# Notes: headphones and earbuds

Research notes for category.json. Written 2026-10-19. Quotes are short. "verbatim" means the fetch returned the sentence as written on the page; "fetched excerpt" means the fetch tool paraphrased and the wording is not guaranteed.

## Seed

Seed run: `python3 scripts/seed.py --category headphones-earbuds`. Example drawn: kitchen-knives steel hardness (a numeric spec that only makes sense as one side of a trade-off, with a community visual grammar putting the trade-off on one axis). Persona: a ship's cook, who thinks in shelf life, storage and what works without the ideal setup.

1. The headphones equivalent of steel hardness. Two candidates. (a) Bass level relative to the Harman target: a single decibel number the measurement community already draws (frequency-response graph against a target line). Push it up and you get the warm, fun sound most people prefer in a shop; push it past about 6 dB and vocals get muddy. (b) Impedance: higher impedance needs an amplifier but is less sensitive to a noisy or high-output-impedance source; lower impedance plays loud from a phone but exposes the source's flaws. Both are numbers nobody can ask a novice about, both belong as annotations with a trade-off attached. Data check: (a) exists for anything RTINGS, Crinacle or a squig.link database has measured, which is the top few hundred products, so it is a researched, top-N facet. (b) is on every wired spec sheet and on most wireless ones. Keep both.

2. What the ship's cook notices first. Three things. (a) Shelf life: true wireless earbuds have a glued-in battery that fades in two to three years and almost no listing says whether it can be replaced; iFixit scores exist for the popular ones. (b) Works without the ideal setup: does a wireless headphone still play with a cable when the battery is dead or Bluetooth is banned on a flight, and does it need an app to turn features on. (c) Storage and wear: are the ear pads and tips replaceable consumables, and is it rated against sweat and rain (IP rating). Data check: IP rating is already in the electronics standard. Wired fallback is on the manufacturer spec page as a 3.5 mm jack or "passive mode". Replaceable pads is on spec pages and in community wikis. Repairability is iFixit only, so top-N researched. Kept all three, with repairability as the most expensive.


## Sources

Format per source: id, URL, kind, what it says (verbatim where marked), the claim it supports, confidence.

### olive_2022_acoustics_today (academic)
URL: https://acousticstoday.org/wp-content/uploads/2022/03/The-Perception-and-Measurement-of-Headphone-Sound-Quality-What-Do-Listeners-Prefer-Sean-E.-Olive.pdf (PDF saved to disk and read with pdftotext; quotes verbatim)
- "A total of 31 different headphone models from 18 manufacturers were evaluated by 130 listeners" ... "ranged in price from $60 to $4,000, including open and closed back designs with dynamic or magnetic planar drivers."
- "Class 1 includes most listeners (64%) who prefer headphones that closely comply with the Harman target. Class 2 listeners (15%) prefer the Harman target with 4-6 dB more bass. Class 3 listeners (21%) prefer the Harman target curve with 2 dB less bass."
- "the more the frequency response of a headphone deviates from the Harman target curve, the lower the listeners rated its sound quality."
- "The correlation between the predicted and measured ratings is 0.86, with an error of 6.7 ratings on a 100-point scale."
- "Two recent studies have measured the variance in frequency response of more than 400 headphones and found no correlation between their retail price and frequency response"
- "there is little correlation between the price of a headphone and its frequency response, the single best indicator of its sound quality."
- "Frequency response measurements of the headphones were made using an ear simulator according to IEC 60318-1 (2009) equipped with a custom pinna"
Supports: mechanism (tuning is the main thing), tuning_vs_target facet, the 64/15/21 segments, bass preference range, "price does not predict sound" judgement. Confidence high (primary source, verbatim).

### soundstage_harman (specialist_review)
URL: https://www.soundstagesolo.com/index.php/features/217-where-are-we-at-with-the-harman-curve (verbatim via fetch)
- "This group, which constitutes 64% of listeners, includes mostly a broad spectrum of people, although they're generally under age 50."
- "This next group, which makes up 15% of listeners, prefers headphones with 3 to 6dB more bass than Harman curve below 300Hz, and 1dB more output above 1kHz."
- "This group, 21% of listeners, prefers 2 to 3dB less bass than the Harman curve and 1dB more output above 1kHz."
Supports: second source for the segment sizes and the dB offsets. Confidence high. Note: Olive says "4-6 dB more" and "2 dB less"; SoundStage says "3 to 6 dB" and "2 to 3 dB". Minor disagreement, both kept; the doc uses 3 to 6 and 2 to 3.

### soundguys_harman_validation (specialist_review)
URL: https://www.soundguys.com/harman-target-soundguys-preference-curve-validated-125420/ (verbatim via fetch)
- "The Har2019 or the SoundGuys target curves are good default choices since they were preferred by 72% of the listeners in this study."
- "In total, 32 test participants aged 21 to 50 years old were selected for this trial" (Olive, AES October 2024)
Supports: in-ear target preference figure (72%), that targets exist for in-ears too. Confidence medium (small panel).

### rtings_noise_isolation (specialist_review)
URL: https://www.rtings.com/headphones/tests/noise-isolation-cancellation-passive-active (page JS-rendered; text pulled from the saved HTML, verbatim)
- "The bass range is where you'll find rumbly bus and plane engines, traffic, and construction. However, these low sounds are some of the hardest to block out passively."
- "most headphones have difficulty with passive isolation in this range. However, this is where ANC tends to shine. Some of the best noise cancelling headphones can attenuate noise by over 15 dB in this range."
- "When it comes to mid-range sounds like ambient speech, some headphones can provide adequate passive isolation in this range alone."
- Test structure: "Noise Isolation - Full Range", "Noise Isolation - Common Scenarios", "Noise Isolation - Voice Handling", "ANC Wind Handling", "Leakage"; measurements shown "with ANC off (dark blue) and on (light blue). Lower means better attenuation."
Supports: anc_attenuation facet (unit dB, >15 dB bass for the best), the bar-by-band visual grammar, ANC is strongest at low frequencies. Confidence high.

