Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Sound Loudness vs Pitch Misconception

  1. Jun 2, 2018 #1
    Hello everybody.
    I am involved in a project aiming to address Physics Misconceptions regarding Sound through the creation of virtual musical instruments.
    One of the major student misconceptions I have found in bibliography is the fact that students perceive sound pitch and loudness as being the same. As I try to formulate educational activities aiming to address these misconceptions, I came by M. Allen's "Misconceptions in Primary Science" which states:
    "Sounds have different pitches because they have different wavelengths and sounds have differing loudness because they have different amplitudes".

    Such a statement is totally wrong from my point of view.
    Pitch depends only on frequency. Loudness on the other hand is an empirical measure of the intensity of sound, ergo, the energy of sound waves arriving at a 1m^2 area per second.
    The energy of sound waves is proportional to the square of the amplitude times the square of the frequency.
    This implies that varying the frequency of a sound while keping the amplitude constant (e.g in a tone generator) we expect to see variations in the sound intensity (confirmed).

    What is your opinion on this?
    Thank you
     
  2. jcsd
  3. Jun 3, 2018 #2

    symbolipoint

    User Avatar
    Homework Helper
    Education Advisor
    Gold Member

    This is sufficient because it is precise and clear enough:

    Pitch is how high or low the tone or sound. Loudness is how big the sound. This is all easy enough to understand.

    You could certainly use a musical instrument to illustrate the meanings. You can also use human voice.

    You probably understand the details of the concept better than I do so you would seem to not have any trouble instructing your students, as accordingly,
     
  4. Jun 3, 2018 #3

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Actually, the quote is correct.

    Frequency and wavelength are connected. One can either say the sound depends on frequency, or on wavelength because the speed of the "wave" for a particular instrument or medium is fixed. So once a wavelength or frequency is established, the other is also known. This means that saying that the pitch depends on wavelength is the same as saying that it depends on frequency. Now, the exactly relationship may not be the same, i.e. increasing pitch means increasing frequency but decreasing wavelength (there is inverse relationship), but nevertheless, pitch depends on frequency and wavelength.

    The amplitude of oscillation determines the "energy" in the wave, and thus, the loudness of the sound wave. One can easily see this if you have a transducer connected to an oscilloscope. Increase the volume of the sound, and the amplitude of the signal you see on the scope display increases in height.

    http://www.phys.uconn.edu/~gibson/Notes/Section2_1/Sec2_1.htm

    This is not a topic on education or pedagogy, but rather on the physics itself. I'll alert the Mentors to have it moved to the appropriate forum.

    Zz.
     
    Last edited: Jun 3, 2018
  5. Jun 3, 2018 #4

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    They are connected by the formula (definition) c=fλ but the one which cannot vary, once the sound (or any wave) has been generated, is the frequency. You can hear a sound under water - say a tone played on a loudspeaker. It will have the same frequency when you take your head out of the water but the speed of sound in water is five times (about 1500m/s) that of of sound in air (about 330m/s. Consequently, the wavelength in water will be proportionately greater (about five times greater). Even in air, the speed of sound can vary by around ten per cent from arctic conditions (0°C) to desert conditions (45°C) but the pitch of the sounds on your radio will be identical.

    On the other hand, the pitch of your flute (a wind instrument) can change as the speed of sound in the air (inside your flute) can be different in the desert. Once the sound has been formed, the frequency that you hear (even when standing in a fridge) will not have changed.

    People often think that frequency and wavelength can be used almost interchangeably but there are often specific times which are more suitable for one than the other.

    I am just waiting for someone to mention the Doppler effect. :wink:
     
  6. Jun 3, 2018 #5

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    On the possible relationship between loudness and pitch, there are many musical instruments that actually change their pitch at high amplitudes because the simple linear relationships that govern their pitch only apply for low amplitudes and you no longer have Simple Harmonic Motion.

    You have to remember that even a simple pendulum does not swing with equal period when it swings by more than a few degrees. I am always aware of a weird change of pitch when a church organ is going full blast. This is really noticeable when an organ plays with an orchestra.
     
  7. Jun 3, 2018 #6

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Let's not turn this into more than what it is. The OP clearly has described the scenario involved in what is being asked, i.e. the sound being transmitted. It is not about the mechanism that created the sound or what is going on within the instrument.

    Once the sound is created in air, then the velocity of sound is often regarded as a constant. Thus, defining f also defines λ.

    Again, look at the sound signal that is being picked up. Increasing the pitch increase the frequency and decreases the wavelength. It really doesn't matter how it was created or what medium it had gone through. The problem is simple. It doesn't require all that complications.

    Zz.
     
  8. Jun 3, 2018 #7

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Good idea - as long as we realise f doesn't change on a path through a variety of c values but that wavelength does.
     
  9. Jun 4, 2018 #8
    I think maybe the direction of the OP's question was missed in some of these responses after #2 (or maybe just a side discussion?)? I don't think he's questioning frequency versus wavelength. This is how I took it...

    My take on this is that the OP is mixing/matching two different things, then saying they conflict. As I understand it:

    "Pitch depends only on frequency." - I'd say "True" (you can get into some 'angels/dancing/pin' details here, since pitch is generally considered perception of frequency, not frequency itself, and things can always get complicated and the perception can possibly deviate from the physical thing, but I don't think OP is questioning that.

    "sounds have differing loudness because they have different amplitudes": I'd say this is also True.

    Now, OP seems to see a conflict, because a high frequency at amplitude X has more energy than a low frequency at amplitude X. But I see that as something separate. It still holds true that loudness is a factor of amplitude. Like anything dealing with perception, you can say it isn't a complete statement. Our perception of loudness varies with frequency, with the environment (other sounds - 'masking'), and relative loudness of different frequencies varies with loudness itself (the Fletcher-Munson curves).

    Just because it isn't complete, doesn't make it "totally wrong", IMO.

    In simple terms, if I increase/decrease the amplitude of a sound, all else being equal, and the sound is in the range of human perception, the sound will be perceived as louder/softer. That is true at any frequency (again, with the person's hearing range).
     
  10. Jun 4, 2018 #9

    256bits

    User Avatar
    Gold Member

  11. Jun 5, 2018 #10
    wave theory

    a specific wave length has a specific frequency in the realm of sound is a vibration heard as tone, or pitch..

    the amplitude or peak to peak displacement of the frequency is heard as loudness.....

    the energy of various vibration frequencies varies predictably with their frequency.. given their amplitudes are all the same.

    in electronics, in optics.... a photon has wavelength, frequency and energy.. they are three different numbers that describe the same thing...

    becareful when discusing the wavelength, frequency, and energy of a phonon

    it takes many phonons distributed over one square meter in one second to derive the "energy" as you describe it.......

    I would think that what you describe as "the energy" is actually sound pressure...

    strive to understand that wavelength, frequency, and energy are three numbers describing the same thing...

    be careful as to what you call energy..... this word can mean different things to different people...

    think phonons, like photons.... photons are packets of electomagnetic vibrating energy... phonons are packets of phisically vibrating energy..

    then go to number of packets arriving on a specific area within a given time....

    The pressue on that area.....can be describe in different ways if you use the concept of "energy" then you must call it power or watts per square meter.. and then adding time it becomes watt seconds - like KWH killowatthours... the first is power, the later ie energy

    K1RSU
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted