The Seed syllable āḥ is associated with Amoghasiddhi and the karma family. It is the long ā with the visarga or aspiration mark. The pronunciation of the visarga is usually described as a soft echo of the vowel. As well as appearing on it's own āḥ occurs in the set of syllables: oṃ āḥ hūṃ.
Other bījas: a | dhīḥ | hūṃ | hrīḥ | maiṃ | oṃ | tāṃ | traṃ | phaṭ | svāhā |
Examples

āḥ in Siddhaṃ

āḥ in Siddhaṃ
alternate form
āḥ in Tibetan
Uchen script
āḥ in Lantsa script
oṃ āḥ hūṃ
The syllables oṃ āḥ hūṃ as a set are associated with body, speech and mind.
Siddhaṃ
Tibetan - Uchen
Lantsa