This paper presents PathChirp, a new active probing tool for estimating the available bandwidth on a communication network path. Based on the concept of "self-induced congestion," PathChirp features an exponential flight pattern of probes we call a chirp. Packet chips offer several significant advantages over current probing schemes based on packet pairs or packet trains. By rapidly increasing the probing rate within each chirp, PathChirp obtains a rich set of information from which to dynamically estimate the available bandwidth. Since it uses only packet interarrival times for estimation, PathChirp does not require synchronous nor highly stable clocks at the sender and receiver. We test PathChirp with simulations and Internet experiments and find that it provides good estimates of the available bandwidth while using up to an order-of-magnitude fewer bytes than current state-of-the-art techniques.