What Is Frequency Response?

Frequency response is the relationship between a speaker's acoustic output level and the frequency of the signal being reproduced.

A loudspeaker does not produce exactly the same acoustic output at every frequency.

For example, a woofer may produce strong output from 40 Hz to several hundred hertz, while its output decreases at higher frequencies.

A tweeter behaves in the opposite region, producing high-frequency output while normally being unsuitable for deep bass.

Frequency Response Graph

A typical frequency response graph has frequency on the horizontal axis and acoustic level on the vertical axis.

SPL
 dB
 100 ┤             ─────────
  95 ┤          ───         ───
  90 ┤       ───                 ──
  85 ┤──────
  80 ┤
     └──────────────────────────────
       20   50  100  500  1k  10k  20k
                 Frequency (Hz)

The horizontal axis is normally logarithmic rather than linear.

Frequency Axis

The frequency axis normally covers a wide range, such as:

20 Hz → 20 kHz

Because human hearing covers a large frequency range, speaker measurements commonly use a logarithmic frequency scale.

This allows low and high frequencies to be displayed on the same graph while preserving the relationship between octaves.

What Is an Octave?

An octave represents a doubling or halving of frequency.

20 Hz
40 Hz
80 Hz
160 Hz
320 Hz
640 Hz
1.28 kHz
2.56 kHz
5.12 kHz
10.24 kHz
20.48 kHz

Each step represents one octave.

This is why frequency response graphs commonly use logarithmic scaling.

SPL on a Frequency Response Graph

The vertical axis normally represents sound pressure level, expressed in decibels.

For example:

80 dB
85 dB
90 dB
95 dB
100 dB

A higher value indicates greater acoustic output under the measurement conditions.

What Does a Flat Frequency Response Mean?

A speaker with a relatively flat frequency response produces similar acoustic output across its specified frequency range.

For example, a response that remains close to 90 dB from 100 Hz to 10 kHz would generally be described as relatively flat over that range.

Perfectly flat response is extremely difficult to achieve in a real loudspeaker.

Why Flat Response Is Desirable

A relatively flat response can make the speaker reproduce the frequency balance of the input signal more accurately.

This is particularly important for:

• Studio monitoring
• Recording
• Measurement systems
• Hi-fi reproduction
• Critical listening

However, the ideal response depends on the application.

Frequency Response Is Not Always Flat

A speaker can intentionally be designed with a particular frequency response.

For example, a PA speaker may be optimized for high efficiency and maximum output rather than extremely flat response.

A subwoofer is intentionally designed to concentrate its output in the low-frequency range.

Frequency Response Bandwidth

Bandwidth describes the range of frequencies over which a speaker operates within a specified response limit.

A specification might state:

50 Hz – 18 kHz

However, this number is incomplete unless the tolerance is also given.

For example:

50 Hz – 18 kHz ±3 dB

is much more informative.

The ±3 dB Specification

A commonly used frequency-response tolerance is ±3 dB.

This means the response remains within approximately 3 dB of the specified reference level over the stated frequency range.

The exact meaning depends on how the manufacturer defines the measurement and reference.

Why the Tolerance Matters

Consider two speakers both advertised as:

50 Hz – 20 kHz

One might have a very smooth response within ±3 dB while another might have large peaks and dips.

The numbers alone therefore do not tell the complete story.

Low-Frequency Extension

The low-frequency extension describes how far down into the bass range the speaker can produce useful output.

A specification of:

45 Hz – 20 kHz

indicates that the speaker reaches lower than one specified as:

70 Hz – 20 kHz

provided the same measurement criteria are used.

High-Frequency Extension

High-frequency extension describes how far upward the speaker continues to reproduce useful output.

Tweeters and compression drivers are designed primarily for this frequency region.

The actual upper limit depends on the driver, diaphragm, horn and crossover.

Frequency Response of a Woofer

A typical woofer has a response that is strongest over the low and mid-bass region and gradually decreases as frequency increases.

The exact response depends on the driver's cone, suspension, motor system and enclosure.

Frequency Response of a Tweeter

A tweeter is designed primarily for high-frequency reproduction.

Its response normally falls rapidly below its usable operating range.

The crossover prevents excessive low-frequency energy from reaching the tweeter.

Frequency Response of a Midrange Driver

A midrange driver is designed to reproduce the frequencies between the woofer and tweeter.

