Evaluation of Studio Microphone Performance Using Time Delay Spectrometry Techniques*

EVALUATION OF STUDIO MICROPHONE PERFORMANCE USING TIME DELAY SPECTROMETRY TECHNIQUES* by Philip White, Bruel&Kjaer Naerum, Denmark Introduction Microphones which are used for related to the performance of the mi- 1. A Time Delay Spectrometry mea- recording music and speech are of- crophone. Correct interpretation of surement system including spe- ten shrouded in mystery in terms of anomalies in the data uncover phe- cial hardware for microphone their electroacoustic performance nomena such as reflections, stand- testing is described. This system and are, to a large extent, judged by ing waves in and around the micro- should be of particular interest to professional users by their "sound" phone cartridge, and grid and hous- microphone designers and major only. In most instances manufactur- ing resonances. Time Delay users such as broadcasting ers' data consist of typical specifica- Spectrometry techniques also shed corporations. tions for a particular microphone much light on the all-important off- ^ , , , ,..,..... n2. The properties and hardware of a type and may include characteristics axis behaviour. M K . , . , flat coherent sound source for such as typical frequency response .\T . , .... microphone testing are de- curves with acceptable tolerances, When measurements are carried , K, y scribed directional characteristics, sensitiv- out on directional microphones, spe- ity, inherent noise values, etc. Such cial attention has to be paid to the 3. JQQ mappings are discussed. data do not necessarily yield enough sound field. In many anechoic cham- information about the performance bers it may still prove difficult to es- 4. Correlation between the objec- of the microphone in an actual re- tablish a proper free field which is tive measurement domain and cording situation. free of pressure-gradient errors. The the subjective "listening" domain applicability of Time Delay Spec- is discussed and illustrated with The Time Delay Spectrometry trometry has been investigated, and measurements on the test (TDS) measurement method, which even in highly reflective environ- microphones. has been in use for several years for ments, a sound field without gradient loudspeaker and monitoring-room errors can be simulated. TDS analysis opens up a number measurements, is also an excellent of possibilities for editing and dis- tool for the design and evaluation of This Application Note discusses playing both time and frequency do- studio microphones. TDS techniques the evaluation of studio microphone main information. This displayed yield a comprehensive set of data performance using TDS techniques data lends itself well to interpreta- which includes amplitude and phase and presents measurements of the tion and can be related to phenome- responses, energy time curves characteristics of five studio micro- na that one can hear in the subjec- (ETCs) and various directional char- phone types. More specifically: tive domain. acteristics. This information can be Conventional Mapping of Studio Microphone Characteristics Frequency Response graph does not reveal the extent to frequencies. The resulting polar The frequency response of a stu- which the energy in the frequency chart shows the response of the mi- dio microphone is normally mea- band is coherently transduced with crophone as a function of angle of sured in an anechoic chamber using respect to time, i.e. no allowance is incidence. At each fixed frequency sinusoidal test signals. The response made for reflections, resonances the response is usually normalized to at each frequency is averaged over a and diffraction effects in and around OdB at 0° angle of incidence. sufficient length of time for the mi- the microphone. crophone to reach "steady state". Implication: No information about Polar Response the behaviour of the microphone at Implication: In a conventional fre- Measurements of the directional frequencies other than the fixed fre quency response graph, the concept characteristics of a microphone are quencies is obtained. Any irregular- of time and delay is obscured. The normally carried out at a few fixed ities in frequency response are lost. * Based on a paper presented at the 72nd convention of the Audio Engineering Society Oct. 23-27, 1982. 