xu	CONT E:-'.JT S
CHAPTER			PAGE
2.	Line Microphone: Useful Directivity on the Line
	Axis. Line with Progressive Delay ............			324
3.	Line Microphone: Useful Directivity on the Line
	Axis. Two Lines and a Pressure Gradient Ele­
	ment .......................................			325
4.	Ultradirectional	Microphone ..................		327
8.7 THROAT MICROPHONE .•..		••••.••.••••.•..•••••....	•••••	329

8.8LAPEL, LAVALIER AND BOOM MICROPHONES ••..•••.•.•.••• 329

8.9	HOT-WIRE MICROPHONE •....	••.•....	•.••.......	•••...	• 330
8.10	RADIO MICROPHONE .•••.•.....	•••....	•.•.•...	•.•••.....	331

8.11DIRECTIONAL EFFICIENCY OF A SOUND COLLECTING SYSTEM 331

8.12WIND EXCITATION AND SCREENING OF MICROPHONES •••••• 332

8.13	NONLINEAR DISTORTION IN MICROPHONES •........	••.....	333
8.14	TRANSIENT RESPONSE OF MICROPHONES ...••.•.•....	••••.	334
8.15	NOISE IN A SOUND PICKUP SYSTEM ••••....••••.••.••••••		335
	A. Ambient Noise in the Studio ......................		335

B.Noise Due to Thermal Agitation of the Air Molecules 335

C.Noise Due to Thermal Agitation of the Atoms in the

	Vibrating		System		................................						336
	D. Noise Due to Thermal Agitation of the Electrons in the
	Conductor		.......................................								336
	E. Noise Due to Barkhausen Effect in the Transformer									..	337
	F. Noise in the Vacuum Tube ........................										337
	G. Noise Due to Thermal Agitation of the Electrons in the
	Plate Resistor ....................................										337
	H. Example of Noise in a Sound Pickup System ......									,.	337
8.16	SHAPES OF MICROPHONES					•.•.•.•.•••..•••...		•.•••..	••..	•	339
IX. MISCELLANEOUS TRANSDUCERS
9.1	INTRODUCTION		•.•••••.•.•••••....				••••.•...	•.••....	•..	•.	340
9.2	TELEPHONE RECEIVERS				.......	••......	••.•....	••......		••	340
	A. Magnetic Telephone Receiver ......................										340
	B. Crystal Telephone Receiver ........................										346
	C. Dynamic Telephone					Receiver ......................					347
	D. Inductor Telephone					Receiver ......................					348
9.3	PHONOGRAPHS		.• • • • • • • • • . • • • • . • • • • • • • • . • • • • • • • . . .						• • • ••		350
	A. Recording		Systems			...............................					351
	1.	Recorders ...................................									351
	2.	Lateral		Cutter		...............................					351
	3.	Vertical Cutter				...............................					355
	4.	Recording Characteristics					.....................				355
	5.	Heated		Stylus		...............................					357
	B. Reproducing			Systems .............................							357
	1.	Record		Player		...............................					357
		a. Electrical Record Player .......						. . . . . .	. . .	..	357
		b. Mechanical				Phonograph .................					358
	2.	Phonograph			Pickups .........................						359
		a. Crystal			Pickup ........................					•.	359

XIV

CHAPTER

CONTENTS

PAGE

	C. Headphone-Type Noise Reducer ....................									417
	D. Electronic Vibration Reducer ......................									418
9.19	NOISE REDUCTION			CIRCUITS		.•.••••.•....		•..•....	••••.••	420
X. MEASUREMENTS
10.1	INTRODUCTION		.•.•.....		••..	•......	•..•...........		••••••	423
10.2	CALIBRATION OF MICROPHONES						..•..••.........	•••...•.••		423
	A. Response		Frequency Characteristic .................							423
	1.	Pressure		Response ...........................						423
		a. Pistonphone				.............................				424
		b. Thermophone ...........................								425
		c. Electrostatic				Actuator ....................				426
		d. Reciprocity				.............................				427
	2.	Field	Response ..............................							428
		a. Rayleigh Disk ...........................								429
		b. Reciprocity				.............................				430
	3.	Secondary Calibration ........................								433
	4.	Artificial		Voice ..............................						433
	5.	Artificial		Throat		.............................				433
	B. Directional Characteristic						.........................			434
	C. Nonlinear Distortion Characteristic .................									435
	D. Phase Distortion				Characteristic ....................					436
	E. Electrical Impedance Frequency Characteristic ......									437
	F. Transient Response Characteristic ..................									437
	G. Measurement of Wind Response . . . .							. . . . . . . .	. . . . . . ..	437
10.3	TESTING	OF LOUDSPEAKERS				•..	••••••••..	•••.••••.•••.•••		438
	A. Response		Frequency Characteristic .................							438
	1.	Pressure		Response ...........................						438
	2.	Apparatus for Measuring the Sound Pressure Fre­
		quency Relationship of a Sound Source							........	439
	3.	Calibration of the Sound Measuring Equipment ..								444
	4. Free Field Sound Room ......................									445
	5.	Outdoor		Response		............................				450
	6.	Small and Partially Deadened Room ... .							. . . . . . ..	450	•
	7. Arrangement of Loudspeakers for Test .........									451
	8.	Living Room Measurements ...................								451
	9.	Theater		Measurements .......................						451
	10.	Automobile			Measurements ....................					451
	B. Directional Characteristic						.........................			452
	C. Nonlinear		Distortion Characteristic .................							452
	D. Efficiency		Frequency Characteristic					................		460
	1. Direct Determination of Radiated Power								.......	461
	2.	Indirect Determination of Radiated Power ......								463
	E. Phase Distortion				Characteristic ....................					464
	F. Electrical Impedance Frequency Characteristic ......									464
	G. Transient Response Characteristic ..................									465
	H. Subjective Measurements						.........................			466
	1.	Loudspeaker			Environment ....................					466
	2.	Loudspeaker Housing, Placement and Mounting .								466