Its usable bandwidth depends on the driver design and the enclosure.

The crossover determines the final operating range within the complete loudspeaker.

Frequency Response of a Full-Range Driver

A full-range driver attempts to reproduce a relatively wide frequency range using a single driver.

The response is influenced by cone size, cone material, mechanical design and enclosure loading.

Very wide bandwidth is difficult to achieve without compromises.

Peaks in Frequency Response

A peak is a region where the speaker produces more output than the surrounding frequencies.

For example:

90 dB ────────────────
95 dB             /──\
90 dB ───────────/    \────

A strong peak can make a speaker sound brighter, harsher, boomy or otherwise tonally unbalanced depending on its frequency.

Dips in Frequency Response

A dip is a region where the acoustic output is lower than the surrounding frequencies.

Dips can be caused by:

• Driver behaviour
• Crossover interaction
• Acoustic cancellation
• Cabinet diffraction
• Room reflections
• Phase relationships

Resonances

A resonance occurs when a mechanical or acoustic system responds strongly at a particular frequency.

Resonances can occur in:

• Driver cones
• Diaphragms
• Cabinets
• Ports
• Horns
• Enclosures

A resonance can appear as a peak or irregularity in the measured response.

Cone Breakup

A loudspeaker cone does not always move as a perfectly rigid piston.

At sufficiently high frequencies, different parts of the cone can begin moving differently.

This behaviour is known as cone breakup.

It can produce peaks and irregularities in the frequency response.

Cabinet Diffraction

Sound waves interact with the edges of the loudspeaker cabinet.

These interactions can produce constructive and destructive interference called diffraction.

Cabinet shape, baffle width and edge geometry can therefore influence the measured frequency response.

Baffle Step

A loudspeaker mounted on a finite baffle behaves differently at low frequencies than at high frequencies.

At higher frequencies, the baffle helps direct sound forward.

At lower frequencies, the sound wraps around the cabinet more easily.

This transition is commonly called the baffle-step region.

Crossover Effects on Frequency Response

In a multi-way loudspeaker, the woofer, midrange and tweeter responses overlap around their crossover frequencies.

The crossover determines how these drivers are combined.

Incorrect crossover design can produce:

• Peaks
• Dips
• Phase problems
• Uneven directivity
• Reduced output

Acoustic Crossover vs Electrical Crossover

The electrical crossover describes the filters created by the electronic components.

The acoustic crossover is the actual combined response of the drivers, enclosures and crossover network.

These two are not necessarily identical.

A driver already has its own natural frequency response, so the final acoustic slope depends on both the driver and electrical filter.

Crossover Region

The crossover region is where two or more drivers contribute to the overall output.

Woofer          Tweeter
  │                │
  └──────┐  ┌──────┘
         │  │
         ▼  ▼
      Crossover
         region

A properly designed crossover should produce a smooth combined response.

Phase and Frequency Response

Two drivers can have similar amplitude responses but still combine poorly if their acoustic phase relationship is unsuitable.

The relative phase between drivers can create constructive or destructive interference.

This can produce a peak or dip around the crossover frequency.

Off-Axis Frequency Response

A speaker's response changes as the microphone moves away from the acoustic axis.

This is called off-axis response.

A speaker can have a smooth on-axis response but a poor off-axis response.

For this reason, professional loudspeaker development often includes measurements at multiple angles.

On-Axis Response

On-axis response is normally measured directly in front of the speaker along its acoustic reference axis.

It is useful for evaluating the primary frequency response but does not fully describe how the speaker behaves throughout a room.

Off-Axis Response

Off-axis measurements are taken at angles away from the acoustic axis.

For example:

0°
15°
30°
45°
60°
90°

These measurements reveal how the speaker's directivity changes with frequency.

Why Off-Axis Response Matters

Listeners rarely remain exactly on the acoustic axis of a loudspeaker.

Reflections from walls, ceilings and floors also contribute to what is heard.

A smooth off-axis response can therefore be important for natural sounding reproduction.

Directivity

Directivity describes how acoustic output is distributed spatially.

At low frequencies, many loudspeakers radiate relatively widely.

As wavelength becomes smaller relative to the driver or horn dimensions, the radiation pattern can become narrower.

Directivity and Frequency

A woofer may radiate broadly at low frequencies but become increasingly directional at higher frequencies.

