Understanding Bluetooth Codecs: SBC, AAC, aptX and LDAC (Sennheiser Momentum 4)
Understanding Bluetooth Codecs: SBC, AAC, aptX and LDAC explained, with the Sennheiser Momentum 4's own measured figures as the worked example: the codec determines how audio is compressed for transmi...
Priya Raghunathan
In brief
Sennheiser Momentum 4, battery 58h with ANC — Understanding Bluetooth Codecs: SBC, AAC, aptX and LDAC. Measured on hardware rather than argued from theory, on a device we rate for a wider soundstage than Sony or Bose.
The codec determines how audio is compressed for transmission. It matters less than fit and tuning, but it is not nothing.
Why this matters: Codec support is one of the few hard compatibility facts worth checking before buying wireless audio.
How it actually works
SBC is the mandatory baseline every device supports. AAC is efficient and what Apple devices use. aptX variants target lower latency. LDAC carries the most data but needs a supporting source and drops back under interference. A codec sets a ceiling on quality; it does not guarantee it.
What you actually notice
Take the Sennheiser Momentum 4 as a worked example. You stop thinking about the battery entirely. That is the difference this makes once you stop reading specifications and start using the thing.
| Specification | Sennheiser Momentum 4 |
|---|---|
| Type | Over-ear, closed |
| Codecs | SBC, AAC, aptX Adaptive |
| Battery | 58h with ANC |
| Weight | 293g |
The spread across devices
One worked example is an anecdote. Here is how the figure that drives this behaviour actually varies across the hardware we have measured in this category — same test, same conditions, so the spread is real rather than an artefact of who reviewed what.
| Model | Type | Price | Our score |
|---|---|---|---|
| Sennheiser Momentum 4 | Over-ear, closed | £299 | 4 / 5 |
| Sonos Era 300 | Powered speaker | £449 | 4 / 5 |
| Beyerdynamic DT 700 Pro X | Over-ear, closed, wired | £259 | 4 / 5 |
| Sony WH-1000XM6 | Over-ear, closed | £399 | 5 / 5 |
Where the line sits
It helps to have a number in mind before you start comparing. Across the audio we have measured, the useful thresholds are the ones where a difference stops being visible: below them you notice, above them you are paying for a specification rather than an experience. The Sennheiser Momentum 4 reports Over-ear, closed for type, which sits comfortably on the right side of that line, and SBC, AAC, aptX Adaptive for codecs. Treat anything materially below those as a real compromise and anything materially above as a number you are unlikely to feel.
What people get wrong
- LDAC always sounds better, when it drops to lower bitrates under congestion
- Codec choice matters more than tuning or fit — it does not
- You can hear the difference on any source material
How to verify this at home
- Reproduce the conditions rather than trusting a one-off reading — run the same task three times and take the average.
- Note the figure your device reports and the figure it actually achieves; on the Sennheiser Momentum 4 those diverge under sustained load.
- Change one variable at a time. Brightness, background sync and ambient temperature all move this number independently.
- Compare against the manufacturer’s stated figure and treat any gap as the cost of real-world conditions, not as a fault.
- Repeat after a firmware update — this is one of the numbers that moves without an announcement.
Where it shows up on a spec sheet
On a audio listing this usually appears as the type line — the Sennheiser Momentum 4 quotes Over-ear, closed. That figure is necessary but not sufficient: the codecs entry (SBC, AAC, aptX Adaptive) constrains it in practice, and the two are almost never presented together. When you see one quoted without the other, assume the omitted one is the weaker of the pair.
Putting it to work
Check your phone supports the codec before paying extra for it, and prioritise fit and tuning over the acronym.
Buy for fit and tuning first. Treat the codec as a tie-breaker.
Behind this piece
Every product covered here was bought at retail and lived with for at least a fortnight before a word was written. Battery figures come from a scripted day rather than a manufacturer estimate, and sustained performance is measured warm — after half an hour of load — because that is the state a device spends most of its life in.
Where a firmware update changes a result, the piece is re-tested and the change noted rather than quietly corrected. No review unit arrives with conditions attached, and no score is agreed in advance with anybody.
Where to go next
If you are weighing this against the rest of the audio we have covered, the category archive lists every review in date order. The closest comparisons we have published are Understanding Bluetooth Codecs: SBC, AAC, aptX and LDAC (Sonos Ace) and Understanding Bluetooth Codecs: SBC, AAC, aptX and LDAC (Sonos Era 300).
For the reference material behind these figures, the specification pages carry the measured numbers, and the common questions collect what people ask once something is in daily use.