### rtings_fr_consistency (specialist_review)
URL: https://www.rtings.com/headphones/tests/sound-quality/frequency-response-consistency (saved HTML, verbatim)
- "if you wear glasses, you may hear a different frequency response due to variables like fit and seal."
- "In-ear headphones and earbuds can yield variance in sound between people, but glasses, hair length, and head size don't factor into their frequency response consistency. The emphasis here is to choose the correct ear tip size and position your earbuds carefully"
- Uses "the industry standard, Brüel & Kjær 5128 rig".
Supports: seal and fit change the bass you hear; ear_tip_options facet; judgement that graphs are rig-specific. Confidence high.

### rtings_latency_research (specialist_review)
URL: https://www.rtings.com/headphones/learn/research/latency (saved HTML, verbatim)
- "The average SBC latency of truly wireless earbuds is 259.8 ms. However, this is only obtained from five products"
- "the average SBC latency of the seven we've tested so far for non-gaming non-earbuds is 257.6 ms."
- "The six gaming headsets we've retested so far have an average SBC latency of 170.3 ms"
- Audeze Maxwell: "We measured 13 ms via wired USB and 23 ms via wireless dongle"; Audeze's own figures "14.31 ms of latency via wired USB and 24.85 ms via wireless dongle."
- AirPods Pro 2: "around 45ms on macOS compared to 160ms on SBC/W11" (they suspect sync compensation on macOS).
Supports: latency facet ranges (dongle ~23 ms, wired ~13 ms, Bluetooth SBC 160 to 260 ms). Confidence high.

### soundguys_android_latency (specialist_review)
URL: https://www.soundguys.com/android-bluetooth-latency-22732/ (verbatim via fetch)
- "We ran 100 tests on each handset for each codec, so our results produced 2,800 data points."
- SBC and aptX "offer the lowest average latency – 308ms" and "316ms respectively"; LDAC "324ms"; AAC "Average latency clocks in at 369ms".
- fetched excerpt: "individuals can't perceive latency below 20ms and many struggle to discern a delay between 40 to 80ms either."
Supports: second measured source for Bluetooth latency; perception thresholds. Confidence high for the numbers, medium for the perception line (excerpt). Note the Android figures are higher than RTINGS's; both kept, doc gives the span.

### feasycom_codecs (primer)
URL: https://www.feasycom.com/bluetooth-audio-codecs-comparison/ (verbatim via fetch)
- Bitrates: SBC "~200–328 kbps"; AAC "256–320 kbps"; aptX "~352 kbps"; aptX HD "~576 kbps"; aptX Adaptive "~279–420 kbps"; LDAC "up to ~990 kbps"; LC3 "~160–345 kbps music".
- Latency: SBC "150–220 ms"; AAC "120–180 ms"; aptX "100–120 ms"; aptX Adaptive "~80 ms / ~40 ms LL mode"; LC3 "can be under 50 ms".
- "if either side falls back to SBC, latency jumps back above 100 ms"
Supports: codec bitrates, codec latency by spec, fallback-to-SBC rule. Confidence medium (vendor primer; spec figures agree with Wikipedia).

### wikipedia_aptx (primer)
URL: https://en.wikipedia.org/wiki/AptX (verbatim via fetch)
- aptX "352 kbit/s (@ 44.1 kHz)"; aptX HD "576 kbit/s"; aptX LL "The technology offers an end-to-end latency of 32 ms over Bluetooth."; "aptX Adaptive's bitrate scales dynamically between 279 kbit/s and 420 kbit/s."
Supports: second source for aptX bitrates and the 32 ms low-latency figure. Confidence high.

### wikipedia_ldac (primer)
URL: https://en.wikipedia.org/wiki/LDAC_(codec) (verbatim via fetch)
- "LDAC audio bitrate settings are set to Best Effort, which switches between discrete bitrate steps (CBR) 330/660/990 kbps depending on connection strength"
- "On 17 September 2019, the Japan Audio Society (JAS) certified LDAC with their Hi-Res Audio Wireless certification."
Supports: LDAC bitrates; that "Hi-Res Wireless" is a certification mark. Confidence high.

### soundguys_ldac_guide (specialist_review)
URL: https://www.soundguys.com/ldac-ultimate-bluetooth-guide-20026/ (verbatim via fetch)
- "990kbps and 660kbps data rates push Bluetooth to the limits of stability."
- "it's quite unlikely that your phone will opt for 990kbps LDAC unless you manually force the settings via the developer options."
- "Sony's LDAC technology doesn't technically provide true Hi-Res audio over Bluetooth." ... "its resolution and noise floor are nowhere near 24 bits, and are worse than 16 bits above 15kHz."
- "it's entirely possible that many won't notice the difference"
- "the risk of missed packages increases substantially as bandwidth requirements increase."
Supports: codec judgement (high bitrate buys stability problems, not audible gains), bitrate versus stability trade-off. Confidence high.

### hydrogenaudio_codec_blind_test (enthusiast_community) NOT FETCHED (403). Search excerpt only: "the perceivable difference to SBC is small in blind tests for aptX Classic"; SBC, AAC and aptX "offer high fidelity on common settings". Used only to set the codec-audibility judgement at medium confidence, not cited as a reference.

