Amixer Returning Wrong Audio Value: A Deep Dive into PulseAudio Volume Discrepancies #

At revWhiteShadow, we understand the frustration when your audio controls don’t behave as expected. A common point of confusion arises when using the amixer command-line utility, particularly in conjunction with PulseAudio, and observing discrepancies between the set volume and the reported volume. You might execute a command like amixer -D pulse sset Master 30%, expecting your audio to be precisely at 30%, only to later query with amixer get Master and see a value like 52% reported. This stark difference can be perplexing, leading to questions about the accuracy of amixer and the underlying audio system. This article aims to provide a comprehensive explanation and actionable solutions for these common amixer reporting anomalies, empowering you to regain precise control over your audio playback.

Understanding the PulseAudio Volume Control Mechanism #

To effectively diagnose and resolve the issue of amixer returning incorrect audio values, it is crucial to first understand how PulseAudio manages audio volume. PulseAudio, a sophisticated sound server, operates as an intermediary between your applications and your hardware sound drivers. Unlike simpler ALSA (Advanced Linux Sound Architecture) setups where amixer might directly interact with the sound card’s hardware controls, PulseAudio introduces a layer of abstraction.

When you use amixer with the -D pulse flag, you are explicitly telling amixer to interact with the PulseAudio sound server, not directly with the ALSA device. PulseAudio doesn’t simply pass through raw percentage values to the hardware. Instead, it employs a more nuanced volume scaling system. This system involves several components that contribute to the observed volume levels.

PulseAudio’s Software Volume Control #

PulseAudio manages volume at a software level. This means that even if your sound card’s hardware volume control is set to its maximum, PulseAudio can still attenuate or amplify the audio signal. The percentage values you see reported by amixer (when interacting with PulseAudio) often represent the software gain applied by PulseAudio to the audio stream.

This software volume control allows for finer granularity and independent volume adjustments for different applications and sink inputs. For instance, you can have one application at 70% volume while another is at 30%, all within the overall master volume setting. The amixer sset Master 30% command instructs PulseAudio to set the master sink’s software volume to 30%.

The Role of PulseAudio Modules #

PulseAudio is highly modular. Various modules can be loaded to provide specific functionalities, including volume control and mixing. When amixer interacts with PulseAudio, it’s essentially communicating with these modules and their exposed controls. The reported volume might reflect the aggregated effect of multiple modules or internal scaling factors within PulseAudio’s processing chain.

Decibels (dB) vs. Percentages in PulseAudio #

A key aspect of PulseAudio’s volume management is its use of decibels (dB) for internal representation and control, even though amixer often presents these values in percentages for user convenience. PulseAudio internally calculates the relationship between its dB scale and the percentage representation.

A 0 dB level typically corresponds to 100% volume, meaning no attenuation or amplification. As the dB value decreases, the volume is attenuated. Conversely, a positive dB value would indicate amplification, though this is often capped or handled differently to prevent clipping.

The conversion between dB and percentage isn’t linear. A 6 dB reduction, for example, roughly halves the perceived loudness, but this doesn’t translate to a direct 50% reduction in the reported percentage. This non-linearity can contribute to the seemingly “off” percentage values when switching between dB and percentage representations or when observing changes across different volume levels.

When you set a volume using amixer sset Master 30%, PulseAudio translates this percentage into a corresponding dB value. Subsequently, when you query amixer get Master, it retrieves the current dB level and converts it back into a percentage for display. The perceived difference might arise from how this conversion is handled or the presence of other factors influencing the final output.

Investigating the amixer Output: Unpacking the Details #

Let’s dissect the information provided by amixer when querying the Master control under PulseAudio. Understanding the nuances of this output is critical to pinpointing the root cause of the discrepancy.

