Short answer: the Optimal Audio Cuboid 15 on measured sound quality, the Optimal Audio Cuboid 5 on price. The Optimal Audio Cuboid 15 ranks #16 of 32 surface-mount and background music speakers, rated 3.0/5, at $1,035. The Optimal Audio Cuboid 5 ranks #26, rated 2.5/5, at $110, $925 cheaper.
The Optimal Audio Cuboid 15 is the more accurate, deeper-bass choice. A larger member of Optimal Audio’s Cuboid installation range, with a 15-inch woofer that reaches 45.0Hz at -3dB in vendor measurements, deep for an installation box, and a cabinet matching the smaller Optimal Audio Cuboid 5. Its measured preference score, a predictor of how neutral it sounds, is 1.94 against 0.27 for the Optimal Audio Cuboid 5. Its bass reaches about 45Hz before falling 3dB, against 122Hz for the Optimal Audio Cuboid 5. The tradeoff: Directivity is uneven (4.54dB variation) and the on-axis response is uneven (0.84 band deviation), so it scores 1.94 on its own and 3.71 with an ideal subwoofer.
Why pick the Optimal Audio Cuboid 5: A compact installation speaker from Optimal Audio with a 5” coaxial driver. Watch for: Bass is very limited, so it needs a subwoofer, and the on-axis response is uneven (1.15 average deviation).
Buy the Optimal Audio Cuboid 15 if sound quality is the priority over the price, or the Optimal Audio Cuboid 5 if the price is the priority over sound quality.
Looking for alternatives? See the complete PA & Live Sound rankings.
Specifications Side By Side
| Optimal Audio Cuboid 15 | Optimal Audio Cuboid 5 | |
|---|---|---|
| Price | $1,035 each Passive | $110 each Passive |
| Ranked | #16 of 32 in Surface-Mount and Background Music Speakers | #26 of 32 in Surface-Mount and Background Music Speakers |
| Passive | Yes | Yes |
| Bluetooth | No | No |
| Drivers | 15” woofer, 2-way | 5” coaxial driver |
| Bass Extension (-3dB) | 45.0Hz (measured, manufacturer data via spinorama.org) | 122.0Hz (measured, manufacturer data via spinorama.org) |
| Bass Extension (-10dB) | about 30.9Hz (derived, log-interpolated between spinorama.org -9dB/-12dB points) | about 91.5Hz (measured, manufacturer data via spinorama.org) |
| Spinorama Preference Score | 1.94 (spinorama.org) | 0.27 |
| Spinorama Preference Score with Perfect Sub | 3.71 (spinorama.org) | 3.94 |
| DIY Kit | No | No |
| Maximum SPL | - | 111dB peak (manufacturer specified) |
| Weight | - | 4.7 lbs (2.15 kg) |
Frequency Response Compared
Both speakers’ spinorama measurements, overlaid. A flatter line is more neutral: peaks sound forward or harsh, dips sound recessed. Each curve is shifted so its average from 300 Hz to 3 kHz sits at 0 dB, so loudness differences between the two are removed and only the shape is compared.
Measurement data: Optimal Audio Cuboid 15 (measured by the manufacturer) and Optimal Audio Cuboid 5 (measured by the manufacturer), via spinorama.org.
Related Speaker Comparisons
How Optimal Audio Cuboid 15 compares with other leading options:
- Optimal Audio Cuboid 10 vs Optimal Audio Cuboid 15
- Optimal Audio Cuboid 8 vs Optimal Audio Cuboid 15
- Optimal Audio Cuboid 12 vs Optimal Audio Cuboid 15
- Optimal Audio Cuboid 15 vs Optimal Audio Cuboid 3
- Optimal Audio Cuboid 12 vs Optimal Audio Cuboid 5
Prices on this page were last checked October 14, 2026 at 2:38 PM UTC. Product prices and availability are accurate as of that date/time and are subject to change; the price and availability shown on Amazon at the time of purchase will apply to the purchase of this product.
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The Two Speakers In Full
Optimal Audio Cuboid 15: $1,035 each Passive eBay
Pros: A larger member of Optimal Audio’s Cuboid installation range, with a 15-inch woofer that reaches 45.0Hz at -3dB in vendor measurements, deep for an installation box, and a cabinet matching the smaller Optimal Audio Cuboid 5. Measured bass extension of 30.9Hz (-10dB).
Cons: Directivity is uneven (4.54dB variation) and the on-axis response is uneven (0.84 band deviation), so it scores 1.94 on its own and 3.71 with an ideal subwoofer. It is sold through Allen & Heath and Optimal Audio installers with no US retail listing.
Full Optimal Audio Cuboid 15 review and specifications →
| Bass Extension (-10dB) | about 30.9Hz (derived, log-interpolated between spinorama.org -9dB/-12dB points) |
|---|---|
| Bass Extension (-3dB) | 45.0Hz (measured, manufacturer data via spinorama.org) |
| Bluetooth | No |
| DIY Kit | No |
| Drivers | 15” woofer, 2-way |
| Passive | Yes |
| Spinorama Preference Score | 1.94 (spinorama.org) |
| Spinorama Preference Score with Perfect Sub | 3.71 (spinorama.org) |
- Reviews, Comparisons and Sound Samples
- Head-to-Head Comparisons
Optimal Audio Cuboid 5: $110 each Passive eBay
Same family: the Optimal Audio Cuboid 3 (smaller model, ranked lower, rated 2.3/5).
Pros: A compact installation speaker from Optimal Audio with a 5” coaxial driver. Optimal Audio’s measurements show a preference score of 0.27 that reaches 3.94 with an ideal subwoofer, with 0.74 listening window deviation.
Cons: Bass is very limited, so it needs a subwoofer, and the on-axis response is uneven (1.15 average deviation). That data is low quality and comes from Optimal Audio. Bass reaches 122.0Hz (-3dB) and about 92Hz (-10dB). That data is low quality.
Full Optimal Audio Cuboid 5 review and specifications →
| Released | Oct 2021 (5 years ago) |
|---|---|
| Bass Extension (-10dB) | about 91.5Hz (measured, manufacturer data via spinorama.org) |
| Bass Extension (-3dB) | 122.0Hz (measured, manufacturer data via spinorama.org) |
| Bluetooth | No |
| DIY Kit | No |
| Drivers | 5” coaxial driver |
| Maximum SPL | 111dB peak (manufacturer specified) |
| Passive | Yes |
| Spinorama Preference Score | 0.27 |
| Spinorama Preference Score with Perfect Sub | 3.94 |
| Weight | 4.7 lbs (2.15 kg) |
- Reviews, Comparisons and Sound Samples
- Head-to-Head Comparisons