I=Rezlv=(taP.@2/2c)V2 (46.18) Most transducers are displacement limited, so for a direct-radiating transducer Vin Eq. (4618)is limited. To obtain the most output power the piston should have the largest possible surface area consistent with keeping the transducer omnidirectional(the transducer will become directional when a 2n). This is easy to do in air but difficult in water since it is hard to make pistons which are both lightweight and stiff enough to hold their shape in water. Alternatively, the driver can be placed at the apex of a horn. For a conical horn, the fluid velocity at the end of the horn( where the radius is a)will be reduced to v(ala) but the radiating piston will now hav an effective radius of a, so the radiated power will increase by a factor of (a /a)2. For high-power operation at a single frequency, the driver can be placed at the end of a quarter wave resonator. 46.7 Directivity It is often desirable for transducers to be directional. Directional sound sources are needed in diagnostic and therapeutic medical ultrasonics, for acoustic depth sounders; and to reduce the power requirements and reverberation in active sonars, etc. Directional microphones are useful to reduce unwanted noise(e.g, to pick up the voice of a speaker and not the audience ); directional hydrophones or hydrophone arrays increase signal to-noise and aid in target localization. One way to achieve directionality is to make the radiating surface large a baffled circular piston has a directivity given by D,= 2),(ka sine)/ka sin 0 (46.19) D equals unity for 0=0 and 1/2 when ka sin 0=2. 2. For small values of ka, D is near unity for all angles Some transducers respond to the gradient of the acoustic pressure rather than pressure, for example, the ribbon microphone which works by detecting the motion of a thin conducting strip orthogonal to a magnetic ield. Such transducers have a directivity which is dipole in nature, i.e D=cos日 (46.20) Note that since the force in this case is proportional not to Po but to kPo, a ribbon microphone(which like a moving coil microphone is electrodynamic) will have flat receiving sensitivity when its impedance is mass controlled. By combining a dipole receiver with a monopole receiver one obtains a unidirectional cardioid receiver with (46.21) Defining Terms Electroacoustics: Concerned with the transduction of acoustical to electrical energy and vice versa. Microphones: Devices which convert acoustical signals into electrical signals Related Topic References J.A. Bucaro, H D. Dardy, and E.F. Carome,"Fiber optic hydrophone,J. Acoust. Soc. Am., vol. 62,P. 1302, 1977. R J. Bobber, New types of transducer, " in Underwater Acoustics and Signal Processing, L. Bjorno(Ed. ),Dor drecht, Holland: D. Riedel. 1981 R ]. Bobber, Underwater Electroacoustic Measurements, Washington, D. C. Government Printing Office, 1969© 2000 by CRC Press LLC p = ReZr*V* 2 = (pa4r0w2/2c)V2 (46.18) Most transducers are displacement limited, so for a direct-radiating transducer V in Eq. (46.18) is limited. To obtain the most output power the piston should have the largest possible surface area consistent with keeping the transducer omnidirectional (the transducer will become directional when a ³ l). This is easy to do in air but difficult in water since it is hard to make pistons which are both lightweight and stiff enough to hold their shape in water. Alternatively, the driver can be placed at the apex of a horn. For a conical horn, the fluid velocity at the end of the horn (where the radius is ae) will be reduced to V(a/ae) but the radiating piston will now have an effective radius of ae so the radiated power will increase by a factor of (ae/a)2 . For high-power operation at a single frequency, the driver can be placed at the end of a quarter wave resonator. 46.7 Directivity It is often desirable for transducers to be directional. Directional sound sources are needed in diagnostic and therapeutic medical ultrasonics, for acoustic depth sounders; and to reduce the power requirements and reverberation in active sonars, etc. Directional microphones are useful to reduce unwanted noise (e.g., to pick up the voice of a speaker and not the audience); directional hydrophones or hydrophone arrays increase signal￾to-noise and aid in target localization. One way to achieve directionality is to make the radiating surface large. A baffled circular piston has a directivity given by De = 2J1(ka sinq)/ka sin q (46.19) De equals unity for q = 0 and 1/2 when ka sin q = 2.2. For small values of ka, De is near unity for all angles. Some transducers respond to the gradient of the acoustic pressure rather than pressure, for example, the ribbon microphone which works by detecting the motion of a thin conducting strip orthogonal to a magnetic field. Such transducers have a directivity which is dipole in nature, i.e., De = cos q (46.20) Note that since the force in this case is proportional not to P0 but to kP0, a ribbon microphone (which like a moving coil microphone is electrodynamic) will have flat receiving sensitivity when its impedance is mass controlled. By combining a dipole receiver with a monopole receiver one obtains a unidirectional cardioid receiver with De = (1 + cos q) (46.21) Defining Terms Electroacoustics: Concerned with the transduction of acoustical to electrical energy and vice versa. Microphones: Devices which convert acoustical signals into electrical signals. Related Topic 49.1 Introduction References J.A. Bucaro, H.D. Dardy, and E.F. Carome, “Fiber optic hydrophone,” J. Acoust. Soc. Am., vol. 62, p. 1302, 1977. R.J. Bobber, “New types of transducer,” in Underwater Acoustics and Signal Processing, L. Bjorno (Ed.), Dor￾drecht, Holland: D. Riedel, 1981. R.J. Bobber, Underwater Electroacoustic Measurements, Washington, D.C.: Government Printing Office, 1969