When you run amixer get Master -D pulse, the output typically looks something like this:

Simple mixer control 'Master',0
  Capabilities: pvolume
  Range: -65.50 dB - 0.00 dB
  Playback channels: Front Left
  Limits: Playback 0 to 127
  Mono: Front Left
  Front Left: 10.25 dB 52%

In this example, several key pieces of information are presented:

• Simple mixer control ‘Master’,0: Identifies the control you are querying. ‘Master’ is the common name for the primary volume control in PulseAudio.
• Capabilities: pvolume: Indicates that this control supports playback volume.
• Range: -65.50 dB - 0.00 dB: This is the effective range of the volume control in decibels. 0.00 dB usually represents the maximum possible volume (unity gain), and -65.50 dB represents the minimum (silence or near silence).
• Playback channels: Front Left: Shows which channels are controlled. In this case, it’s the front left channel. For stereo, you might see “Front Left; Front Right”.
• Limits: Playback 0 to 127: This refers to the underlying ALSA control’s raw integer range. PulseAudio maps its dB values to this range.
• Mono: Front Left: Indicates how mono sources are handled.
• Front Left: 10.25 dB 52%: This is the crucial part. It shows the current volume setting in both decibels (10.25 dB) and percentage (52%).

The discrepancy you’re observing—setting to 30% and getting 52%—suggests that the reported 52% is the current state of the PulseAudio master volume. The initial command amixer -D pulse sset Master 30% attempts to set this value. However, what happens between the command execution and the reporting can involve several factors.

The dB Conversion Factor and Perceived Loudness #

As mentioned earlier, PulseAudio’s internal representation and user-facing percentages are not directly proportional. A 30% setting in PulseAudio’s internal dB scale might translate to a different percentage when interpreted by amixer’s display logic.

Consider this: if 0.00 dB is 100%, and the range extends down to -65.50 dB, then each decibel represents a step in loudness. However, the perception of loudness is logarithmic. A 3 dB change is roughly perceived as a doubling or halving of loudness.

Let’s assume, for demonstration, that PulseAudio internally represents 30% volume as a specific dB value. When you query amixer get Master, it retrieves that dB value and converts it back to a percentage. The 52% you are seeing is the current reported percentage corresponding to the actual dB level. The discrepancy means that the 30% you set might have been interpreted by PulseAudio in a way that, when converted back, results in a different displayed percentage.

Default Sink and Volume Levels #

PulseAudio manages multiple audio sinks (output devices) and sources (input devices). The “Master” control typically refers to the default sink. If you have multiple audio output devices connected, or if PulseAudio has switched the default sink, the amixer command might be affecting a different device than you intend, or the perceived “master” volume might be influenced by other settings.

PulseAudio Configuration Files and Defaults #

PulseAudio’s behavior is heavily influenced by its configuration files, primarily located in /etc/pulse/ and ~/.config/pulse/. These files contain settings for default volumes, volume curves, and module loading. It’s possible that certain default values or profiles within these configurations are affecting how volume is interpreted and reported.

Troubleshooting Common amixer Volume Discrepancies #

When amixer returns a value that doesn’t match what you intended, several avenues of investigation can help resolve the issue. These solutions range from simple command adjustments to deeper system configuration checks.

Understanding the amixer Syntax for PulseAudio #

The -D pulse flag is essential for targeting PulseAudio. If you omit it, amixer will attempt to control the ALSA default device, bypassing PulseAudio entirely. This can lead to very different behavior and is usually not what you want when managing modern desktop audio.

The command amixer sset Master 30% is the correct syntax to set the PulseAudio master volume to 30%. The issue is likely in how PulseAudio interprets and reports this value.

Directly Setting Volume in Decibels (dB) #

Since PulseAudio operates internally with decibels, sometimes setting the volume directly in dB can provide a more predictable outcome and allow for more precise control. You can find the acceptable dB range using amixer get Master -D pulse.

For example, if the range is -65.50 dB - 0.00 dB, you could try setting a specific dB value. However, without knowing the exact dB equivalent of your desired 30% loudness, this is still an approximation.

A more practical approach is to find the dB value that corresponds to your desired loudness and then set that. You can experiment by observing the dB value reported by amixer get Master as you adjust the volume using graphical tools (like your desktop environment’s volume slider).