### bluetooth_sig_le_audio (standards_body)
URL: https://www.bluetooth.com/learn-about-bluetooth/feature-enhancements/le-audio/ (verbatim via fetch)
- "Extensive listening tests have shown that LC3 will provide improvements in audio quality over the SBC codec included with Classic Audio, even at a 50% lower bit rate."
- "LE Audio will also add broadcast audio, enabling an audio source device to broadcast one or more audio streams to an unlimited number of audio sink devices." (Auracast)
- "LE Audio enhances the performance of Bluetooth® audio, adds support for hearing aids, and introduces Auracast™ broadcast audio."
Supports: vocabulary for LC3, LE Audio, Auracast; codec values. Confidence high.

### soundguys_multipoint (specialist_review)
URL: https://www.soundguys.com/how-to-bluetooth-multipoint-headphones-set-up-62931/ (verbatim via fetch)
- "Multipoint extends this capability to more than one device at a time"
- "The Bose Noise Canceling 700 keeps a list of up to eight devices, but only two will work with Bluetooth Multipoint simultaneously."
- "Keep in mind that your ears can only listen to one stream of information at a time."
- "Bluetooth Multipoint only works with the AAC Bluetooth codec." (dated; see notebookcheck below)
Supports: multipoint facet definition and the two-device limit. Confidence high for the definition, low for the AAC-only line.

### notebookcheck_sony_ldac_multipoint (specialist_review) NOT FETCHED; search excerpt: Sony firmware 2.0.2 for WH-1000XM5 "allows LDAC to be available during a multipoint Bluetooth connection"; before that owners had to choose between LDAC and multipoint. URL: https://www.notebookcheck.net/Sony-WH-1000XM5-receive-Multipoint-LDAC-and-head-tracking-support-via-new-update.736850.0.html
Supports: the judgement that multipoint and high-bitrate codecs were once exclusive and now depend on firmware. Confidence medium (excerpt). Sony help guide (below) confirms multipoint exists on the XM5.

### sony_wh1000xm5_helpguide (spec_sheet)
URL: https://helpguide.sony.net/mdr/wh1000xm5/v1/en/print.pdf (PDF saved and read with pdftotext; verbatim)
- Battery table: "LDAC / Noise canceling function: ON / Max. 26 hours"; "LDAC / OFF / Max. 36 hours"; "AAC / Noise canceling function: ON / Max. 30 hours"; "AAC / OFF / Max. 40 hours".
- "Impedance: 48 Ω (1 kHz) (when connecting via the headphone cable with the headset turned on) 16 Ω (1 kHz) (when connecting via the headphone cable with the headset turned off)"
- "Sensitivity: 102 dB/mW (when connecting via the headphone cable with the headset turned on) 100 dB/mW (when connecting via the headphone cable with the headset turned off)"
- "Mass: Approx. 250 g (8.82 oz)"; "Bluetooth Specification version 5.2"; "Supported Codec: SBC AAC LDAC"; "Connecting the headset to 2 devices simultaneously (multipoint connection)".
- Cable: "you can use the headset as noise canceling headphones while the headset is connected to a device via the supplied headphone cable and the headset is turned on." Hint: "You can listen to music even if the headset is turned off. In this case, the noise canceling function cannot be used."
Supports: battery_life ranges and that ANC and LDAC cost hours; impedance and sensitivity being on spec sheets; wired_fallback; multipoint on listing; codec list. Confidence high.

### apple_airpods_pro3_specs (spec_sheet)
URL: https://www.apple.com/airpods-pro/specs/ (verbatim via fetch)
- "Up to 8 hours of listening time on a single charge with Active Noise Cancellation enabled"; "Up to 24 hours of listening time with Active Noise Cancellation enabled" (with case); "5 minutes in the case provides around 1 hour of listening time".
- "Dust, sweat, and water resistant (IP57): AirPods Pro and charging case"; "Bluetooth 5.3 wireless technology"; weight "0.20 ounce (5.55 grams)" each.
- "Silicone ear tips in five sizes: XXS, XS, S, M, L"; features listed: Adaptive Audio, Transparency mode, Conversation Awareness, Hearing Test, Hearing Aid, Loud Sound Reduction, Personalized Spatial Audio.
- No impedance, sensitivity, driver size or codec list on the page.
Supports: true wireless battery ranges, IP values, tip sizes on listing, what a leading brand does and does not publish. Confidence high.

### apple_water_resistance (spec_sheet)
URL: https://support.apple.com/en-us/105046 (verbatim via fetch)
- "AirPods Pro 3, the MagSafe Charging Case (USB-C) for AirPods Pro 3, and AirPods 5 (both models)" have IP57; AirPods Pro 2 (USB-C) and AirPods 4 have IP54.
- "AirPods Pro, AirPods 3, AirPods 4 (both models), and AirPods 5 (both models) are sweat and water resistant, but they are not waterproof or sweatproof."
- "They aren't designed for use in water sports like swimming, or for showering."
- "Sweat, water, and dust resistance aren't permanent conditions and can diminish over time."
Supports: ip_rating facet, its limits. Confidence high.

### standardclarity_iec60529 (primer)
URL: https://standardclarity.com/standards/iec-60529/ (verbatim via fetch)
- "IP + solids digit (0-6) + water digit (0-9)"; "X is a placeholder meaning that characteristic is not specified by that position in the designation; it does not mean zero protection"
- Water level 4 "Splashing water from any direction"; level 5 "Water jets from a 6.3 mm nozzle"; level 7 "Temporary immersion up to about 1 m".
Supports: ip_rating value order and vocabulary. Confidence high (matches the search-summary explanations of IEC 60529).