This can create a transition problem when the woofer is crossed to a tweeter with a different radiation pattern.

Good crossover and driver selection attempt to manage this transition.

Frequency Response and Speaker Size

Driver size affects frequency response.

Large woofers can move significant amounts of air and are well suited to low-frequency reproduction.

Small drivers can generally operate to higher frequencies more easily, although the exact limits depend on the driver design.

Frequency Response and Cone Material

Cone material affects stiffness, mass and damping.

These properties influence the driver's frequency response and breakup behaviour.

Materials used in loudspeaker cones include paper, polypropylene, aluminium, carbon fibre and other composite materials.

Frequency Response and Enclosure Type

The enclosure changes the driver's acoustic loading and therefore its frequency response.

Different enclosure types produce different low-frequency behaviour.

Frequency Response of a Sealed Speaker

A sealed enclosure normally produces a relatively smooth low-frequency roll-off determined by the driver and enclosure alignment.

The system Q affects the shape around the system resonance.

See the Sealed Speaker Box article for more information.

Frequency Response of a Bass Reflex Speaker

A bass reflex enclosure produces additional acoustic output around its tuning frequency.

The tuning frequency and enclosure alignment therefore have a strong effect on the low-frequency response.

See the Bass Reflex article for more information.

Frequency Response of a Horn

A horn can increase acoustic output over its intended operating range.

Its frequency response depends on the driver, horn profile, throat, mouth, length and acoustic loading.

See the Horn Speakers article for more information.

Frequency Response and Sensitivity

Sensitivity indicates how much acoustic output a speaker produces for a specified electrical input under defined measurement conditions.

Frequency response shows how that output changes with frequency.

The two specifications therefore provide different information.

Sensitivity vs Frequency Response

Frequency Response and Impedance

The electrical impedance of a loudspeaker also changes with frequency.

The impedance curve can reveal:

• Driver resonance
• Enclosure tuning
• Crossover behaviour
• Electrical resonances

Frequency response and impedance are therefore complementary measurements.

Frequency Response Measurement

A typical speaker frequency-response measurement requires:

• A measurement microphone
• An audio interface or measurement system
• A suitable amplifier
• Measurement software
• A controlled measurement environment

The microphone receives the acoustic output while the system sweeps through the desired frequency range.

Sine Sweep Measurement

A frequency sweep sends a continuously changing frequency through the speaker.

The measurement system records the acoustic output at each frequency.

The result is a frequency-response curve.

20 Hz ───────────────────────► 20 kHz

        Frequency sweep
              ↓
       Microphone measures
              ↓
       Frequency response

Measurement Microphone

A measurement microphone should have a known and reasonably flat response over the frequency range being measured.

Ordinary microphones may introduce their own frequency-response errors.

For accurate loudspeaker development, a calibrated measurement microphone is preferable.

Measurement Distance

The microphone distance affects the measurement.

Common loudspeaker measurements may use a standardized distance such as 1 metre, although other distances can be appropriate depending on the application and measurement method.

The important requirement is to use a known and repeatable setup.

Room Effects

A normal room contains reflections from:

• Walls
• Floor
• Ceiling
• Furniture
• Other objects

These reflections combine with the direct sound and can produce peaks and dips in the measured response.

This is why indoor measurements can be difficult at low frequencies.

Near-Field Measurement

A microphone placed close to the driver can reduce some room-related effects.

Near-field measurements are particularly useful for examining low- frequency driver and port behaviour.

However, near-field measurements are not directly interchangeable with a far-field listening-position measurement.

Outdoor Measurements

Outdoor measurements can provide a relatively reflection-free environment when performed appropriately.

They are useful for loudspeaker development because they reduce the effect of nearby room boundaries.

Ground reflections and environmental conditions still need to be considered.

Anechoic Measurements

An anechoic chamber is designed to minimize acoustic reflections.

It allows highly controlled loudspeaker measurements.

Professional loudspeaker manufacturers may use anechoic or other controlled measurement environments.

Frequency Response Smoothing

Measurement software can apply smoothing to a frequency-response curve.

For example:

1/1 octave
1/3 octave
1/6 octave
1/12 octave
1/24 octave

More smoothing makes the graph easier to read but can hide narrow resonances and irregularities.

The smoothing setting should therefore be considered when comparing measurements.