For instance, if you want roughly 30% volume, and you observe that your graphical tool sets the volume to -20 dB and amixer reports this as 30% for its own purposes (which is not always the case), you could then try:

amixer -D pulse sset Master -20dB

Note: The exact dB value for a specific percentage can vary depending on the PulseAudio configuration, installed plugins, and the specific sound card.

Identifying and Setting the Correct Sink #

PulseAudio can manage multiple output devices (sinks). The “Master” control typically targets the default sink. If your system has multiple audio outputs (e.g., HDMI audio, analog audio, Bluetooth), the “Master” might be referring to a device you’re not actively using.

To identify your default sink, you can use:

pactl info | grep "Default Sink"

This will output something like Default Sink: alsa_output.pci-0000_00_1f.3.analog-stereo.

To list all available sinks and their indices, use:

pactl list sinks

If the “Master” control isn’t behaving as expected, you might need to target a specific sink. You can do this by referring to the sink name or index. For example, to set the volume of a sink named alsa_output.pci-0000_00_1f.3.analog-stereo:

amixer -D pulse sset 'alsa_output.pci-0000_00_1f.3.analog-stereo' 30%

Or, if you know the sink index (e.g., sink 0):

amixer -D pulse sset 'Monitor of alsa_output.pci-0000_00_1f.3.analog-stereo',0 30%

(Note: the exact naming for amixer can sometimes be a “monitor” of the sink, which is a virtual source providing the sink’s audio output. This can be confusing. It’s often easier to use pactl for specific sink control).

A more direct way using pactl to set the volume of the default sink:

pactl set-sink-volume @DEFAULT_SINK@ 30%

And to get the volume of the default sink:

pactl get-sink-volume @DEFAULT_SINK@

The output of pactl get-sink-volume will show the volume in both percentage and dB format, giving you a clearer picture.

Checking PulseAudio Volume Levels for Specific Applications #

The Master volume affects the overall output. However, individual applications also have their own volume controls within PulseAudio. If you’re experiencing issues, it’s worth checking the volume of the specific application you’re using.

To list all playback streams (applications) and their volumes:

pactl list sink-inputs

This output will show you the “Application Name” or “Media Stream” and its “Volume” in both dB and percentage. You can then use pactl set-sink-input-volume <sink-input-index> <volume> to adjust specific application volumes.

For example, if the second sink-input in the list (index 1) is too low, you might set it like this:

pactl set-sink-input-volume 1 70%

Resetting PulseAudio Configuration #

Sometimes, corrupted or misconfigured PulseAudio settings can lead to unexpected behavior. A simple way to test this is to restart PulseAudio or clear its configuration.

Restarting PulseAudio:

pulseaudio -k
pulseaudio --start

This will kill the running PulseAudio daemon and then restart it. Any applications that were actively playing audio might need to be restarted or their audio streams reconnected.

Clearing PulseAudio Configuration:

You can remove your user-specific PulseAudio configuration directory. Back this up first!

mv ~/.config/pulse ~/.config/pulse_backup
pulseaudio -k
pulseaudio --start

This will force PulseAudio to recreate its default configuration files. You will lose any custom settings you’ve made in ~/.config/pulse/, so you might need to reapply them if necessary.

Investigating ALSA Mixer Settings (When Not Using -D pulse) #

If you were to run amixer sset Master 30% without the -D pulse flag, you would be interacting directly with the ALSA layer. In this scenario, the values reported by amixer get Master would reflect the hardware controls of your sound card as exposed by ALSA.

The output would look different, typically showing a range in numerical values (e.g., 0 to 100) or dB, and the capabilities might list pmix (for physical mixer) instead of just pvolume.

If you are not using -D pulse and amixer reports different values, it means you are directly manipulating the ALSA controls. In such cases, the discrepancy could be due to:

• Hardware Volume Limits: The sound card itself might have limitations.
• ALSA Mixer Controls: Multiple ALSA controls might be linked or affect the final output volume in ways not immediately obvious.
• Software/Hardware Mix: Some applications might bypass PulseAudio and directly use ALSA, leading to a different volume perception.

However, for most modern desktop Linux systems, PulseAudio is the default and intended way to manage audio.

Addressing the 30% vs. 52% Discrepancy Specifically #

The common scenario you’ve described—setting to 30% and seeing 52%—often points to how PulseAudio is interpreting and scaling the volume. It’s not uncommon for the amixer percentage to represent a specific point within PulseAudio’s internal dB scale that doesn’t directly map to a linear 0-100% scale from your perspective.

Here are some targeted thoughts on why this might occur and what to do:

The “Amplification” Factor #

In some PulseAudio configurations, the range of the master volume control might be set such that 0% corresponds to the lowest possible volume, and 100% corresponds to a nominal maximum (e.g., 0 dB). However, if the underlying ALSA control has a higher maximum output level, PulseAudio might introduce software amplification.

For instance, if your sound card’s native maximum output is actually higher than what PulseAudio considers its “100%” (0 dB), then a setting of 30% in PulseAudio might be internally mapped to a value that, when represented as a percentage by amixer, turns out to be higher than your initial expectation. This is particularly true if amixer’s percentage display is tied to the raw ALSA integer range rather than a normalized PulseAudio percentage.

PulseAudio’s ability to amplify signals above 0 dB (which would correspond to 100% in a simple system) is sometimes enabled. This means that amixer might report a value higher than 100% for some sink inputs if amplification is active and the volume is pushed beyond unity gain. In your case, 52% might represent a state where PulseAudio is applying some boost.

Understanding PulseAudio’s Volume Control Range #

Let’s re-examine the amixer get Master -D pulse output. The Range: -65.50 dB - 0.00 dB is critical. This indicates that the control is designed for attenuation (decreasing volume) from a maximum of 0 dB. If amixer sset Master 30% is executed, PulseAudio attempts to translate this 30% into a dB value within this range.

The percentage displayed by amixer is often derived from the current dB value divided by the maximum possible dB value (0 dB in this case), then converted to a percentage. However, the mapping isn’t always straightforward due to the non-linear nature of perceived loudness and PulseAudio’s internal scaling.

If amixer reports 52%, it implies the current setting is approximately 0.52 * (0 dB - (-65.50 dB)) = 34.06 dB below the maximum, or simply a value that translates to 52% of the available range.

The core issue is that the 30% you provided to amixer sset Master 30% might not be interpreted by PulseAudio as a direct 30% reduction from its maximum. PulseAudio might be applying its own scaling factors or internal representations.

The pactl Alternative for Precision #

Given the inconsistencies, using pactl is often more reliable for precise volume control with PulseAudio. The pactl command directly interacts with the PulseAudio daemon and allows for more granular control.

To set the master volume to 30% using pactl:

pactl set-sink-volume @DEFAULT_SINK@ 30%

Let’s test this command and then check the value using pactl get-sink-volume @DEFAULT_SINK@.

# Set volume to 30%
pactl set-sink-volume @DEFAULT_SINK@ 30%

# Check the reported volume
pactl get-sink-volume @DEFAULT_SINK@

The output of pactl get-sink-volume will typically show the volume as a percentage and in decibels. For example:

Volume: front-left: 30% 0 dB,   front-right: 30% 0 dB

Or, if it’s not precisely at 0 dB for 30%:

Volume: front-left: 30% -2.50 dB, front-right: 30% -2.50 dB

If using pactl set-sink-volume @DEFAULT_SINK@ 30% also results in a different reported percentage (e.g., 52% when you query again with amixer or pactl), it points to a deeper PulseAudio configuration or behavior.

Customizing PulseAudio’s Volume Scaling #

PulseAudio has configuration options that can influence how volume levels are interpreted and displayed. The default.pa file (usually in /etc/pulse/ or ~/.config/pulse/) loads modules and sets parameters.

One such parameter is flat-volumes. If flat-volumes = yes, PulseAudio attempts to keep all sink inputs at the same volume as the sink itself. If flat-volumes = no, individual applications can have different volumes than the master sink. The flat-volumes setting can impact how volume changes are perceived and reported.

You might also find settings related to the “volume curve” or “rate”. These advanced configurations can modify how PulseAudio maps user-facing volume controls to internal gain stages.

Checking alsamixer Directly (Without -D pulse) #

To isolate whether the issue lies with PulseAudio or the underlying ALSA setup, you can use alsamixer directly.

1. Open a terminal.
2. Run alsamixer.
3. Press F6 to select your sound card (usually a Realtek, HDA Intel PCH, or similar).
4. Navigate using arrow keys to the “Master” or “PCM” volume controls.
5. Observe the levels and try adjusting them directly.

If alsamixer shows a different behavior or reports different values for the hardware controls, it might indicate an ALSA-level issue. However, remember that PulseAudio typically controls these ALSA devices, so changes made in alsamixer might be overridden by PulseAudio if it’s running.

Advanced Troubleshooting: PulseAudio Daemon Configuration #

PulseAudio’s behavior is governed by its daemon settings, which are configured in files like daemon.conf and default.pa. These files are located in /etc/pulse/ (system-wide defaults) and can be overridden by files in ~/.config/pulse/ (user-specific overrides).

Examining daemon.conf and default.pa #

• daemon.conf: This file contains general daemon settings. Look for parameters related to volume scaling, e.g.:

  • flat-volumes: As discussed, setting this to yes or no can affect volume behavior.
  • enable-lfe-remixing, default-sample-channels, default-sample-rate: While not directly volume, these can influence audio processing.
  • max-volume-percent: This parameter sets the maximum volume PulseAudio will allow, expressed as a percentage of the underlying hardware volume. Setting this too low could lead to unexpected behavior.

• default.pa: This file loads PulseAudio modules and sets default configurations. Crucially, it loads the module-alsa-sink and module-alsa-source modules, which bridge PulseAudio to ALSA. You might find specific volume settings or module parameters here. For example, the alsa_card_... configuration might have its own volume mapping.

When debugging volume issues, temporarily removing or renaming your user-specific PulseAudio configuration directory (~/.config/pulse/) can help determine if the problem is with custom settings. After removing it, restart PulseAudio:

rm -rf ~/.config/pulse
pulseaudio -k
pulseaudio --start

This will revert to system-wide defaults. If the issue disappears, you can then selectively reintroduce your custom settings from the backup to find the problematic one.

Understanding Volume Steps and Ramps #

PulseAudio uses “volume steps” and “volume ramps” to define how volume changes occur when you press volume keys or use sliders. These are configured in daemon.conf and can influence the granularity and perceived effect of volume adjustments.

For example, default-volume-step-multiplier and default-sample-step-size can affect how quickly the volume changes. While not directly the cause of your reported value mismatch, these settings can influence the overall experience and how accurately you can hit a specific volume target.

Conclusion: Towards Accurate Audio Control #

The discrepancy between setting an audio volume with amixer and the value reported is a common point of confusion when working with PulseAudio. At revWhiteShadow, we’ve established that this often stems from PulseAudio’s sophisticated software volume management, its internal dB-based scaling, and the way it translates these values into user-friendly percentages.

To effectively resolve this, we recommend the following:

1. Prioritize pactl: For precise and reliable volume control with PulseAudio, pactl set-sink-volume @DEFAULT_SINK@ <percentage> is generally more direct than amixer -D pulse sset Master <percentage>.
2. Verify Sink Identity: Ensure you are controlling the intended audio output device by checking your default sink with pactl info.
3. Examine dB Values: Use pactl get-sink-volume to see both the percentage and dB values, which can offer more insight into the underlying setting.
4. Check Application Volumes: If the master volume seems correct but a specific application is too quiet or loud, use pactl list sink-inputs to manage individual stream volumes.
5. Configuration Review: If the issue persists, consider reviewing your PulseAudio configuration files (daemon.conf, default.pa) for any non-standard settings that might be influencing volume scaling.

By understanding the layers of audio control in modern Linux distributions and utilizing the appropriate tools like pactl, you can overcome these amixer reporting anomalies and achieve the accurate audio experience you desire. We hope this comprehensive guide from revWhiteShadow has provided clarity and actionable solutions for your audio control needs.