### sennheiser_hd600 (spec_sheet)
URL: https://us.sennheiser-hearing.com/products/hd-600 (verbatim via fetch)
- Impedance "300 Ω"; Sensitivity "97 dB (1 V)"; THD "< 0,1 % (1 kHz, 1 V)"; "dynamic, open"; weight 260 g; "3.5 mm stereo jack plug"; replacement earpads ($54.95) and cable ($19.95) sold as spare parts.
Supports: impedance and sensitivity ranges, that sensitivity is quoted in dB/V by some brands and dB/mW by others, serviceability (replaceable pads and cable), open-back exists as a stated spec. Confidence high.

### sennheiser_momentum4 (spec_sheet)
URL: https://us.sennheiser-hearing.com/products/momentum-4-wireless (verbatim via fetch)
- "Up to 60 hrs music playtime via Bluetooth and with ANC (test condition: iPhone, mid volume level)"; codecs "SBC, AAC, aptX™, aptX adaptive™"; Bluetooth 5.2; impedance "Active 470 ohms / Passive 60 ohms"; sensitivity "106 dB SPL (1 kHz / 0 dB FS)"; 3.5 mm cable included; replacement earpads $34.95.
Supports: battery_life upper end for over-ears, codec values, wired_fallback, serviceability. Confidence high.

### amazon_wh1000xm5_listing (retail_listing)
URL: https://www.amazon.com/dp/B0BXYCS74H. Fetch returned only the page head. The meta description read "Auto NC Optimizer, Up to 30 Hrs of Battery" among feature claims. Two further Amazon pages, Best Buy, Crutchfield, B&H, Adorama, Walmart, Target and Newegg all returned 403, 404, 500, a captcha page or a shell without the product table. So the retail-listing survey is incomplete. Supports only: battery hours and ANC appear in the listing title. Confidence low. The listing_field_hints in category.json lean on the three manufacturer spec pages above and should be checked against the scraper's own field fill rates.

### ifixit_airpods5 (specialist_review)
URL: https://www.ifixit.com/News/119587/airpods-5-are-the-least-unrepairable-airpods-yet (verbatim via fetch)
- Earlier AirPods: "Apple's wireless earbuds have earned a well-deserved 0 out of 10 for repairability"; AirPods 5 "still earn just a 2 out of 10"
- "for the first time, we think skilled technicians and even intrepid DIYers will probably be able to replace AirPod batteries with a decent success rate"
- Fairbuds received "the first 10/10 repairability score for wireless earbuds"
Supports: serviceability facet values and that scores exist for the popular products. Confidence high.

### ifixit_airpods_pro3 (specialist_review)
URL: https://www.ifixit.com/News/113507/airpods-pro-3-teardown-still-a-tragedy (verbatim via fetch)
- "when it comes to repairability, they're still a 0 out of 10."
- "the coin-cell battery is glued down so firmly that only heat, tweezers, and a lot of luck will free it."
- "the battery stops holding a charge after about 18 months" (quoting a user-facing claim), "slowly become unusable" and can't be repaired; "essentially disposable by design".
Supports: sealed batteries end the product's life; serviceability trade-off. Confidence high for the score, medium for the 18-month figure (one source; MakeUseOf says two to three years).

### makeuseof_earbud_batteries (primer)
URL: https://www.makeuseof.com/wireless-earbuds-all-break-over-the-same-thing-almost-nobody-sells-fix/ (verbatim via fetch)
- "each time they charge and discharge, some capacity is lost. After a few hundred full cycles, your earbud battery that once lasted 12 hours could be cut in half"
- batteries fail around the "two- or three-year mark"; "most earbuds were never designed to be opened."
- Fairbuds: "a standard coin-cell battery behind a silicone gasket, swappable with a fingernail", iFixit 10/10, IP54.
Supports: lifespan trade-off of true wireless, second source for Fairbuds. Confidence medium (general-audience primer).

### nwavguy_impedance (primer)
URL: http://nwavguy.blogspot.com/2011/02/headphone-amp-impedance.html (verbatim via fetch)
- "most headphones work best when the output impedance is less than 1/8th the headphone impedance."
- "The greater the output impedance, the greater the frequency response deviations."
- "With some headphones, especially balanced armature or multi driver designs, these variations can be rather extreme." (SuperFi 5: "total, and very audible, variation of 12 dB" at 43 Ω source)
Supports: impedance facet (why low impedance earphones are source-sensitive), the 1/8 rule, trade-off. Confidence high (engineer's measurements, widely cited).

### wikipedia_headphone_amp (primer)
URL: https://en.wikipedia.org/wiki/Headphone_amplifier (verbatim via fetch)
- "High output impedance can result in frequency response fluctuations, due to varying load impedance at different frequencies."
- "a 120-Ohm output impedance could cause a 5-dB error in frequency response with certain types of headphones."
- Damping example: "a 32 Ω headphone driven by a headphone amp with a <1 Ω output impedance would have a damping factor of >32, whereas the same headphone driven with an iPod touch 3G (7 Ω output impedance) would have a damping factor of just 4.6."
- 120 Ω headphones from an iPod "would most likely not benefit from the addition of a lower impedance headphone amplifier."
Supports: second source for output impedance effects; that moderate-impedance headphones often do not need an amp. Confidence high.

### themasterswitch_impedance (primer)
URL: https://www.themasterswitch.com/headphone-impedance-sensitivity-explained (verbatim via fetch)
- "Any impedance up to 32Ω can be played quite happily off a simple portable device, like a smartphone."
- "Impedance from 33-100Ω is a little bit of a grey area"; "Anything above 100Ω is almost certainly going to require an amplifier."
- "Sensitivity is a measure of how loud a pair of headphones will play at a given power level."
Supports: amp rule of thumb (one side of a disagreement; Wikipedia's 120 Ω example says otherwise). Confidence medium.