Raw vs Smoothed Response

A raw response contains more detailed information but can look irregular because of reflections, interference and narrow resonances.

A smoothed response makes the overall tonal trend easier to see.

Both views can be useful.

Frequency Response and Room Response

A speaker's measured response in a room is not necessarily the same as its intrinsic free-field response.

Room modes can produce strong low-frequency peaks and dips.

Speaker placement and listening position can therefore have a major effect on what is heard.

Room Modes

A room supports resonances at particular frequencies determined by its dimensions.

These resonances can strongly affect bass response.

Consequently, a speaker that measures smoothly in a controlled environment may show substantial peaks and dips in a particular room.

Speaker Placement

Speaker placement influences frequency response because nearby boundaries alter acoustic radiation.

Moving a speaker closer to a wall can increase some low-frequency output.

Corner placement can produce even stronger boundary loading.

Listening Position

The listening position also affects the measured and perceived bass response.

Moving the listener only a small distance can sometimes change the relative strength of room modes.

This is why speaker evaluation should distinguish between the speaker's intrinsic response and the response of the complete room system.

Frequency Response and Equalization

Equalization can be used to correct some frequency-response irregularities.

For example, a digital equalizer can reduce a broad frequency peak.

However, EQ cannot completely compensate for every acoustic problem.

Deep cancellation caused by destructive interference generally cannot be solved simply by adding large amounts of amplifier power at the same frequency.

Boosting vs Cutting

When using equalization, cutting a strong peak is often safer than attempting to boost a deep cancellation.

A large boost increases amplifier power and driver excursion while not necessarily producing the desired acoustic output.

Frequency Response and Maximum SPL

Frequency response should not be considered separately from maximum output capability.

A speaker may have a very smooth response at low measurement level but develop compression or distortion at high SPL.

Professional speaker evaluation therefore includes measurements at different output levels.

Power Compression

As a voice coil heats during high-power operation, its electrical and mechanical behaviour can change.

The acoustic output may therefore increase less than expected as electrical power continues to rise.

This behaviour is commonly referred to as power compression.

Frequency Response at Different SPL Levels

It can be useful to measure the same speaker at several output levels.

Low SPL     ────────────────
Medium SPL  ────────────────
High SPL    ────────────────
                  ↓
          response changes

Differences between the curves can reveal thermal compression, nonlinear behaviour or mechanical limitations.

Frequency Response and Distortion

A frequency-response graph shows amplitude behaviour but does not by itself describe distortion.

Two speakers can have similar frequency responses while producing very different levels of harmonic distortion.

Distortion measurements are therefore an important complement to frequency-response measurements.

Harmonic Distortion

When a speaker is driven with a pure tone, nonlinearities can create additional harmonic frequencies.

For example, a 1 kHz input can produce components at:

1 kHz
2 kHz
3 kHz
4 kHz
...

These additional components are harmonic distortion.

Frequency Response and Phase

Amplitude response and phase response provide different information.

A frequency-response graph normally shows the magnitude of the response.

Phase response shows how the phase of the acoustic signal changes with frequency.

Both can be useful when designing multi-way loudspeakers.

Group Delay

Group delay describes how the timing of different frequency components changes through a system.

Enclosures, crossovers and acoustic structures can all affect group delay.

It can be particularly useful when analyzing low-frequency systems and complex acoustic alignments.

Reading a Frequency Response Graph

When examining a speaker response graph, look for:

1. Overall frequency range
2. Low-frequency extension
3. High-frequency extension
4. Average response level
5. Large peaks
6. Deep dips
7. Crossover irregularities
8. High-frequency roll-off
9. Low-frequency roll-off
10. Off-axis behaviour

Example of a Good Response

dB

 95 ┤
 94 ┤    ─────────────────────
 93 ┤  ──                    ──
 92 ┤──                        ──
 91 ┤
 90 ┤
    └────────────────────────────
      50 Hz             15 kHz

A relatively smooth curve generally indicates consistent output across the measured range.

The acceptable variation depends on the application.

Example of a Problematic Response

dB

100 ┤           /\
 95 ┤──────────/  \───────
 90 ┤       __/          \__
 85 ┤──────/                ──
    └──────────────────────────
      50 Hz             15 kHz

Large peaks and dips can indicate resonances, crossover problems, diffraction, driver breakup or measurement effects.