3 Mapping of Microphone Characteristics using TDS Fig. 1. Configuration of TDS system for measurement of microphone characteristics Time Delay Spectrometry tech- (100VRMS) Power Amplifier Type sound source. The transmitting fre- niques offer several features which 2713. quency response of this microphone, are particularly beneficial for micro- in conjunction with de-emphasis fil- phone testing: 2. The microphone under test is ter WB0504, is shown in Fig.2. The connected to a special de- usable frequency range is from ap- 1. The sine sweep, in conjunction emphasis filter WB0504 proximately 700 Hz to 40 kHz. (Using with a narrow band tracking filter, (-12dB/oct.) from which the sig- One-inch Condenser Microphone yields a very high signal-to-noise nal is fed into the front end of the Type 4145, a frequency range of ap- ratio (the test signal can be as TDS system (Heterodyne Analyz- proximately 400 Hz to 16 kHz is pos- much as 40 dB below the level of er Type 2010). sible.) It should be noted that it is the the broad-band ambient noise). excellent ambient noise rejection The Band Pass Filter Type 1617 properties of the TDS system which 2. The analysis time is no longer was not used in the measurements enable condenser microphones to than the duration of the excitation decribed herein. be used as transmitters at relatively signal (typically 0,5 to 2,0 low frequencies where the transmit- seconds). The Sound Source121 ted SPL approaches the threshold of A Bruel&Kjaer Condenser Micro- hearing (at a distance of 1 meter). 3. Very flexible time-domain editing phone Type 4133 was used as the possibilities (segmentation). Additional features of a general TDS system and a thorough descrip tion of TDS techniques are given in B&K Type 5842 Product Data 129- 81 W. Configuration of the Measurement System The measurement system is shown in Fig. 1. Special features of the sys tem are: 1. The sound source. A B&K mea surement microphone Type 4133 with 0° incidence free field re sponse from 4 Hz to 40kHz ±2dB was used as the transmitter Fig.2. Transmitting frequency response of B&K Condenser Microphone Type 4133 in conjunc- This is driven by a high voltage tion with de-emphasis filter WB 0504 4 TDS Mapping Possibilities A general response measurement on an electroacoustical transducer such as a studio microphone can be depicted as shown in Fig. 3. The dashed line in Fig. 3a shows the test signal which is a fast, linear sine sweep. The response of the trans ducer is the curve shown inside the Time Window, T, of the measure ment. The Time Window is set using the TDS system by choosing an ap propriate sweeprate, S, and band width, B, of the sweeping filter. T is then given by the relationship T = B/S. B and S are chosen such that: 1. T is longer than the duration of the impulse response of the mi crophone under test. 2. T is of short enough duration to exclude room reflections. Having fixed T, the frequency res olution and lower limiting frequency are given by Af = 1/T. Various mapping possibilities can be seen by looking at the response shown in Fig. 3a from three different directions: 1. Fig. 3b: Energy as a function of time (Energy Time Curve). 2. Figs. 3c and 3d: Amplitude and phase as a function of frequency (Complex Frequency Response). Fig.3. Various mapping possibilities using TDS techniques 3. Fig.3e: Frequency as a function of time (smearing)[3]. The "ideal" versions of these curves are shown in Fig. 4. Note that the typical response curves shown in Fig. 3 are irregular (less than ideal): 1. The ETC has a finite width and is jagged (Fig. 3b). 2. The amplitude response (Fig. 3c) is not flat. The phase response (Fig. 3d) is not a straight line which passes through the origin. 3. A certain amount of frequency versus time smearing is exhibited (Fig.3e). Smearing is not dis cussed in this Application Note. Fig. 4. "Ideal" response curves of an electroacoustic transducer 5 Correlation Between the Subjective and Objective Domains of Microphone Performance There is most certainly no simple such response data can be mapped mapping possibilities that TDS tech- and direct link between the objective to physically identifiable processes niques offer enable relations be- and subjective domains of micro- or phenomena such as housing reso- tween the objective and subjective phone performance. In the subjec- nances or reflections. We can then domains to be identified more easily. tive domain we listen to the sound begin to correlate these processes image and this is judged individually with the subjective impressions we in the following sections micro- and subjectively: "We each hear associate with the "sound" of a mi- phone performance is depicted in what we hear". When presented with crophone. For example, we may be terms of the various TDS mappings a hard-copy measurement, however, able to identify particular reflections available, and illustrations are given we are given a set of objective data (indicated by peaks in the Energy of TDS measurements on five differ- which is not as open to free and indi- Time Curve) with a harsh colouration ent studio microphone types. vidual interpretation. Anomalies in of the sound image. The various Wide-band Energy Time Curves - A Simple, Overall Description of Microphone Performance The Energy Time Curve (ETC) is an "clean" sound but one which more, the wide bandwidth of the ideal starting point in the evaluation lacks definition of transients and microphone ensures proper tonal of electroacoustic transducers. The the ability to separate different balance between a wide range of ETC yields a straightforward indica- instruments in terms of timbre instruments.

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