### headphones_com_forum_impedance (enthusiast_community)
URL: https://forum.headphones.com/t/impedance-vs-loud-sound/25764 (verbatim via fetch)
- "There isn't always a precise relationship between a headphone's input impedance and its loudness. It depends alot on the headphone's design."
- "you also need to look at its signal voltage sensitivity (dB/V) and its power efficiency (dB/mW)."
- "For most headphones, a damping factor between 10 and 20 is considered ideal"; "Some sources suggest a damping factor of 8 or more is a good general rule."
Supports: sensitivity facet; the derived dB/V metric; damping factor vocabulary. Confidence medium.

### headphones_com_drivers (primer)
URL: https://headphones.com/blogs/features/headphone-driver-types-explained (verbatim via fetch)
- Dynamic: heavier diaphragm and coil are "harder to move quickly to reproduce high frequencies."
- Planar: "a very thin tension sheet of material between a grid or array of magnets"; more expensive to make; "can't usually move as much as some dynamic drivers"; seal-sensitive.
- Electrostatic: bias "of either 230 or 580 volts"; "often aren't able to output low frequencies at high SPLs"; need "dedicated electrostatic energizers".
- Balanced armature: "A highly magnetically permeable U-shaped reed or armature"; single BA can't match a dynamic's range (paraphrase).
Supports: driver_type facet and families. Confidence high.

### campfire_drivers (primer)
URL: https://www.campfireaudio.com/blogs/insights/iem-driver-types-balanced-armature-dynamic-planar (verbatim via fetch)
- Dynamic: "Bass that feels physical in a way other driver types struggle to match"; "Generally slower to respond to transients than balanced armatures or planars".
- BA: "Respond to signal changes extremely quickly"; "BA bass is typically tighter and faster but less visceral".
- Planar: "Control distortion extremely well"; "Can also be more power-hungry than BA or dynamic models".
- Hybrid: "The challenge is crossover design"; "A poorly designed crossover creates audible dips, peaks, or phase issues".
Supports: second source for driver trade-offs. Confidence medium (manufacturer blog, but consistent with headphones.com).

### campfire_tips (primer)
URL: https://www.campfireaudio.com/blogs/insights/iem-ear-tips-fit-seal-guide (verbatim via fetch)
- "the gap between a sealed and unsealed fit can be 10 dB or more in the sub-bass region."
- "A properly inserted foam tip blocks 15 to 25 dB of ambient noise, compared to 10 to 15 dB for silicone."
- Silicone tips last 6 to 12 months; foam 1 to 3 months (paraphrase).
Supports: seal matters, ear_tip_options facet, passive isolation numbers, foam versus silicone trade-off. Confidence medium (one manufacturer; RTINGS FR consistency page agrees on the mechanism).

### headphones_com_open_closed (primer)
URL: https://headphones.com/pages/open-vs-closed-headphones (verbatim via fetch)
- Open: cannot "minimize the music leaking out of the earpieces - which will happen with open-back headphones"; "The feeling of openness afforded by an open headphone is noticeable even when no music is playing"
- Closed: "isolate you from external ambient noise to varying degrees"; mentions the "closed-back dip" as a tuning challenge.
Supports: noise_handling split, open versus sealed trade-off. Confidence high.

### razer_anc (primer)
URL: https://www.razer.com/technology/hybrid-anc-active-noise-cancelling (verbatim via fetch)
- Feedforward: "an externally facing microphone that is positioned towards the environment around you"; Feedback: "an internally facing microphone that is positioned closer to your ears"; hybrid combines both.
Supports: ANC families in the mechanism. Confidence medium (manufacturer).

### soundcore_anc (primer)
URL: https://www.soundcore.com/blogs/earbuds/how-does-active-noise-cancelling-work (verbatim via fetch)
- "Low-frequency sound waves are longer, more consistent, and predictable, giving the ANC processor ample time to analyze them and generate an accurate anti-noise wave."
- "High-frequency sounds are shorter and more abrupt, changing too quickly for even the fastest processors to counteract perfectly."
- Feedforward mics "on the outside of the earbud cup"; feedback mics "inside the earbud cup, near the speaker".
Supports: second source for ANC families and the low-frequency limit. Confidence medium (manufacturer; agrees with RTINGS).

### soundguys_anc_205 (specialist_review)
URL: https://www.soundguys.com/you-can-have-the-best-anc-headphones-with-one-simple-trick-139427/ (verbatim via fetch)
- "We've tested 205 noise canceling products in the last couple years"
- "There's a very slight positive (but incredibly weak) relationship between ANC performance and price."; "price is a terrible way to predict ANC performance on its own."
- Unsealed designs: Galaxy Buds3 "unsealed, and offer very little effective ANC"; AirPods 4 "destroy about 54% of outside noise"; "there are no contemporary ANC headphones that score less than 5 here".
Supports: judgement that price does not predict ANC; that seal (passive isolation) is the precondition for ANC. Confidence high.

### soundguys_how_we_test (specialist_review)
URL: https://www.soundguys.com/how-we-test/ (verbatim via fetch)
- "we use a Bruel & Kjaer 5128 head simulator"; "the pink line represents our headphone preference curve, which we believe is the response shape most people will like the sound of."
- Isolation: "we first play a sample of shaped noise over loudspeakers and record with our artificial head. Then, we put the headphones on the head and record again."
- Battery: "We set the product to play back music peaking at 75dB(SPL), a safe and realistic listening level."
Supports: that reviewers measure against a target (their own house curve, not Harman), how ANC is measured, that battery claims depend on volume. Confidence high.

