Quarterly Progress and Status Report. An ionophone for acoustical measurements

Transcription

1 Dept. for Speech, Music and Hearing Quarterly Progress and Status Report An ionophone for acoustical measurements Fransson, F. journal: STL-QPSR volume: 3 number: 4 year: 1962 pages:

2

3

4 Fig. I1-1. a. The S.T.L. Ionophone generator. 6V6G - - Mod voltage a a 6V6G -- - Osc. for rect~f~er f~lament I - - Sound source Fig. I1-2. a. Test of an open pipe. b. Test of an oboe Fig b. Oscillogram of the Ionophone current. Mod. frequency 1000 c/s.

5 The electrical data for the iron gap working in free air was approximately: Udc-2.5 kv Idc-,I2 md ImodJu2 mb pertaining to free air testing of open pipes. In experiments with closed pipes and musical instruments: In some cases considerably lower values have been employed. The frequency of the HF-oscillator was 160 kc/s. Two types of airgap are shown in Fig, , Measurements The properties of musical instruments are to a great extent depending upon their resonance frequencies and bandwidths. For a flutc, for instance, the second resonance frequency should bo near to twice the first allowing a number of tones in the second octave to be produced by means of overblowing with the same fingering as ccrresponding tones in tho fi~st octave, The facility of blowing and the flexibility of a tone depend to some extent upon the bandwidth. The bandwidth for an oboe is greater than the corresponding bandwidth for a flute but the significance of this difference is not well understood. As an introduction to the investigation of different wind instruments the resonance frequencies and bandwidths of some simple pipes and some wood wind instruments have been measured by means of the ionopho~and some few cxamples are given below. The resonance frequency for a pipe can be written: where f = the resonance fruquency with the number n, f, = the n frequency of the first resonance, k a constant (for a circ. pipe 0 open at both ends: k = 1 and for a circ. pipe closed at one end 0 ko = 2) and A is the deviation factor. O) '1 depends to a great extent on the radiation impedance of the pipe,

6 The following empirical expressions for the relative frequency deviation A and the bandwidth B have been found to be useful. and b B=b +f'k2 0 Curves of A and B and some recordings with a gliding sine wave are shown in Figs to As seen from Fig (pipe L*), where the source is placed at 2.3 cm from the closed end, a zero is found at about 3800 c/s. This pipe can be treated as one short 1/4 wavelength (h) pipe and one longer open pipe. The resonance frequency of the 1/4 A pipe is = 3740 c/s corresponding to the zero on the recording. A similar condition. exists for the flute where the 1/4h the embrochure. pipe is between the cork and The conical pipe in Fig is something between an open and closed pipe, i.e, the ccnstant k is neither 1 or 2. 0 The flute No. 1 is an old system instrument of box-wood. An attach- ment with electrodes for the excitation was fixad at the embrouchure. This attachment consists of a plaster-moulding of the lips of a, flute player. As seen from the recording a zero is found at about 6500 c/s corresponding to the 1/4A pipe at the cork. The curve for the bandwidth B seems to be fairly continuous but A varies ayound a mean value. The old system oboe No. 1 is made of cocus-wood and the oboe-reed is substituted for a small glass tube with the electrodes. Even here the bandwidth curve is fairly continuous buta varies considerably more than for the flute, These variations in A depe~d upon the cavities at the closed holes and keys (3),

7 Resonance number x102 c/s a. Deviation factor A and bandwidth B. Fig. I1-3. Cylindrical pipe L1. b. Frequency response for sine wave (closed pipe). c. Frequency response for sine wave (open pipe).

8 RESONANCE NUMBER L 15 16n RESONANCE NUMBER Fig. I1-4. Cylindrical pipe La closed at one end. a. Deviation factor A and bandwidth B. b. Frequency response for sine wave. Fig. I1-5. Conical pipe closed at narrow end. a. Deviation factor A and bandwidth B. b. Frequency response for sine wave.

9 RESONANCE NUMBER " L n RESONANCE NUMBER Fig Flute No. I. a. Deviation factor A and bandwidth B. b. Frequency response for sine wave. Fig. I1-7. Oboe No. 1. a. Deviation factor a and bandwidth B. b. Freauency response for sine wave.

10 Pipe I B c/s I! I I b k0 fl C/S kl k2 b toc 1 : Cyl. L, closed :: i2 I d 2 Conlcal Flute I,d' Oboe 1.b" / *10-~ *~ ~ I ~ I *I LA.- In Tablo 11-leare shown the results of these measurements of the pipes and instruments. Acknowledgsments A large dept of gratitude is due to Professor Torbern Laurent, the Head of the Division of Telegraphy-Telephony, and Docent Gunnar Fant for granting the facilities of the Speech Transmission Laboratory. The author wishes to acknowledge the guidance and helpful criticism of Docent Fant and valuable assistance of civ.ing. G. Garpendahl in designing the Ionophone generator. F. Fransson References: (1 ) Backus, J. : "Vibrations of the Reed and Air Column in the Clarinet", J.Acoust.Soc.Am., 2 (I 961 )? pp (2) Benado, A.H. : "On Woodwind Instrument BoresN, J.Acoust. Soc.Jim., 31 (1959), PP (3) Benade, A.H.: "3n the Mathematical Thvory of Woodwind Finger H3lesIt, J.Acoust.Soc.Am., 32 (1 960), pp (4) Fant, G.: uacoustic Analysis and Synthesis of Spcech with Applications to Swedisht1, (19591, PP Ericsson Technics, 2, No. 1 (5) Fant, G.: Acoustic Theory of Speech Production, Mouton & Co., ' s-gravenhage 1 960, 323 pp,

11 (6) Igarashi, J. and Masaru Koyasu: llacoustical Properties of Trumpets", J.Acoust.Soc.Am., 3 (1 953), pp (7) Klein, S. r "Un Nouveau Traducteur Electrostatistique: L' I~nophone~~, Bcustica, 4 (1 954), pp (8) ltionovac, super&onic Speaker", Electronic Industries April 1957, pp. 77, (9) Martin, D.W.: "Lip Vibrations in a Cornet MouthpieceI1, J.Acoust.Soc.Am,, 9, (1 942), pp (10) Meyer, J.: 13Unharmonische Komponenten im Klang der labialen Orgelpfeifen1!, Mitteilung aus der Physikalisch- Technischen Bundesanstalt, Braunschweig 1962 (1 1 ) Meyer, J. : IJ?J"ber die Stirnrnung von Klarinet ton", Mitteilung aus der Physikalisch-Technischen Bundesanstalt, Braunschweig 1962 (1 2) Tombs, D.M. : "Corona Wind LoudspeakerI1, Nature ondo don), a, No (19551, P a 923