ステレオ音声をモノラル変換した際の位相打ち消し（コムフィルタ）修復法

The Physics of Phase Cancellation and Comb Filtering

Sound travels through air as pressure waves composed of alternating compressions (positive voltage peaks) and rarefactions (negative voltage troughs). In a dual-channel stereo recording, both channels record these pressure fluctuations.

When the left and right audio channels contain identical frequencies but opposite phase angles (e.g. 180° out of phase), summing them together causes destructive interference: the positive peak of channel A adds to the negative trough of channel B (+1 + (-1) = 0), completely erasing the acoustic signal.

Constructive vs Destructive Interference in Spoken Dialogue

In spoken voice recordings, phase cancellation rarely deletes the entire vocal track evenly across all frequencies. Instead, because different vocal formant frequencies have different physical wavelengths, certain frequencies cancel out while others reinforce each other.

This creates a series of sharp, repeating notches across the frequency spectrum known as comb filtering. The speaker's chest warmth (150Hz–300Hz) and core speech presence (2kHz–4kHz) disappear, leaving a hollow, tinny sound reminiscent of speaking inside a metal pipe.

Why Single-Speaker Smartphones Expose Hidden Stereo Flaws

More than 70% of spoken-word podcasts and YouTube videos are consumed on mobile devices, smart speakers, or single-earbuds. These devices routinely collapse stereo audio into a single physical mono driver.

A recording that sounds pleasantly "wide" in studio stereo headphones may completely collapse into an unintelligible whisper on a smartphone speaker if phase cancellation is left uncorrected.

Phase Diagnostic Matrix: Polarity Flips vs Channel Isolation

Use this diagnostic table to determine the correct technical repair for your stereo recording:

Step-by-Step Method to Eliminate Phase Cancellation

ステップ 1：Audition the Raw Track in Mono Sum Mode

Load your audio or video file into AutoSilenceCut Stereo to Mono Converter. Enable the Mono preview toggle and listen to spoken dialogue. If speech volume drops noticeably or sounds hollow compared to stereo, phase cancellation is present.

ステップ 2：Toggle 180° Polarity Inversion (Ø)

Click the Invert Polarity (Ø) switch on the Right channel. If the issue was electrical polarity reversal, vocal body and low-end punch will immediately snap back into focus with full presence.

ステップ 3：Isolate the Cleanest Dominant Channel (L or R Only)

If inverting polarity does not completely resolve hollow comb filtering, the issue is acoustic arrival-time delay (such as a lapel mic and a room shotgun mic recording the same voice). Select Left Channel Only or Right Channel Only to audition each microphone independently and isolate the cleanest, closest signal.

ステップ 4：Apply -6 dB Equal-Power Mono Downmixing

When summing two clean, coherent channels, ensure the converter applies standard -6 dB gain attenuation (S_mono = 0.5 × (S_L + S_R)) to prevent signal voltages from exceeding 0 dBFS and causing digital clipping.

ステップ 5：Export Lossless Mono 16-Bit WAV Master

Render the repaired audio directly to a single-channel 16-bit 48kHz PCM WAV file. Your finished track is now 100% phase-aligned, perfectly mono-compatible across all mobile devices, and uses half the bandwidth of stereo files.

実践ケーススタディ：Dual-Mic Interview with Acoustic Phase Bleed

A documentary filmmaker recorded an interview using a lavalier microphone on Channel 1 and a camera-mounted shotgun microphone on Channel 2. When imported into the editing timeline as a stereo track, the shotgun mic captured the speaker's voice with a 4.2 millisecond delay compared to the chest lavalier.

Summing both tracks created deep notches at 119Hz, 357Hz, and 595Hz, destroying the subject's vocal resonance.

The Fix: Rather than attempting complex manual sample shifting, the editor loaded the file into the Stereo to Mono converter, selected Left Channel Only (Lavalier), and exported a clean mono WAV. The dialogue instantly regained full clarity and presence.

5 Mistakes When Converting Stereo Dialogue to Mono

• Summing Channels at 0 dB (No Attenuation): Adds +6 dB of voltage gain, causing instant digital distortion on loud vocal passages. Always use -6 dB downmixing.
• Mixing Up Polarity and Stereo Width: Artificial stereo widening plugins (Haas effect) introduce deliberate phase delay. Converting widened tracks to mono without isolating a single channel causes extreme cancellation.
• Leaving Dialogue in Stereo for Podcast RSS Feeds: Wastes 50% of listener mobile data with zero benefit in spatial fidelity. Single-host spoken word belongs in mono.
• Ignoring Phase Issues Until Final Video Render: Always check mono compatibility in the preflight stage before cutting, pacing, and mastering dialogue.
• Using Lossy MP3 Intermediates During Repair: Re-encoding phase-cancelled audio to MP3 before repairing causes generation loss and smear. Always work in uncompressed WAV.