### crinacle_ief_system (enthusiast_community)
URL: https://crinacle.com/2021/06/04/the-tonal-technical-dichotomy-the-ief-evaluation-system/ (verbatim via fetch)
- "tonality is derived from the word 'tone', hence it is a metric primarily based within the frequency domain."
- "'Technicalities', 'technical ability', or 'technical performance' is an umbrella term that encompass basically all non-tonal aspects of sound reproduction."
- "tonality may be the most easily measurable aspect of audio but the interpretation of the measurements is probably one of the most subjective aspects."
Supports: vocabulary (tonality, technicalities); judgement that the graph shows the flavour, not the quality. Confidence high.

### cringraph_readme (enthusiast_community)
URL: https://raw.githubusercontent.com/mlochbaum/CrinGraph/master/README.md (verbatim via fetch)
- "CrinGraph is the tool which allows readers to compare measurements against each other, and save easily readable images to share around the internet."
- Graph window with "Standard logarithmic frequency (Hz) and sound pressure level (dB) axes", target curves, normalization; "the code here can be used freely by anyone".
Supports: the frequency-response-against-target visual grammar and that squig.link sites are built on it. Confidence high.

### shokz_bone_conduction (primer)
URL: https://shokz.com/blogs/guides/the-technical-difficulties-of-bone-conduction-headphones (verbatim via fetch)
- "Bone conduction headphones often struggle to produce deep, rich bass."
- "the earphone core vibrates, causing the surrounding air to vibrate and form sound waves that leak."
- "the vibrations that are transmitted through the temporal bones can cause some discomfort, and even headaches, after extended periods of time."
- "the open-ear design allows users to maintain situational awareness since the ear canal remains unobstructed."
Supports: open-ear family trade-offs. Confidence medium (manufacturer admitting limits).

### soundguys_bone_vs_open (specialist_review)
URL: https://www.soundguys.com/bone-conduction-vs-open-earbuds-160735/ (verbatim via fetch)
- Bone conduction: "bass is very quiet, so kick drums and bass lines barely register"; "why they're the go-to for swimmers"; "the only option in this whole category that works for certain kinds of hearing loss".
- Open earbuds: "they leak enough sound that the person beside you on the bus can hear what you're listening to"; sound "considerably better than bone conduction".
Supports: second source for open-ear trade-offs; form_factor values. Confidence high.

### Not reachable (recorded so nobody repeats the attempt)
- r/headphones wiki and r/HeadphoneAdvice: reddit cannot be fetched by this tool.
- Head-Fi: returned a security challenge page.
- Audio Science Review: not fetched directly; search summary says they use the GRAS 45CA-10 and measure three placements per channel. Not cited.
- Sony product pages (sony.com, sony.co.uk, electronics.sony.com): 403; the help-guide PDF on helpguide.sony.net worked and carries the full spec table.
- Audio-Technica codec explainer and ATH-M50x page: 403. The M50x figures (38 Ω, 99 dB/mW, 285 g) come from the search summary only and are not used as evidence in the JSON.

## Against the existing add-on schema

Current add-on (`machines/electronics/add-ons/headphone_earphone.jsonc`): form_factor, headset_type, anc_present, single_charge_hours, total_battery_with_case_hours, codec_support. Standards above it: connectivity.connection_types, bluetooth_version, power.battery_life_hours, compatibility, ingress_protection, sizing.weight, high_signal_markdown. Classification uses AGENTS.md's four kinds. Proposals only; nothing under data/ was edited.

| Facet in the doc | In add-on or standard today | Proposed kind | Note |
|---|---|---|---|
| form_factor | yes, add-on form_factor | hard filter (vertical anchor for this leaf) | Keep. Enum already matches the doc's values. |
| noise_handling | partly: anc_present | derived, not a field | Compute from anc_present + form_factor + title words. The add-on lacks any open-back/closed-back signal; add `enclosure: enum [open_back, closed_back, semi_open, not_applicable, unspecified]` as a hard filter for over-ears and on-ears, since it is a literal fit-for-purpose fact buyers filter by and it is in nearly every wired over-ear title. |
| connection | yes, electronics-standard connection_types | hard filter | Keep. A `wireless_dongle_2_4ghz` value is missing from connection_types; gaming headsets need it. |
| tuning_vs_target | no | quality currency, researched | Candidate for the one number this cohort is compared on, but only if we commit to backfilling it from a measurement database for the top N. If we would not, it is not currency; then it lives in high_signal_markdown as "measured close to Harman target" when a review says so. |
| anc_attenuation | no | researched annotation | Not a schema field: fill rate would be tiny. Jev markdown material ("RTINGS: 18 dB bass attenuation"). |
| battery_life | yes, add-on single_charge_hours and total_battery_with_case_hours, plus power.battery_life_hours in the standard | quality currency | Keep the add-on pair; the standard's battery_life_hours duplicates single_charge_hours for this leaf. Add a condition flag `battery_hours_condition: enum [anc_on, anc_off, unspecified]` or the number is not comparable across brands. |
| latency | no | derived class, not a field | Class is free from connection + codec. Measured ms is Jev material. |
| multipoint | no | hard filter (boolean) | Add `multipoint: boolean | null`. Buyers filter on it literally; on spec pages for anything mid-range. |
| ip_rating | yes, electronics-standard ingress_protection | hard filter | Keep. |
| wired_fallback | partly: connection_types includes audio_3_5mm | hard filter | A 3.5 mm value in connection_types covers the cable-input half. Power-off playback is Jev material from the manual. |
| serviceability | no | researched annotation | Jev material plus iFixit for the top N. Replaceable pads could be a boolean on wired over-ears but fill rate is low; leave in markdown. |
| codec_support | yes, add-on codec_support | hard filter for Android buyers (LDAC/aptX), annotation otherwise | Keep. Add `lc3` is already present; fine. |
| driver_type | no | annotation; sizing-adjacent | For in-ear monitors a `driver_config_raw: string` ("1DD+4BA") is cheap and on every IEM spec sheet. For wireless products it is almost always dynamic, so a full enum would be mostly one value. Leave as Jev material unless the IEM sub-catalogue grows. |
| impedance | no | quality currency for wired only | Add `impedance_ohm: number | null` and `sensitivity_db: number | null` plus `sensitivity_basis: enum [db_per_mw, db_per_v, unspecified]`. These are on every wired spec sheet, dense for the wired cohort, and the dB/V derived metric needs the basis. Null for true wireless. |
| sensitivity | no | quality currency for wired only | See impedance. |
| ear_tip_options | no | sizing-adjacent annotation | `ear_tip_sizes_included: number | null` fits the sizing rule (bought-by-number, in the box). Low priority. |

