loaded into compatible hardware or software.

The microphone is part of the IR

A cabinet IR should not be thought of as merely a digital copy of a speaker.

If a microphone was used to create the IR, the microphone is part of the measurement.

Its contribution includes:

Frequency response
Polar response
Proximity effect
Position relative to the speaker
Angle
Distance

Consequently, an IR labeled with the same cabinet and speaker can sound dramatically different when created with another microphone or microphone position.

Microphone position

Microphone position has a major influence on cabinet sound.

Moving a microphone across the face of a guitar speaker can substantially change the tonal balance.

Typical variables include:

Center versus edge of the cone
Distance from the speaker
On-axis versus off-axis placement
Position relative to individual speakers in a multi-speaker cabinet

For this reason, commercial and community IR libraries frequently contain many variations of the same cabinet.

They are not necessarily redundant. Each may represent a genuinely different microphone configuration.

Multiple microphones

Cabinets are often recorded using more than one microphone.

An IR library may therefore contain:

Individual microphone IRs
Pre-mixed microphone combinations
Multiple positions of each microphone
Different microphone distances
Different blend ratios

A mixed IR contains the characteristics of the selected microphone combination as it existed when the IR was created.

If greater control is desired, separate IRs can sometimes be loaded and mixed during playback instead.

IR length

Impulse responses can be stored at different lengths.

Longer IRs can preserve more low-frequency and time-domain information, but they also require more processing and storage.

Some hardware devices impose maximum IR lengths or automatically truncate or resample imported IRs.

For guitar cabinet applications, the practical effect of IR length depends on the equipment, software, sample rate, and the material represented by the IR.

Longer is therefore not automatically better in every application.

Sample rate

IR files are digital audio data and have a sample rate.

Playback software or hardware may:

Require a particular sample rate
Resample the IR during import
Resample it during playback
Support several rates

Follow the requirements of the device or software loading the IR.

An IR should not be assumed to operate differently merely because its file sample rate differs if the playback system performs appropriate conversion.

Minimum-phase and phase information

IR libraries may offer files described as minimum phase, minimum phase transformed (MPT), or similar terminology.

Minimum-phase processing changes the phase relationship of the response while retaining its general magnitude response.

This can make IRs easier to combine in some circumstances because timing differences between independently measured files are reduced.

Other IRs preserve more of the original measured timing and phase information.

Neither form is universally superior. The appropriate choice depends on how the IR will be used.

Combining IRs

Two or more IRs can be blended to create a composite cabinet sound.

This is commonly used to simulate multi-microphone recording.

However, phase and timing relationships matter.

Two individually good IRs may produce unexpected cancellations when combined.

Possible results include:

Reduced low frequencies
Comb filtering
Hollow or thin sound
Changes in attack
Unexpected frequency peaks or dips

IR mixing software may provide alignment, delay, phase, or polarity controls to manage these interactions.

IRs and physical guitar cabinets

A cabinet IR is normally intended for playback through a relatively full-range system such as:

Studio monitors
Headphones
PA systems
FRFR speakers
Suitable full-range power amplifiers and speakers

When a model is played through a traditional physical guitar cabinet, an additional cabinet IR may not be desirable because the physical cabinet already provides strong speaker and cabinet coloration.

A common arrangement is therefore:

Direct amplifier capture → power amplifier → physical guitar cabinet

rather than:

Direct amplifier capture → cabinet IR → power amplifier → physical guitar cabinet

There are exceptions and creative uses, but understanding the signal chain prevents accidental duplication.

FRFR playback

FRFR generally means full-range, flat-response.

The goal of an FRFR playback system is to reproduce the complete modeled signal, including the cabinet and microphone response, without deliberately adding the strong coloration associated with a traditional guitar cabinet.

In practice, no loudspeaker system is perfectly flat, and room acoustics also affect the result.

Nevertheless, the distinction is useful:

Traditional guitar cabinet: provides its own strong cabinet/speaker character.

FRFR/full-range system: intended to reproduce the cabinet/microphone character already contained in the model or IR.

Cabinet modeling

Not every digital cabinet system uses static IR files in the same way.

Some hardware and software provides cabinet modeling with controls for variables such as:

Cabinet
Speaker
Microphone
Microphone position
Distance
Angle
Resonance

The implementation may involve IRs, interpolation between measurements, algorithmic processing, more advanced modeling, or combinations of techniques.

Therefore, the presence of adjustable cabinet controls does not necessarily mean that a single conventional IR is simply being modified.

Capturing an amplifier with an IR in the chain

It is technically possible for an IR or cabinet simulation to be active while creating a capture.

If that processing is present in the measured return path, its effect may become part of the resulting model.

This can be intentional.

It can also happen accidentally.

Before capturing, verify whether any of the following are active:

Hardware cabinet simulation
Load-box cabinet processing
DAW cabinet plugin
Interface DSP
IR loader
Amplifier direct-output speaker simulation

If the objective is a direct amplifier capture, these should normally be disabled or bypassed.

Identifying what a capture contains

Before adding an IR to a downloaded capture, determine whether cabinet processing is already included.

Useful clues may come from:

Capture metadata
Filename
Creator notes
Platform tags
Documentation
Listening tests

However, filenames alone should not be considered definitive.

A model named after an amplifier may be an amplifier-only capture or a complete amplifier/cabinet/microphone capture.

Good capture documentation should state this explicitly.

Choosing an IR

There is no universally correct cabinet IR for an amplifier capture.

Choice depends on the desired result.

Useful considerations include:

Cabinet type
Speaker
Microphone
Microphone position
Musical style
Instrument
Pickup type
Mix context
Playback system

An IR that sounds impressive in isolation may not be the best choice in a mix.

Likewise, a relatively bright or mid-focused IR that seems less impressive alone may work extremely well with other instruments.

Evaluate IRs in the context in which they will actually be used.

Organizing IR libraries

IR collections can become very large.

Useful organization may include:

Manufacturer
Cabinet
Speaker
Microphone
Position
Mix
Sample rate
Personal favorites

Do not assume that owning thousands of IRs necessarily improves results.

A smaller, well-understood collection can be considerably easier to use than a huge library of poorly documented files.

Documenting captures and IRs

When sharing a capture that includes a physical cabinet, useful information includes:

Cabinet manufacturer and model
Speaker manufacturer and model
Cabinet configuration
Microphone
Microphone position
Distance
Microphone preamp
Whether multiple microphones were used

When sharing a direct capture, state clearly that the cabinet is not included.

This single piece of information can prevent a great deal of confusion.

Practical rule

Before adding cabinet processing, ask:

Does this capture already contain a cabinet and microphone response?

If the answer is yes, additional cabinet processing is normally unnecessary.

If the answer is no, a cabinet IR, cabinet model, or physical cabinet will generally be required for conventional guitar-amplifier playback.

Knowing what is already present in the model is more important than following a particular preset or signal-chain convention.