Frequency Response of a Complete Loudspeaker

A finished multi-way speaker's response is the combination of:

• Woofer response
• Midrange response
• Tweeter response
• Enclosure response
• Crossover network
• Cabinet diffraction
• Driver interaction

The final response must therefore be evaluated as a complete system.

Frequency Response and Crossover Design

Before designing a passive crossover, the actual frequency responses of the drivers should ideally be measured in the intended cabinet or baffle.

Using only nominal driver specifications can produce an inaccurate crossover.

The acoustic response of the complete system should be the design target.

Why Manufacturer Frequency Graphs Matter

Manufacturer graphs can provide useful information about a driver's operating range and behaviour.

However, measurements from different manufacturers may use different conditions, smoothing and scales.

They should therefore be compared carefully.

Frequency Response Does Not Tell Everything

A frequency-response graph alone cannot tell you:

• Maximum SPL
• Distortion
• Power compression
• Directivity in all directions
• Mechanical reliability
• Transient behaviour
• Thermal performance

A complete loudspeaker evaluation requires several different measurements.

Useful Speaker Measurements

• Frequency response
• Impedance
• Phase response
• Distortion
• Directivity
• Maximum SPL
• Power compression
• Waterfall or decay response

Waterfall Measurements

A waterfall plot shows how acoustic energy changes with both frequency and time.

It can reveal resonances that continue after the original excitation has stopped.

This is particularly useful when investigating cabinet, cone or horn resonances.

Frequency Response and Speaker Design

Frequency response should be considered at every stage of speaker development.

The general process is:

Driver selection
       ↓
Enclosure design
       ↓
Crossover design
       ↓
Prototype
       ↓
Measurement
       ↓
Adjustment
       ↓
Final speaker

Measurement allows the designer to verify whether the physical speaker behaves as predicted.

Common Frequency Response Problems

Common Frequency Response Measurement Mistakes

• Ignoring room reflections.
• Using an uncalibrated microphone for precision measurements.
• Comparing measurements made at different distances.
• Ignoring measurement smoothing.
• Comparing graphs with different vertical scales.
• Judging a speaker only from on-axis response.
• Ignoring speaker placement.
• Ignoring listening position.
• Using EQ to compensate for deep acoustic cancellation.
• Assuming frequency response alone determines speaker quality.

Practical Frequency Response Measurement Workflow

1. Place the speaker in a suitable measurement position.
2. Connect the measurement system and amplifier.
3. Position the measurement microphone.
4. Set a safe measurement level.
5. Run a frequency sweep.
6. Record the response.
7. Repeat the measurement at different angles.
8. Apply appropriate smoothing when analyzing the graph.
9. Identify peaks and dips.
10. Investigate their likely causes.
11. Modify the enclosure or crossover if required.
12. Repeat the measurement.

Frequency Response Measurement Checklist

• Measurement microphone selected
• Microphone calibration considered
• Measurement distance defined
• Speaker position fixed
• Measurement level controlled
• Frequency sweep configured
• On-axis response measured
• Off-axis response measured
• Low-frequency behaviour checked
• Crossover region checked
• High-frequency response checked
• Measurement smoothing documented
• Room effects considered
• Distortion measured separately
• Impedance measured separately

Key Takeaways

• Frequency response describes how a speaker's acoustic output changes with frequency.
• A frequency-response graph normally shows frequency on the horizontal axis and SPL on the vertical axis.
• The frequency axis is normally logarithmic.
• An octave represents a doubling or halving of frequency.
• A relatively flat response indicates consistent output over a specified frequency range.
• Frequency bandwidth should ideally include a tolerance such as ±3 dB.
• Peaks can be caused by resonances, cone breakup, enclosure effects or crossover behaviour.
• Dips can result from cancellation, crossover interaction, diffraction or room effects.
• Enclosure design has a major effect on low-frequency response.
• Multi-way speaker response is the combination of several drivers and the crossover network.
• Off-axis response is important because listeners and room reflections are not restricted to the acoustic axis.
• Directivity changes with frequency.
• Frequency response and sensitivity describe different properties.
• Frequency response alone does not describe distortion or maximum SPL.
• Room measurements can contain strong reflections and room-mode effects.
• Near-field measurements are useful for examining low-frequency behaviour.
• Impedance, phase, distortion and directivity measurements complement frequency-response measurements.
• Measurement is an essential part of developing a high-quality loudspeaker.