Add-on fields the doc considers noise: none of the six are noise. `headset_type` is a use-case enum the doc did not need as a facet because `connection` (dongle) and the microphone question cover it; keep it only if call quality becomes a facet. The standard's `power.battery_capacity_mah` is noise for this leaf (nobody compares headphones by mAh); `wattage` likewise.

What the scraper should check: the listing_field_hints in category.json rest on Apple, Sony and Sennheiser spec pages because every retail listing fetch was blocked. Compare against actual fill rates for form_factor, anc_present, single_charge_hours and codec_support before trusting the "on the listing" claims for multipoint and ip_rating.

### sennheiser_mtw4 (spec_sheet) — added after the blind check
URL: https://us.sennheiser-hearing.com/products/momentum-true-wireless-4 (verbatim via fetch)
- "Up to 7.5 hours of playtime, extendable to 30 hours with the charging case, and quick charging for an hour of playtime in just 8 minutes."
- No codec list and no IP rating on the page; Bluetooth 5.4 and Auracast via firmware.
Supports: second true wireless battery figure alongside Apple's 8/24. Confidence high.

Correction to the Sony WH-1000XM5 note above: the battery table has three codec rows, not two. SBC is 28 h with cancelling on and 38 off, alongside AAC 30/40 and LDAC 26/36. Cancelling costs 10 h on each codec.

## Blind check fixes (2026-10-19)

Applied to category.json: (1) mechanism now separates Harman's 31-model listening test from the two 400-headphone price surveys (Breebaart 2017; Olive 2018); (2) the connection cold-start question now asks where the sound comes from and whether delay matters, instead of cable-or-wireless; (3) uncited range bounds dropped or replaced: anc_attenuation and battery_life keep only a typical sentence, latency is 13 to 369 ms (RTINGS cable, SoundGuys AAC), impedance is 16 to 470 ohms (Sony passive, Momentum 4 active); (4) the 23 ms dongle and 160 ms Bluetooth floor are labelled single measurements in the latency and connection facets and in the latency judgement; (5) ear tips use the source's "sub-bass region"; (6) the bass trade-off uses Olive's "may alienate many listeners"; (7) the Sony table is described as six figures across three codecs; (8) multipoint, impedance and sensitivity reclassified as researched with low backfill cost, driver_type as derived, codec_support kept as listing with the justification in its backfill note, and the listing judgement now states that every listing label rests on three manufacturer pages plus existing add-on fields; (9) the CrinGraph reference URL now matches the raw README that was fetched.

Not changed: wired_fallback stays "listing" because the cable-input half is already a connection_types value (audio_3_5mm) in the electronics standard; the power-off half is marked manual-only in its note. Tradeoff on delay still says "the only ways under 40 ms" because the aptX Low Latency 32 ms figure is a spec from two sources, not a single measurement.

Also applied, by the same rule though not flagged: sensitivity and tuning_vs_target had uncited min/max bounds (90 to 115 dB; -6 to 12 dB). Both now carry only a typical sentence built from cited figures.

## Branches (2026-10-19)

Each branch is a claim about how an expert narrows the choice after one answer. The sentence it follows from is quoted from category.json, with the field named, or from these notes.

### Split 1, wearing style (form_factor)

1. wired in-ear, true wireless in-ear: then ask ear_tip_options. From splits[0].rationale, "tips only for in-ears", and ear_tip_options.listing_field_hint, "Only applies to in-ear and true wireless."
2. open-ear (clip or hook), bone conduction: then ask ip_rating; noise_handling, anc_attenuation and ear_tip_options stop mattering. From noise_handling.backfill_note, "If the form factor is open-ear or bone conduction, or the title says open-back, it is open" (so question 2 is already answered); the "Noise reduction by band" grammar, "open designs show as near zero"; ear_tip_options.backfill_note, "Leave blank for over-ears and open-ear."; the mechanism family "Bone conduction and open-ear", typical for "Running, cycling and swimming", with ip_rating.why_experts_care, "It separates gym and rain use from desk use."

No branch for over-ear, on-ear or neckband: the next question is the same (noise handling) and the register says nothing that drops a facet for them.

### Split 2, outside noise (noise_handling)

3. active noise cancelling: then ask anc_attenuation, battery_life. From anc_attenuation.why_experts_care, "Listings say noise cancelling as if it were one thing. It is not"; the anc_attenuation against battery_life trade-off, "Cancelling costs battery, 10 hours of 40 on Sony's flagship"; battery_life.definition, "Hours of playback on one charge with noise cancelling on".
4. sealed without cancelling, open (hears everything): anc_attenuation stops mattering. From anc_attenuation.definition, measured "with cancelling on"; anc_attenuation.backfill_note, "Backfill the top N cancelling products ... For the long tail, show the noise_handling class instead of a number"; splits[1].rationale, "cancelling is wasted at a quiet desk".

