Distortion in Music and Speech…. The Good, Bad and the Ugly Marshall Chasin, AuD., M.Sc., Reg. CASLPO, Aud(C) Musicians’ Clinics of Canada

Distortion- good,bad, or ugly… Definition of distortion: – The creation of other energy that was not there in the original signal. A linear system has no distortion A non-linear system has distortion

Distortion- good,bad, or ugly… Linear: – F(a + b) = F(a) + F(b)

Distortion- good,bad, or ugly… Non-linear: – F(a + b) = F(a) + F(b) + other stuff …

Distortion- good,bad, or ugly… “Other stuff” – Harmonic energy (multiples of the signal primary) = harmonic distortion – Intermodulation distortion = multiples of the signal and also combinations of the two or more primaries (adding, subtracting…)

No Distortion ( perfect sinusoid)

Odd Order Distortion… (symmetrical)

Odd and Even Distortion… (asymmetrical)

Distortion- good,bad, or ugly… Limited bandwidth (eg. telephone) will limit distortion. Why? … the “other stuff” does not get transmitted. So… distortion in the 1970s was probably less noticeable than distortion of 2005…

Example of harmonic distortion and bandwidth… 25 dB amplify/15% normalization/LP 3400 passive

Spirit in the Sky 1970s vs Fuzz box, tube amplifier, and finger picking… Can’t quite remember the original reason for the distortion…

Norman Greenbaum

Rocket ‘88 Malfunctioning amplification stage (tube) in Sam Philips’ studio 1951 (Sun Records) Turned up other 3 stages to compensate and worked well, except at high levels… Non-existent “bass line”… rock and roll…

Types of distortion in music Tequilla induced distortion – 1/3 tequilla and 2/3 grapefruit juice. Peak clipping – Square wave (odd multiples of primary) – Tube distortion (depends on the tube) Phase distortion – Can be almost anything – … ugly

Music and Distortion Intentional tube distortion. – Screamers Overloading the amplifier at high levels. – Rocket ‘88 Highly distorting amplifier at all levels. – Spirit in the Sky

Music and Distortion Peak input limiting level being too low for high inputs of modern music. This was not an issue in the 1950s and 1960s since the bandwidth of hearing aids was very limited (up to 3800 Hz) so distortion was not heard… today it is heard! Modern telephones have 25% distortion but not heard since limited bandwidth ( Hz).

Speech and Distortion The normal cochlea is a highly non-linear (distorting) transducer. (Cubic) distortion products are quite dominant. – Especially 2f 1 -f 2 An aging cochlea is “more linear”…

Speech and Distortion A-C Lindblad (1987) “Influence of nonlinear distortion on speech intelligibility” KTH, TA116. – Cubic distortion is better than quadratic or other degrees of distortion. – For severe and profoundly hard of hearing people, a cubically distorting amplifier was better than a linear one…. … WHY?

Speech and Distortion 2f 1 – f 2: In speech, many sounds have resonances. One type is the “quarter wavelength resonator” (… same as ear mold tubing) Odd numbered multiples of the fundamental (BTE aids: 1000 Hz, 3000 Hz, 5000 Hz,…)

BTE and tubing resonances…

Speech and Distortion In speech, the vowel [a] ‘father’, and the nasals [m], [n], and [ng] are also quarter wavelength resonators…

Vowel [a] (‘father’)

2f 1 – f 2 calculations … If first resonance is at 500 Hz, and second is at 1500 Hz, and…., … then 2f 1 – f 2 is at –500 Hz… well, its actually the “absolute value” so its 500 Hz. Cubic distortions can enhance lower frequency sounds for the hard of hearing person.

Vowel [u] (‘booze’)

2f 1 – f 2 calculations… If first resonance is at 400 Hz, and second resonance is at 700 Hz… … then 2f 1 – f 2 is at 100 Hz. Cubic distortions, can generate new low frequency sound energy for the hard of hearing person.

(Cubic) Distortions Audibility of non-resonant speech sounds can also be helped by distortions. … [s] has energy only above 3500 Hz, but inter-modulation distortions can generate lower frequency cues that are audible.

[s] No energy below 3500 Hz

Distortion and Ageing A presbycusic cochlea is more linear (poorer vascular supply- ie., its stiffer) Fewer distortion products Less “additional” information … Poorer speech discrimination than “expected”

Summary-Distortion and Speech Can enhance low frequency sound energy for hard of hearing people. Can create additional low frequency sound energy for hard of hearing people. Peak clipping can be useful to create these distortions… Too much can be as bad as too little…

Summary-Distortion and Music Low frequency energy as a result of high level sounds … rock and roll… Distortion = 1970s … after all, the 1970s needed something after the 1960s! Lead Zeplin