### Split 3, connection

5. true wireless with charging case: then ask battery_life, serviceability, multipoint, codec_support; impedance, sensitivity, driver_type and wired_fallback stop mattering. From connection.why_experts_care, "Wireless brings battery life, codec, multipoint and a glued-in battery that ages"; the connection against serviceability trade-off, "True wireless earbuds are the most convenient form ever sold and the shortest-lived"; impedance.backfill_note, "Leave blank for true wireless; it does not apply"; sensitivity.backfill_note, "Leave blank for true wireless"; driver_type.listing_field_hint, "Wireless listings give only a driver size in millimetres, which means dynamic"; wired_fallback.why_experts_care, "Most true wireless earbuds and some premium over-ears have no input at all."
6. Bluetooth headphones: then ask battery_life, multipoint, wired_fallback, serviceability, codec_support; impedance, sensitivity and driver_type stop mattering. From connection.why_experts_care as above; summary, "check battery hours, repairability and cable playback"; mechanism, "impedance and sensitivity matter only there" (wired); driver_type.listing_field_hint as above.
7. 2.4 GHz dongle: then ask latency, battery_life; impedance, sensitivity and driver_type stop mattering. From splits[2].rationale, "A gaming or video answer routes to 2.4 GHz dongles and wired"; latency.listing_field_hint, "wired or dongle is low"; connection.why_experts_care, "Wireless brings battery life"; mechanism and driver_type hint as in 6.
8. wired 3.5 mm, wired USB-C or Lightning: then ask impedance, sensitivity, driver_type; battery_life, codec_support, multipoint, wired_fallback and latency stop mattering. From connection.why_experts_care, "Wired brings impedance and sensitivity and never runs out"; splits[2].rationale, "wired brings impedance and the amplifier question"; codec_support.definition, formats "the headphone can receive over Bluetooth"; multipoint sits in the list of what "Wireless brings"; wired_fallback.definition, "Whether a wireless headphone has an analogue input"; latency.listing_field_hint, "wired or dongle is low"; driver_type.listing_field_hint, "Spec sheets of wired headphones and in-ear monitors state the type and count".

Not branched: tuning_vs_target and ip_rating apply on every path (ip_rating is only promoted on the open-ear branch). multipoint and codec_support are left alone on the dongle branch because the register does not say whether dongle headsets also pair over Bluetooth.

## Trimmed detail (2026-10-19)

Sentences cut from category.json to meet the validator's word caps. The field named is where each came from. No number, name or claim in the register changed; these are the parts that now live only here.

- mechanism.plain: the two surveys of more than 400 headphones were "by Breebaart in 2017 and by Olive's team in 2018". Also cut, no fact: "The ear-side design then decides what else you hear."
- splits[0].rationale (form_factor): wearing style cuts the catalogue into "three or four pools"; "bone conduction only to open-ear" sat alongside tips-to-in-ears and open-back-to-over-ears.
- splits[2].rationale (connection): "Most of the catalogue is now wireless"; the dongle routing was justified "since ordinary Bluetooth measured 160 to 370 ms of delay in the two labs we read" (RTINGS and SoundGuys; the figures stay in the latency facet and the latency judgement).
- battery_life.why_experts_care: the codec glosses, AAC "the everyday" codec, LDAC "Sony's higher-bitrate" codec, SBC "the baseline" codec.
- latency.why_experts_care: SBC is "the baseline codec every Bluetooth product has"; "Audeze's own figure for it was 25 ms" (the Maxwell over its dongle; rtings_latency_research above quotes Audeze at 24.85 ms); "The codec only helps when the phone supports it too; otherwise the link falls back to SBC" (still stated in codec_support.definition).
- codec_support.why_experts_care: 990 kbps LDAC "is not the high-resolution audio it is marketed as, and many listeners will not hear the difference" (SoundGuys; still in judgements[1] and in the Hi-Res Audio Wireless vocabulary entry); LC3 runs "under LE Audio"; Auracast is "a broadcast from one source to any number of listeners" (still in vocabulary).
- judgements[7].note: the blocked retailers were "Best Buy, Crutchfield, B&H, Adorama, Walmart, Target and Newegg", which "returned 403, 404, 500 or a captcha"; the listing hints rest on spec pages, "not on observed retail product tables".
- judgements[8].note: RTINGS's 260 ms average is measured "with its own rig".
- references: olive_2022_acoustics_today supports[0] dropped "of listeners"; feasycom_codecs supports[0] shortened "aptX Adaptive" to "Adaptive" and "up to" to "max"; sony_wh1000xm5_helpguide supports[0] was reordered. No figure changed.
- summary, splits[1].rationale, tuning_vs_target and impedance why_experts_care, and judgements[1] and [6] notes were shortened by rewording only; no fact, number or source was cut.

## Corrections log

### 2026-10-19: branches, questions, caps

- Branches added: 8 (form_factor 2, noise_handling 2, connection 4). Each is traced to a register sentence under "Branches" above.
- Questions rewritten: 12 fields, the 3 split questions and 9 facet cold_start_questions, all now at most 12 words with no preamble and the answers offered where there are two to four. ip_rating's question already met the rule and was kept verbatim. Each split question and its facet's cold_start_question carry the same text.
- Fields trimmed to cap: 17 (summary, mechanism.plain, 3 split rationales, 5 why_experts_care, 4 judgement notes, 3 reference supports lines). Cut facts recorded under "Trimmed detail (2026-10-19)".
- generated_at, facets, judgements, references, evidence ids and priority scores untouched. Validator clean; category.md and viewer.html regenerated.